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PRINCIPLES OF MANURING
MANURES
AND THE
PRINCIPLES OF MANURING
BY
C. M. AIKMAN, M.A., D.Sc., F.R.S.E., F.I.C.
FORMERLY PROFESSOR OF CHEMISTRY, GLASGOW VETERINARY COLLEGE, AND
EXAMINER IN CHEMISTRY, GLASGOW UNIVERSITY; AUTHOR OF 'FARMYARD MANURE,'
ETC.
THIRD IMPRESSION
WILLIAM BLACKWOOD AND SONS EDINBURGH AND LONDON MCMX
D. VAN NOSTRAND COMPANY NEW YORK
_All Rights reserved_
TO
SIR JOHN BENNET LAWES, BART., D.C.L., LL.D., F.R.S.,
OF ROTHAMSTED,
AND
SIR J. HENRY GILBERT, M.A., LL.D., F.R.S.,
FORMERLY SIBTHORPIAN PROFESSOR OF RURAL ECONOMY,
UNIVERSITY OF OXFORD,
WHOSE FAMOUS INVESTIGATIONS DURING THE LAST FIFTY YEARS
HAVE SO LARGELY CONTRIBUTED TO BUILD UP
THE SCIENCE OF MANURING,
THIS WORK,
EMBODYING MANY OF THE ROTHAMSTED RESULTS,
IS DEDICATED.
PREFACE.
When the present work was first undertaken there were but few works in
English dealing with its subject-matter, and hardly any which dealt with
the question of Manuring at any length. During the last few years,
however, owing to the greatly increased interest taken in agricultural
education, the demand for agricultural scientific literature has called
into existence quite a number of new works. Despite this fact, the
author ventures to believe that the gap which the present treatise was
originally designed to fill is still unfilled.
Of the importance of the subject all interested in agriculture are well
aware. It is no exaggeration to say that the introduction of the
practice of artificial manuring has revolutionised modern husbandry.
Indeed, without the aid of artificial manures, arable farming, as at
present carried out, would be impossible. Fifty years ago the practice
may be said to have been unknown; yet so widespread has it now become,
that at the present time the capital invested in the manure trade in
this country alone amounts to millions sterling. It need scarcely be
pointed out, therefore, that a practice in which such vast monetary
interests are involved is worthy of the most careful consideration by
all students of agricultural science, as well as, it may be added, by
political economists.
The aim of the present work is to supply in a concise and popular form
the chief results of recent agricultural research on the question of
soil fertility, and the nature and action of various manures. It makes
no pretence to be an exhaustive treatise on the subject, and only
contains those facts which seem to the author to have an important
bearing on agricultural practice. In the treatment of its subject it may
be said to stand midway between Professor Storer's recently published
elaborate and excellent treatise on 'Agriculture in some of its
Relations to Chemistry'--a work which is to be warmly recommended to all
students of agricultural science, and to which the author would take
this opportunity of acknowledging his indebtedness--and Dr J. M. H.
Munro's admirable little work on 'Soils and Manures.'
In order to render the work as intelligible to the ordinary agricultural
reader as possible, all tabular matter and matter of a more or less
technical nature have been relegated to the Appendices attached to each
chapter.
The author's somewhat wide experience as a University Extension
Lecturer, and as a Lecturer in connection with County Council schemes of
agricultural education, during the last few years, induces him to
believe that the work may be of especial value to those engaged in
teaching agricultural science.
He has to express the deep obligation he is under, in common with all
writers on Agricultural Chemistry, to the classic researches of Sir John
Bennet Lawes, Bart., and Sir J. Henry Gilbert, now in progress for more
than fifty years at Sir John Lawes' Experiment Station at Rothamsted.
His debt of gratitude to these distinguished investigators has been
still further increased by their kindness in permitting him to dedicate
the work to them, and for having been good enough to read portions of
the work in proof. In addition to the free use which has been made
throughout the book of the results of these experiments, the last
chapter contains, in a tabular form, a short epitome of some of the more
important Rothamsted researches on the action of different manures.
To the numerous German and French works on the subject, more especially
to Professor Heiden's encyclopaedic 'Lehrbuch der Duengerlehre' and the
various writings of Dr Emil von Wolff, the author is further much
indebted.
Among English works he would especially mention the assistance he has
derived from the writings of Mr R. Warington, F.R.S., Professor S. W.
Johnson, Professor Armsby, the late Dr Augustus Voelcker, and others. He
would also tender his acknowledgments to the new edition of Stephens'
'Book of the Farm,' and he has to thank its editor, his friend Mr James
Macdonald, Secretary to the Highland and Agricultural Society of
Scotland, for having read parts of his proof-sheets.
It is also his pleasing duty to thank his friends Dr Bernard Dyer, Hon
Secretary of the Society of Public Analysts, Dr A. P. Aitken, Chemist to
the Highland and Agricultural Society of Scotland; Professor Douglas
Gilchrist of Bangor; Mr F. J. Cooke, late of Flitcham; Mr Hermann Voss
of London; and Professor Wright of Glasgow, for having assisted him in
the revision of proof-sheets.
ANALYTICAL LABORATORY,
128 WELLINGTON STREET, GLASGOW,
_January_ 1894.
CONTENTS.
PART I.--HISTORICAL INTRODUCTION.
PAGE
Beginning of agricultural chemistry 4
Early theories regarding plant-growth 4
Van Helmont 4
Digby 6
Duhamel and Stephen Hales 8
Jethro Tull 9
Charles Bonnet's discovery of source of plants' carbon 11
Researches of Priestley, Ingenhousz, Senebier, on assimilation
of carbon 11-12
Publication of first English treatise by Earl Dundonald 13
Publication of Theodore de Saussure, 'Chemical Researches on
Vegetation,' 1804 14
Theories on source of plant-nitrogen 15
Early experiments on this subject 16
Sir Humphry Davy's lectures (1802-1812) 17
State of agricultural chemistry in 1812 17
Beginning of Boussingault's researches (1834) 21
Publication of Liebig's first report to the British Association 24
Refutation of "humus" theory 26
Liebig's mineral theory 26
Liebig's theory of source of plants' nitrogen 27
Publication of Liebig's second report to British Association 30
Liebig's services to agricultural chemistry 31
Development of agricultural research in Germany 32
The Rothamsted Experiment Station 33
Sir J. B. Lawes and Sir J. H. Gilbert, the nature and value of
their experiments 33
Review of the present state of our knowledge of plant-growth 36
Proximate composition of the plant 36
Fixation of carbon by plants 37
Action of light on plant-growth, Dr Siemens' experiments 38
Source of oxygen and hydrogen in the plant 39-40
Source of nitrogen in the plant 40
Relation of the free nitrogen to leguminous plants 42-44
Relation of nitrogen in organic forms, as ammonia salts,
and nitrates to the plant 46-50
Nitrification and its conditions 51
Ash constituents of the plant 53
Methods of research for ascertaining essentialness of ash
constituents of plants 53
(_a_) Artificial soils, (_b_) water-culture 53-55
Method in which plants absorb their food-constituents 55
Endosmosis 55
Retention by soils of plant-food 57
Causes of retention by soils of plant-food 59
Manuring 60
"Field" and "pot" experimentation 60
PART II.--PRINCIPLES OF MANURING.
CHAPTER I.--FERTILITY OF THE SOIL.
What constitutes fertility in a soil 65
I. Physical properties of a soil 66
Kinds of soils 67
Absorptive power for water of soils 67
Absorptive power for water of sand, clay, and humus 68
Fineness of particles of a soil 69
Limit of fineness of soil-particles 69
Importance of retentive power 70
Power of plants for absorbing water from a soil
experiments by Sachs 73
How to increase absorptive power of soils 74
Amount of water in a soil most favourable for plant growth 75
Hygroscopic power of soils 75
Capacity of soils for absorbing and retaining heat 76
Explanation of dew 77
Heat of soils 78
Heat in rotting farmyard manure 78
Causes of heat of fermentation 79
Influence of colour on heat-retaining power 80
Power of soils for absorbing gases 81
Gases found in soils 81
Variation in gas-absorbing power of soils 82
Absorption of nitrogen by soils 82
Requirements of plant-roots in a soil 83
Influence of tillage on number of plants in a certain area 86
Comparison of English and American farming 86
II. Chemical composition of a soil 87
Fertilising ingredients of a soil 87
Importance of _nitrogen, phosphoric acid_, and _potash_ in a
soil 88
Chemical condition of fertilising ingredients in soils 89
Amount of soluble fertilising ingredients in soils 90
Value of chemical analysis of soils 90
III. Biological properties of a soil 92
Bacteria of the soil 92
Recapitulation of Chapter I 96
APPENDIX TO CHAPTER I.
NOTE
I. Table of absorptive power of soil substances by Schuebler 98
II. Table of rate of evaporation of water in different soils
by Schuebler 99
III. Table of hygroscopic power of soils dried at 212 deg. F.
(Davy) 99
IV. Gases present in soil 100
V. Amount of plant-food in soils 100
VI. Chemical composition of the soil 101
VII. Forms in which plant-foods are present in the soil 107
CHAPTER II.--FUNCTIONS PERFORMED BY MANURES.
Etymological meaning of word manure 109
Definition of manures 110
Different classes of manures 111
Action of different classes of manures 113
CHAPTER III.--POSITION OF NITROGEN IN AGRICULTURE.
The Rothamsted experiments and the nitrogen question 115
Different forms in which nitrogen exists in nature 116
Relation of "free" nitrogen to the plant 117
Combined nitrogen in the air 118
Amount of combined nitrogen falling in the rain 119
Nitrogen in the soil 120
Nitrogen in the subsoil 121
Nitrogen of surface-soil 121
Amount of nitrogen in the soil 123
Soils richest in nitrogen 123
Nature of the nitrogen in the soil 124
Organic nitrogen in the soil 125
Differences of surface and subsoil nitrogen 126
Nitrogen as ammonia in soils 127
Amount of ammonia in soils 127
Nitrogen present as nitrates in the soil 128
Position of nitric nitrogen in soil 128
Amount of nitrates in the soil 129
Amount of nitrates in fallow soils 129
Amount of nitrates in cropped soils 130
Amount of nitrates in manured wheat-soils 131
The sources of soil-nitrogen 131
Accumulation of soil-nitrogen under natural conditions 133
Accumulation of nitrogen in pastures 134
Gain of nitrogen with leguminous crops 135
The fixation of "free" nitrogen 136
Influence of manures in increasing soil-nitrogen 136
Sources of loss of nitrogen 137
Loss of nitrates by drainage 137
Prevention of loss of nitrogen by permanent pasture and
"catch-cropping" 138
Other conditions diminishing loss of nitrates 139
Amount of loss of nitrogen by drainage 140
Loss of nitrogen in form of "free" nitrogen 141
Total amount of loss of nitrogen 142
Loss of nitrogen by retrogression 142
Artificial sources of loss of nitrogen 144
Amount of nitrogen removed in crops 144
Losses of nitrogen incurred on the farm 146
Loss in treatment of farmyard manure 146
Nitrogen removed in milk 147
Economics of the nitrogen question 147
Loss of nitrogen-compounds in the arts 148
Loss due to use of gunpowder 148
Loss due to sewage disposal 149
Our artificial nitrogen supply 150
Nitrate of soda and sulphate of ammonia 150
Peruvian guano 151
Bones 151
Other nitrogenous manures 152
Oil-seeds and oilcakes 153
Other imported sources of nitrogen 153
Conclusion 153
APPENDIX TO CHAPTER III.
NOTE
I. Determination of the quantity of nitrogen supplied by
rain, as ammonia and nitric acid, to an acre of land
during one year 155
II. Nitrogen in soils at various depths 156
III. Nitrogen as nitrates in cropped soils receiving no
nitrogenous manures, in lb. per acre (Rothamsted soils) 157
IV. Nitrogen as nitrates in Rothamsted soils 157
V. Examples of increase of nitrogen in Rothamsted soils
laid down in pasture 158
VI. Loss by drainage of nitrates 158
VII. Examples of decrease of nitrogen in Rothamsted soils 159
VIII. Amount of drainage and nitrogen as nitrates in
drainage-water from unmanured bare soil, 20 and 60
inches deep 160
CHAPTER IV.--NITRIFICATION.
Process of nitrification 161
Occurrence of nitrates in the soil 162
Nitre soils of India 162
Saltpetre plantations 163
Cause of nitrification 165
Ferments effecting nitrification 167
Appearance of nitrous organisms 168
Nitric organism 169
Difficulty in isolating them 169
Nitrifying organisms do not require organic matter 169
Conditions favourable for nitrification--
Presence of food-constituents 170
Presence of a salifiable base 171
Only takes place in slightly alkaline solutions 172
Action of gypsum on nitrification 173
Presence of oxygen 173
Temperature 175
Presence of a sufficient quantity of moisture 176
Absence of strong sunlight 176
Nitrifying organisms destroyed by poisons 176
Denitrification 177
Denitrification also effected by bacteria 178
Conditions favourable for denitrification 178
Takes place in water-logged soils 179
Distribution of the nitrifying organisms in the soil 179
Depth down at which they occur 180
Action of plant-roots in promoting nitrification 181
Nature of substances capable of nitrification 181
Rate at which nitrification takes place 183
Nitrification takes place chiefly during summer 183
Process goes on most quickly in fallow fields 184
Laboratory experiments on rate of nitrification 185
Certain portions of soil-nitrogen more easily
nitrifiable than the rest 187
Rate of nitrification deduced from field experiments 187
Quantity of nitrates formed in the soils of fallow fields 188
Position of nitrates depends on season 188
Nitrates in drainage-waters 188
Amount produced at different times of year 189
Nitrification of manures 190
Ammonia salts most easily nitrifiable 191
Sulphate of ammonia the most easily nitrifiable manure 191
Rate of nitrification of other manures 192
Soils best suited for nitrification 192
Absence of nitrification in forest-soils 193
Important bearing of nitrification on agricultural
practice 193
Desirable to have soil covered with vegetation 194
Permanent pasture most economical condition of soil 194
Nitrification and rotation of crops 195
APPENDIX TO CHAPTER IV.
NOTE
I. Old theories of nitrification 196
II. Nitrification takes place in solutions devoid of organic
matter 196
III. Oxidising power of micro-organisms in soils 197
IV. Effect of urine on nitrification in soils 197
V. Solution used by Professor Frankland in cultivating
nitrificative micro-organisms 198
VI. Experiments by Boussingault on rate of nitrification 198
VII. Nitrogen as nitrates in Rothamsted soils after bare
fallow in lb. per acre 198
CHAPTER V.--POSITION OF PHOSPHORIC ACID IN AGRICULTURE.
Occurrence of phosphoric acid in nature 199
Mineral sources of phosphoric acid 200
Apatite and phosphorite 200
Coprolites 201
Occurrence of phosphoric acid in guanos 202
Universal occurrence in common rocks 202
Occurrence in the soil 203
Condition in which phosphoric acid occurs in the soil 203
Occurrence in plants 204
Occurrence in animals 205
Sources of loss of phosphoric acid in agriculture 205
Loss of phosphoric acid by drainage 206
Artificial sources of loss of phosphoric acid 206
Amount of phosphoric acid removed in milk 207
Loss of phosphoric acid in treatment of farmyard manure 208
Loss of phosphoric acid in sewage 208
Sources of artificial gain of phosphoric acid 208
APPENDIX TO CHAPTER V.
NOTE
I. Composition of apatite (Voelcker) 210
II. Percentage of phosphoric acid in the commoner rocks 211
CHAPTER VI.--POSITION OF POTASH IN AGRICULTURE.
Potash of less importance than phosphoric acid 212
Occurrence of potash 213
Felspar and other potash minerals 213
Stassfurt salts 214
Occurrence of saltpetre 215
Occurrence of potash in the soil 215
Potash chiefly in insoluble condition in soils 216
Percentage of potash in plants and plant-ash 216
Occurrence of potash in animal tissue 217
Sources of loss of potash 217
Amount of potash removed in crops 218
Amount of potash removed in milk 218
Potash manures 218
APPENDIX TO CHAPTER VI.
NOTE
I. Amount of potash in different minerals 220
II. Quantity of potash obtained from 1000 lb. of different
kinds of vegetation in the manufacture of potashes 220
PART III.--MANURES.
CHAPTER VII.--FARMYARD MANURE.
Variation in its composition 223
Made up of three classes of constituents 224
_Solid excreta_--
Its nature 224
Difference in composition of the solid excreta of the
different farm animals 224
Causes of this difference 225
Percentage of manurial ingredients in solid excreta
of different animals 226
_Urine_--
Its nature 228
Variation in its composition 229
Causes of this variation 229
Manurial value of the urine of the different farm
animals 230
Percentage of the _organic matter_, _nitrogen_, and
_mineral substances_ in the food, voided in the
solid excreta and urine 232
Comparison of manurial value of total excrements of
the different farm animals 234
Nature of changes undergone by food in process of
digestion 235
_Litter_--
Its uses 236
_Straw_ as litter, and its qualifications 237
Composition of different kinds of straw 238
_Loam_ as litter 239
_Peat_ as litter 240
Comparison of properties of _peat-moss_ and _straw_ 241
The _bracken-fern_ as litter 241
_Dried leaves_ as litter 242
Manures produced by the different animals--
_Horse-manure_--
Amount produced 243
Its nature and composition 243
Amount of straw used for litter 244
Sources of loss on keeping 245
How to prevent loss 245
Use of "fixers," and the nature of their action 245
_Cow-manure_--
Amount produced 248
Its nature and composition 248
Amount of straw used as litter 248
Sources of loss on keeping 249
Advantages of _short dung_ 249
_Pig-manure_--
Amount produced 250
Its nature and composition 250
Amount of straw used as litter 251
_Sheep-manure_--
Amount produced 251
Nature and composition 251
Amount of straw used as litter 252
Methods of calculating amount of manure produced on
the farm 252, note
Fermentation of farmyard manure--
Action of _micro-organic_ life in producing fermentation 255
Two classes of _bacteria_ active in this work, _aerobies_
and _anaerobies_ 255
Conditions influencing fermentation--
_Temperature_ 256
_Openness to the air_ 256
_Dampness_ 257
_Composition of manure_ 257
Products of fermentation 257
Analyses of farmyard manure--
Dr Voelcker's experiments 259
Variation in composition 259
Amounts of _moisture_, _organic matter_
(containing _nitrogen_), and _mineral matter_ 260
Its manurial value compared with _nitrate of soda_,
_sulphate of ammonia_, and _superphosphate_ 260
Comparison of fresh and rotten manure--
The nature and amount of loss sustained in the process of
_rotting_ 261
Ought manure to be applied _fresh_ or _rotten_? 262
Relative merits of _covered_ and _uncovered_ manure-heaps 263
Methods of application of farmyard manure to the field--
Merits and demerits of the different methods 265
Setting it out in _heaps_ 265
Spreading it _broadcast_, and letting it lie 266
Ploughing it in immediately 267
Value and function of farmyard manure--
As a supplier of the necessary elements of plant-food 268
As a "universal" manure 269
Proportion in which _nitrogen_, _phosphoric acid_, and
_potash_ are required by crops 269
Proportion in which they are present in farmyard manure 270
Farmyard manure _poor in nitrogen_ 270
Lawes' and Gilbert's experiments 271
How it may be best reinforced by the use of "artificials" 271
Indirect value of farmyard manure as a supplier of
_humus_ to the soil 273
Its influence on soil-texture 273
Its influence in setting free inert fertilising matter
in the soil 274
Rate at which farmyard manure ought to be applied 275
Lasting nature of farmyard manure 276
Its economic value 276
APPENDIX TO CHAPTER VII.
NOTE
I. Difference in amount of excreta voided for food consumed 279
II. Solid excreta voided by sheep, oxen, and cows 279
III. Urine voided by sheep, oxen, and cows 280
IV. Percentage of food voided in the solid and liquid
excrements 281
V. Pig excrements 281
VI. Manurial constituents in 1000 parts of ordinary foods 282
VII. Analyses of stable-manure, made respectively with
peat-moss litter and wheat-straw 283
VIII. Analyses of bracken 283
IX. Analyses of horse-manure 283
X. The nature of the chemical reactions of ammonia "fixers" 284
XI. Analyses of cow-manure 286
XII. Composition of fresh and rotten farmyard manure 286
XIII. Comparison of fresh and rotten manure 288
XIV. Lord Kinnaird's experiments 289
XV. Drainings of manure-heaps 290
XVI. Amounts of potash and phosphoric acid removed by
rotation from a Prussian morgen (.631 acre) 290
XVII. Composition of farmyard manure (fresh) 291
XVIII. The urine (quantity voided) 291
CHAPTER VIII.--GUANO.
Importance in agriculture 293
Influence on British farming 294
Influence of guano not wholly good 295
Value of guano as a manure 296
Origin and occurrence of guano 297
Variation in composition of different guanos 299
I. Nitrogenous guano--
(_a_) Peruvian guano 300
Different deposits of Peruvian guano 301
Appearance, colour, and nature of Peruvian guano 303
Composition of Peruvian guano 304
(_b_) Other nitrogenous manures: Angamos, Ichaboe 306
II. Phosphatic guanos--
Occurrence of phosphatic guanos 308
Inequality in composition of phosphatic guanos 309
"Dissolved" phosphatic guano 310
"Equalised" or "rectified" guano 311
The action of phosphatic guanos as manures 312
Proportion of fertilising constituents in guano 314
Mode of application of guanos 315
Quantity of guano to be used 317
Adulteration of guano 318
So-called guanos--
Fish-guano 320
Value of fish-guano 322
Meat-meal guano 324
Value of meat-meal guano 324
Bat guano 325
Pigeon and fowl dung 325
APPENDIX TO CHAPTER VIII.
NOTE
I. Peruvian guano imported into United Kingdom, 1865-1893 327
II. Guano deposits of the world 327
III. Composition of concretionary nodules 328
IV. Table showing gradual deterioration of Peruvian guano,
1867-1881 329
V. Composition of different guanos 329
VI. Liebig's theory as to the action of oxalic acid in guano 330
VII. Analyses of dung of fowls, pigeons, ducks, and geese 331
CHAPTER IX.--NITRATE OF SODA.
Amount of exports 332
Date of discovery of nitrate deposits 333
The origin of nitrate deposits 334
Forbes and Darwin on the theory of their origin 335
Source of nitric acid in nitrate of soda 337
Guano theory of origin of nitrate of soda 337
Nitric acid in nitrate of soda probably derived from sea-weed 339
Appearance of nitrate-fields 340
The method of mining the nitrate of soda 341
Composition of _caliche_ 342
Extent of the nitrate deposits 342
Composition and properties of nitrate of soda 343
Nitrate applied as a top-dressing 344
Nitrate of soda encourages deep roots 344
Is nitrate of soda an exhausting manure? 345
Crops for which nitrate of soda is suited 346
Method of application of nitrate of soda 347
Importance of having a sufficiency of other fertilising
constituents 348
Conclusions drawn 349
APPENDIX TO CHAPTER IX.
Total shipments from South America, 1830-1893 351
Total imports into Europe and United Kingdom, 1873-1892 351
CHAPTER X.--SULPHATE OF AMMONIA.
Value of ammonia as a manure 352
Sources of sulphate of ammonia 353
Ammonia from gas-works 353
Other sources 354
Composition, &c., of sulphate of ammonia 355
Application of sulphate of ammonia 356
APPENDIX TO CHAPTER X.
Production of sulphate of ammonia in United Kingdom, 1870-1892 358
CHAPTER XI.--BONES.
Early use of bones 359
Different forms in which bones are used 360
Composition of bones 362
The organic matter of bones 363
The inorganic matter of bones 363
Treatment of bones 364
Action of bones 365
Dissolved bones 368
Crops suited for bones 368
Bone-ash 369
Bone-char or bone-black 369
APPENDIX TO CHAPTER XI.
NOTE
I. Analysis of bone-meal 371
II. Analysis of dissolved bones 371
III. Composition of bone-ash 372
IV. Composition of bone-char 372
CHAPTER XII.--MINERAL PHOSPHATES.
Coprolites 373
Canadian apatite or phosphorite 374
Estremadura or Spanish phosphates 375
Norwegian apatite 376
Charlestown or South Carolina phosphate 376
Belgian phosphate 377
Somme phosphate 378
Florida phosphate 378
Lahn phosphate 379
Bordeaux or French phosphate 379
Algerian phosphate 379
Crust guanos 379
Value of mineral phosphates as manures 380
APPENDIX TO CHAPTER XII.
Imports of phosphates 381
CHAPTER XIII.--SUPERPHOSPHATES.
Discovery of superphosphate by Liebig 382
Manufacture of superphosphate 383
Nature of the reaction taking place 385
Phosphates of lime 385
Reverted phosphate 389
Value of reverted phosphate 391
Composition of superphosphates 391
Action of superphosphates 392
Action of superphosphate sometimes unfavourable 395
Application of superphosphate 395
Value of insoluble phosphates 396
Rate at which superphosphate is applied 397
APPENDIX TO CHAPTER XIII.
NOTE
I. The formulae, and molecular and percentage composition,
of the different phosphates 398
II. Reactions of sulphuric acid and phosphate of lime 398
III. Table for conversion of soluble phosphate into insoluble
phosphate 399
IV. Action of iron and alumina in causing reversion 399
V. Relative trade values of phosphoric acid in different
manures 400
CHAPTER XIV.--THOMAS-PHOSPHATE OR BASIC SLAG.
Its manufacture 401
Not at first used 403
Discovery of its value as a manure 403
Composition of basic slag 404
Processes for preparing slag 406
Solubility of basic slag 408
Darmstadt experiments with basic slag 410
Results of other experiments 413
Soils most suited for slag 414
Rate of application 414
Method of application 416
APPENDIX TO CHAPTER XIV.
Analysis of basic slag 417
CHAPTER XV.--POTASSIC MANURES.
Relative importance 418
Scottish soils supplied with potash 419
Sources of potassic manures 419
Stassfurt potash salts 420
Relative merits of sulphate and muriate of potash 421
Application of potash manures 422
Soils and crops suited for potash manures 423
Rate of application 423
CHAPTER XVI.--MINOR ARTIFICIAL MANURES.
Scutch 427
Shoddy and wool-waste 427
Soot 428
CHAPTER XVII.--SEWAGE AS A MANURE.
Irrigation 431
Effects of continued application of sewage 433
Intermittent irrigation 434
Crops suited for sewage 434
Treatment of sewage by precipitation, &c. 436
Value of sewage sludge 439
CHAPTER XVIII.--LIQUID MANURE 442
CHAPTER XIX.--COMPOSTS.
Farmyard manure a typical compost 446
Other composts 447
CHAPTER XX.--INDIRECT MANURES.
Lime 449
Antiquity of lime as a manure 449
Action of lime 449
Lime a necessary plant-food 450
Lime of abundant occurrence 452
Lime returned to the soil in ordinary agricultural
practice 452
Different forms of lime 453
Caustic lime 453
Lime acts both mechanically and chemically 455
I. Mechanical functions of lime 455
Action on soil's texture 455
Lime renders light soils more cohesive 457
II. Chemical action of lime 457
III. Biological action of lime 459
Action of lime on nitrogenous organic matter 460
Recapitulation 461
CHAPTER XXI.--INDIRECT MANURES--GYPSUM, SALT, ETC.
Gypsum 462
Mode in which gypsum acts 462
Salt 465
Antiquity of the use of salt 465
Nature of its action 465
Salt not a necessary plant-food 466
Can soda replace potash? 466
Salt of universal occurrence 467
Special sources of salt 468
The action of salt 468
Mechanical action on soils 470
Solvent action 470
Best used in small quantities along with manures 472
Affects quality of crop 472
Rate of application 473
CHAPTER XXII.--THE APPLICATION OF MANURES.
Influence of manures in increasing soil-fertility 474
Influence of farmyard manure on the soil 475
Farmyard manure _v._ artificials 476
Farmyard manure not favourable to certain crops 477
Conditions determining the application of artificial manures 477
Nature of the manure 478
Nitrogenous manures 478
Phosphatic manures 480
Potash manures 480
Nature of soil 481
Nature of previous manuring 482
Nature of the crop 483
Amounts of fertilising ingredients removed from the soil by
different crops 484
Capacity of crops for assimilating manures 486
Difference in root-systems of different crops 488
Period of growth 489
Variation in composition of crops 490
Absorption of plant-food 490
Fertilising ingredients lodge in the seed 491
Forms in which nitrogen exists in plants 491
Bearing of above on agricultural practice 492
Influence of excessive manuring of crops 492
CHAPTER XXIII.--MANURING OF THE COMMON FARM CROPS.
Cereals 493
Especially benefited by nitrogenous manures 494
Power of absorbing silicates 494
Barley 495
Period of growth 495
Most suitable soil 496
Farmyard manure not suitable 497
Importance of uniform manuring of barley 497
Norfolk experiments on barley 497
Proportion of grain to straw 498
Wheat 499
Rothamsted experiments 500
Continuous growth 500
Flitcham experiments 500
Oats 501
A very hardy crop 502
Require mixed nitrogenous manuring 502
Arendt's experiments 503
Avenine 503
Quantities of manures 504
Grass 504
Effect of manures on herbage of pastures 505
Influence of farmyard manure 506
Influence of soil and season on pastures 507
Manuring of meadow land 508
Bangor experiments 508
Norfolk experiments 509
Manuring of permanent pastures 509
Roots 510
Influence of manure on composition 512
Nitrogenous manures increase sugar 512
Amount of nitrogen recovered in increase of crop 513
Norfolk experiments 513
Manure for swedes 514
Highland Society's experiments 515
Manuring for rich crops of turnips 516
Experiments by the author on turnips 516
Potatoes 517
Highland Society's experiments 518
The Rothamsted experiments 519
Effect of farmyard manure 520
Manuring of potatoes in Jersey 521
The influence of manure on the composition 521
Leguminous crops 522
Leguminous plants benefit by potash 523
Nitrogenous manures may be hurtful 523
Clover sickness 524
Alternate wheat and bean rotation 524
Beans 525
Manure for beans 525
Relative value of manurial ingredients 526
Gypsum as a bean manure 526
Effect of manure on composition of crop 527
Peas 527
Hops 528
Cabbages 528
APPENDIX TO CHAPTER XXIII.
Experiments on bean-manuring 530
CHAPTER XXIV.--ON THE METHOD OF APPLICATION, AND ON THE MIXING OF MANURES.
Equal distribution of manures 531
Mixing manures 532
Risks of loss in mixtures 533
Loss of ammonia 533
Effects of lime on ammonia 535
Loss of nitric acid 536
Reversion of phosphates 537
Manurial ingredients should be applied separately 538
CHAPTER XXV.--ON THE VALUATION AND ANALYSIS OF MANURES.
Value of chemical analysis 539
Interpretation of chemical analysis 539
Nitrogen 540
Phosphoric acid 541
Importance of mechanical condition of phosphate 542
Potash 542
Other items in the chemical analysis of manures 543
Fertilisers and Feeding Stuffs Act 543
Different methods of valuing manures 544
Unit value of manurial ingredients 544
Intrinsic value of manures 545
Field experiments 545
Educational value of field experiments 547
Value of manures deduced from experiments 548
Value of unexhausted manures 549
Potential fertility of a soil 549
Tables of value of unexhausted manures. 551
APPENDIX TO CHAPTER XXV.
NOTE
I. Factors for calculating compounds from manurial
ingredients 553
II. Units for determining commercial value of manures and
cash prices of manures 554, 555
III. Manurial value of nitrogen and potash in different
substances 556
IV. Comparative manurial value of different forms of
nitrogen and potash 557
V. Lawes' and Gilbert's tables for calculating unexhausted
value of manures 559
CHAPTER XXVI.--THE ROTHAMSTED EXPERIMENTS.
Nature of experiments on crops and manures 561
Soil of Rothamsted 561
Table I. List of Rothamsted field experiments 562
Wheat experiments--
Unmanured plots 562
Wheat grown continuously on same land (unmanured) 562
Table II. Results of first eight years 562
Table III. Results of subsequent forty years 562
Table IV. Wheat grown continuously with farmyard
manure (14 tons per annum) 564
Table V. Wheat grown continuously with artificial
manures 565
Table VI. Experiments on the growth of barley, forty years,
1852-91 566
Table VII. Experiments on the growth of oats, 1869-78 567
Table VIII. Experiments on root crops--Swedish turnips 568, 569
Table IX. Experiments on mangel-wurzel 568, 569
Table X. Experiments with different manures on permanent
meadow-land, thirty-six years, 1856-91 570
Table XI. Experiments on the growth of potatoes--average for
five seasons, 1876-80 571
Table XII. Experiments on growth of potatoes (continued)--
average for twelve seasons, 1881-92 572
* * * * *
INDEX 573
PART I.
HISTORICAL INTRODUCTION
MANURES AND THE PRINCIPLES OF MANURING.
HISTORICAL INTRODUCTION.
Agricultural Chemistry, like most branches of natural science, may be
said to be entirely of modern growth. While it is true we have many old
speculations on the subject, they can scarcely be said to possess much
scientific value. The great questions which had first to be solved by
the agricultural chemist were,--What is the food of plants? and,--What
is the source of that food? The second of these two questions more
easily admitted of answer than the first. The source of plant-food could
only be the atmosphere or the soil. As the composition of the
atmosphere, however, was not discovered till the close of last century,
and the chemistry of the soil is a question which is still requiring
much work ere we shall be in possession of anything like a full
knowledge of it, it will be at once obvious that the very fundamental
conditions for a solution of the question were awanting. The beginning,
then, of a true scientific agricultural chemistry may be said to date
from the brilliant discoveries associated with the names of Priestley,
Scheele, Lavoisier, Cavendish, and Black--that is, towards the close of
last century.
_Early Theories on Source of Plant-food._
While this is so, and while we must regard the early attempts made
towards solving this question as being, for the most part, of little
scientific value, it is not without interest, from the historical point
of view, to glance briefly at some of these old interesting
speculations.
The Aristotelian doctrine, regarding the possibility of dividing matter
into the so-called four primary elements, _fire_, _air_, _earth_, and
_water_, which obtained in one form or another till the birth of modern
chemistry, had naturally an important influence on these early theories.
_Van Helmont's Theory._
Among the earliest and most important attempts made to solve the problem
of plant-growth was that by Jean Baptiste Van Helmont, one of the best
known of the alchemists, who flourished about the beginning of the
seventeenth century. Van Helmont believed that he had proved by a
conclusive experiment that all the products of vegetables were capable
of being generated from water. The details of this classical experiment
were as follows:--
"He took a given weight of dry soil--200 lb.--and into this soil he
planted a willow-tree that weighed 5 lb., and he watered this carefully
from time to time with pure rain-water, taking care to prevent any dust
or dirt falling on to the earth in which the plant grew. He allowed this
to go on growing for five years, and at the end of that period, thinking
his experiment had been conducted sufficiently long, he pulled up his
tree by the roots, shook all the earth off, dried the earth again,
weighed the earth and weighed the plant. He found that the plant now
weighed 169 lb. 3 ounces, whereas the weight of the soil remained very
nearly what it was--about 200 lb. It had only lost 2 ounces in
weight."[1]
The conclusion, therefore, come to by Van Helmont was that the source of
plant-food was _water_.[2]
_Digby's Theory._
Some fifty years later an extremely interesting book was published
bearing the following title: 'A Discourse concerning the Vegetation of
Plants, spoken by Sir Kenelm Digby, at Gresham College, on the 23d of
January 1660. (At a meeting of the Society for promoting Philosophical
Knowledge by Experiments. London: Printed for John Williams, in Little
Britain, over against St Botolph's Church, 1669.)' The author attributes
plant-growth to the influence of a _balsam_ which the air contains. This
book is especially interesting as containing the earliest recognition of
the value of saltpetre as a manure. The following is an extract from
this interesting old work:--
"The sickness, and at last the death of a plant, in its natural course,
proceeds from the want of that balsamick saline juice; which, I have
said, makes it swell, germinate, and augment itself. This want may
proceed either from a destitution of it in the place where the plant
grows, as when it is in a barren soil or bad air, or from a defect in
the plant itself, that hath not vigour sufficient to attract it, though
it be within the sphere of it; as when the root has become so hard,
obstructed and cold, as that it hath lost its vegetable functions. Now,
both these may be remedy'd, in a great measure, by one and the same
physick.... The watering of soils with cold hungray springs doth little
good; whereas muddy saline waters brought to overflow a piece of ground
enrich it much. But above all, well-digested dew makes all plants
luxuriate and prosper most. Now what may it be that endues these liquors
with such prolifick virtue? The meer water which is common to them all,
cannot be it; there must be something else enclosed within it, to which
the water serves but for a vehicle. Examine it by spagyric art, and you
will find that it is nothing else than a _nitrous salt_, which is
dilated in the water. It is this salt which gives foecundity to all
things: and from this salt (rightly understood) not only all vegetables,
but also all minerals draw their origine. By the help of plain
_salt-peter_, dilated in water and mingled with some other fit earthy
substance, that may familiarize it a little with the corn into which I
endeavoured to introduce it, I have made the barrenest ground far out-go
the richest, in giving a prodigiously plentiful harvest. I have seen
hemp-seed soaked in this liquor, that hath in due time made such plants
arise, as, for the tallness and hardness of them, seemed rather to be
coppice-wood of fourteen years' growth at least, than plain hemp. The
fathers of the Christian doctrine at Paris still keep by them for a
monument (and indeed it is an admirable one) a plant of barley
consisting of 249 stalks, springing from one root or grain of barley; in
which they counted above 18,000 grains or seeds of barley. But do you
think that it is barely the salt-peter, imbibed into the seed or root,
which causeth this fertility? no: that would be soon exhausted and could
not furnish matter to so vast a progeny. The salt-peter there is like a
magnet, which attracts a like salt which foecundates the air, and gave
cause to the Cosmopolite to say there is in the air a hidden food of
life."[3]
_Duhamel and Hales._
The names of the French writer, Duhamel, and of the English, Stephen
Hales, may be mentioned in passing as authors of works bearing on the
question of vegetable physiology. Both of these writers flourished about
the middle of the eighteenth century. The writings of the former
contained much valuable information on the effects of grafting, motion
of sap, and influence of light on vegetable growth, and also the results
of experiments which the author had carried out on the influence of
treating plants with certain substances. 'Statical Essays, containing
Vegetable Staticks; or an Account of some Statical Experiments on the
Sap of Vegetables, by Stephen Hales, D.D.' (2 vols.), was published in
London in 1738; and contained, as will be seen from its title, records
of experiments of very much the same nature as those of Duhamel.
_Jethro Tull's Theory._
Some reference may be made to a theory which created a considerable
amount of interest when it was first published--viz., that of Jethro
Tull. The chief value of Tull's contribution to the subject of
agricultural science was, that he emphasised the importance of tillage
operations by putting forward a theory to account for the fact,
universally recognised, that the more thoroughly a soil was tilled, the
more luxuriant the crops would be. As Tull's theory had a very
considerable influence in stirring up interest in many of the most
important problems in agricultural chemistry, and as it contained in
itself much, the value of which we have only of late years come to
understand, a brief statement of this theory may not be without
interest.
According to Tull the food of plants consists of the particles of the
soil. These particles, however, must be rendered very minute before they
become available for the plant, which absorbs them by means of its
rootlets. This pulverisation of the soil goes on in nature independently
of the farmer, but only very slowly, and the farmer has therefore to
hasten it on by means of tillage operations. The more efficiently these
operations are carried on, the more abundant will the supply of
plant-food be rendered in the soil. He consequently introduced and
advocated the system of horse-hoe husbandry. This theory, he informs us,
was suggested to him by the custom, which he had noticed on the
Continent, of growing vines in rows, and hoeing the intervals between
these rows from time to time. The excellent results which followed this
mode of cultivation induced him to adopt it in England for his farm
crops. He accordingly sowed his crops in rows or ridges, wide enough
apart to admit of thorough tillage of the intervals by ploughing as well
as by hand-hoeing. This he continued until the plant had reached
maturity. As to the exact width of the interval most suitable, he made a
large number of experiments. At first, in the cultivation of wheat, he
made this interval six feet wide; but latterly he adopted an interval of
lesser width, that finally arrived at being between four and five feet.
He likewise experimented on each separate ridge as to which was the best
number of rows of wheat to be sown, latterly adopting, as most
convenient, two rows at ten inches apart. The great success which he met
with in this system of cultivation induced him to publish the results of
his experiments in his famous work, 'Horse-Hoeing Husbandry.'
While Tull's theory was based on principles at heart thoroughly sound,
he was carried away by his personal success into drawing unwarrantable
deductions. Thus he came to the conclusion that rotation of crops was
unnecessary, provided that a thorough system of tillage was carried out.
Manures also, according to him, might be entirely dispensed with under
his system of cultivation, for the true function of all manures is to
aid in the pulverisation of the soil by fermentation.
The first really valuable scientific facts contributed to the science
were made by Priestley, Bonnet, Ingenhousz, and Senebier.
_Discovery of the Source of Plants' Carbon._
To Charles Bonnet (1720-1793), a Swiss naturalist, is due the credit of
having made the first contribution to a discovery of very great
importance--viz., the true source of the _carbon_, which we now know
forms so large a portion of the plant-substance. Bonnet, who had devoted
himself to the question of the function of leaves, noticed that when
these were immersed in water bubbles were seen, after a time, to collect
on their surface. De la Hire, it ought to be pointed out, had noticed
this same fact about sixty years earlier. It was left to Priestley,
however, to identify these bubbles with the gas he had a short time
previously discovered--viz., oxygen. Priestley had observed, about this
time, the interesting fact that plants possessed the power of purifying
air vitiated by the presence of animal life.[4] The next step in this
highly interesting and important discovery was taken by John Ingenhousz
(1730-1799), an eminent physician and natural philosopher. In 1779,
Ingenhousz published a work in London entitled 'Experiments on
Vegetables.' In it he gives the results of some important experiments he
had made on the question already investigated by Bonnet and Priestley.
These experiments proved that plant-leaves only gave up their oxygen in
the presence of sunlight. In 1782 he published another work on 'The
Influence of the Vegetable Kingdom on the Animal Creation.'[5]
The source of the gas, which Bonnet had first noticed to be given off
from plant-leaves, Priestley had identified as oxygen, and Ingenhousz
had proved to be only given off under the influence of the sun's rays,
was finally shown by a Swiss naturalist, Jean Senebier[6] (1742-1809),
to be the _carbonic acid gas_ in the air, which the plant absorbed and
decomposed, giving out the oxygen and assimilating the carbon.
_Publication of First English Treatise on Agricultural Chemistry._
In 1795, a book dealing with the relations between chemistry and
agriculture was published. This work was written by a Scottish nobleman,
the Earl of Dundonald, and possesses especial interest from the fact
that it is the first book in the English language on agricultural
chemistry. The full title is as follows: 'A Treatise showing the
Intimate Connection that subsists between Agriculture and Chemistry.'
In his introduction the author says: "The slow progress which
agriculture has hitherto made as a science is to be ascribed to a want
of education on the part of the cultivators of the soil, and to a want
of knowledge, in such authors as have written on agriculture, of the
intimate connection that subsists between the science and that of
chemistry. Indeed, there is no operation or process not merely
mechanical that does not depend on chemistry, which is defined to be a
knowledge of the properties of bodies, and of the effects resulting from
their different combinations."
In quoting this passage Professor S. W. Johnson remarks:[7] "Earl
Dundonald could not fail to see that chemistry was ere long to open a
splendid future for the ancient art that had always been and always will
be the prime supporter of the nations. But when he wrote, how feeble
was the light that chemistry could throw upon the fundamental questions
of agricultural science! The chemical nature of the atmosphere was then
a discovery of barely twenty years' standing. The composition of water
had been known but twelve years. The only account of the composition of
plants that Earl Dundonald could give was the following: 'Vegetables
consist of mucilaginous matter, resinous matter, matter analogous to
that of animals, and some proportion of oil.... Besides these,
vegetables contain earthy matters, formerly held in solution in the
newly-taken-in juices of the growing vegetables.' To be sure, he
explains by mentioning in subsequent pages that starch belongs to the
mucilaginous matter, and that on analysis by fire vegetables yield
soluble alkaline salts and insoluble phosphate of lime. But these salts,
he held, were formed in the process of burning, their lime excepted; and
the fact of their being taken from the soil and constituting the
indispensable food of plants, his lordship was unacquainted with. The
gist of agricultural chemistry with him was, that plants 'are composed
of gases with a small proportion of calcareous matter; for although this
discovery may appear to be of small moment to the practical farmer, yet
it is well deserving of his attention and notice.'"
_De Saussure._
The year 1804 witnessed the publication of by far the most important
contribution made to the science up till this time. This was
'Recherches Chimique sur la Vegetation,' by Theodore de Saussure, one of
the most illustrious agricultural chemists of the century. De Saussure
was the first to draw attention to the mineral or ash constituents of
the plant; and thus anticipate, to a certain extent, the subsequent
famous "mineral" theory of the great Liebig. The French chemist
maintained that these ash ingredients were essential; and that without
them plant-life was impossible. He also adduced fresh experiments of his
own in support of the theory, based on the experiments of Bonnet,
Priestley, Ingenhousz, and Senebier, that plants obtain their carbon
from the carbonic acid gas in the air, under the influence of the
sunlight. He was of opinion that the _hydrogen_ and _oxygen_ of the
plant were, probably, chiefly derived from water. He showed that by far
the largest portion of the plant's substance was derived from the air
and from water, and that the ash portion was alone derived from the
soil. To Saussure we owe the first definite statement on the different
sources of the plant's food. It may be said that the lapse of nearly a
century has shown his views to be, in the main, correct.
_Source of Plant-nitrogen._
There was one question, which, even at that remote period in the history
of the subject, engaged the attention of agricultural chemists--viz.,
the question of the source of the plant's _nitrogen_--a question which
may be fitly described at the present hour as still the burning
question of agricultural chemistry.[8]
As soon as it was discovered that nitrogen was a constituent of the
plant's substance; speculations as to its source were indulged in. The
fact that the air furnished an unlimited storehouse of this valuable
element, and the analogy of the absorption of carbon (from the same
source by plant-leaves), naturally suggested to the minds of early
inquirers that the free nitrogen of the air was the source of the
plant's nitrogen. As, however, no direct experiments could be adduced to
prove this theory, and as, moreover, nitrogen was found in the soil, and
seemed to be a necessary ingredient of all fertile soils, the opinion
that the soil was the only source gradually supplanted the older theory.
Little value, however, must be attached to these early theories, as they
can scarcely be said to have been based on experiments of serious value.
Indeed it may be safely affirmed, in the light of subsequent
experiments, that it was impossible for this question to be decided at
this early period, from the fact that analytical apparatus, of a
sufficiently delicate nature, was then wholly unknown. Indeed it is only
within the last few years that it has been possible to carry out
experiments which may be regarded as at all crucial. A short sketch of
the development of our knowledge of the relation of nitrogen to the
plant will be given further on.
_Sir Humphry Davy's Lectures._
A series of lectures on agricultural chemistry, delivered by Sir Humphry
Davy during the years 1802-1812, for the Board of Agriculture, and
subsequently published in book form in the year 1813,[9] affords us an
opportunity of gauging, pretty accurately, the state of knowledge on the
subject at the time.
_Position of Agricultural Chemistry at beginning of Century._
In his opening lecture Davy says: "Agricultural chemistry has not yet
received a regular and systematic form. It has been pursued by competent
experimenters for a short time only. The doctrines have not as yet been
collected into any elementary treatise, ... and," he adds, "I am sure
you will receive with indulgence the first attempt made in this country
to illustrate it by a series of experimental demonstrations."
He further on remarks: "It is evident that the study of agricultural
chemistry ought to be commenced by some general inquiries into the
composition and nature of material bodies, and the law of their changes.
The surface of the earth, the atmosphere, and the water deposited from
it, must either together, or separately, afford all the principles
concerned in vegetation, and it is only by examining the chemical
nature of these principles that we are capable of discovering what is
the food of plants, and the manner in which this food is supplied and
prepared for their nourishment."
Davy goes on further to say: "No general principles can be laid down
respecting the comparative merits of the different systems of
cultivation and the various systems of crops adopted in different
districts, unless the chemical nature of the soil, and the physical
circumstances to which it is exposed, are fully known."
He recognises the enormous importance of experiments. "Nothing is more
wanting in agriculture than experiments, in which all the circumstances
are minutely and scientifically detailed."
In dealing with the composition of plants he says: "It is evident that
the most essential vegetable substances consist of hydrogen, carbon, and
oxygen, in different proportions, generally alone; but in some few cases
combined as carbon and nitrogen. The acids, alkalies, earths, metallic
oxides, and saline compounds, though necessary in the vegetable economy,
must be considered as of less importance, particularly in their relation
to agriculture, than the other principles."
Further on: "It will be asked, Are the pure earths in the soil merely
active as mechanical or indirect chemical agents, or do they actually
afford food to the plant?"
This question he answers by saying that "water, and the decomposing
animal and vegetable matter existing in the soil, constitute the true
nourishment of plants; and as the earthy parts of the soil are useful in
retaining water, so as to supply it in the proper proportion to the
roots of the vegetables, so they are likewise efficacious in producing
the proper distribution of the animal or vegetable matter. When equally
mixed with it, they prevent it from decomposing too rapidly; and by
their means the soluble parts are supplied in proper proportions."
_Value of Davy's Lectures._
The chief value of these lectures is due to the fact that they form the
first attempt to connect in a systematic manner the various scattered
facts, up to that time ascertained, and to interpret their bearing on
agricultural practice. We have in them, it is true, a strange mixture of
facts belonging rather to botany and physiology than to agricultural
chemistry; still they undoubtedly furnished a great impetus to inquiry,
and at the same time they did much to popularise the science.
But not merely did Davy summarise and systematise the various results
arrived at by others, he also made many valuable contributions to the
science himself. The conclusions he drew from the results he obtained
were, no doubt, in many cases false, and in other cases exaggerated;
still the results possess a permanent interest. He may be said to have
worked out many of the most important _physical_ or _mechanical_
properties of a soil, although exaggerating the importance of the
influence of these properties on the question of fertility.[10]
These experiments had to do with the heat- and water-absorbing powers of
a soil. He experimented on a brown fertile soil, and a cold barren clay,
and found at what rate they lost heat. "Nothing," he says, "can be more
evident than that the genial heat of the soil, particularly in spring,
must be of the highest importance to the rising plant; ... so that the
temperature of the surface, when bare and exposed to the rays of the
sun, affords at least one indication of the degree of the fertility."
Again he says: "The power of soils to absorb water from air is much
connected with fertility.... I have compared the absorbent powers of
many soils, with respect to atmospheric moisture, and I have always
found it greatest in the most fertile soils; so that it affords one
method of judging of the productiveness of land."
Where he erred was in overestimating the functions of the mechanical
properties of a soil, and in considering fertility to be due to them
alone.
During the next thirty years or so, little progress seems to have been
made in the way of fresh experimentation.
_Boussingault._
In 1834, Boussingault,[11] the most distinguished French agricultural
chemist of the century, began that series of brilliant chemico-agricultural
experiments on his estate at Bechelbronn, in Alsace, the results of which
have added so much to agricultural science. It was the first instance of
the combination of "science with practice," of the institution of a
laboratory on a farm; a combination peculiarly fitted to promote the
interests of agricultural science, and an example which has been since
followed with such magnificent results in the case of Sir John Lawes's
famous Rothamsted Experiment Station, and other less known research
stations.
Boussingault's first paper appeared in 1836, and was entitled, "The
amount of nitrogen in different kinds of foods, and on the equal value
of foods founded on these data."
In the year following other papers were published on such subjects as
the amount of gluten in different kinds of wheat; on the meteorological
considerations of how far various agricultural operations--such as
extensive clearings of wood, the draining of large swamps,
&c.--influence of climate on a country; and on experiments on the
culture of the vine.
Boussingault was the first observer to study the scientific principles
underlying the system of _rotation of crops_. In 1838 he published
the results of some very elaborate experiments he had carried out on
this subject. He also was the first chemist to carry out elaborate
experiments with a view to deciding the question of the assimilation by
plants of free atmospheric nitrogen. His first contribution to the
subject was published in 1838, but can scarcely be regarded as
possessing much scientific value, except in so far as it stimulated
further research. Some thirteen years later he returned to this
question; and during the years 1851-1855 carried out most elaborate
experiments, the results of which, until quite recently, were generally
regarded as having, along with the experiments of Messrs Lawes, Gilbert,
and Pugh, definitely settled the question.[12]
In 1839 Boussingault was elected a member of the French Institute, an
honour paid to him in recognition of his great services to agricultural
chemistry.[13]
The foregoing is a brief epitome of the history of the development of
agricultural chemistry up to the year 1840, the year which witnessed the
publication of one of the most memorable works on the subject, which has
appeared during the present century--Liebig's first report to the
British Association, a work which may be described as constituting an
epoch in the history of the science. Liebig's position as an
agricultural chemist was so prominent, and his influence as a teacher so
potent, that a few biographical facts may not be out of place before
entering upon an estimate of his work.
_Liebig._
Liebig was born at Darmstadt in the year 1803. He was the son of a
drysalter, and early devoted himself to the study of chemistry in the
only way at first at his disposal--viz., in an apothecary's shop. Soon
finding, however, his opportunities of study limited, he left the
apothecary's shop for the University of Bonn. He did not remain long at
Bonn, but in a short time left that university for Erlangen, where he
studied for some years, taking his Ph.D. degree in 1822. His subsequent
studies were carried on at Paris under Gay-Lussac, Thenard, Dulong, and
other distinguished chemists. Through the influence of A. Humboldt, who
was at that time in Paris, and whose acquaintance he was fortunate
enough to make, he was received into Gay-Lussac's private laboratory.
In 1824--that is, when he was only twenty-one years of age--he was
appointed Professor _Extraordinarius_ of Chemistry at the University of
Giessen. Two years later he was appointed to the post of Professor
_Ordinarius_--an appointment which he held for twenty-five years. In
1845 he was created Baron, and in 1852 appointed Professor at Munich. He
died in 1873.
_His First Report to British Association._
The report above referred to was made by Liebig at the request of the
Chemical Section of the British Association. It was read to a meeting of
the Association held in Glasgow in 1840, and was subsequently published
in book form, under the title of 'Chemistry in its Application to
Agriculture and Physiology,' Liebig's position, past training and
experience were such as to peculiarly fit him for the part of pioneer in
the new science. As Sir J. H. Gilbert has remarked,[14] "In the
treatment of his subject he not only called to his aid the previously
existing knowledge directly bearing upon his subject, but he also turned
to good account the more recent triumphs of organic chemistry, many of
which had been won in his own laboratory."
In his dedication to the British Association at the beginning of the
book, Liebig says: "Perfect agriculture is the true foundation of all
trade and industry--it is the foundation of the riches of States. But a
rational system of agriculture cannot be formed without the application
of scientific principles; for such a system must be based on an exact
acquaintance with the means of nutrition of vegetables, and with the
influence of soils and actions of manure upon them. This knowledge we
must seek from chemistry, which teaches the mode of investigating the
composition and of studying the characters of the different substances
from which plants derive their nourishment."
_His criticism of the "Humus" Theory._
The first subject which Liebig discusses is the scientific basis of the
so-called "humus" theory. The humus theory seems to have been first
promulgated by Einhof and Thaer towards the close of last century. Thaer
held that humus was the source of plant-food. He stated in his published
writings that the fertility of a soil depended really upon its humus;
for this substance, with the exception of water, is the only source of
plant-food. De Saussure, however, by his experiments--the results of
which he had published in 1804--had shown the fallacy of this humus
theory; and his statements had been further developed and substantiated
by the investigations of the French chemist Braconnot and the German
chemist Sprengel. Despite, however, the experiments of Saussure,
Braconnot, and Sprengel, the belief that plants derived the
carbonaceous portion of their substance from humus still seemed to be
commonly held in 1840.
While Liebig, therefore, can scarcely be said to have been the first to
controvert the humus theory, he certainly dealt it its death-blow. He
reasserted de Saussure's conclusions, and by some simple calculations
showed very clearly that it was wholly untenable. One of the most
striking of the arguments he brought forward was the fact that the humus
of the soil itself consisted of the decayed vegetable matter of
preceding plants. This being so, how, he asked, could it be the original
source of the carbon of plants? To reason thus was simply to reason in a
circle. He pointed out, further, that the comparative insolubility of
humus in water, or even in alkaline solutions, told against its
acceptance as correct.
_His Mineral Theory._
Having thus controverted the humus theory, he then goes on to deal with
the question of the source of the various plant constituents. In
treating of the relation of the soil to the plant, he puts forward his
"mineral" theory. It cannot be doubted that, while the advance of
science since Liebig's time has induced us to considerably modify his
mineral theory, it contained the statement of one of the most important
facts in the chemistry of plant physiology. He was the first to fully
estimate the enormous importance of the mineral portion of the plant's
food, and point the way to one of the chief sources of a soil's
fertility. Up to this period the ash constituents had been generally
considered to be of minor importance. By emphasising the contrary
opinion, and insisting upon their essentialness to plant-life, he gave
to agricultural research a fresh impetus upon the right lines. His
statement of his mineral theory was in the main true, but was not the
whole truth.
De Saussure, as has already been pointed out, to a certain extent,
anticipated Liebig's mineral theory. He was of the opinion that whatever
might be the case with some of the mineral constituents of plants,
others were necessary, inasmuch as they were always found in the ash. Of
these he instanced the alkaline phosphates. "Their small quantity does
not indicate their inutility," he sagaciously remarks. Sir Humphry Davy,
as has already been pointed out, missed recognising the true importance
of the ash constituents. It was left to Liebig, then, to restate the
important doctrine of the essentialness of the mineral matter, already
implied to some extent by de Saussure.
Liebig says: "Carbonic acid, water, and ammonia are necessary for the
existence of plants, because they contain the elements from which their
organs are formed; but other substances are likewise necessary for the
formation of certain organs destined for special functions, peculiar to
each family of plants. Plants obtain these substances from inorganic
nature."
While insisting on the importance of the mineral constituents, he did
so in a more or less general way not sufficiently distinguishing one
mineral constituent from another.
As all plants contained certain organic acids, and as these organic
acids were nearly always found in a neutral state--_i.e._, in
combination with bases, such as potash, soda, lime, and magnesia--the
plant must be in a position to take up sufficient of these alkaline
bases to neutralise these acids. Hence the necessity of these mineral
constituents in the soil. According to him, however, the exact nature of
the bases was a point of not so much importance. He assumed, in short,
as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual
replaceability amongst the bases than can be now admitted.
Passing on to a consideration of the difference of the mineral
composition of different soils, he attributes this to the difference in
the rocks forming the soils. "Weathering" is the great agent at work in
rendering available the otherwise locked-up stores of fertility. He
attributes the benefits of fallow exclusively to the increased supply of
these incombustible compounds which were thus rendered available to the
plant. Treating of this subject, he says: "From the preceding part of
this chapter" (in which he has been explaining weathering) "it will be
seen that fallow is that period of culture when the land is exposed to
progressive disintegration by the action of the weather, for the purpose
of liberating a certain quantity of alkalies and silica, to be absorbed
by future plants."
_His Theory of Manures._
Treating of manures, he showed how the most important constituents of
manures were _potash_ and _phosphates_. In the first edition of his work
he also insisted on the value of _nitrogen_ in manures, condemning the
want of precautions, in the treatment of animal manures, against loss of
nitrogen.
In the later editions of his work he seems to have receded from that
opinion, and considered that there was no necessity for supplying
nitrogen in manures, since the ammonia washed down in rain was a
sufficient source of all the nitrogen the plant required. It was here
that Liebig went astray, first in denying the importance of supplying
nitrogen as a manure; and secondly, in overestimating the amount of
ammonia washed down in rain, which has subsequently been shown to be
entirely inadequate to supply plants with the whole of their
nitrogen.[15]
_His Theory of Rotation of Crops._
In explaining the benefits of the rotation of crops, Liebig propounded a
very ingenious theory, but one which was largely of a speculative
nature, and which has since been shown to be unfounded on any scientific
basis. It was to the effect that one kind of crop excreted matters which
were especially favourable to another kind of crop. He did not say
whether he considered such excretion positively injurious to the crop
which excreted them; but he inferred that what was excreted by the crop
was what was not required, and what could, therefore, be of little
benefit to a crop of the same nature following it.
The second portion of Liebig's report dealt with the processes of
fermentation, decay, and putrefaction.
_Publication of Liebig's Second Report to British Association._
In 1842 Liebig contributed his second famous report to the British
Association, subsequently published under the title of 'Animal
Chemistry; or, Organic Chemistry in its Applications to Physiology and
Pathology.' The publication of this report created even greater interest
than the publication of his first work. In it he may be said to have
contributed as much to animal physiology, as, in his first, he did to
agricultural chemistry. His subsequent principal works on agricultural
chemistry were--'Principles of Agricultural Chemistry,' published in
1855, and 'On Theory and Practice in Agriculture,' 1856.
_Liebig's services to Agricultural Chemistry._
An attempt has been made to sketch in the very briefest manner some of
the main points in Liebig's teaching, as contained in his famous report
to the British Association in 1840. Agricultural chemistry up till that
year can scarcely be described as having a distinct existence as a
branch of chemistry. Much valuable work, it is true, had already been
done, especially by his two great predecessors, de Saussure and
Boussingault; but it was, down to the year 1840, a science made up of
isolated facts. Liebig's genius formed it into an important branch of
chemistry, supplied the necessary connection between the facts, and by a
series of brilliant generalisations formed the principles upon which all
subsequent advance has been built.
As has already been indicated, Liebig's chief claim to rank as the
greatest agricultural chemist of the century does not rest upon the
number or value of his actual researches, but on the formative power he
exercised in the evolution of the science. His master-mind surveyed the
whole field of agricultural chemistry, and saw laws and principles where
others saw simply a confusion of isolated, and, in many cases, seemingly
contradictory facts.
But great as the direct value of Liebig's work was, it may be questioned
whether its indirect value was not even greater. The publication of his
famous work had the effect of giving a general interest to questions
which up till then had possessed a special interest, and that for
comparatively few. Both on the Continent and in England a very large
amount of discussion took place regarding his various theories.
_Development of Agricultural Research in Germany._
It was especially in Germany, however, that Liebig's work bore its
greatest and most immediate fruit. Thanks to the great chemist, the
German Government recognised the importance of forwarding scientific
research by State aid. Agricultural Departments were added to some of
the universities, largely at State expense, while agricultural research
stations were, one after another, instituted in different parts of the
country.
The first of the agricultural research stations to be founded was the
now famous one of Moeckern, near Leipzig. It was instituted in the year
1851. Others followed, until at the present day there are some seventy
to eighty of these _Versuchs-Stationen_ scattered throughout Germany,
all well equipped and doing excellent work. Some idea of the activity of
the German stations may be inferred when it is stated that up to the
year 1877 the total number of papers embodying the results of their
experiments published by them amount to over 2000.[16]
To trace the development of agricultural chemistry, subsequent to
Liebig's time, in the way it has been done prior to the year 1840, is no
longer possible. This is due to the enormous increase in the number of
workers in the field, as also to the overlapping nature of their work,
which renders a strict chronological record wellnigh an impossibility.
It will be better, therefore, to attempt to give a brief statement of
our present knowledge on the subject, naming the chief workers in the
various departments of the subject.
_The Rothamsted Experiments._
Before doing so, it is fitting that reference should be made to the work
and experiments of two living English chemists, who have done much to
contribute to our knowledge in every branch of the science--viz., Sir
John Lawes, Bart., and Sir J. H. Gilbert, F.R.S.
The fame of the Rothamsted experiments is now world-wide; and no single
experiment station has ever produced such an amount of important work as
the magnificently equipped research station at Rothamsted. The
Rothamsted station may be said to date from 1843, although Sir John
Lawes was engaged in carrying out field experiments for ten years
previous to that date.[17] In 1843 Sir John Lawes associated with
himself the distinguished chemist Sir J. H. Gilbert, and the numerous
papers since published have almost invariably borne the two names. The
expense of working the station has been borne entirely by Sir John Lawes
himself; who has further set aside a sum of L100,000, the Laboratory,
and certain areas of land, for the continuance of the investigations
after his death. The fields under experimentation amount to about fifty
acres. By a Trust-deed, which was signed on February 14, 1889, Sir John
Lawes has made over the Rothamsted Experimental Station to the English
nation, to be managed by trustees.
It is impossible to enter, in any detail, into the nature and scope of
the Rothamsted experiments.[18] It may be stated that, since the year
1847, some eighty papers have been published on field experiments, and
experiments on vegetation; while thirty papers have been published
recording experiments on the feeding of animals.[19]
What has all along characterised these valuable experiments has been
their practical nature. While their aim has been entirely scientific,
the scale of the experiments and the conditions under which they have
been carried out, have been such as to render them essentially
_technical_ experiments. For this reason their results possess, and will
always possess, a peculiar interest for every practical farmer.
The greatest services the Rothamsted experiments have rendered
agricultural chemistry have been the valuable contributions they have
made to our knowledge of the function of nitrogen in agriculture; its
relation in its different chemical forms to plant-life; and the sources
of the nitrogen found in plants. Researches of a most elaborate nature
have been carried out on what is still one of the most keenly debated
questions of the present hour--viz., the relation of the "free" nitrogen
in the atmosphere to the plant. Of the very highest value also have been
the elaborate researches of Mr R. Warington, F.R.S., on the important
question of _Nitrification_, which have been in course in the Rothamsted
Laboratory for the last fifteen years, and to which full reference will
be made in the chapter on Nitrification.
To the Rothamsted experiments also we owe the refutation of Liebig's
mineral theory. In fact it may safely be said that no experimenters in
the field of agricultural chemistry have made more numerous or valuable
contributions to the science than these illustrious investigators.
_Review of our present Knowledge of Agricultural Chemistry._
Some attempt may now be made to indicate briefly our present knowledge
of the more important facts regarding plant physiology, agronomy, and
manuring.
_Proximate Composition of the Plant._
The great advance made in the direction of the improvement of the
accuracy of old analytical processes and the discovery of numerous new
ones have furnished us with elaborate analyses of the composition of
plants. We now know that the plant-substance is made up of a large
number of complex organic substances, formed out of carbon, hydrogen,
oxygen, and nitrogen,[20] and that these substances form, on an average,
about 95 per cent of the dry vegetable matter; the other 5 per cent
being made up of mineral substances. As to the source of these different
substances, our knowledge is, on the whole, pretty complete. With regard
to the carbon of green-leaved plants, which amounts to from 40 to 50 per
cent, subsequent research has confirmed Senebier and de Saussure's
conclusions, that its source is the carbonic acid gas of the air. The
decomposition of the carbonic acid gas is effected by the leaves under
the influence of sunlight. That a certain quantity of carbon may be
obtained from the carbonic acid absorbed by plant-roots, is indeed
probable. Especially during the early stages of plant-growth this source
of carbon may be of considerable importance. Generally speaking,
however, it may be said of all green-leaved plants, that the chief
source of their carbon is the carbonic acid gas in the atmosphere.
_Carbon Fixation by Plants._
The exact way in which this decomposition of carbonic acid gas is
effected by the leaves is not yet clear. It seems to be directly
dependent, in some way or other, on the chlorophyll, or green colouring
matter. This decomposition of carbonic acid, and the fixation of the
carbon by the plant with the formation of starch, takes place only under
the influence of sunlight. During the night a reflex action takes place,
which is commonly known as _respiration_, and which is exactly analogous
to animal respiration.[21] The rate at which the fixation of carbon
takes place depends on the strength of the sun's rays. It seems to take
place very rapidly under a strong tropical sun.[22] The action of
sunlight on the absorption of carbon has been studied by a number of
observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet,
Prillieux, Lommel, &c.
_Action of Light on Plant-growth._
Experiments made by several observers, more especially Pfeffer, have
shown that the yellow rays of the solar spectrum are the most potent in
inducing this decomposition.
Some interesting experiments have been carried out by different
observers on the possibility of growing plants under the influence of
artificial light. While it would seem that the light from oil-lamps or
gaslight is unable to promote growth, except in very exceptional cases,
the electric light, or other strong artificial light, seems to be
capable of taking the place of sunlight. Heinrich was the first to show
that sunlight could be replaced by the magnesium light.
Experiments with the electric light have been carried out by
Herve-Mangon in France and Dr Siemens in England. The plants grown under
the influence of the electric light were observed to be of a lighter
green colour than those grown under normal conditions, thus indicating a
feebler growth; in fact, Siemens was of the opinion that the electric
light was about half as effective as daylight.[23]
These experiments are interesting from an industrial point of view; for
it is conceivable that at some distant time electricity might be called
to the aid of the agriculturist.
_Source of Plants' Oxygen._
With regard to the source of the oxygen, which, next to carbon, is the
element most largely present in the plant's substance--amounting to,
roughly speaking, about 40 per cent--all evidence seems to indicate that
it is chiefly derived from water, which is also the source of the
plant's hydrogen. In addition to water, carbonic acid and nitric acid
may also furnish small quantities. It has been pretty conclusively
proved that the atmospheric oxygen, while necessary to plant-growth, and
promoting the various chemical vital processes, is not a direct source
of the plant's oxygen. The important function played by atmospheric
oxygen in certain stages of the plant's growth has been long recognised.
Malpighi, nearly two hundred years ago, observed that for the process of
germination atmospheric air was necessary; and shortly after the
discovery of the composition of the air was made, oxygen was identified
as the important gas in promoting this process. Oxygen is also
especially necessary during the period of ripening.
_Source of Plants' Hydrogen._
Hydrogen, which amounts to about 6 per cent, is, as has already been
pointed out, chiefly derived from water. It is possible that ammonia
also may form a source.
_Source of Plants' Nitrogen._
When we come to treat of the source of the nitrogen, which is found in
the plant's substance to an extent varying from a fraction of a per cent
to about 4 per cent, we enter on a much more debated question.
What is the source, or, what are the sources, of plant-nitrogen? is a
question to the solution of which more time and more research have been
devoted than to the solution of any other question connected with
agricultural chemistry.
The most obvious source is the free nitrogen, which forms four-fifths of
the atmospheric air. Reference has already been made to this
question.[24] Priestley was the first of the long list of experimenters
on this interesting question.
As far back as 1771 he affirmed that certain plants had the power of
absorbing free nitrogen; and this opinion he supported by the results of
certain experiments he had made on the subject. Eight years
later,--viz., in 1779--Ingenhousz further supported this conclusion, and
stated that all plants could absorb, within the space of a few hours,
noticeable quantities of nitrogen gas. The first to oppose this theory
was de Saussure, who, in 1804, carried out experiments which showed that
plants were unable to utilise free nitrogen.
Subsequent experiments, carried out by Woodhouse and Senebier, supported
de Saussure's conclusions. Mention has already been made of
Boussingault's elaborate researches on the subject.[25] His first
experiments were carried out in 1838. He concluded that plants did not
absorb free nitrogen. Georges Ville was the first to reassert the older
theory, put forward by Priestley and Ingenhousz. His opinion was founded
on experiments he had carried out during the years 1849-52. The subject
created so much interest at the time, that a committee of the French
Academy--consisting of Dumas, Regnault, Peligot, Chevreul, and
Decaisne--were appointed to investigate Ville's experiments. The result
of the investigation of the Commission was to confirm Ville's
experiments. It is a significant fact, however, that the plant
experimented with by the Commission was _cress--a non-leguminous
plant_. It has been commonly assumed that the results of recent
experiments have confirmed Ville's experiments. It is only proper to
point out that this is not a necessary inference. The assimilation of
free nitrogen by the _leguminosae_, so far as modern research has
revealed, only takes place under the influence of micro-organic life.
Ville's experiments, however, were supposed to be conducted under
_sterilised_ conditions.
In the meantime the results of Boussingault's second series of
experiments, carried out between the years 1851 and 1855, were
published, and confirmed his earlier experiments.
The results of a large number of experiments subsequently carried out
were in support of Boussingault's conclusions. Among them may be
mentioned Mene, Harting, Gunning, Lawes, Gilbert and Pugh, Roy,
Petzholdt, and Bretschneider.
Such an amount of overwhelming evidence might naturally have been
regarded as conclusively proving that the free nitrogen of the air is
not an available source of nitrogen to the plant. The question, however,
was not decided. In 1876 Berthelot reopened it. From experiments he had
carried out, he concluded that free nitrogen was fixed by various
organic compounds, under the influence of silent electric discharges. In
1885 he carried out further experiments, from which he concluded that
argillaceous soils had the power of fixing the free nitrogen of the
atmosphere. This they effected, he was of opinion, through the agency of
micro-organisms. Schloesing has recently shown that this fixation of
free nitrogen by soils is extremely doubtful.[26] The gain of nitrogen
observed under such conditions can be explained by the absorption by the
soil of combined nitrogen--viz., ammonia--from the air.
Berthelot's early experiments in 1876 had the effect of stimulating a
number of other experiments, with the result that we now possess the
solution of this long-debated and most important problem.
The names of the better known investigators on this subject, in addition
to Berthelot's, are those of Hellriegel, Wilfarth, Deherain, Joulie,
Dietzell, Frank, Emil von Wolff, Atwater, Woods, Nobbe, Ward, Breal,
Boussingault, Wagner, Schultz-Lupitz, Fleischer, Pagnoul, Schloesing,
Laurent, Petermann, Pradmowsky, Beyrenick, Lawes, and Gilbert.
It is impossible to enter into the details of these most important
experiments. An attempt may be made, instead, briefly to epitomise them.
_Recent Experiments on Nitrogen question._
In the first place, it may be asked, How is it possible that the
previous elaborate experiments, published prior to 1876, should now
prove unreliable? A satisfactory explanation may be found in the fact,
as Lawes and Gilbert have recently pointed out, that the fixation of the
free nitrogen by the plant, or within the soil, takes place, if at all,
through the agency of electricity or of micro-organisms, or of both.
The earlier experiments, however, were so arranged as to exclude the
influence of either of those agencies.
The question has further been limited in its scope. It is now supposed
that only plants of the _leguminous_ order have the power of drawing
upon the free atmospheric nitrogen. Of the experiments above referred
to, those of Hellriegel and Wilfarth are the most striking and
important. They found in their experiments, that while the legumes have
the power of obtaining their nitrogen from the air, cereals have not.
Similar experiments by Atwater in America, and others, support this
conclusion.
Their conclusions may be briefly epitomised as follows:--
(_a_) That the leguminous plants--such as peas, &c.--have the power of
drawing their nitrogen supplies from the free nitrogen of the air in a
way not possessed by other plants; and that they thus possess two
sources of nitrogen--the soil and the air.
(_b_) That this absorption of free nitrogen is not effected directly by
the plant, but is the result, so to speak, of the joint action of
certain micro-organisms present in certain soils and in the plant
itself, (_symbiosis_).
(_c_) That this fixation is connected with the formation of minute
tubercles on the roots of the plants of the leguminous class; and that
these tubercles may be the home of the fixing organism.
(_d_) That these fixing micro-organisms are not present in all
soils.[27]
While the relation of free nitrogen to the plant has long been, and
still is, a very obscure problem, it was early recognised that the
combined nitrogen present in soils and manures was an important source
of plant-food. Reference has already been made to the early theory of
Sir Kenelm Digby regarding the value of nitrates.[28] De Saussure, as we
have also already seen, was fully impressed with the importance of
applying nitrogen to the soil as a manure. Liebig's early attitude on
this question was to the effect, that to apply nitrogen in manures was
quite unnecessary, as the plant had a sufficient source in the ammonia
present in the air, which he erroneously supposed was sufficient in
quantity to supply all the needs of the crops. Despite this early
recognition of the value of combined nitrogen to the plant, it is only
of recent years that we have obtained any definite knowledge as to the
respective value of its different compounds as manures, or as to the
form in which it is assimilated by the plant. It exists in three
forms--(1) as organic nitrogen; (2) as ammonia salts; (3) as nitrates
and nitrites. Much experimental work has during late years been devoted
to studying the comparative action and merits of these three forms.
_Relation of Organic Nitrogen to the Plant._
First, as to the relation of organic nitrogen to the plant. There is a
large number of different organic compounds which contain nitrogen. That
the plant is able to assimilate certain of these organic compounds,
seems, from several experiments, to be extremely probable. From certain
researches, carried out as far back as the year 1857, Sir Charles
Cameron concluded that the plant could assimilate one of them--viz.,
_urea_. From what, however, we have subsequently learned regarding the
process of "nitrification," it is quite probable that the nitrogen in
these experiments was first converted into nitrates before being
assimilated. At any rate, as the plants were not tested for urea, the
experiments must be regarded as leaving the problem unsolved.
Other experiments were carried out of a similar nature by Professor S.
W. Johnson, the different kinds of nitrogen experimented with being
_uric acid_, _hippuric acid_, and _guanine_. But here, again, no
definite conclusion can be drawn, as no analyses were made of the
plants. More recently, however, Dr Hampe has carried out experiments
with _urea_, _uric acid_, _hippuric acid_, and _glycocoll_. These
experiments may be held as demonstrating the fact that at least one
organic compound of nitrogen is capable of being assimilated, as urea
was actually identified as being present in the plants experimented
with. From further experiments, carried out by Dr Paul Wagner and
Wolff, _glycin_, _tyrosin_, and _kreatin_ are able to be assimilated by
the plant.
_Plants able to absorb certain Forms of Organic Nitrogen._
We may conclude, then, from these interesting experiments, that plants
are able to absorb certain organic forms of nitrogen. That they do so in
nature to any extent is extremely improbable, such organic forms of
nitrogen being rarely present in the soil, or if present, being
converted into ammonia or nitrate salts before assimilation.
_Nature of Humus in the Soil._
While on the subject of organic nitrogen, reference may be briefly made
to that substance known as _humus_,--the name applied to the organic
portion of soils,--a substance which figures so largely in early
theories of plant-nutrition. The most elaborate investigation of the
composition of humus has been carried out by Mulder. According to
Mulder, it is composed of a number of organic bodies, and he has
identified the following substances--ulmin, humin, ulmic, humic, geic
acids, &c. These bodies are composed of carbon, hydrogen, and oxygen,
which are invariably associated with nitrogen. Detmer and Simon have
further investigated the subject. The true function of humus, it would
seem, in addition to its numerous mechanical properties, is to furnish,
by its decomposition, carbonic acid and nitrogen--in the form of
ammonia and nitric acid--to the soil; the former acting as a solvent of
the mineral food, the latter as the source of the plant's nitrogen. The
old theory, therefore, that the presence of humus in a soil is a
condition of fertility, is not so far removed from the truth. Where
there is an abundance of humus in the soil there is likely also to be an
abundance of nitrogen.
_Relation of Ammonia to the Plant._
It seems to be beyond doubt that nitrogen is directly absorbed by plants
in the form of ammonia. Liebig, as we have seen, concluded that this was
the great source of nitrogen for the plant, and that the ammonia
compounds present in the air were an all-sufficient supply. Subsequent
research, while confirming his belief so far as regards the capability
of plants to assimilate nitrogen in the form of ammonia, has proved that
the amount of ammonia present in the air is very minute, and utterly
inadequate to supply the plant with the whole of its nitrogen.
Investigations have been made on this subject by Graeger, Fresenius,
Pierre, Bineau, and Ville. According to Ville's researches, which are
among the most recent, the amount does not exceed 30 _parts per thousand
million parts of air_.[29] Some conception of the value of this source
of nitrogen may be gained by estimating the quantity falling, dissolved
in rain, on an acre of soil throughout the year. Various estimations of
the total amount of combined nitrogen, which is in this way brought to
the soil, have been made. A certain amount of discrepancy, it is true,
is to be found in these various estimations, no doubt largely due to the
difference in the circumstances under which the investigations were
carried out. Mr Warington has made several investigations at Rothamsted,
and, according to his most recently published figures, the total
quantity only amounts to 3.37 lb. per acre per annum--of which only 2.53
lb. is as ammonia itself.[30]
As already mentioned, there can be little doubt that plants can absorb
nitrogen in the form of ammonia. The question of how far plant-leaves
are able to absorb ammonia is a much debated one. It is probable that if
they can do so, it is only to a very small extent.[31] The question as
to whether the plant's roots can absorb ammonia or not, is also a very
keenly debated one. The point is a very difficult one to decide, and is
much complicated by the consideration that ammonia, when applied to the
the soil, is so speedily converted into nitric acid. Despite, however,
these difficulties, and the vast amount of controversy on the point, the
experiments of Ville, Hosaeus and Lehmann, seem to indicate beyond doubt
that ammonia is a direct source of nitrogen. Lehmann's experiments would
seem, further, to indicate that there are certain periods of a plant's
growth when its preference for ammonia salts seems to be greater than at
other times. The point, however, it must be confessed, is still an
obscure one. The great difficulty in deciding it, as has just been said,
lies in the fact that ammonia salts, when applied to a soil, are, by the
process of nitrification, converted into nitrates. In experimenting,
therefore, with ammonia, and noting the results, it is wellnigh
impossible to say, except by subsequent analyses, whether the nitrogen
in the ammonia salts has not been converted into nitrates before
assimilation.
_Relation of Nitric Acid to the Plant._
Thirdly, as to nitrogen in the form of nitrates. While it is true that
plants can absorb nitrogen in certain organic forms and as ammonia
salts, it is now a well-known fact that the chief, and by far the most
important, source of nitrogen is nitric acid. Probably more than 90 per
cent of the nitrogen absorbed by green-leaved plants from the soil is
absorbed as nitrates. The tendency of all nitrogen compounds in the soil
is towards conversion into nitric acid. It is the final form of nitrogen
in the soil. The precise method in which this conversion takes place is
a discovery of only a few years' standing. The great economic importance
of this discovery, made by the French chemists Schloesing and Muentz, and
associated in this country with the names of Warington, Munro, and P. F.
Frankland, is only gradually being appreciated. It is without doubt one
of the most interesting made in the domain of agricultural chemistry of
late years.
_Nitrification._
It was in the year 1877 that the two French chemists above referred to
published the results of some experiments they had carried out, which
proved that nitrification--the name given to the process by which
ammonia or other nitrogen salts are converted in the soil into nitric
acid--was due to the action of micro-organic life.
The basis of the theory rests upon the fact that dilute solutions of
ammonia salts or urine, containing all the necessary constituents of
plant-food, if previously sterilised, may be kept for an indefinitely
long period of time, provided the air supplied be filtered through
cotton wool,--so as to prevent the entrance of micro-organisms--without
any formation of nitrates. Introduce, however, into such a solution a
little fresh soil, and nitrification will soon follow.
The conditions under which the nitrification ferment acts, as well as
the nature of the ferment, or rather ferments, have subsequently been
carefully studied by Schloesing and Muentz, Winogradsy, Deherain,
Kellner, and other Continental observers, and especially by Warington,
Munro, and P. F. Frankland in this country. These conditions cannot be
gone into here. They will be fully discussed in the chapter on
Nitrification. Briefly stated, they are a certain range of temperature
(between slightly above freezing-point and 50 deg. C., the maximum activity
taking place, according to Schloesing and Muentz, at about 30 deg. C.); a
plentiful supply of atmosphere oxygen (hence the fact observed by
Warington, that nitrification is chiefly limited to the surface-soil); a
certain amount of moisture; and the presence of certain of the necessary
mineral plant constituents, and the presence of carbonate of lime.
The light which these discoveries throw upon the extremely complicated
question of the fertility of the soil is considerable, as it follows
that no soil can be regarded as really a fertile one in which the
process of nitrification does not freely take place. They furthermore
explain many facts, hitherto observed but not well understood, with
regard to the action of different nitrogenous manures.
_Ash Constituents of the Plant._
We now come to consider the present state of our knowledge on the
essentialness of the ash or mineral portion of the plant. While a
portion of the plant's substance which, up to Liebig's time, had
obtained little notice, it has, since the publication of his famous
"mineral" theory, obtained an ever-increasing amount of investigation.
Up till 1800 practically nothing was known of the function of the ash
constituents. In 1802 de Saussure wrote that it was unknown whether the
constituents of many plants were due to the soils on which they grew, or
whether they were the products of vegetable growth. Some two years
later, however, he was enabled to carry out a number of experiments
which really placed the subject on a firm scientific basis. The
essentialness of the ash constituents was only, however, placed beyond
all doubt by Wiegmann and Polstorff's researches, carried out in 1840.
Reference has already been made to the great stimulus given to research
by the promulgation of Liebig's mineral theory.
_Methods of Research._
In epitomising the vast amount of work carried on since 1840, with the
view of ascertaining the essentialness of the various substances found
in the ash of plants, two methods of experimentation have been
followed.
_Artificial Soils._
The first of these two methods was that adopted in the famous
experiments, carried out by Prince Salm-Horstmar, which have done so
much to further our knowledge on this question. It consisted in growing
plants on an artificial soil--formed out of sugar-charcoal, pulverised
quartz or purified sand--to which were added the different food
constituents.
_Water-culture._
While the results obtained by Prince Salm-Horstmar by this method were
of a most valuable nature, subsequent experimenters have abandoned his
method for the other method--viz., "water-culture." The medium used in
this process is pure water; and it is from experiments carried out in
water-culture that much of our present knowledge, in regard to the
relation of the ash constituents to the plant, is due.
The names of those who have worked in this department are very numerous.
Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg,
Kuehn, Lucanus, W. Wolff, Hampe, Beyer, E. Wolff, P. Wagner,
Bretschneider and Lehmann. The results obtained by these and other
experimenters have demonstrated the following facts.
The substances which have been found in the ash of plants are: _potash_,
_soda_, _lime_, _magnesia_, _oxide of iron_, _oxide of manganese_,
_phosphoric acid_, _sulphuric acid_, _silica_, _carbonic acid_,
_chlorine_, _lithia_, _rubidia_, _alumina_, _oxide of copper_,
_bromine_, _iodine_, and occasionally even other substances. Of these,
however, only six are probably absolutely necessary for
plant-growth--viz., _potash_, _lime_, _magnesia_, _oxide of iron_,
_phosphoric acid_, and _sulphuric acid_. Three other substances seem
also to be almost invariably present, and may possibly be essential--in
very minute quantities at any rate--viz., _chlorine_, _soda_, and
_silica_. With regard to _alumina_ and _oxide of copper_, these
constituents must be regarded as accidental; while _iodine_ and
_bromine_ only occur in the ash of marine plants.
_Method of Absorption of Plant-food._
A department of vegetable physiology which has had much work devoted to
it is the method in which plant-roots absorb their food. The plant's
nourishment is absorbed in solution by means of the roots. Its
absorption takes place, according to Fischer and Dutrochet, who have
investigated the subject at great length, by the process known as
_endosmosis_. It has also been established by numerous experiments, that
different plants require different constituents in different
proportions.
_Water as a Carrier of Plant-food._
The function performed by water, as the carrier of plant-food, and the
motion of the sap of the plant, are questions which have also received
much attention. The motion of the plant's sap seems to have attracted a
great deal of attention at a very early stage of the study of plant
physiology. As far back as 1679, Marriotte studied it. Among other old
experimenters were Hales, Guettard, Senebier, Saint-Martin, de Candolle,
and Miguel. In more recent times, it has been investigated by Schuebler,
Lawes and Gilbert, Knop, Sachs, Unger, and Hosaeus. Some idea of the
enormous amount of water transpired by plant-leaves may be gained by the
statement that from 233 lb. to 912 lb. of water are transpired for every
pound of plant-tissue formed.[32]
_Agronomy._
When we come to deal with questions relating to the chemistry of the
soil, we find that so much investigation has been devoted to this one
branch of agricultural chemistry as to constitute it a special branch by
itself--known in France under the name of _agronomie_--and being taught
in the large agricultural colleges by special professors of the subject.
The value of studying the properties of soils was recognised at an early
period. This study was for long largely confined to their _physical_,
or, what are popularly known as their _mechanical_ properties. Thus Sir
Humphry Davy ascertained many important facts with regard to the heat
and water absorbing and retaining properties of soils.
_Retention by Soil of Plant-food._
It was not till a later period that the power soils possess of fixing
from their watery solutions various plant-foods, both organic and
inorganic, was discovered. The earliest recognition of this most
important property of soils was made by Gazzeri, who, in 1819, called
attention to the fact that the dark fluid portion of farmyard manure was
purified on passing through clay. He concluded that soils, more
especially clayey soils, possessed the property of being able to fix
from their watery solutions the necessary plant-food constituents, and
fix them beyond risk of loss, only affording a gradual supply to the
plant as required.
The first experiments carried out on this subject were those by Huxtable
and Thompson in 1850. The liquid portion of farmyard manure was filtered
through soil and subsequently examined, when it was found to have not
only lost its colour, but also to have lost its smell. Ammonia and
ammonia salts were also experimented with, and it was found that soils
possessed the power of fixing ammonia.
To Thomas Way, however, we are indebted for the most valuable
contribution on this important subject made by any one single
investigator. His experiments were not merely carried out with regard
to ammonia, but also with regard to other bases--such as potash, lime,
magnesia, soda, &c. Since Way's experiments much work has been done by
Liebig, Stohmann, Henneberg, and Heiden, as also by Voelcker, Eichhorn,
Knop, Rautenberg, Pochwissnew, Warington, Beyer, Bretschneider, Sestini,
Laskowsky, Strehl, Pillnitz, Peters, W. Wolff, Lehmann, and Biedermann.
_Bases and Acids fixed by Soil._
From these experiments it may be taken as proved beyond doubt that soils
have the power of fixing, to a greater or less extent, the following
bases: ammonia, potash, lime, magnesia and soda; as well as the two
acids, phosphoric and silicic. The order in which the different bases
are fixed is an important point. It would seem that the soil has a
greater affinity for the more valuable manurial substances, such as
ammonia, potash, and lime, and that these substances are first fixed.
That in fixing any one of the above-mentioned bases from its solution,
it can only do so at the expense of another base. Thus, in fixing
potash, either lime, magnesia, or soda must be given up. Further, when a
base in solution, as sulphate or chloride, is absorbed by a soil, the
base is alone fixed, while the sulphuric acid or chlorine is left in
solution. Lastly, the amount of base absorbed by a soil depends on the
concentration of its solution, on the nature of its combination, and the
temperature. Way found in his experiments that a clay soil has more
power than a peaty soil, and that a peaty soil has more power than a
sandy soil.
_Causes of this Fixation._
So much for the fact of soil absorption; as to the cause or causes of
this absorption, a great number of theories have been put forward. Those
may be divided into two classes--those accounting for it as due to
physical properties of the soil; and those, on the other hand,
explaining it as due to chemical action.
To the latter class Way's belonged. He explained it as due to the
formation in the soil of hydrated double silicates, consisting of a
silicate of alumina, along with a silicate of the base fixed. Bruestlein
and Peters, on the other hand, were of the opinion that it was purely
physical in its nature. A theory has been advanced that it is due to the
formation of insoluble ulmates and humates, formed by the union of ulmic
and humic acids, along with the bases fixed. Among others who devoted
investigation to this interesting question, may be mentioned Rautenberg
and Heiden.
On reviewing the evidence, it seems to be pretty well established that
it really is mainly a chemical act, due chiefly to the formation of
double silicates, and doubtless to a certain extent to the formation of
insoluble humates and ulmates. Heiden's experiments would seem to
indicate, however, that it is also partly of a physical nature.
With regard to the absorption of phosphoric acid, this has been shown to
be a chemical act, and depends on the formation of insoluble phosphates
of calcium, iron, aluminium, and magnesium, the percentage of iron
especially determining this.
Much analytical work has been accomplished of late years with a view of
ascertaining the amount of ash in different kinds of plants, and in the
different parts of the plant.
_Action of Manures._
The department of agricultural chemistry which has been most largely
developed of late years is that connected with the problems of
_manuring_. It is, from a practical point of view, of most value. It is
some considerable time since we have recognised that the only three
ingredients it is, as a rule, expedient to apply as artificial manures,
are _nitrogen_, _phosphoric acid_, and _potash_. The nature, mode of
action of the different compounds, and properties of these three
substances, and their comparative influence in fostering plant-growth,
together with the economic question of which form is, under various
circumstances, the most economical for the farmer to use, have together
given rise to a large number of "field" and "pot" experiments. As the
principles underlying this practice form the subject of the following
treatise, any further discussion of the question must be left to the
following chapters.
_Note._--The reader interested in the historical development of
agricultural chemistry is referred to Sir J. H. Gilbert's
Presidential Address to the Chemical Section of the British
Association, 1880.
FOOTNOTES:
[1] The History of the Chemical Elements. By Sir Henry E. Roscoe, F.R.S.
(Wm. Collins, Sons, & Co.)
[2] Van Helmont's science was, however, of an extremely rudimentary
nature, as may be evidenced by the belief he entertained that the smells
which arise from the bottom of morasses produce frogs, slugs, leeches,
and other things; as well as by the following recipe which he gave for
the production of a pot of mice: "Press a dirty shirt into the orifice
of a vessel containing a little corn, after about twenty-one days the
ferment proceeding from the dirty shirt, modified by the odour of the
corn, effects a transmutation of the wheat into mice." The crowning
point in this recipe, however, lay in the fact that he asserted that he
had himself witnessed the fact, and, as an interesting and corroborative
detail, he added that the mice were born full-grown. See 'Louis Pasteur:
His Life and Labours.' By his Son-in-law. Translated by Lady Claud
Hamilton. (Longmans, Green, & Co.) P. 89.
[3] He then goes on to relate a number of experiments by Cornelius
Drebel and Albertus Magnus, showing the refreshing power of this balsam,
and then those of Quercitan with roses and other flowers, and his own
with nettles.
[4] Priestley, however, did not realise that _carbonic acid gas_ was a
necessary plant-food; on the contrary, he considered it to have a
deleterious action on plant-growth. Percival was really the first to
point out that carbonic acid gas was a plant-food.
[5] It is recorded as an instance of the scientific enthusiasm of the
man, that he was wont to carry about with him bottles containing oxygen,
which he had obtained from cabbage-leaves, as also coils of iron wire,
with which he could illustrate the brilliant combustion which ensued on
burning the latter in oxygen gas.
[6] For a full account of Senebier's researches, see 'Physiologie
vegetale, contenant une description des organes des plantes, et une
exposition des phenomenes produits par leur organisation, par Jean
Senebier.' (5 tomes. Geneve, 1800.)
[7] How Crops Grow. By Professor S. W. Johnson. Macmillan & Co.
(Introduction, p. 4.)
[8] See p. 40 to 45.
[9] Elements of Agricultural Chemistry, in a course of Lectures for the
Board of Agriculture. By Sir Humphry Davy. (London, 1831.)
[10] This department of agricultural research was subsequently carried
on by Sprengel, Schuebler, and others.
[11] Born in Paris, 1802; died 11th May 1887.
[12] See p. 40.
[13] While much of Boussingault's work was carried out previous to the
year 1840, he continued to enrich agricultural chemistry with numerous
valuable contributions up till the time of his death. It may be well
here to mention the names of his most important contributions to
agricultural science, made subsequent to 1840.
In 1843 he published, in a work entitled 'Economie Rurale,' the results
of his numerous experiments and researches. This work is well known to
English agriculturists from an English translation which appeared in
1845 (Boussingault's 'Rural Economy,' translated by G. Law. H. Balliere,
London).
In 1860 appeared the first volume of his last great work, 'Agronomie
Chimie Agricole et Physiologie' This work, which consisted of seven
volumes, was not finished till 1884. He died on the 11th of May 1887. It
may be added that the Royal Society of London awarded him the Copley
medal in 1887.
[14] See British Association Proceedings, 1880, p. 511.
[15] It may be pointed out that, while the amount of ammonia washed down
by the rain is small, Schloesing has found in some recent experiments
that a damp soil may absorb from the air in the course of a year 38 lb.
of combined nitrogen, chiefly ammonia, per acre. See p. 132.
[16] The example, set by Germany, has been followed by other countries
in which well-equipped research stations now exist. Perhaps the most
striking example of the rapid development of the means of agricultural
research is furnished by the United States of America. At present over
fifty agricultural experiment stations, more or less well equipped,
exist at present in that country, all liberally supplied by State aid.
The earliest to be founded, it may be added, was that at Middletown,
Connecticut, the date of its institution being 1875.
[17] It may thus claim to be the second oldest experimental station,
that instituted by Boussingault at Bechelbronn in Alsace being the
oldest.
[18] For an account of the Rothamsted experiments, and a short biography
of Sir John Lawes, the reader is referred to a pamphlet by the present
writer, entitled 'Sir J. B. Lawes, Bart., LL.D., F.R.S., and the
Rothamsted Experiments' ('Scottish Farmer' Office, 93 Hope Street,
Glasgow).
[19] Of these numerous elaborate experiments, perhaps those which have
attracted the most widespread interest amongst agriculturists have been
those carried out on the growth of wheat on the same land year after
year for a period of nearly fifty years. The important light which this
series of experiments has thrown upon the theory of the rotation of
crops, and the subject of the manuring of cereals, is very great.
[20] Associated in some cases with phosphorus and sulphur.
[21] It must be pointed out that plant-respiration does not take place
_only_ during the night-time. It probably goes on at all times, but it
is only during the night-time that its action is apparent, as the
reverse process of carbon assimilation, which goes on at an incomparably
greater rate, masks its action during the daytime.
[22] The length of the day has an important influence on plant-growth,
as is evidenced by the rapid growth of vegetation in Norway and Sweden.
In these countries there is a late spring, and a short and by no means
hot summer, but a very long period of daylight.
[23] A point of great interest which these experiments elucidated is
that nocturnal repose is not absolutely necessary for the growth and
development of all plants.
[24] See pp. 15 and 22.
[25] See p. 22.
[26] See Chapter III., pp. 120 and 131.
[27] Further reference is made to this subject in Chapter III., p. 136.
[28] See p. 6.
[29] See Phil. Trans., Part II., 1861, pp. 444-446. Lawes & Gilbert.
Schloesing has found in the air in the neighbourhood of Paris 1 lb. of
ammonia in 26,000,000 cubic yards; while Muentz found only about half
that amount in a similar quantity of air on the top of the Pic du Midi.
[30] See Chapter III., pp. 119, 120; Appendix, p. 155.
[31] Some recent experiments by Dyer and Smetham would seem to show that
comparatively small quantities of ammonia in the air prove actually
hurtful to plant-life. Thus they found that one volume of ammonia in
1000 volumes of air was fatal to hardy plants; while one volume in 3000
volumes killed tender ones.
[32] According to the experiments of Hellriegel and Wollny. The
quantity, it may be added, varies with the leaf-surface and the length
of the period of growth of the plant. It is greatest with clovers and
grasses, and least in the potatoes and roots.
PART II.
PRINCIPLES OF MANURING
CHAPTER I.
FERTILITY OF THE SOIL.
It is necessary to clearly understand to what the fertility of a soil is
due ere we can hope to master the theory of manuring.
_What constitutes Fertility in a Soil._
The question, What constitutes fertility in a soil? is by no means an
easy one to answer. If we say, The presence of a plentiful supply of the
constituents which form the plant's food, our answer will be incomplete.
Similarly, if we reply, A certain physical condition of the soil--here,
again, it will be found equally unsatisfactory; for fertility of a soil
depends both on its physical condition and on its chemical composition,
and indeed even on other circumstances. It may be well, then, before
proceeding to treat of the nature and action of the different manures,
to offer a brief statement of the conditions of fertility so far, at any
rate, as we at present know them. For it may be well to warn the reader
that, despite the great amount of work carried out on this subject by
experimenters, we still have much to learn before we shall be in a
position fully and clearly to understand the subject of soil-fertility
in all its bearings.
Apart altogether from the influence exerted by climate, latitude,
altitude, and exposure, the fertility of a soil may be said to depend on
the following properties. These we may divide into three groups or
classes:--
1. Physical or mechanical.
2. Chemical.
3. Biological.
=I. Physical Properties of a Soil.=--The physical properties of a soil
are generally admitted to have a very important bearing on its
fertility. This has been long practically recognised, and perhaps has in
the past been unduly exalted in importance, at the expense of the no
less important functions of the chemical.[33] The reason of this is
doubtless to be ascribed to the fact that it is much easier to study
the physical properties of a soil than it is to study the chemical; and
that, while we are in possession of a very large amount of useful
information with regard to the former, we are at present only on the
threshold of our knowledge of the latter.
_Variety of Soils._
It is a matter of common observation that soils differ widely in their
mechanical nature. The early recognition of this fact is evidenced by
the large number of technical terms which have been long in vogue among
farmers descriptive of these differences. Thus soils are in the habit of
being described as "heavy," "light," "stiff," "strong," "warm," "cold,"
"wet," "damp," "peaty," "clayey," "sandy," "loamy," &c., &c.
_Absorptive Power for Water._
One of the most important of the physical properties of a soil is its
power to absorb water.
Water to the plant economy is just as important and necessary as it is
to the animal economy. Consequently it is of primary importance to
examine into the conditions which regulate the absorption of this
important plant-food by the soil.
By the absorptive power of a soil is meant its capacity for drinking in
any water with which its particles may come in contact. This power
depends, first, on the predominance of its proximate constituents--viz.,
_sand_, _clay_, _carbonate of lime_, and _humus_; and secondly on the
fineness of the soil-particles.
_Absorptive Power of Sand, Clay, Humus._
First, then, with regard to the absorptive power of sand, clay, and
humus. Of these, sand possesses this power to the least extent, clay to
a greater extent, while humus possesses it most of all.[34]
The extent, therefore, of the absorptive power of a soil depends very
much on the proportions in which it possesses these three ingredients.
The more sandy a soil is, the less will its power be of absorbing water;
and this, there is little doubt, is one of the reasons why a sandy soil
is, as a rule, an unfertile soil. Of course there are other and even
more important reasons; but that this absorptive power has an important
bearing on the question is conclusively proved by the fact that sandy
soils are more fertile in a climate where rain is frequent than in one
where much dry weather prevails. The incapacity of a sandy soil to
absorb a large quantity of moisture is not fraught with such evil
effects to the crops in the former case, because it is counteracted by
the climatic conditions, which obviate the necessity, in a soil, of
possessing great absorptive powers.
The converse, of course, we may mention in passing, holds good of clayey
soils.
_Fineness of Soil-particles._
The second quality in a soil on which its absorptive power depends is
the fineness of its particles. The great benefit which a soil derives
from a good tilth, in this respect, was one of the reasons why Tull's
system of horse-hoeing husbandry was so successful in its results.[35]
The finer the soil-particles, it may be said generally, the greater is
the absorptive power of the soil.
_Limit to Fineness._
There is, however, a limit to the fineness to which the particles of a
soil ought to be reduced; for it has been found by experiment that when
a certain degree of fineness is reached, the absorptive power decreases
with any further pulverisation. A German experimenter found, for
example, that a garden loam, capable of absorbing 114 per cent of water
in its natural state, when pulverised very fine was able to absorb only
62 per cent of water. Here, clearly, the limit to which it is advisable
to pulverise a soil had been exceeded.
_Reason of the above._
It is not difficult to see why this should be so. The amount of water
that a soil can soak up is due to the number of pores, or air-spaces, it
contains of a certain size. If these pores are large and few in number,
the amount of water absorbed will be naturally less than when they are
numerous and smaller in size. Up to a certain extent, the more a soil is
broken the greater will be the number of pores created, of a size to
permit the water to soak in. Beyond that point the pores become too
minute, and the soil becomes too compact, each particle clinging
together too closely.
_Retentive Power of Soils for Water._
Now closely connected with this absorptive power of soils, which we have
just been considering, is the power soils possess of holding or
retaining the water they absorb. This power, it will be seen at a
glance, must have an important bearing on the fertility of a soil.
_Importance of Retentive Power._
As a considerable interval often elapses between the periods of
rainfall, soils, if they are to support vegetable growth, must be able
to store up their water-supply against periods of drought. This is all
the more necessary when we remember that, in the case of heavy crops,
the rainfall would often be inadequate to supply the water necessary for
their growth. In fact, it has been estimated that the average
evaporation from soils bare of any cultivation is equal to the rainfall.
That the evaporation from soils covered with vegetation is very much
greater, has been strikingly shown by a calculation made by the late
eminent American botanist, Professor Asa Gray, who calculated that a
certain elm-tree offered a leaf-surface, from which active transpiration
constantly went on, of some five acres in extent; while it has further
been calculated that a certain oak-tree, within a period of six months,
transpired during the daytime eight and a half times more water than
fell as rain on an area equal in circumference to the tree-top.[36] Just
as the state of the fineness of the soil-particles has an important
influence on the absorptive power of soils, so, too, it is found, it has
an important bearing on the rate at which evaporation takes place.
Evaporation goes on to the greatest extent in soils whose particles are
compacted together, capillary action in this case taking place more
freely, and effecting evaporation from a greater depth of soil. The
stirring of the surface portion of the soil, as for example by hoeing or
harrowing, has for this reason an important influence in lessening the
amount of evaporation, and minimising the risks of drought, by breaking
the capillary attraction. The amount of evaporation which takes place
from a soil covered with a crop, depends largely on the nature of the
crop; a deep-rooted crop, since it draws its moisture from a wider area
of soil, being more effective in drying a soil than a shallow-rooted
crop. The difference in the amounts evaporated from a cropped and a bare
fallow soil has been shown at Rothamsted to equal a rainfall of nine
inches, the crop being barley. The increase, of course, is due to the
water which the crop transpires.[37]
It may be generally said that the greater the absorptive power of a
soil, the greater is its retentive power; for soils that most largely
absorb water are the most reluctant to part with it.
While these properties are undoubtedly necessary for fertile soils, it
is needless to add that they may be possessed by a soil to too great an
extent. The soil that is unable to throw off any excess of water becomes
cold and damp, and does not admit of proper tillage. Its pores become
entirely choked up, and the circulation of air, which, as we shall see,
is of so much importance, is rendered impossible. Plants in such a soil
are apt to sicken and die, the water becomes stagnant, and certain
chemical actions are caused which give rise to poisonous gases, such as
sulphuretted hydrogen, &c. A stiff clayey soil offers a good example of
the disadvantage of over-retentiveness. Owing to the difficulty such
soils experience in throwing off their excessive water, they are
extremely difficult to till; and sowing operations are on that account
apt to be delayed.
_Power Plants have of absorbing Water from a Soil._
It is a strange fact, and one worth noticing in this connection, that
the power plant-roots have of drawing their moisture from a soil, seems
to depend on the retentive power of the soil. By this is meant that
plants have not the means of exhausting the water in a retentive soil to
such an extent as in a non-retentive soil.
In some extremely interesting experiments, carried out by the well-known
German botanist Sachs, it was found that plants wilted in a loamy soil,
whose water-holding capacity was 52 per cent, when its moisture reached
8 per cent; while in a sandy soil--water-holding capacity 21 per
cent--the same species of plant did not wilt until its moisture reached
1-1/2 per cent. Here, then, we see that on one kind of soil the plant
was able to live, and obtain sufficient water for its needs, while it
died of thirst in another soil, although that soil contained quite as
much moisture.
Speaking generally, we may say that Hellriegel's experiments have shown
that any soil can supply plants with all the water they need so long as
its moisture is not reduced below one-third of the whole amount it can
hold.[38]
_How to increase Absorptive Power of Soils._
The absence or presence, in excess, of the above properties, suggests a
word or two on how these natural defects may, to a certain extent, be
remedied artificially. It stands to reason, that if organic matter in a
soil renders its absorptive power greater, a simple method of improving
a soil defective in this property is by the addition of organic matter.
One of the benefits of ploughing-in green crops on sandy soils is
undoubtedly due to this fact; the addition of farmyard manure having
also a similar effect. The absence of a sufficient amount of
retentiveness, such as is found in sandy soils, in the same way
suggests, as a remedy, the addition of clay; and, _vice versa_, where
the soil is too clayey, the natural method of improvement will be the
addition of sand.[39]
_Shrinkage of Soils._
In drying, soils shrink. Those which shrink least are sandy and chalky
soils. Humus soils, on the other hand, shrink most.
_Most favourable Amount of Water in a Soil._
The amount of water in a soil most favourable for plant-growth is a
question of considerable difficulty. Too great an amount of moisture
renders the land cold; air cannot obtain access to the soil-particles,
and the plants sicken and die. Hellriegel has found that as much as 80
per cent of what the soil can hold is hurtful to plants, and that from
50 to 60 per cent is the best amount.[40]
_Hygroscopic Power._
A property possessed by soils in relation to water, which is quite
distinct from absorptive power, is their hygroscopic power. By this is
meant their power of absorbing water from the air where it is present in
the gaseous form. This property is identical with the property which
will be adverted to immediately--viz., capacity for absorbing gases. The
extent to which soils possess this hygroscopic property seems to be
regulated very much by the same conditions as regulate their ordinary
absorptive power.[41] This property is considered to be of great
importance in the case of soils in hot climates, where their
agricultural value may be said to depend to a large extent upon it. The
amount of water, however, absorbed in this way is, comparatively
speaking, insignificant. Lastly, it may be observed that there are
certain methods of drying soils afflicted with too much moisture. These
consist in making open ditches, and thus relieving them of their
superabundance of water, or in planting certain kinds of trees, such as
willows and poplars. The amount of green surface presented by the large
number of leaves of trees, from which the constant evaporation of water
goes on, is very great. The consequence is that trees may be regarded as
pumping-engines. It is from this cause that foresters have noticed that
clay lands are apt to become wetter after the trees growing upon them
have been cut down.[42]
_Capacity for Heat in Soils._
A property which depends largely on those we have just been considering
is the capacity soils possess of absorbing and retaining heat.[43] The
temperature of a soil, of course, largely depends on the temperature of
the air; but this, we must not forget, depends also on the soil itself.
The heat given forth by the sun's rays strikes the soil, with the result
that, while so much of its heat is absorbed, a certain portion--and this
will vary according to the nature of the soil--of its heat is radiated
into the air.
The changes in the temperature of the soil naturally take place more
slowly than the changes in the temperature of the air; the depth of soil
thus affected by those changes varies also in different climes. It has
been calculated that in temperate climes the changes of temperature
occurring from day to night are not felt much below three feet down.
_The Explanation of Dew._
We have, it may be stated, generally two processes going on. During the
day the soil is engaged in absorbing its heat from the sun's rays; when
night comes, and the sun goes below the horizon, the air is chilled
below the temperature of the soil, which radiates out its stored-up heat
into the air. The result is that the temperature of the soil is soon
reduced below the temperature of the air, and the moisture, present in
the air in the form of vapour, coming in contact with the cold surface
of the earth, is condensed into dew, which is deposited, and is seen
best early in the morning before the sun has had time to evaporate it
again. Dew is most abundant in summer-time, for the reason that the
difference in temperature of the day and night is then greatest. In
winter-time it is seen as hoar-frost.
_Heat of Soils._
The temperature of a soil, however, is due to other sources than the
sun's rays. Whenever vegetable matter decays, there is always a certain
amount of heat generated. Soils, therefore, in which there is a large
amount of decaying vegetable matter, are certain to receive more heat
from this source than soils of more purely mineral nature.
_Heat in Farmyard Manure._
A good example of the amount of heat that accompanies fermentation, or
decay of vegetable matter, is seen in the case of rotting farmyard
manure. The danger of loss of the volatile ammonia from this cause is
often great, and care must be taken to prevent fermentation going on too
quickly, and the temperature from becoming too high.[44] The actual
increase in the temperature of a soil effected by the addition of
certain bulky organic manures, such as farmyard manure, may thus be
considerable. In some experiments carried out at Tokio, Japan, it was
found that the application of 20 tons of farmyard manure per acre
increased the temperature of the soil to a depth of five inches, for a
period of nearly a month, on an average, one and a half degrees
Fahrenheit. The amount of water present in a soil, it may be noticed in
passing, will have a considerable effect in regulating its temperature,
a damp soil being, as a rule, a cold soil.
_The Cause of the Heat of Fermentation._
It may be asked, How is the decay, or fermentation, of vegetable matter,
such as farmyard manure, caused? or rather, To what is it due? Decay of
any substance is just its slow combustion or burning. When a substance
unites with the active chemical element in air--the oxygen gas--it is
said to be oxidised. Now, this union of a substance with oxygen is the
explanation of burning, and the phenomena of burning and decay are
explained by the same chemical operation. When bodies decay, or when
they burn, they unite with oxygen: when this union of a body and oxygen
takes place very quickly, and the result is a flame and very great heat,
then we call it burning; when, however, it takes place slowly, it is not
called burning, but simply oxidation or decay. The ultimate products are
the same, however, whether the body burns or decays; and the process of
decay is always accompanied by heat, as well as the process of
burning.[45] It is not, of course, only the vegetable or organic matter
in a soil that decays, but also the mineral matter. The oxidation,
however, of the mineral matter in the soil takes place so slowly, and
the amount of heat generated by this oxidation is so slight, that the
temperature of the soil can scarcely be said to be much affected by it.
_Influence of Colour of a Soil._
There is still another quality of a soil on which its temperature
depends, and that is its colour. This may seem at first sight to be
scarcely worth taking into account, and yet it has been shown to have a
very striking influence on the temperature of a soil. This naturally is
best seen in climates where there is a good deal of sun. Dark-coloured
soils have a greater heat-absorbing capacity than light-coloured soils;
and experiments carried out for the purpose of determining the extent of
this influence have shown that under certain conditions the difference
between a soil covered with a black substance, and one covered with a
white substance, amounted to from 13 deg. to 14 deg. Fahr. Other things being
equal, a crop on a dark-coloured soil will be sooner ripened than one on
a light-coloured soil. A soil covered by a crop is cooler than one
without any crop.
_The Power Soils have for absorbing Gases._
We have just seen that one cause of the heat of soils is the oxidation
which is constantly going on in all soils, but more rapidly in soils
containing a large quantity of vegetable matter. This suggests a word or
two on the power soils have of absorbing gases.
The chief gases in the atmosphere are oxygen and nitrogen. Both these
gases are absorbed by soils, although not in similar proportions.[46]
With regard to the former, it is well known that a plentiful supply of
oxygen in the pores of the soil is a necessary condition of fertility.
This was long ago experimentally proved by de Saussure, who showed that
plants absorbed oxygen through their roots. At certain periods of their
growth this demand for oxygen on the part of the plant is greater than
at other times. For example, seeds in the process of germination require
to have free access to a plentiful supply of oxygen. This fact
emphasises the enormous importance of providing a good seed-bed, and of
seeing that the seed is not buried too deeply.
_Carbonic Acid and Ammonia._
In addition to oxygen and nitrogen, the air contains other gases which
are absorbed by the soil. Of these, carbonic acid is the most abundant.
By far the largest portion of the carbonic acid which the soil obtains
from the air, is washed down in solution in the rain.[47] Of the other
constituents of the atmosphere, the combined forms of nitrogen--viz.,
_ammonia_, _nitric_, and _nitrous acids_--are the most important. These
are all absorbed by the soil, but, like carbonic acid, they are chiefly
washed down by the rain. The amount of ammonia which may be absorbed by
a soil from the air, is very much greater than was formerly supposed.
Some recent experiments by Schloesing, referred to in a following
chapter,[48] show this. A damp soil may in the course of a year absorb
far more ammonia than that washed down in rain.
_Gas-absorbing Power of Soils varies._
The power of different soils to absorb these gases varies. This
variation depends not only on their physical properties, but also on
their chemical as well. Soils containing much organic matter have a
greater capacity for absorbing gases than the more purely mineral ones.
_Absorption of Nitrogen._
The absorption of nitrogen by the soil is a question of considerable
importance. It will be referred to later on under the heading of the
biological properties of soils, as it is fixed by the agency of
micro-organisms.[49]
To recapitulate, the chief physical or mechanical properties of a soil
are its absorptive and retentive powers for water; its capacity for
heat; and its power of absorbing gases. It will be easily seen how
tillage operations are calculated to influence these physical properties
of a soil. Thus, in the case of a stiff soil, tillage increases its
power for absorbing the atmospheric gases, chiefly oxygen, which are so
necessary for rendering its fertilising matters available. On the other
hand, in a light and too open soil it may exert quite a contrary effect.
It may be also well to refer here to the important influence these
physical properties exercise on the growth of the plant.
_Plant-roots require a certain Openness in the Soil._
One of the functions of the soil is to support the plant in an upright
position, and this is a function which requires in the soil a certain
amount of compactness or firmness. On the other hand, however, a soil
must not possess too great compactness, otherwise the plant-roots will
experience a difficulty in pushing their way downwards. This is
especially the case during the earlier periods of growth, when the
plant-roots are as yet extremely tender, and experience great difficulty
in overcoming much resistance. The importance of preparing a mellow
seed-bed will be thus at once seen to be based on sound scientific
principles; and this for a double reason. Not only does the young plant
require every facility for developing its roots, but also, as has just
been pointed out, an abundant supply of oxygen is of paramount
importance during the process of germination.
_Soil and Plant-roots._
The whole question of the influence of the mechanical condition of the
soil on the development of plant-roots is one of the highest importance
and interest, and is not so generally recognised as it ought to be.
_Natural tendency of Plant-roots to grow downwards._
It may be taken as certain that the tangled condition of plant-roots is
due to the resistance offered by the soil-particles, and that the
natural tendency of the plant-root is to grow downwards. The roots, in
short, would probably grow in as symmetrical a form as do the stalks or
branches, were it not that they are hindered from so doing by the
soil-particles. Where, then, the soil is such as to offer much
hindrance, the growth of the plant cannot but be retarded. Some
extremely interesting experiments have been performed by the eminent
German chemist Hellriegel on the influence which the closeness of the
soil-particles has on root-development. In these experiments peas and
beans were grown in moistened sawdust, more or less compactly
compressed. It was found that when the sawdust Was compressed to any
extent, plant-growth took place very slowly, or entirely ceased.
The importance of having plant-roots as widely developed in the soil as
possible, will be at once seen when we reflect that this means that the
area of soil from which the plant derives its soil-food is thereby
greatly increased. Another important consideration is, that the deeper
plant-roots can penetrate in a soil, the more able--other conditions
being equal--is the plant to withstand the action of drought, as it can
draw water for its needs from the deeper layers of the soil, long after
a plant, whose roots do not penetrate so deeply, has wilted.
_Plants require Room._
Another important bearing tillage has on plant-growth may here be
discussed. A problem of considerable difficulty is presented in the
question, How many individual plants will a certain piece of soil
support in a healthy way? For as plants require room, it is imperative
that they be not too closely crowded together.
The question resolves itself pretty much into one of quality against
quantity.
Experiments on this subject have shown that a certain area of soil is
only able to support the healthy growth of a certain number of plants.
If the limit be exceeded, the result is imperfect development.
_Number of Plants on certain Area increased by Tillage._
It is obvious, however, that the more thoroughly tilled a soil is, the
greater will be the number of plants it will be possible to grow on it.
The roots, instead of being forced to spread themselves along the
surface-soil, and thus take up a large amount of room, will find no
difficulty in striking downwards. Two or three plants may thus be
enabled to grow in a thoroughly tilled soil in the same space as only
one could before tillage.
_American and English Farming._
The above considerations throw considerable light on what seems to many
farmers a strange anomaly--viz., the fact that the return of farm
produce per acre on American farms is, as a rule, very much less than
that from our own impoverished soils in this country. To many, at first
sight, this seems to be in direct contradiction to our common belief,
and to point to the conclusion that the virgin soils of America are,
after all, actually inferior in fertility to the soils of Britain.
It is not, however, necessary to draw this conclusion, as the facts of
the case admit of another explanation. The inferior returns obtained
from American farms are due, not to the fact that the American soil is
less fertile than the British--for this is not true--but to the fact
that it is less _intensively_ cultivated.
In America land is cheap and labour is dear; it is consequently found to
be more economical to cultivate a large tract of land less thoroughly
than a small area more thoroughly. In Britain the reverse is the case,
labour being cheap and land being dear. It is thus necessary to make the
land go as far as possible, and produce as heavy a crop as it is
possible to produce. There can be little doubt, that were American
farming to be carried on as intensively as is British farming, the
present yield would be at least probably doubled.
We have now to consider the second class of properties which influence
the fertility of a soil. These are _chemical_.
=II. Chemical Composition of a Soil.=--Chemically considered, the soil
is a body of great complexity. It is made up of a great variety of
substances. The relations existing between these substances and the
plant are not all of equal importance; some--and these form by far the
largest proportion of the soil-substance--are concerned in acting simply
as a mechanical support for the plant, and in helping to maintain those
physical properties in the soil which, as we have just seen, exercise
such important functions in the plant's development.
_Fertilising Ingredients._
A small portion of the soil-substance, however, takes a very much more
active part in promoting plant-growth, by acting as direct food of the
plant. As we have already seen in the Introductory Chapter,[50] the
substances which have been found in the ash of plants are the following:
_potash_, _lime_, _magnesia_, _oxide of iron_, _phosphoric acid_,
_sulphuric acid_, soda, silica, chlorine, oxide of manganese, lithia,
rubidia, alumina, oxide of copper, bromine, and iodine. The general
presence of some of these substances is doubtful; the presence of
others, again, probably purely accidental; while some are only found in
plants of a special nature, as, for instance, iodine and bromine, which
are only found in the ash of marine plants.
Of these ash constituents, only the first six substances--those marked
in italics--are absolutely necessary to plant-growth. In addition to
these six ash constituents, the plant also derives its _nitrogen_, which
is a necessary plant-food, chiefly from the soil.[51]
_Importance of Nitrogen, Phosphoric Acid, and Potash._
But of these seven constituents of the soil which are necessary to
plant-growth, some have come to be regarded by the agriculturist with
very much greater interest than others. This is due to the fact that
they are normally present in the soil in very much smaller quantities
than is the case with the other equally necessary food ingredients;
that, in short, they are nearly invariably present in the soil, in a
readily available form, in lesser quantities than the plant is able to
avail itself of, and often, as in impoverished or barren soils, in
quantities too small for even normal growth. These ingredients are
_nitrogen_, _phosphoric acid_, and _potash_.[52]
The importance of seeing that all the necessary plant ingredients are
present in a soil in proper quantities will be at once properly
estimated when it is stated that the absence or insufficiency in amount
of one single ingredient is capable of preventing the growth of the
plant, although the other necessary ingredients may be even abundantly
present.
With lime, magnesia, iron, and sulphuric acid, most soils are abundantly
supplied. The substances with which the farmer has to concern himself,
then, are nitrogen, phosphates, and potash. It is these substances
therefore, that, as a rule, are alone added as manures.
_Chemical Condition of Fertilising Ingredients in Soil._
But in considering the chemical properties of a soil, a simple
consideration of the quantity of the different ingredients present is
not enough. A very important consideration is their chemical condition.
Ere any plant-food can be assimilated by the plant's roots, it must
first be rendered soluble. The quantity of soluble, or, as it is known,
_available_, plant-food in a soil is very small. It is, of course, being
steadily added to each day by the process of disintegration constantly
going on in soils.
_Amount of Soluble Fertilising Ingredients._
The exact nature and dissolving capacity of the soil-water, charged as
it is, to a greater or less extent, with different acids and salts, as
well as the dissolving power of the sap of the rootlets of the plant
itself, render the exact estimation of the available fertilising
constituents wellnigh impossible. An approximate estimate, however, may
be obtained by treating the soil with pure water and dilute acid
solutions. The treatment of the soil with dilute acid solutions is for
the purpose of simulating, as nearly as may be done, the conditions it
is submitted to in the soil. By treating a soil with water, we obtain a
certain amount of plant-food dissolved in the water. This can only be
regarded as indicating approximately the amount available at that moment
to the plant. But every day, thanks to the numberless complicated
reactions going on in the soil, this soluble plant-food is constantly
being added to. Considerations such as the above, together with our
ignorance as to the exact combinations in which the necessary minerals
enter the plant, will serve to indicate the great difficulty of this
part of the subject.[53]
_Value of Chemical Analysis of Soils._
It is largely for these reasons that a chemical analysis of a soil is
from one point of view of little value in giving evidence of its actual
fertility. What it demonstrates more satisfactorily is its potential
fertility. It is useful in revealing what there is present in it, not
necessarily, however, in an available condition. Under certain
circumstances it may be made of great value, as, for example, when we
are anxious to know what will be the result of certain kinds of
treatment, such as the application of lime, &c.
It is hardly advisable, therefore, to place before the reader a number
of soil analyses. That he may obtain an approximate idea of the
composition of a soil, one or two representative analyses will be found
in the Appendix,[54] along with a short account of the chief minerals
out of which soils are formed.
A point of considerable interest is the quantity per acre different
soils contain of nitrogen, phosphoric acid, and potash. Although the
amount of these ingredients when stated in percentage seems very
trifling, yet when calculated in lb. per acre, it is seen to be in large
excess of the amount removed by the different crops. This question will
be dealt with in succeeding chapters.
A point of further interest is the chemical form in which the necessary
plant constituents are present in the soil. For information on this
point the reader is referred to the Appendix.[55]
The third class of properties which affect the fertility of a soil are
those which have been termed the _biological_.
=III. Biological Properties of a Soil.=--The important functions which
modern discoveries have shown to be discharged by minute organic life in
the terrestrial economy are nowhere more strikingly exemplified than in
the important _role_ they perform in the soil.
_Bacteria of the Soil._
The soil of every cultivated field is teeming with bacteria whose
function is to aid in supplying plants with their necessary food. The
nature of, and the functions performed by, these organisms differ very
widely. Regarding many of them we know very little; every day, however,
our knowledge is being extended by the laborious researches of
investigators in all parts of the world, and it is to be anticipated
that ere long we shall be in possession of many facts regarding the
nature and the method of the development of these most interesting
agents in terrestrial economy. That they are present, however, in
enormous numbers in all soils we have every reason to believe, one class
of organism connected with the oxidation of carbonic acid gas being
estimated to be present to the extent of over half a million in one
gramme of soil[56] (Wollny and Adametz). One class--and their
importance is very great in agriculture--prepare the food of plants by
decomposing the organic matter in the soil into such simple substances
as are easily assimilated by the plant. The so-called "ripening" of
various organic fertilisers is effected, we now know, entirely through
the agency of bacteria of this class. Plant-life is unable to live upon
the complex nitrogenous compounds of the organic matter of the soil, and
were it not for bacteria these substances would remain unavailable.
Attention will be drawn in the Chapter on Farmyard Manure to this
question more in detail. Of these bacteria, among the most important are
those which are the active agents in the process known as
"nitrification"--_i.e._, the process whereby organic nitrogen and
ammonia salts are converted into nitrites and nitrates. The presence of
these organisms, it would appear, is indispensable to the fertility of
any soil. There are organisms, on the other hand, which have the power
of reversing the work of the nitrification bacteria by converting
nitrates into other forms of nitrogen. The reduction of nitrates in the
soil is often the source of much loss of valuable nitrogen, which
escapes in the free state, so that the action of bacteria is not
altogether of a beneficial nature.
_Three Classes of Organisms in the Soil._
So far as the subject has been at present studied, the micro-organisms
in the soil may be divided into three classes.[57]
_First Class of Organisms._
We have, first of all, those whose function it is to oxidise the soil
ingredients. Organisms of this class may act in different ways. They may
assimilate the organic matter of the soil and convert it into carbonic
acid gas and water; or, on the other hand, they may oxidise it by giving
off oxygen. Some of these organisms, whose action is of the first kind,
choose most remarkable materials for assimilation. One has been found to
require ferrous carbonate for its development, which it oxidises into
the oxide (Winogradsky); while another,[58] the so-called sulphur
organism, converts sulphur into sulphuretted hydrogen according to some,
and according to others into sulphates. To this class of organism the
nitrifying organisms belong. As will be seen more fully in a subsequent
chapter, two distinct organisms connected with this process have already
been isolated and studied--one of these effecting the formation of
nitrites from organic nitrogen or ammonia salts, and the other the
conversion of nitrites into nitrates. The second method in which these
oxidising organisms act is by giving off oxygen. There is much interest
attaching to this fact, as it was supposed till quite recently that all
evolution of oxygen in vegetable physiology was dependent on the
presence of light, and also intimately connected with chlorophyll, or
the green colouring matter of plants. It would seem, however, that among
the soil organisms these conditions are not necessary, and the evolution
of oxygen may be carried on in the case of colourless organisms as well
as in the case of light. With organisms of this kind every soil is
probably teeming. A typical example is the organism which is the active
agent in the oxidation of carbonic acid gas, and which has already been
referred to as existing in the soil in such numbers.[59]
_The Second Class of Organisms in the Soil._
The second class of organisms are those which reduce or destroy the soil
constituents. The most important of these, from the agricultural point
of view, are those which effect the liberation of nitrogen from its
compounds. In the putrefaction of organic matter the organisms chiefly
act, it is probable, in the entire absence of atmospheric oxygen; but
it would seem, however, that they may also act in the presence of
oxygen. It is through their agency that the soil may lose some of its
nitrogen in the "free" form. To this class belong the denitrifying
organisms already referred to which reduce the nitrates and nitrites in
the soil.[60]
_Third Class of Organisms._
The third class of organisms are those by whose agency the soil is
enriched. Of this class those fixing the free nitrogen from the air are
the most important. The nature of these organisms is still somewhat
obscure, but that leguminous plants have the power of drawing upon this
source of nitrogen is now a firmly established fact. Further reference
to these interesting organisms may be delayed to another chapter.
The important point to be emphasised is, that for the healthy
development of these organisms, which are so necessary in every fertile
soil, certain conditions must exist. These necessary conditions will be
treated more in detail later on. It is sufficient to notice that they
have to do with the physical properties as well as the chemical
composition of the soil. This furnishes a further reason for the
necessity of having the mechanical condition of a soil satisfactory.
_Recapitulation._
From what we have said, it will be seen that the question of
soil-fertility is a very complicated one, and depends on numerous and
varied conditions; that the properties which constitute fertility, while
seemingly very widely different in their nature, in reality influence
one another to a very great extent; that not merely is the presence in a
soil of the necessary plant constituents necessary to fertility, but
that the possession by the soil of certain physical or mechanical
properties is equally necessary; while, lastly, we have seen that the
presence of certain micro-organic life is bound up with the problem of
fertility in a very direct and practical manner.
The importance of the conditions, other than those of a purely chemical
nature, have been thus far somewhat prominently emphasised, for the
reason that in what follows attention will be almost exclusively devoted
to the purely chemical conditions of fertility. It is well, then, to
realise that, while the latter conditions are by far the most important,
so far as the farmer is practically concerned, inasmuch as they are most
under his control, they are not the only conditions, and are not by
themselves able to control fertility.
FOOTNOTES:
[33] This statement perhaps needs qualification. While the important
_role_ played by the physical qualities of the soil were in the early
years of the science recognised, of more recent years the chemical
composition of the soil has been engaging almost exclusive
investigation. Physical properties of the soil have recently acquired a
further importance in the eyes of the agricultural chemist, from the
important influence they exert on what we have here called the
biological properties of a soil--viz., the development of those
fermentative processes whereby plant-food is prepared to a large extent.
[34] A good example of the absorptive capacity of a soil containing a
large quantity of vegetable matter is furnished by peat-bogs, which,
sponge-like, can absorb enormous quantities of water. (See Appendix,
Note I., p. 98.)
[35] Jethro Tull, an early well-known agricultural writer, who lived
about the middle of last century, propounded the theory, that as the
food of plants consisted of the minute earthy particles of the soil, all
that was required by the skilful farmer was to see that his soil was
properly tilled. He accordingly published a work entitled 'Horse-hoeing
Husbandry,' in which he advocated a system of thorough tillage. (See
Historical Introduction, p. 10.)
[36] See Introduction, p. 55.
[37] See Introductory Chapter, p. 55.
[38] It is not exactly known why excess of water should prevent normal
growth in the plant. Probably it is on account of the fact that free
access of oxygen is hindered in such a case. The roots are thus not
freely enough exposed to this necessary gas, and fermentative processes
of the nature of nitrification are not promoted. It may be also due to
the fact that the solution of plant-food is too dilute when such excess
of water prevails.
[39] See Appendix, Note II., p. 98.
[40] Some experiments by E. Wollny show this. He found, when
experimenting with _summer rape_, that the best results were obtained
when the soil contained only 40 per cent of its total water-holding
power; when the amount was either lessened or increased the results
obtained fell off. The effect of either too little or too much water is
seen in the development of the different organs of the plant as well as
on its period of growth, much water seeming to retard the growth. The
quality of the plant seems also to be influenced by this condition.
Experiments on cereal grains by Wollny show that not merely is the
texture of the grain influenced, but that much moisture lessens the
percentage of nitrogen. Wollny is of the opinion that for crops
generally, the best amount is from 40 to 75 per cent of the total
water-holding capacity of the soil.
[41] See Appendix, Note III., p. 99.
[42] See p. 55.
[43] The effect of the temperature of the soil on the development of the
plant is most important. This is especially marked at the period of
germination, but is felt at subsequent periods of growth. Up to a
certain temperature the warmer the soil the more rapid the plant's
development. In this country the temperature most favourable to growth
is rarely exceeded, or indeed reached.
[44] See Chapter on Farmyard Manure.
[45] As will be seen further on, the fermentation of organic substances
is caused by the action of micro-organic life.
[46] See Appendix, Note IV., p. 100.
[47] Of course it must be remembered that a large amount of carbonic
acid in soils comes from the decay of vegetable matter. Soils are twenty
to one hundred times richer in carbonic acid than the air.
[48] See Chapter III., p. 119.
[49] See Introduction, p. 40.
[50] See Introductory Chapter, p. 54.
[51] See pp. 44 and 135.
[52] Occasionally also _lime_.
[53] See Appendix, Notes V. and VI., pp. 100, 101.
[54] Note VI., p. 101.
[55] Note VII., p. 107.
[56] Even larger estimates of the number of germs in a gramme of soil
have been made--from three-quarters to one million (Koch, Fuelles, and
others).
[57] These organisms consist of molds, yeast, and bacteria, the
last-named being most abundant. In the surface-soil, among the bacteria,
bacilli are most abundant. Micrococei are not abundant.
[58] Investigated by Winogradsky, Olivier, De Rey Pailhade, and others.
[59] Organisms of this kind have been investigated among others by
Heraues, Hueppe, and E. Wollny. According to the two first-mentioned
investigators, certain colourless bacteria effect the formation in the
absence of light from humus and carbonates a body resembling in its
nature cellulose.
[60] Investigated by Springer, Gayon and Dupetit, Deherain, and
Marguenne.
APPENDIX TO CHAPTER I.
NOTE I. (p. 68).
The following determinations by Schuebler show the absorptive power of
different kinds of soil-substances. These were obtained by soaking
weighed quantities of the soil in water, and allowing the excess of
liquid to drain away, and weighing the wet earth.
Per cent of water
absorbed by 100
parts of earth.
Siliceous sand 25
Gypsum 27
Calcareous sand 29
Sandy clay 40
Strong clay 50
Arable soil 52
Fine calcareous 85
Garden-earth 89
Humus 190
It has been calculated that the absorptive power of a mixture of
different substances is not simply equal to the sum of their separate
ingredients.
NOTE II. (p. 74).
EVAPORATION.
The retentive property of a soil for water tends to retard evaporation.
The following table by Schuebler shows the rate at which evaporation
proceeds in different soils. The experiment was conducted in the
following way. The soil experimented upon was saturated with water and
spread over a disc, and allowed to evaporate for four hours, when it was
weighed. The amount of time required for the evaporation of 90 per cent
of the water was also estimated. Of 100 parts of water in the wet soil
there evaporated, at 60 deg. Fahr.--
Time required to
In four hours-- evaporate 90 per cent.
From-- per cent. Hours. Minutes.
Quartz 88 4 4
Limestone 76 4 44
Sandy clay 52 5 1
Stiffish clay 46 6 55
Loamy clay 46 7 52
Pure grey clay 32 11 17
Loam 32 11 15
Fine calcium carbonate 28 12 51
Humus 21 17 33
Magnesium carbonate 11 33 20
NOTE III. (p. 76).
HYGROSCOPIC POWER OF SOILS.
Davy found the hygroscopic power of soils to be as follows. He found
that 100 parts by weight of three samples of different sands absorbed 3,
8, and 11 parts of water, respectively, in one hour; while three loams
absorbed similarly 1.3, 1.6, and 1.8 parts.
The following samples of soil were dried at 212 deg. Fahr., and exposed to
an atmosphere saturated with water and a temperature of 62 deg. Fahr., when
it was found they absorbed the following amounts in twelve hours'
time:--
Quartz sand 0.0
Limestone sand 0.3
Lean clay 2.1
Fat clay 2.5
Clay soil. 3.0
Pure clay. 3.7
Garden-loam 3.5
Humus 8.0
NOTE IV. (p. 81).
GASES PRESENT IN SOILS.
The air which we find enclosed in the pores of the soil is distinctly
_poorer_ in oxygen than ordinary air. Boussingault found the percentage
of oxygen in a sandy soil, freshly manured and wet with rain, to be as
low as 10.35 per cent; while the air in forest-soil contained 19.5 per
cent of oxygen, and .93 per cent of carbonic acid. The percentage of
oxygen in soils depends on the rate of decay of the organic portions.
The depth of the soil-layer also determines the quantity. This is owing
to the fact that diffusion takes place more slowly deep down than near
the surface.
NOTE V. (p. 90).
AMOUNT OF SOLUBLE PLANT-FOOD IN THE SOIL.
Two of the most reliable methods of ascertaining an approximation of the
quantity of soluble soil constituents are (1) by treating the soil with
distilled water, and (2) by analysing the drainage-water. With regard to
the former of these two methods, it has been found that even the amount
of fertilising matter dissolved out by pure distilled water varies. This
variation depends on the amount of distilled water used, as well as the
length of time the soil is left in contact with the solvent. By washing
the soil with different quantities of water, different amounts of
soluble soil ingredients will be found to have been washed out; for
although the first washings contain by far the greater portion of the
soluble matter, each subsequent washing will be found to contain further
quantities.
A number of experiments have shown that 1000 parts of distilled water
dissolved out from different soils from one half to one and a half parts
of soluble constituents; or from .05 to .15 per cent. Of this soluble
matter from 30 to 67 per cent is mineral in its nature, and from 33 to
70 per cent organic. Poor sandy soils yield the minimum quantity, while
peaty soils yield the maximum. The quantity of soluble matter in a
regular peaty soil may vary from .4 to 1.4 per cent; this consists
chiefly, however, of organic matter. (See Johnson's 'How Crops Feed,' p.
312.)
Perhaps a more satisfactory method is by analysing the drainage-water of
a soil. This has been found to vary very considerably in composition.
The average of a large number of analyses are .04 to .05 per cent of
dissolved matter. Of this dissolved matter the largest proportion is
made up of organic matter, nitric acid, lime, and soda salts. It must be
borne in mind, however, that even the drainage-water does not furnish an
exact indication of the amount of dissolved matter in a soil. Much,
perhaps the largest proportion of dissolved matter, never finds its way
into the drainage-water. That contained by the drainage-water really
represents the surplus quantity of dissolved matter which the soil is
unable to retain, and which is thus washed by the rain into the drains.
The composition of drainage-water is interesting, as it shows that,
practically speaking, all the necessary plant ingredients are in a state
of solution in the soil.
NOTE VI. (p. 90).
CHEMICAL COMPOSITION OF THE SOIL.
The most important substances present in soils are as follows: silica,
alumina, lime, magnesia, potash, soda, ferric oxide, manganese oxide,
sulphuric acid, phosphoric acid, and chlorine. Of these substances the
presence of alumina, silica, lime, and, in certain cases, magnesia,
along with the organic portion of the soil--the humus--has the chief
influence in determining the nature and the physical properties of a
soil.
In order to clearly understand to what it is soils owe the nature of
their chemical composition, it is necessary to consider the composition
of some of the chief minerals out of the disintegration of which soils
are formed.
While we know of some seventy elements present in the earth's crust, it
is practically made up of only some sixteen. These sixteen are--oxygen,
silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, iron,
aluminium, calcium, magnesium, sodium, potassium, fluorine, manganese,
and barium.[61] Of these, oxygen is by far the largest constituent,
forming, roughly speaking, about 50 per cent.
The main mass of the rocks consists of silica, and this is generally
combined with alumina, as in clay, forming aluminium silicate, and with
the commoner alkalies and alkaline earths. Another extremely abundant
compound is carbonate of lime, which, as limestone, chalk, and marl,
forms one-sixth of the earth's total rocks.
The word "mineral" means a definite chemical compound of natural
occurrence. The number of minerals is very great, and it is impossible
to go into the subject here. Reference can only be made to a few of the
more prominent ones, which are chiefly concerned in the formation of
soils.
Those formed out of silicates are, from the agricultural point of view,
the most important, as they form a very large group; and it is by their
disintegration that soils are chiefly formed. They consist of silica and
alumina, along with various other substances, chiefly alkalies and
alkaline earths. It is important to note one peculiarity about the
solubility of silicates. We have two classes of silicates: the one,
which is called "acid," and contains an excess of silica; the other,
"basic," and which contains an excess of base. Now, while the former of
these is more or less insoluble, the second is soluble. This fact has an
important signification in the process of the disintegration of the
silicate minerals we are about to consider.
The first and most important class are the _Felspars_. Felspar is not
really a definite mineral, with a definite chemical composition, but
rather the name of a class of minerals of which there are several
different kinds. The felspars are composed of silica and alumina, along
with potash, soda, and lime, with traces of iron and magnesia. Their
principal constituents, however, are silica and alumina, along with
either potash, soda, or lime. According as the base potash, soda, or
lime predominates, the felspar is known as Orthoclase, Albite, and
Oligoclase, respectively.
The following are the analyses of the three minerals (by the late Dr
Anderson):--
--------------------+-----------------+-----------------+-----------------
| Orthoclase. | Albite. | Oligoclase.
+--------+--------+--------+--------+--------+--------
| 1. | 2. | 1. | 2. | 1. | 2.
--------------------+--------+--------+--------+--------+--------+--------
Silica | 65.72 | 65.00 | 67.99 | 68.23 | 62.70 | 63.51
Alumina | 18.57 | 18.64 | 19.61 | 18.30 | 23.80 | 23.09
Peroxide of iron | traces | 0.83 | 0.70 | 1.01 | 0.62 | none
Oxide of manganese | traces | 0.13 | none | none | none | none
Lime | 0.34 | 1.23 | 0.66 | 1.26 | 4.60 | 2.44
Magnesia | 0.10 | 1.03 | none | 0.51 | 0.02 | 0.77
Potash | 14.02 | 9.12 | none | 2.53 | 1.05 | 2.19
Soda. | 1.25 | 3.49 | 11.12 | 7.99 | 8.00 | 9.37
--------------------+--------+--------+--------+--------+--------+--------
| 100.00 | 99.47 | 100.08 | 99.83 | 100.79 | 101.37
--------------------+--------+--------+--------+--------+--------+--------
According as these various felspars are present in a soil, so will the
quality of the soil be. It stands to reason that as the presence of
potash in a soil is one of the distinguishing features of its fertility,
much will depend on the extent to which the orthoclase felspar is
present; and also, not only on the extent, but on the state and degree
of its disintegration. It is important to note the method of this
disintegration. It is effected by the absorption of water. This water is
not merely absorbed mechanically, but actually enters into the
composition of the mineral. It is not present as moisture merely,
capable of being expelled at ordinary boiling temperature, but it forms
what is known as water of composition. In this process of hydration, the
mineral loses its lustre and crystalline appearance, crumbles away into
a more or less--according to its state of disintegration--powdery mass.
A very great change is also effected in its chemical composition; it
loses nearly all its base. This is effected in the following way. As
water enters into the mineral's composition, it sets free a certain
portion of the base; there is thus formed a basic silicate, which, being
soluble in water, is washed away in solution. This change may be
illustrated by quoting the analysis of a kaolin clay formed by the
disintegration of orthoclase felspar.
_Kaolin Clay formed by disintegration of Orthoclase._
Silica 46.80
Alumina 36.83
Peroxide of iron 3.11
Carbonate of lime 0.55
Potash 0.27
Water 12.44
------
100.00
------
The chief difference here is the almost total loss of potash and a
portion of the silica, and the gain of water. The other constituents
practically remain insoluble.
Another important mineral is _Mica_. Its composition is not unlike
felspar. It contains silica, alumina, and iron, in considerable
quantities, also magnesia and potash. There are two kinds of mica--that
containing potash, and that containing magnesia, in excess. The analyses
of these two kinds are as follows (by the late Dr Anderson):--
MICAS.
(_a_) Potash. (_b_) Magnesia.
Silica 46.36 42.65
Alumina 36.80 12.96
Peroxide of iron 4.53 none
Protoxide of iron none 7.11
Oxide of manganese 0.02 1.06
Magnesia none 25.75
Potash 9.22 6.03
Hydrofluoric acid 0.70 0.62
Water 1.84 3.17
----- -----
99.47 99.35
----- -----
The decomposition of mica is very slow, however, as it is a peculiarly
hard mineral.
Other important minerals are _Hornblende_ and _Augite_. These are
composed of silica, alumina, iron oxide, manganese oxide, lime and
magnesia. These are the chief minerals out of which soils are formed. It
is scarcely necessary to say that few soils are made up out of any of
these three minerals alone. Nearly all rocks are formed out of a mixture
of these minerals. Where, however, any one mineral predominates over the
rest, the nature of the soil will be thereby affected. In order to
illustrate this, it may be well to mention the composition of one or two
of the commoner rocks.
1. _Granite_, which is so abundant in certain parts of the north of
Scotland, and which gives rise to the soils in the neighbourhood of
Aberdeen, is made up of a mixture of quartz, felspar, and mica. It
depends on the felspar present--_i.e._, whether it is orthoclase,
oligoclase, or albite--whether the soil will be rich in potash or not.
Granite containing orthoclase felspar produces a fairly fertile soil. An
important consideration, which is apt to complicate this question, is
the situation of such soils. They are generally so high above sea-level,
that their fertility is seriously impaired on these grounds.
2. _Gneiss_, another common rock, is similar in composition, only that
it contains very little felspar, and a correspondingly greater amount of
mica.
3. _Syenite_ contains quartz, felspar, and hornblende.
The rocks of which greenstone and trap are types, are found very largely
scattered over the country. They are of two kinds, diorite and dolorite.
4. _Limestone_ is of two great classes. We have (1) Common, (2)
Magnesian. The following are the analyses of these two classes by Dr
Anderson:--
----------------------+---------------------------+------------------------
| Common. | Magnesian.
+-------------+-------------+-------------+----------
| Mid-Lothian | Sutherland. | Sutherland. | Dumfries.
----------------------+-------------+-------------+-------------+----------
Silica | 2.00 | 7.43 | 6.00 | 2.31
Iron oxide and alumina| 0.45 | 0.76 | 1.57 | 2.00
Carbonate of lime | 93.61 | 84.11 | 50.21 | 58.81
Carbonate of magnesia | 1.62 | 7.45 | 41.22 | 36.41
Phosphate of lime | 0.56 | - | - | -
Sulphate of lime | 0.92 | - | - | -
Organic matter | 0.20 | - | - | -
Water | 0.50 | - | - | -
| ----- | ----- | ----- | -----
| 99.86 | 99.75 | 99.00 | 99.53
----------------------+-------------+-------------+-------------+----------
Clays are formed by the disintegration of any of the crystalline rocks;
the purest clays being formed from felspar. A pure clay consists simply
of silica and alumina, all the other constituents having been washed
out. Disintegration, however, seldom reaches such an extent; otherwise
clay soils would be completely barren, which they are notably not. The
impurities present in clay, which consist of alkalies, especially potash
and other mineral ingredients of the plant, are what confer on clay
soils their fertility. Clays differ, however, very considerably in
their composition. The following is an analysis of a clay soil by Dr
Anderson:--
Silica 60.03
Alumina 14.91
Peroxide of iron 8.94
Lime 2.08
Magnesia 4.22
Potash 3.87
Soda 0.06
Water and carbonic acid 5.67
-----
99.72
-----
NOTE VII. (p. 91).
FORMS IN WHICH PLANT-FOODS ARE PRESENT IN SOIL.
The forms in which the bases necessary for plant-food are present in the
soil, are chiefly as _hydrated silicates_, and in combination with
organic acids, forming humates, &c., as well as in the form of sulphates
and chlorides.
Phosphoric acid is present in combination with iron, alumina, or lime,
or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is
generally present in a more or less insoluble condition, in combination
with iron and lime; whereas chlorine is combined with the alkali bases
in an easily soluble form. An important point is as to the form in which
the plant absorbs these food constituents. In this connection reference
may be made to a theory put forward by a very distinguished French
agricultural chemist, Professor Grandeau. His theory is that the
necessary ingredients of plant-food are absorbed into the plant as
humates, or, at any rate, that the medium of this transference is humic
acid, and organic acids of a similar nature. This theory, however, while
ingenious, has not yet been supported by sufficient evidence to make its
acceptance advisable. It is probable that it is only in the form of
soluble salts that the plant can absorb its food. It is quite probable,
however, at the same time, that the exact form in which the different
food substances enter the plant may be largely determined by
circumstances. According to Nobbe, chloride of potassium is the most
suitable form of potassium salts, although the plant may absorb its
potassium as sulphate, phosphate, or even silicate.
FOOTNOTES:
[61] Composition of the earth's solid crust in 100 parts by weight:--
Oxygen 44.0 to 48.7 | Calcium 6.6 to 0.9
Silicon 22.8 to 36.2 | Magnesium 2.7 to 0.1
Aluminium 9.9 to 6.1 | Sodium 2.4 to 2.5
Iron 9.9 to 2.4 | Potassium 1.7 to 3.1
(Roscoe's 'Lessons in Elementary Chemistry,' p. 8.)
CHAPTER II.
FUNCTIONS PERFORMED BY MANURES.
Having now considered the general conditions on which fertility of soil
depends, we are in a position to deal with the nature and function of
manures.
Manures may be classified in several different ways, and a considerable
amount of confusion is sometimes caused by the variety of classification
adopted by different writers on this subject.
_Etymological meaning of the word Manure._
Let us, in the first place, clearly understand what we mean by a manure.
The word manure comes from the French word _manoeuvrer_, which simply
means "to work with the hand," hence "to till," and this etymological
meaning of the word illustrates the old belief in the function of
manures. We have already seen in the historical introduction that,
according to Tull, the true and only function of manures was to aid in
the pulverisation of the soil by fermentation. In advancing his system
of _thorough tillage_, he claimed that since tillage effected the
pulverisation of the soil, where it was practised, manures could be
dispensed with.
_Definition of Manures._
We no longer, of course, attach this old meaning to the word. The word
manure is now applied to any substance which by its application
contributes to the fertility of a soil. As has been shown in the
previous chapter, the substances necessary for plant-growth which are
apt to be lacking in a soil, are only generally three in number--viz.,
_nitrogen_, _phosphoric acid_, and _potash_. A manure, therefore, is
understood to be any substance containing these ingredients, either
singly or together, and its commercial value is determined by the amount
it contains of these substances. But while this is so, it must not be
forgotten that if we define a manure to be a substance which contributes
in any way to the fertility of the soil, substances other than these
above mentioned may be fairly regarded as manures. The fertility of a
soil, we have seen, depends not merely on the presence of certain
constituents, but also on their chemical condition--_i.e._, whether they
are easily soluble or not. It further depends, as we have also seen, on
the possession by the soil of certain mechanical and biological
properties. Thus there are substances which act upon the soil's inert
fertilising matter, and by their action convert it into a more speedily
available form. There are other substances which by their application
exert a considerable effect on the texture of the soil, and thereby
influence its physical and biological properties. All such substances,
according to the above definition of a manure, must be included under
the term. It will thus be seen that since fertility in a soil can be
promoted in a variety of ways, and the functions performed by manures
are of different kinds, we can divide them into different classes,
according to their respective action.
_Different Classes of Manures._
In the first place, we can divide manures into two great classes,--(1)
those supplying to the soil necessary plant-food constituents, and thus
contributing directly to fertility; and (2) those influencing
soil-fertility in an indirect manner. The first class we may call
_direct_ manures, and the second _indirect_. Those two classes admit
further of being subdivided into other smaller classes. Among the direct
manures we have a number of subdivisions in use. They may be divided
into _general_ manures and _special_ manures, according as they contain
all the elements necessary for plant-growth, or only some of them; or
they may be divided according to their source into _natural_ and
_artificial_, _mineral_ and _vegetable_. Similarly we have a number of
subdivisions among the second class, depending on the special nature of
the action they exert. Some manures act in both capacities--both
directly and indirectly--and in order that their value be fully
appreciated must be studied under both heads. The most striking example
of such a manure is farmyard manure. There are other manures which may
in certain circumstances act in two different ways. Such a substance is
lime. There are soils which are actually lacking in a sufficiency of
lime for the needs of crops. On such soils an application of lime would
act both as a direct and also as an indirect manure. There may also be
cases of an exceptional nature, in which magnesia salts or even iron
salts may act as direct manures. Many manures commonly regarded as
purely direct manures would exert an indirect influence were the
quantities in which they were applied sufficiently large. This is the
case, indeed, with many artificial manures, such as guano, bones,
nitrate of soda, and basic slag. It has been claimed for nitrate of soda
that it not merely promotes fertility by supplying nitrogen in its most
available form to the soil, but that the soda it contains exerts a
valuable indirect influence in consolidating the soil and increasing its
absorptive powers. When we reflect, however, on the small quantity of
this manure which is applied per acre, its mechanical influence must be
insignificant. The same applies to basic slag, which contains a
considerable quantity of free lime in its composition. As this manure,
however, is sometimes applied in considerable quantities, it is
reasonable to suppose that its indirect value may not be altogether
insignificant. Indeed we have proof of this in the fact that its most
favourable action has been found to be on soils rich in organic
matter.[62] The action of bones and guano, and indeed of all other
manures containing a large percentage of decomposable organic matter, is
likewise of a double nature, inasmuch as their decomposition or
putrefaction in the soil gives rise to the formation of carbonic and
organic acids, which are capable of exerting a chemical action on the
soil ingredients. There is one point in connection with the action of
these manures which is worthy of notice, and it is that, however slight
their indirect value may be, their action as a direct manure is very
much accelerated by the way in which their organic matter putrefies. In
short, they may be described as providing, to a certain extent, the
solvents which render them available for the requirements of the plant.
It may be here convenient to classify the manures which we intend
subsequently to deal with.
I. Manures, action of which is both direct and indirect--_e.g._, _green
manures_, _farmyard manure_, _composts_, and _sewage_.
II. Manures which may be regarded as having only a direct
action--_e.g._, _guano_ of all kinds, _bones_ in all forms, _nitrate of
soda_, _sulphate of ammonia_, _dried blood_, _superphosphates_, _mineral
phosphates_ of all kinds, _horns_ and _hoofs_, _shoddy_, _wool-waste_,
_fish-guano_, _muriate of potash_, _sulphate of potash_, and _kainit_.
III. Manures which may be regarded as having only an indirect
value--_e.g._, _lime_, _mild_ and _caustic_, _marl_, _gypsum_, _salt_,
&c.
We shall now proceed to discuss the nature and action of these different
manures, starting with those exercising both a _direct_ and _indirect_
influence. Before doing so it may be well to consider the occurrence and
natural sources of the three important soil constituents, nitrogen,
phosphoric acid, and potash, with a view of seeing to what extent these
are being removed from our soils by the various natural processes
constantly going on, as well as by the crops, and how far their natural
sources are capable of making good this loss--in short, to clearly
understand the economic reasons for the application of artificial
manures.
FOOTNOTES:
[62] See Chapter on Basic Slag.
CHAPTER III.
THE POSITION OF NITROGEN IN AGRICULTURE.
Of manurial ingredients, nitrogen is by far the most important, and on
the presence and character of the nitrogen it contains, the fertility of
a soil may be said to be most largely dependent. Most soils, as a rule,
are better supplied with available ash ingredients than with available
nitrogen compounds. The expensive nature of most artificial nitrogenous
manures also gives to nitrogen the first position from an economic point
of view. A thorough study, therefore, of the different forms in which it
exists in nature, of the numerous and complicated changes it undergoes
in the soil, by which it is prepared for the plant's needs, of the
relation of its different forms to plant-life, and of the natural
sources of its loss and gain, is of the highest importance if we are to
hope to understand the difficult question of soil-fertility.
_The Rothamsted Experiments and the Nitrogen question._
The position of nitrogen in agriculture is a question of great
difficulty and complexity. It has engaged much attention, and has had
devoted to its elucidation much elaborate and painstaking research. To
the Rothamsted experiments we owe most of the information we possess on
the subject, and the facts contained in this chapter are almost entirely
derived from the results of these famous experiments, as embodied in the
memoirs and writings of Messrs Lawes, Gilbert, and Warington.
_Different forms in which Nitrogen exists in Nature._
We have already referred to the nitrogen question in the historical
introduction. In order, however, to have a comprehensive view of the
subject, it may be well to recapitulate some of the facts there
mentioned.
Nitrogen, as we have already seen, exists in the "free" or elementary
condition, as nitrates and nitrites, as ammonia, and in a large number
of different organic forms.
_Nitrogen in the Air._
It occurs in greatest abundance (amounting to about 80 per cent) in the
first of these forms in the air. That this free nitrogen, which is
practically unlimited in quantity,[63] has originally been the source of
all its other forms, is of course obvious. But this conversion of free
nitrogen into the various compound forms in which it occurs throughout
the mineral, vegetable, and animal kingdoms, has been a process effected
by a variety of indirect methods, and only at the expense of a vast
amount of time. For practical purposes, the free nitrogen of the air may
be regarded chiefly as a non-available source for most bodies containing
it. It may be described as of all forms of nitrogen the least active, as
far as plant-life is concerned.
_Relation of "free" Nitrogen to the Plant._
The relation of the "free" nitrogen to the plant has formed the subject
of much research, more especially during the last few years, and a brief
epitome of the main results arrived at has already been given in the
Introductory Chapter.[64]
That this source of nitrogen is not so inaccessible to the plant as was
formerly believed, has now been abundantly proved. As the considerations
which have led to this conclusion, and have suggested the very recent
elaborate experiments on the fixation of free nitrogen by the plant--the
results of which bid fair, it would seem, to largely revolutionise our
agricultural practice--have been due to the study of the relation of the
soil-nitrogen to the plant, it will be best to defer further discussion
of this question till we have dealt with the other sources of nitrogen.
_Combined Nitrogen in the Air._
In addition to nitrogen in the free state, air contains very small
quantities of this element in combined forms. We have it in minute
traces as nitrates and nitrites, as ammonia,[65] and also in still
smaller traces as organic nitrogen in the minute dust-particles which
modern researches have revealed as being present in such enormous
numbers in our atmosphere. What the sources of these nitrates and
nitrites (which exist in quantities so minute that accurate
determination of their amount is rendered extremely difficult) are is a
disputed point. That nitrogen and oxygen unite together to form nitric
and nitrous oxides under the influence of intense heat, such as the
electric spark, has been proved beyond doubt. One source, therefore, is
probably the electrical discharges which are taking place more or less
frequently on different parts of the earth's surface. Nitrates may also
be formed in the combustion of nitrogenous bodies.[66] In the burning of
coal-gas, for example, it is probable that small quantities of nitrates
may be produced. Similarly the slow combustion or decay of nitrogenous
organic matter, which constantly takes place all over the earth's
surface, may be regarded as another source of this form of combined
nitrogen. Ammonia may be similarly formed by the combustion, either
quick or slow, of nitrogenous organic matter. It exists in the air as
nitrate or nitrite of ammonia, and also as carbonate of ammonia.[67]
_Amount of combined Nitrogen falling in the Rain._
The importance of the combined nitrogen in the air as a source of
soil-nitrogen is best gauged by the amount falling annually on the soil
dissolved in rain. This has been found to vary considerably. In the rain
falling in the vicinity of large towns the amount is greater than in
rain falling in the country. Thus at Rothamsted, in England, the average
amount for several years was only 3.37 lb. nitrogen per annum per acre,
of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln,
in New Zealand, 1.74 lb. fell annually per acre--as ammonia,.74, as
nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which
.93 was as ammonia, and 2.84 as nitric acid.[68] That the combined
nitrogen derived from the air by the soil may be considerably in excess
of this is highly probable. Soils, especially when damp, may absorb much
larger quantities from the air of the combined nitrogen it contains. We
must remember that the air in contact with the soil-surface is
constantly being changed, and that there is thus a constant renewal of
the air passed over the ground. The result is that the amount of air
from which combined nitrogen may be removed is very great.[69]
_Nitrogen in the Soil._
It has been remarked as a fact worthy of notice that nitrogen is
essentially a superficial element. By this is meant that it is only
found, as a rule, on the earth's immediate surface. This statement can
only be admitted to be true within certain limits. The chief source of
nitrogen, in addition to the atmosphere, is, of course, vegetable and
animal tissue.[70] As vegetable and animal tissue are only found to any
extent on the earth's surface, nitrogen is therefore chiefly found
there. The natural deposits of nitrogen salts, such as the
nitrate-fields of Chili and the saltpetre soils of India, &c., also
only occur superficially. Notwithstanding these facts, however, the
amount of nitrogen which exists at probably considerable depths from the
surface must be very great. There are few sedimentary rocks which do not
contain it. At Rothamsted a sample of calcareous clay, taken from a
depth of 500 feet, contained .04 per cent--that is, as much as is found,
on an average, in the Rothamsted clay subsoils.
_Nitrogen in the Subsoil._
On the whole, however, as we have said, nitrogen is chiefly found in the
surface-soil. The amount found in the subsoil at Rothamsted seems to
vary very slightly at different depths, the percentage amounting to from
.06 to .03.[71] Unlike the nitrogen of the surface-soil, that in the
subsoil seems to be of very ancient origin, being probably derived from
the remains of animal and vegetable life in the mud deposited at the
bottom of the ocean. It is more abundant in the case of a clay subsoil
than in a sandy subsoil.
_Nitrogen of Surface-Soil._
Nitrogen has a tendency to collect on the top layers of the
surface-soil, the first 9 inches or foot containing by far the largest
proportion of it. In the table given in the Appendix,[72] the rate at
which it decreases in amount the further down we go is clearly shown.
Determinations of the respective amounts of nitrogen in every 3 inches
of the soil, taken to a depth of one foot of the experimental
wheat-field at Rothamsted, showed that the percentage between the first
3 inches and the second 3 inches varied very slightly. A more marked
difference, however, was shown to exist between the nitrogen in the
second and third 3 inches; while the fourth 3 inches were distinctly
poorer--differing very little in their percentage of nitrogen from the
subsoil. This was the case in unmanured soil. In the case of heavily
manured soil, the increase in the soil's percentage, due to manure, was
shown to be felt to the depth of a foot, but not much below it.[73]
A careful perusal of the tables in the Appendix will show that the
quantity of nitrogen in the case of both arable and pasture soils
steadily decreases for the first 3 feet, but that below this depth
little decrease is seen, the percentage evidently becoming fairly
constant.
_The amount of Nitrogen in the Soil._
Very considerable difference exists in the amount of nitrogen present in
different soils. The majority of analyses refer only to the amount found
in the surface-soil--generally in the first 9 or 12 inches. As the soil,
further, is not a body exactly homogeneous in its character, very
considerable difficulty exists in obtaining reliable results. A great
deal depends, therefore, on the method of sampling and the basis of
calculation adopted; and it may be that this may occasionally explain,
to some extent at least, the great discrepancies in the estimation of
the quantities of nitrogen present in different soils as found by
different investigators.
_Peat-soils richest in Nitrogen._
Of all soils, peat-soils are richest in nitrogen. Professor S. W.
Johnson found the nitrogen in fifty separate samples of peat to range
from .4 per cent to 2.9 per cent, the average being 1.5 per cent. On the
other hand, marls and sandy soils are poorest, the analyses of a number
of these soils showing only from .004 to .083 per cent for the former,
and .025 to .074 for the latter. As a general rule most arable soils
contain over one-tenth per cent of nitrogen, or, say, over 3500 lb. per
acre. A good pasture-soil, taken to a depth of 9 inches at Rothamsted,
was found to contain about a quarter per cent. In ten samples of soil,
taken to a depth of 9 inches, from different parts of Great Britain and
Ireland, Munro found from .128 to .695 per cent of nitrogen, the average
being .3278 per cent. The Rothamsted soils, it may be pointed out, are
probably poor in nitrogen compared with most soils. A. Mueller's
investigations showed that in some of the soils he has analysed, the
nitrogen amounted to little short of one per cent, while for the others
the average was over half a per cent; even the poorer soils he examined
contained about one quarter per cent on an average. Anderson's analyses
of Scottish wheat-soils showed a variation of from .074 to .22 in the
surface-soil, while he found in their subsoil from .15 to .92 per cent.
Boussingault's results are also very much higher. The amount of nitrogen
in a number of loams coming from widely different localities he examined
contained from 6000 to 30,000 lb. per acre--the soil taken to a depth of
17 inches.[74]
_Nature of the Nitrogen in the Soil._
When we compare the amount of nitrogen removed by different crops
(which, even in the case of those most exhaustive of nitrogen, does not
often amount to more than 150 lb. per acre), with the amount contained
in the soil, the former amount seems very insignificant when compared to
the latter. Such being the case, it would seem at first sight that the
addition of nitrogen in the form of manures is quite superfluous. We
must remember, however, that while the _total_ amount of nitrogen is
relatively large when compared to that removed by crops, only a very
small proportion is in a condition _available_ to the plant. This leads
us to consider the different forms in which nitrogen is present in the
soil, and their respective quantities.
_Organic Nitrogen in the Soil._
Nitrogen occurs in the soil as organic nitrogen, nitric acid, nitrous
acid, and ammonia. By far the largest proportion is present in the first
of these forms. This is a wise provision, for otherwise the soil would
be apt to become very speedily impoverished in nitrogen; for that
present as nitrates it has scarcely any power to retain, while that
present as ammonia is soon converted into nitrates by the process of
_nitrification_.
The organic nitrogen of the soil, although we are apt to think of it as
such, is by no means of a homogeneous character, or of equal value as a
source of plant-food. Some of it, it would seem from recent
investigations, is in a condition more susceptible of being converted
into an available form than the rest. Thus in the process of
nitrification, a process which we shall consider at length immediately,
there seems to be generally a certain small proportion more ready to
undergo this change than the rest; so that when this small amount is
used up nitrification proceeds more slowly. In short, although we as yet
know very little of the nature of the organic nitrogen of soils, we
cannot doubt but that there is a constant series of changes in its
composition taking place, resulting in the gradual elaboration of more
available forms, until ultimately these are converted into ammonia and
nitrates.
The great bulk of the organic nitrogen, however, in the soil must be
regarded as in an _inert_ condition, and by no means available for the
crop. What the exact chemical form of this nitrogen is it is extremely
difficult to say. Mulder was of the opinion that a considerable
proportion was in the form of humate of ammonia. This opinion, as we
shall have occasion to see immediately, was based on false grounds. It
is highly probable that it may be in some form approximating to amide
nitrogen. Its inert character is against the belief that it long remains
as albuminoid nitrogen.
_Different Character of Surface and Subsoil Nitrogen._
A point of very considerable importance to notice is, that the
nitrogenous organic matter of the surface-soil is very different from
that found in the subsoil. This difference is shown by the variation in
the ratio of nitrogen to carbon, which points to the fact that, just as
we should naturally suppose, the origin of the latter is very much more
ancient than the origin of the former. Thus in the first 9 inches of old
pasture-soil at Rothamsted, the ratio was 1:13; while in the subsoil, 3
feet from the surface, it was only 1:6. In the surface-soil it thus
approaches more nearly in composition ordinary vegetable matter.
_Nitrogen as Ammonia in Soils._
The second form in which nitrogen is present in soil is as ammonia. A
very considerable misapprehension has existed in the past as to the
amount of nitrogen in this form in soils. This mistake was due to the
method adopted in estimating it, which consisted in treating the soil
with boiling caustic alkalies and counting as ammonia what was given off
as such. It is now known that certain forms of organic nitrogen--as, for
example, amides--if treated in this way are slowly converted into
ammonia. Statements, therefore, which are found in the older text-books,
representing the amount of ammonia in soils as at over a tenth per cent,
must be regarded as utterly unreliable. Indeed it is highly probable
that ammonia only occurs in most soils in very minute traces. From what
we know of the process of nitrification, we see how it is wellnigh
impossible that ammonia should exist to any extent in the soil except
under very exceptional circumstances.
_Amount of Ammonia present in the Soil._
In ordinary soils it probably does not amount to more than from .0002
per cent to .0008 per cent, or an average of .0006 per cent.[75] In
rich soils, or in garden-soils, the amount may be considerably more.
Thus Boussingault found in a garden-soil .002 per cent. In peat and in
peat-mould even a higher percentage has been found--viz.,.018 for the
former and .05 for the latter.
_Nitrogen present as Nitrates in the Soil._
The third form of nitrogen in the soil is nitric acid. It is more
abundant in this form than as ammonia; but still, compared with the
organic nitrogen, its amount is trifling. Probably not more than 5 per
cent of the total nitrogen of a soil is ever present as nitrates. The
reason of this is twofold. First, as we have already remarked, the soil
has very little power to retain nitrogen in this form; and secondly,
where the soil is covered with growing vegetation the nitrates are
quickly assimilated by the plant as they are formed. It is for this
reason that we find the quantity of nitrogen as nitrates very much
greater in fallow soils than in those covered with a crop.
_Position of Nitric Nitrogen in Soil._
As we shall have occasion to see more fully in the following chapter on
Nitrification, the formation of nitrates is chiefly limited to the
surface-soil, the largest proportion being formed within the first 9 or
12 inches. For this reason we find the largest quantity of nitrates in
the surface-soil. But inasmuch as they are easily washed into the lower
layers of the soil after formation, we often find a considerable
proportion beyond the first 9 inches. The position of nitrates in the
soil thus depends very considerably on the season of the year and the
weather. In dry weather, where the evaporation of the soil-water takes
place at a considerable rate, the tendency will be to concentrate the
nitrates in the superficial portion of the soil. In wet weather, on the
other hand, the tendency will be to wash the nitrates into the lower
layers.
_Amount of Nitrates in the Soil._
The determination of the amount of nitrates in a soil is not of very
great economic importance; as this varies so much, and depends on such a
number of different conditions, such as the season, the condition of the
land, and prevailing weather. A point of very much greater economic
importance is the total amount formed in the year, and the rate at which
nitrification takes place. These questions will be discussed elsewhere,
and therefore need not here be referred to. Some interesting analyses
made at Rothamsted, however, of the amount of nitrates in soils at
different depths, merit careful consideration.
_Nitrates in Fallow Soils._
In the Appendix to the chapter on Nitrification,[76] will be found a
table containing the amounts of nitrates found in the first 27 inches of
fallow soils. The amounts vary from 33.7 lb. to 59.9 lb. per acre. The
analyses were made in September or October. In four out of the six
analyses, it will be found that by far the largest proportion is found
in the first 9 inches. In these cases the preceding summer had been dry,
and thus the nitrates had not been washed down to any depth. In the
other two cases the largest amount is found in the second 9 inches of
soil, and a considerable amount is also found in the third 9 inches.
_Nitrates in Cropped Soils._
In the case of cropped soils we find the amount of nitrates very much
less. A table containing an elaborate series of determinations of
nitrates in cropped soils, receiving, however, no manure, and taken to a
depth of 9 feet, will be found in the Appendix.[77] The first 27 inches
only contain some 5 to 14 lb. per acre, and the most of that is found in
the first 9 inches. This shows how speedily nitrates are assimilated by
the growing crop. An interesting point shown by these analyses is that
nitrates almost entirely cease in cropped soils a certain depth down,
but that at a still lower depth they again occur in small quantities.
_Nitrates in manured Wheat-soils._
Lastly, we give in the Appendix[78] the amount of nitrates found in
wheat and barley soils, differently manured, at Rothamsted. From a
perusal of these tables, it will be seen that the amount (under various
conditions of manuring) of nitrates in the first 27 inches varies from
21.2 lb. per acre to 52.2 lb. for the wheat-soils, and 20.1 to 44.1 lb.
per acre for the barley-soils.
THE SOURCES OF SOIL-NITROGEN.
We shall now consider the sources of soil-nitrogen, the conditions which
determine its increase, and the amount of that increase, as well as the
sources of loss, and the conditions which determine this loss.
_That dissolved in Rain._
The natural sources of the soil-nitrogen are several. We have first of
all the atmospheric nitrogen. Of this let us first consider that present
as combined nitrogen. This, as we have already seen, consists chiefly of
nitrates, nitrites, and ammonia, and reaches the soil dissolved in rain
or in other meteoric forms of water, such as snow, hail, fog,
hoar-frost, &c.
_That absorbed by the Soil from the Air._
It is also absorbed by the soil from the air, especially when the soil
is in a damp condition, as has been proved by Schloesing's experiments,
already referred to. The total amount which falls dissolved in the rain,
per acre per annum, varies very considerably in different parts of the
world, but in any case only amounts yearly to a few pounds per acre.[79]
That absorbed by the soil from the air may be probably very much more
considerable. Schloesing in his experiments found that this latter might
amount to 38 lb. per acre per annum. These results, however, were
obtained under circumstances most favourable for absorption--viz., with
a damp soil and in the vicinity of Paris, where the air is presumably
richer in combined nitrogen than it is in the country. The nitrogen
absorbed, it may be mentioned, was almost entirely in the form of
ammonia. It is to be noted that the nitrogen the soil obtains in this
way from the combined nitrogen of the air is not all pure gain. With
regard to the nitrates and nitrites, no doubt most of these are formed
by electrical discharge, although a small portion of them may be formed
by the oxidation of ammonia by means of ozone and peroxide of hydrogen.
With regard to the ammonia and the combined nitrogen present in the
organic particles in the air, a not inconsiderable proportion is
probably derived from the soil. Schloesing considers the chief source of
the ammonia present in the air to be the tropical ocean; but we must
remember that the source of much of the nitrogen in the tropical ocean
is, after all, the soil.
Leaving aside for a moment the question of the availability of the free
nitrogen of the air, let us consider the other sources of soil-nitrogen.
_Accumulation of Soil-nitrogen under Natural Conditions._
The chief source is of course the remains of vegetable and animal
tissue.[80] Plants are the great conservers of soil-nitrogen. By
assimilating such available forms of it as nitrates, and converting them
into organic nitrogen, they prevent the loss of this most valuable of
all soil constituents that would otherwise take place.
They also serve to collect the nitrogen from the lower soil-layers and
concentrate it in the surface portion. In a state of nature, where the
soil is constantly covered with vegetation, the process going on,
therefore, will be one of steady accumulation of nitrogen in the
surface-soil. To what extent this accumulation goes on, and how far it
is limited by the conditions of loss, will be considered immediately.
That it may go on to a very great extent is amply proved by the
existence of the so-called _virgin_ soils of countries like America and
Australia. There are cases, also, where the accumulation of nitrogen is
practically unlimited, although the result in such cases is not
necessarily a fertile soil. Such cases are peat-bogs. But let us pass on
to the accumulation of soil-nitrogen under the ordinary conditions of
husbandry.
_Accumulation of Nitrogen in Pastures._
The case which, under the conditions of ordinary farming, most resembles
a state of nature, is that of permanent pasture. It will be best,
therefore, to study first the conditions under which gain of nitrogen
takes place in this case.
_Increase of Nitrogen in the soil of Pasture-land._
That there is a steady increase of nitrogen in the soil of land under
pasture is a fact of universal experience. The older a pasture is the
richer is its soil in nitrogen. The comparison of the analyses of the
soil of arable land with the soil of pastures of different ages shows
this in a striking way.[81] Thus at Rothamsted it was found that while
the amount of nitrogen in an ordinary arable soil was .140 per cent,
that in pastures eight, eighteen, twenty-one, and thirty years old was
respectively .151, .174, .204, and .241 per cent. In the last two
analyses we have a record of the actual gain in nitrogen made by the
same pasture, this being .04 per cent in nine years' time. From these
statistics it may be inferred that the surface-soil of a pasture may
increase at the rate of 50 lb. per acre per annum. A point of great
interest in connection with this subject is the fact that there seems to
be a limit to the accumulation of nitrogen in pastures; for it would
seem that pastures centuries old are not any richer in nitrogen than
those thirty to forty years old.
_Gain of Nitrogen with Leguminous Crops._
Another case where the gain of nitrogen to the surface-soil is very
striking is in that of leguminous crops, such as clover, beans, peas,
&c. This fact has been long recognised--especially with regard to
clover--by farmers, and has been largely instrumental in leading to the
investigation of the "free" nitrogen question. That a soil bearing a
leguminous crop increases in nitrogen at a very striking rate is a
problem that requires to be solved. A partial explanation of the
phenomenon is found in the extraordinary capacity such a crop as clover
has, by means of its multitudinous and ramifying roots, for collecting
nitrogen from the subsoil. This, however, would only account for the
increase in nitrogen to a certain extent. There must be some other
source, and the only other source is the air. That the free nitrogen of
the air is, after all, available for the plant's needs, is a supposition
which has long seemed extremely probable, and which, within the last
few years, has been proved beyond doubt to be a fact in the case of
leguminous plants.
_The Fixation of "Free" Nitrogen._
The method in which these plants are able to make use of the free
nitrogen is still a point requiring much research. So far as the
question is at present investigated, it would seem that the fixation is
effected by means of micro-organisms present in tubercles or root
excrescences found on the roots of leguminous plants.[82] Not merely has
this been placed beyond doubt, but attempts have been made to isolate
and study the bacteria effecting this fixation. From Nobbe's exceedingly
interesting experiments, recently carried out, it would seem that the
different kinds of leguminous plants have different bacteria. Thus the
bacteria in the tubercle on the pea seems to be of a different order
from the bacteria in the tubercles of the lupin, and so on. This
discovery is of great importance, it need scarcely be pointed out, as it
throws much light on the principles of the rotation of crops.
_Influence of Manures in increasing Soil-nitrogen._
It may be doubted, however, if under any other conditions there is a
positive gain of soil-nitrogen. In other cases the amount in the soil is
only _maintained_ under liberal manuring. In connection with this point
a very striking fact has been observed with regard to the effect of
continuous large applications of farmyard manure. It has been found at
Rothamsted that in such a case, after a while, the manure does not seem
to increase the soil-nitrogen, although where the nitrogen goes to
remains a mystery. In the case of the application of artificial manures,
there does not seem to be almost any appreciable gain to the
soil-nitrogen. The soil-nitrogen is only increased by means of the
residue of crops. In this way, of course, by increasing the amount of
this crop-residue, artificial manures may be said indirectly to increase
the soil-nitrogen.[83]
SOURCES OF LOSS OF NITROGEN.
We now come to consider the sources of loss. The chief source, of
course, is that by drainage. Land under cultivation will suffer very
much more from this source of loss than in a state of nature. Our modern
system of husbandry, involving as it does thorough drainage, can
scarcely fail to very considerably increase this source of loss.
_Loss of Nitrates by Drainage._
The form in which nitrogen is lost in this way is as nitrates. It is a
somewhat striking fact, and one worthy of note, that of the three
important manurial ingredients--nitrogen, phosphoric acid, and potash,
the first of these, in its final and most valuable form, is alone
incapable of being fixed by the soil, and thus retained from loss by
drainage.
As nitrates are constantly being formed in the soil, the loss to its
total nitrogen must be considerable. It is due to the fact of the great
solubility of nitrates, as well as to the fact, as already mentioned, of
the incapacity of the soil-particles to fix them. To this one exception
must be made. According to Knop, small quantities of nitric acid are
held in the _insoluble_ condition in soils in the form of highly _basic
nitrates of iron and alumina_. The quantity, however, of these insoluble
compounds probably amounts to a very minute trace indeed.
_Permanent Pasture and "Catch-cropping" prevents Loss._
The amount of loss varies, and will depend on a number of different
circumstances--thus the nature of the soil, climate, and season of the
year will all influence its quantity. The way in which the soil is
cultivated is also another important factor. Where it is constantly
covered with vegetation, as in the case of permanent pasture, the loss
will be at a minimum. Under such conditions, plant-roots are always
there ready to fix, in the insoluble organic form, the soluble nitrates
as they are formed. A consideration of this fact forms one of the
strongest arguments in favour of the practice of what is known as
"catch-cropping." The practice consists in sowing some quickly-growing
green crop--_e.g._, _mustard_, _vetches_, &c.--so as to occupy the soil
immediately after harvest, and subsequently to plough it in. The
nitrates, which it is known are most abundantly formed towards the end
of summer,[84] and which are allowed to accumulate in the soil from the
period at which the active growth of, and consequently assimilation of
nitrates by, the cereal crop have ceased, are thus fixed in the organic
matter of the plant, and removed from danger of loss by drainage
incidental to autumn rains.
_Other Conditions diminishing Loss of Nitrates._
The nature of the soil is another important condition regulating this
loss. Some soils are very much opener and more porous than others; in
such soils, of course, the loss by drainage will be greatest. We are apt
at first sight, however, knowing the great solubility of nitrates, to
overrate this source of loss. We have to remember that while nitrates
are constantly being washed down to the lower layers of the soil, there
is likewise an upward compensating movement of the soil-water constantly
taking place. This is due to the evaporation of water from the surface
of the soil, which induces an upward capillary movement of water from
its lower to its higher layers.[85] This upward movement of water is
very much increased, in the case of soil covered with vegetation, by the
transpiration of the plants. The climate and the season of the year will
affect the extent of this upward movement. Where there is a heavy
rainfall it will be very much less than in dry climates. After a long
period of drought the nitrates will be found to be concentrated in the
top few inches of the soil; and in hot climates this sometimes takes
place to such an extent that the surface of the soil has been actually
covered with a saline crust, caused by the rapid evaporation of
soil-water under the influence of a burning tropical sun. From this
point of view it will be seen how very much less powerful a single
shower of rain is--even although at the time it is heavy--in causing
loss of nitrates by drainage, than a continuance of wet weather. In the
former case, where the showers are separated by an interval of dry
weather, the nitrates washed down into the lower layers of the soil are
slowly brought up again by the capillary action caused by evaporation.
_Amount of Loss by Drainage._
What the actual amount of loss is which takes place in this way it is
wellnigh impossible to say. What it amounts to under certain definite
circumstances has been discovered by actual experiment at Rothamsted.
Taking the circumstances most favourable to extreme loss--viz.,
unmanured fallow land--the highest amount registered at Rothamsted for a
year is 54.2 lb. per acre from soil 20 inches deep, while the smallest
amount is 20.9 lb. In the former case, the drainage-water was equivalent
to 21.66 inches, while in the latter, to 8.96 inches. The average for
thirteen years on unmanured fallow soil has been 37.3 lb. (for 20
inches), 32.6 lb. (for 40 inches), 35.6 lb. (for 60 inches). The point
of especial interest in this connection is that an annual loss of
nitrogen, equal to over 2 cwt. of nitrate of soda, may take place from a
comparatively poor arable soil lying fallow.
The loss on cropped soils is of course very much less--in short, should
amount to very little--especially in permanent pasture, where it is
reduced to a minimum. Taking an average, Mr Warington is of opinion that
the loss in England may be put at 8 lb. per annum per acre.[86]
_Loss in Form of Free Nitrogen._
The other chief natural source of loss of nitrogen is due to its escape
from the soil in its "free" state. This source of loss is very much less
important than that by drainage, and probably amounts to very little.
That, however, it takes place is beyond a doubt; and that it may--as we
shall see by-and-by--under certain circumstances amount to something
very considerable is also proved. Where large quantities of nitrogenous
organic matter decay, and where, consequently, the supply of atmospheric
oxygen is insufficient to effect complete oxidation, "free" nitrogen may
be evolved in considerable quantities. Similarly, it may be evolved in
the case of vegetable matter decaying under water. In soils rich in
organic matter the reduction of even nitrates may take place,
accompanied with the evolution of free nitrogen, which is thus lost.
_Total Amount of Loss of Nitrogen._
What the rate of total loss of nitrogen is from these different sources
does not admit of easy calculation. Sir John Lawes, in dealing with the
question of soil-fertility, estimated some years ago, by comparing the
soil of old pasture at Rothamsted with that which had been under arable
culture for 250 years, that during that period some 3000 lb. of nitrogen
per acre had disappeared from the arable land. Examples of decrease of
nitrogen in Rothamsted soils, under various conditions of culture, will
be found in the Appendix.[87]
_Loss of Nitrogen by Retrogression._
A source of loss of nitrogen may be here mentioned which has to do with
diminution of amount of available nitrogen, rather than absolute loss of
nitrogen to the soil, and which we may term _loss by retrogression_.
Nitrogen in an available form, such as nitrates, has been found to be
converted into a less available form. This retrogression may be
effected, as in the case of nitrates, by reduction--_i.e._, by removal
of the oxygen in combination with the nitrogen, which in many cases may
be set free, and thus partially although not necessarily entirely lost.
Such reduction is due to the action of bacteria of the denitrifying
order.[88] Or, on the other hand, nitrogen may be converted into some
kind of insoluble form which seems to resist decomposition and lies in
an inert condition in the soil utterly unavailable for the plants'
needs. A striking example of this retrogression of nitrogen seems to be
afforded in the case of farmyard manure. It has been found in the
Rothamsted experiments, as has been pointed out in the preceding pages,
that when farmyard manure is applied, year after year, to the same land
in large quantities, a very considerable percentage of its nitrogen does
not (_i.e._, within a reasonable number of years) become available for
the crop's uses. What, indeed, becomes of the nitrogen is a mystery; but
it is highly probable that some such kind of retrogression as that above
referred to, whereby the nitrogen is converted into some inert organic
form, takes place.
_Artificial Sources of Loss of Nitrogen._
So far, the sources of loss of nitrogen considered have been what we may
term _natural_ sources. By this is meant that the loss of nitrogen from
the above sources takes place in a state of nature, and not merely under
conditions of cultivation. No doubt the loss due to drainage is very
much greater under arable farming than would be the case where
artificial drainage does not obtain; still, under any conditions, this
loss must be reckoned with. On the other hand, by _artificial_ sources
of loss are meant those entirely dependent on our modern system of
agriculture and our modern system of sewage disposal, whereby the
nitrogen contained in that portion of the produce of the farm which goes
to supply our food is not returned to the soil, but is totally lost.
_Amount of Nitrogen removed in Crops._
The modern tendency towards centralisation in large towns has rendered
this loss--despite all that has been said to the contrary--a necessity.
It is extremely difficult, however, to form any estimate of its amount.
We know, of course, the amount of nitrogen removed from the soil by
different crops. We cannot, however, estimate how much of this may find
its way back again to the soil. The amount of nitrogen contained in the
different crops will be fully dealt with in the chapter on the manuring
of different crops. It may be, however, not without interest to give
here some approximate indication of the amount of this loss, in order to
render the view of the subject as comprehensive as possible.
Recent agricultural returns for Great Britain give the total produce of
_wheat_ at over 76 million bushels, that of _barley_ at over 69 million,
and that of _oats_ at over 150 million. Calculating the amount of
nitrogen, these quantities of wheat, barley, and oats respectively and
collectively contain, and calculating also how much _sulphate of
ammonia_ and _nitrate of soda_ these amounts of nitrogen represent, the
following are the results:--
Nitrogen. Sulphate of Nitrate of
Ammonia. Soda.
Bushels. Tons. Tons. Tons.
Wheat 76,224,940 37,432 176,465 227,266
Barley 69,948,266 27,324 128,813 165,896
Oats 150,789,416 56,835 267,936 345,068
----------- ------- ------- -------
Total 296,962,622 121,591 573,214 738,230
=========== ======= ======= =======
Of course these figures, so far as the amounts of nitrogen are
concerned, can only be regarded as approximate, as it is only possible
in such calculations to obtain approximate results. Accepting these
calculations as merely approximate, they are, nevertheless, of the
highest interest and importance. It is of great importance to understand
that in the annual produce of our three common cereal crops--supposing
them to be all consumed off the farm--there is removed from the soil a
quantity of nitrogen equal to that contained in over _half a million
tons of sulphate of ammonia, and three quarters of a million tons of
nitrate of soda._
As has already been remarked, it is impossible to estimate exactly what
proportion of this total nitrogen finds its way back to the soil. In the
case of wheat, it may be pointed out that the portion which is used as a
feeding-stuff--viz., _bran_--is very much richer in nitrogen than the
flour. While, then, we are unable to estimate with any exactitude this
source of loss of nitrogen, it cannot for a moment be doubted that it is
enormous, from what has been already stated. We must remember that the
portion of the crop richest in nitrogen is that which is generally
removed--the straw which is grown in producing a bushel of wheat,
barley, or oats, containing less than half the amount of nitrogen
contained by a bushel of the grain itself.
_Losses of Nitrogen incurred on the Farm._
In addition to the loss due to removal of crops from the farm, there are
one or two other sources of loss which it may be well to briefly refer
to.
_Loss in Treatment of Farmyard Manure._
There can be little doubt that in the past a very considerable source of
loss was the improper treatment of farmyard manure. The way in which
this loss may take place will be fully considered in the chapter on
farmyard manure. Suffice it to say here, that this may take place by
volatilisation of the nitrogen as carbonate of ammonia, caused by
carelessness in allowing the temperature of the manure-heap to rise too
high; or by drainage of the soluble nitrogen compounds, caused by
allowing the rich black liquor of the manure-heap to be washed away, and
not properly conserved.
_Nitrogen removed in Milk._
Another source of loss which is apt to be overlooked is the amount of
nitrogen removed in milk. Professor Storer has calculated that in the
case of a cow giving 2000 quarts, or 4300 lb., of milk in a year, and
the milk being all sold as such, there would be carried away from the
farm 22 lb. of nitrogen.[89]
_Economics of the Nitrogen question._
And here, before concluding our survey of the different sources of loss
of nitrogen, it may be well to regard for a moment the subject from a
somewhat wider standpoint than that from which we have been considering
it. The total supply of nitrogen in a combined form is limited. As we
have pointed out, it may be regarded as the element on which, more than
any other, life, animal as well as vegetable, depends. To animal life it
is alone available in combined form; to vegetable life it is chiefly
also only available in combined form. In the air we have an unlimited
quantity of nitrogen, but it is almost entirely in an _uncombined_ form,
and therefore largely unavailable. The conversion of nitrogen from the
free state to a combined form is a process which takes place only very
slowly. Any source which diminishes the sum-total of our already all too
limited supply of combined nitrogen must be regarded as worthy of most
serious consideration. The question, therefore, of the artificial waste
of nitrogen daily taking place around us, is one which ought to possess
for economists a very great interest indeed. This waste has, of late
years, enormously increased, and would seem to threaten us at no very
distant date with a nitrogen famine. It is incidental to the use of
certain nitrogenous substances in the manufacture of various articles,
and to our present system of sewage disposal.
_Loss of Nitrogen-compounds in the Arts._
The articles referred to are such as explosives, starch, textile
substances, malt liquors, &c. The question is strikingly dealt with in
an able paper on "The Economy of Nitrogen" in the 'Quarterly Journal of
Science.'[90]
_Loss due to Use of Gunpowder._
The explosives--more particularly gunpowder--are the most important of
these articles. Gunpowder contains 75 per cent of saltpetre, which in
its turn contains about 10 per cent of nitrogen. When gunpowder
explodes, practically the whole of this nitrogen is converted into
"free" nitrogen. The loss is thus in a sense irreparable. In the paper
above, referred to, our total annual exports of this substance are
estimated at 19,000,000 lb.; while the total annual production of the
world is estimated at not less than 100,000,000 lb. The annual loss of
nitrogen due to this source alone would amount to about 10,000,000
lb.[91] Similarly, loss of nitrogen, although to a less extent, is
caused by the use of other explosives, as well as in the manufacture of
the other articles above mentioned.
_Loss due to Sewage Disposal._
The loss due to our present system of sewage disposal has been already
taken into account in dealing with the loss due to removal of crops. It
may be well, however, to treat it from the sewage aspect. Taking the
amount of nitrogen in the excreta of every individual as, on an average,
half an ounce, the annual amount voided in the excreta of the total
population of the British Isles would amount to 365,000,000 lb.[92]--of
this, the amount in the London sewage alone being 91,000,000 lb.[93] By
the water system, which is almost universally adopted in this country,
the above quantity of nitrogen is entirely lost to the soil. A small
portion of it, it may be argued, is eventually recovered in sea weed and
fish, which may be used for manure. This, however, is to argue too much
_sub specie aeternitatis_. Not all the nitrogen originally present in the
excreta finds its way into the sea; for it is highly probable that a
considerable quantity escapes in the process of the decomposition of the
sewage as "free" nitrogen.
From the above statement of the sources of loss and gain of nitrogen
taking place in the soil, it may be pretty safe to conclude that while
in a state of nature the gain balances the loss, if indeed it does not
do more, under conditions of arable farming such is very far from being
the case; and that if fertility of the land is to be maintained,
recourse to nitrogenous manures must be had,--in short, that the
application of artificial nitrogenous manures is a necessary condition
of modern husbandry.
_Our Artificial Nitrogen Supply._
Before concluding this chapter, it may be interesting to enumerate very
briefly the chief sources of our artificial nitrogen supply.
_Nitrate of Soda and Sulphate of Ammonia._
The most important artificial nitrogenous manures in use at present are
nitrate of soda and sulphate of ammonia. Of the former, the annual
exportation from Chili is close on one million tons, of which quantity
about 120,000 tons is imported into the United Kingdom. Of sulphate of
ammonia, on the other hand, the total production in this country is
about 130,000 tons per annum,[94] the greater proportion of which is
exported, leaving only from 30,000 to 40,000 tons for consumption.
Nitrate of soda, it must be remembered, is not entirely used for
manurial purposes, a small proportion of the above imports being used
for chemical manufacturing purposes.
_Peruvian Guano._
Peruvian guano is another important nitrogenous manure very much less
abundant now than formerly, as the different guano-beds have become
nearly exhausted. While the imports of this important manure into the
United Kingdom amounted in 1870 to nearly 250,000 tons, at present not
more than 11,000 tons are being imported.
_Bones._
A further source of nitrogen is bones, which, of course, are chiefly
valuable as a phosphatic manure, but which contain also some 3 to 4 per
cent of nitrogen. Of this valuable manure we import at present about
30,000 tons, while about 60,000 tons are collected in this country,
bringing up our total consumption to 100,000 tons.
_Other Nitrogenous Manures._
The above mentioned are the most important of nitrogenous manures; there
are, however, a number of other nitrogenous manures used in this country
in very much smaller quantities. As most of these substances are made in
this country, it is very difficult to estimate the amount of their
annual production with exactness. These substances are as follows:
fish-guano, meat-meal guano, dried blood, shoddy, scutch, horns and
hoofs, hair, bristles, feathers, leather-scrap, &c. Of fish-guano, the
total consumption per annum may be put down at about 8000 tons, of which
a fourth is imported into this country, the remaining 6000 tons being
manufactured at home. Of meat-meal guano, dried blood, hoof-guano, &c.,
about 2500 tons are annually imported, the home production bringing up
the total amount to some 10,000 tons. Of shoddy, some 12,000 tons are
manufactured in this country; while scutch--the name given to a manure
manufactured from the waste products incidental to the manufacture of
glue and the dressing of skins--is produced only to the extent of a few
thousand tons annually.
It is a fact worthy of notice, that while the use of phosphatic manures
has increased very considerably of late years, the same cannot be said
of nitrogen. According to Mr Hermann Voss, some 34,000[95] tons of
nitrogen were used in the form of artificial manures in 1873, while now
only about 28,000 tons are used--_i.e._, some 6000 tons less.
_Oil-seeds and Oilcakes._
There still remains a very important source of nitrogen which has not
yet been mentioned, in the shape of oil-seeds and oilcakes, used for
feeding purposes. Oilcakes are both manufactured in this country and
imported in large quantities. Recent Agricultural Returns show the total
imports of oilcakes at 256,296 tons; that of linseed at 370,000 tons;
that of rape-seed at 80,000 tons; and that of cotton-seed at 289,413
tons.
_Other imported Sources of Nitrogen._
We have further, in considering this question, to take into account the
large amount of maize, peas, beans, wheat, and oats which are imported
into this country, a certain quantity of which is used as cattle-food,
and will therefore go to enrich their manure. Also the imported straw
used for purposes of litter must not be forgotten. In 1887 this amounted
to 52,393 tons.
_Conclusion._
In conclusion, it may be asked how far are the artificial sources of
nitrogen able to make good the loss? In the opinion of such a reliable
authority as Sir John Lawes, they do not. There are some soils which
depend almost entirely upon imported fertility, and could not be
cultivated without it. Upon some of them it is possible that the imports
of nitrogen are in excess of the exports. Taking the agricultural
acreage as a whole, however, he is of opinion that there is a decided
loss of nitrogen, which he estimates at _from 15 lb. to 20 lb. per acre
per annum_.[96]
FOOTNOTES:
[63] The total amount of nitrogen in the air has been estimated
approximately at four million billion tons.
[64] See Introductory Chapter, pp. 40 to 45.
[65] Although ammonia is more abundant than nitrates and nitrites, it
only amounts to a few parts per million of air. According to Muentz, the
air at great heights contains more ammonia than in its lower strata. The
opposite, however, is the case with regard to nitrates, which are only
found in air near the surface of the earth. See p. 49.
[66] Nitric acid may also be formed by the oxidation of ammonia by
ozone, or peroxide of hydrogen.
[67] According to Schloesing, the chief source of the ammonia present in
the air is the tropical ocean, which yields gradually to the atmosphere,
under the action of the powerful evaporation constantly going on, a
large amount of nitrogen in this form. The sources of the nitrogen of
the ocean are the nitrates which it receives from the drainage of land,
animal and vegetable matter, sewage, &c.
[68] See Appendix, Note I., p. 155.
[69] To illustrate this point, it may be mentioned that on the least
windy of days, when the wind is only moving at the rate of two miles an
hour--and this, it may he added, is so slow as to be scarcely
noticeable--the air in a space of 20 feet is changed over five hundred
times in an hour. The combined nitrogen thus absorbed is probably
entirely in the form of ammonia. It would seem so at any rate, from some
experiments by Schloesing. See p. 132.
[70] No vegetable or animal cell exists which does not contain nitrogen.
[71] This is less on the whole than what has been found in subsoils by
Continental investigators. Thus, for example, A. Mueller found the
average of a number of analyses of subsoils to be .15 per cent., and the
late Dr Anderson found the nitrogen in the subsoil of different Scottish
wheat-soils to run from .15 per cent to .97 per cent.
[72] See Appendix, Note II., p. 156.
[73] "Under prolonged kitchen-garden culture the subsoil becomes
enriched with nitrogenous matter to a far more considerable depth; this
has been shown by the analyses of the soil of the old kitchen-garden at
Rothamsted. This is doubtless due to the practice of deep trenching
employed by gardeners."--R. Warington, 'Lectures on Rothamsted
Experiments.' U.S.A. Bulletin, p. 24.
[74] The comparatively insignificant effect the addition of various
nitrogenous manures have in increasing the total soil-nitrogen is
strikingly illustrated in the tables given in the Appendix, Note IV., p.
157.
[75] See Storer's Agric. Chem., vol. i. p. 357.
[76] See Chapter IV., Appendix, Note VII., p. 198.
[77] See Appendix, Note III., p. 157.
[78] See Appendix, Note IV., p. 157.
[79] See Appendix, Note I., p. 155.
[80] The original source of the nitrogen in the soil must have been the
nitrogen in the air. When plants first begin to grow on a purely mineral
soil, they must obtain nitrogen from some source. The small traces
washed down in the rain will supply sufficient nitrogen to enable a
scanty growth of the lower forms of vegetable life; whereas these by
their decay furnish their successors with a more abundant source, which
rapidly increases, until we have a fair percentage of humus accumulated.
[81] See Appendix, Note V., p. 158.
[82] See Historical Introduction, pp. 40-45.
[83] The evidence demonstrating this is to be found in the fact that the
amount of carbon found in different soils rises or falls in proportion
to the nitrogen. See p. 126.
[84] See Chapter IV. on Nitrification.
[85] Diffusion as well as capillary attraction is a means of bringing
nitrates again to the surface-soil after rain.
[86] See Appendix, Note VI., p. 158, and Note VIII., p. 160; also p.
154.
[87] See Appendix, Note VII., p. 159.
[88] See following Chapter on Nitrification, p. 178.
[89] According to the Agricultural Returns for 1888, the number of cows
in milk in Great Britain amounted to 2,450,444. If we multiply this
number by 22 the result is 54,000,000 lb., or in tons 24,107. This
quantity represents 154,067 tons of ordinary commercial nitrate of soda.
[90] For 1878 (p. 146 _et seq._) The reader interested in the subject is
referred to the paper itself.
[91] In tons 4464, and represents 28,530 tons of nitrate of soda.
[92] This in tons 162,946, which represents 1,041,384 tons of nitrate of
soda.
[93] This in tons 40,625, which represents 259,633 tons of nitrate of
soda. See paper in 'Journal of Science' already referred to.
[94] Europe's total production may be stated at 200,000 tons.
[95] 10,500 tons of which were as guano.
[96] Mr Warington estimates this at about 8 lb. See p. 141.
APPENDIX TO CHAPTER III.
NOTE I. (p. 119).
DETERMINATIONS OF THE QUANTITY OF NITROGEN SUPPLIED BY RAIN, AS
AMMONIA AND NITRIC ACID, TO AN ACRE OF LAND, DURING ONE YEAR.
(_From Dr Fream's 'Soils and their Properties_,' p. 62.)
----------------------+----------------+---------+----------------+--------
| | | Nitrogen per |
| | | million, as | Total
| | +--------+-------+Nitrogen
| Year. |Rainfall.| | | per
| | |Ammonia.| Nitric| acre.
| | | | Acid. |
----------------------+----------------+---------+--------+-------+--------
| | | | | lb.
Kuschen | 1864-65 | 11.85 | 0.54 | 0.16 | 1.86
" | 1865-66 | 17.70 | 0.44 | 0.16 | 2.50
Insterburg | 1864-65 | 27.55 | 0.55 | 0.30 | 5.49
" | 1865-66 | 23.79 | 0.76 | 0.49 | 6.81
Dahme | 1865 | 17.09 | 1.42 | 0.30 | 6.66
Regenwalde | 1864-65 | 23.48 | 2.03 | 0.80 | 15.09
" | 1865-66 | 19.31 | 1.88 | 0.48 | 10.38
" | 1866-67 | 25.37 | 2.28 | 0.56 | 16.44
Ida-Marienhuette, mean | | | | |
of six years | 1865-70 | 22.65 | | | 9.92
Proskau | 1864-65 | 17.81 | 3.21 | 1.73 | 20.91
Florence | 1870 | 36.55 | 1.17 | 0.44 | 13.36
" | 1871 | 42.48 | 0.81 | 0.22 | 9.89
" | 1872 | 50.82 | 0.82 | 0.26 | 12.51
Vallombrosa | 1872 | 79.83 | 0.42 | 0.15 | 10.38
Montsouris, Paris | 1877-78 | 23.62 | 1.91 | 0.24 | 11.54
" | 1878-79 | 25.79 | 1.20 | 0.70 | 11.16
" | 1879-80 | 15.70 | 1.36 | 1.60 | 10.52
| | | | |
| +---------+--------+-------+--------
| | | | |
|Mean of 22 years| 27.63 | | | 10.23
| | | | |
----------------------+----------------+---------+--------+-------+--------
NOTE II. (p. 122).
NITROGEN IN SOILS AT VARIOUS DEPTHS.
(1) _Rothamsted Soils._
------------------+---------------------------+---------------------------
Depth. | Arable soil. | Old pasture soil.
------------------+-----------+---------------+------------+--------------
| per cent. | lb. per acre. | per cent. | lb. per acre.
1st 9 inches | 0.120 | 3,015 | 0.245 | 5,351
2d 9 " | 0.068 | 1,629 | 0.082 | 2,313
3d 9 " | 0.059 | 1,461 | 0.053 | 1,580
4th 9 " | 0.051 | 1,228 | 0.046 | 1,412
5th 9 " | 0.045 | 1,090 | 0.042 | 1,301
6th 9 " | 0.044 | 1,131 | 0.039 | 1,186
| ----- | ----- | ----- | -----
Total, 54 inches | - | 9,554 | - | 13,143
|-----------+---------------+------------+--------------
7th 9 inches | 0.042 | 1,049 | - | -
8th 9 " | 0.041 | 1,095 | - | -
9th 9 " | 0.044 | 1,173 | - | -
10th 9 " | 0.043 | 1,076 | - | -
11th 9 " | 0.043 | 1,112 | - | -
12th 9 " | 0.045 | 1,198 | - | -
| ----- | ------ | ----- | -----
Total, 9 feet | | 16,257 | |
------------------+-----------+---------------+------------+--------------
(2) _Manitoba Soils._
----------+----------|-------------+-----------+----------
Depth. | Brandon. | Niverville. | Winnipeg. | Selkirk.
----------+----------+-------------+-----------+----------
| per cent.| per cent. | per cent. | per cent.
1st foot | 0.187 | 0.261 | 0.428 | 0.618
2d " | 0.109 | 0.169 | 0.327 | 0.264
3d " | 0.072 | 0.069 | 0.158 | 0.076
4th " | 0.019 | 0.038 | 0.107 | 0.042
----------+----------+-------------+-----------+----------
NOTE III. (p. 130).
NITROGEN AS NITRATES IN CROPPED SOILS RECEIVING NO NITROGENOUS
MANURE, IN LB. PER ACRE (_Rothamsted Soils_).
-------------+----------------+---------+---------+---------+--------
| Wheat. | | | |
+-------+--------+ Bokhara | | | White
| After | After | clover, | Vetches,| Lucern, | clover,
Depth. |fallow,|clover, | 1882. | 1883. | 1885. | 1885.
| 1883. | 1883. | | | |
-------------+-------+--------+---------+---------+---------+--------
| lb. | lb. | lb. | lb. | lb. | lb.
1st 9 inches| 3.4 | 6.1 | 3.4 | 10.2 | 8.9 | 11.5
2d 9 " | 3.1 | 4.4 | 1.0 | 2.7 | 1.1 | 1.4
3d 9 " | 0.8 | 1.6 | 0.6 | 1.1 | 0.8 | 0.9
4th 9 " | 1.0 | 1.3 | 1.0 | 1.5 | 0.8 | 1.9
5th 9 " | 0.8 | 1.5 | 0.8 | 2.5 | 1.0 | 7.1
6th 9 " | 0.6 | 0.8 | 1.7 | 4.4 | 0.9 | 11.3
7th 9 " | 0.8 | 2.2 | | 4.5 | 0.6 | 13.1
8th 9 " | 0.9 | 1.7 | | 4.9 | 0.8 | 12.6
9th 9 " | 0.7 | 2.4 | | 4.8 | 0.7 | 11.2
10th 9 " | 2.0 | 2.1 | | 5.1 | 0.6 | 10.7
11th 9 " | 1.5 | 2.1 | | 6.4 | 0.4 | 11.1
12th 9 " | 3.8 | 2.8 | | 6.5 | 0.4 | 10.0
-------------+-------+--------+---------+---------+---------+--------
NOTE IV. (p. 124 and p. 131).
NITROGEN AS NITRATES IN WHEAT-SOILS VARIOUSLY MANURED, OCTOBER 1881,
IN LB. PER ACRE (_Rothamsted Soils_).
----+----------------------------------+------+------+------+--------+-------
| | | | | |Excess
| | | | | |over
Plot| Manuring. |1st 9 |2nd 9 |3rd 9 |Total 27|plots
| |inches|inches|inches|inches |3 and 4
----+----------------------------------+------+------+------+--------+-------
| | lb. | lb. | lb. | lb. | lb.
3 |No manure, 38 years | 9.7 | 5.3 | 2.8 | 17.8 |
4 | " 30 " | 9.2 | 4.0 | 1.8 | 15.0 |
16a | " 17 " | 10.6 | 5.0 | 2.3 | 17.9 | 1.5
5a |Ash constituents, 30 years | 12.6 | 7.1 | 4.6 | 24.3 | 7.9
17a | " " 1 year | 10.3 | 7.5 | 3.4 | 21.2 | 4.8
6a | " and ammonium salts, 200 lb. | 16.5 | 7.5 | 4.7 | 28.7 | 12.3
7a | " " 400 " | 22.8 | 11.3 | 5.7 | 39.8 | 23.4
8a | " " 600 " | 21.1 | 13.9 | 7.8 | 42.8 | 26.4
9a |Ash and sodium nitrate, 550 " | 19.7 | 10.0 | 8.2 | 37.9 | 21.5
9b |Sodium nitrate, " " | 16.3 | 20.1 | 17.7 | 54.1 | 37.7
10a |Ammonium salts, 400 " | 14.2 | 11.9 | 7.3 | 33.4 | 17.0
11a |Superphosphate and ammonium salts,| | | | |
| 400 lb. | 17.9 | 9.3 | 3.6 | 30.8 | 14.4
19 |Rape-cake, 1700 lb. | 14.1 | 13.0 | 7.1 | 34.2 | 17.8
2 |Farmyard manure, 14 tons--38 years| 30.0 | 15.4 | 6.8 | 52.2 | 35.8
----+----------------------------------+------+------+------+--------+-------
NITROGEN AS NITRATES IN BARLEY-SOILS VARIOUSLY MANURED, MARCH 1892,
IN LB. PER ACRE (_Rothamsted Soils_).
-------+------------------------+-------+-------+-------+-------+--------
| | | | | Total | Excess
| | 1st 9 | 2d 9 | 3d 9 | 27 | over
Plot. | Manuring. |inches.|inches.|inches.|inches.|plot 10.
-------+------------------------+-------+-------+-------+-------+--------
| | lb. | lb. | lb. | lb. | lb.
10 |No manure | 5.9 | 4.7 | 5.1 | 15.7 | -
20-40 |Ash constituents (mean) | 6.7 | 7.0 | 6.4 | 20.1 | 4.4
1 A |Ammonium salts, 200 lb. | 6.1 | 8.3 | 7.0 | 21.4 | 5.7
2A-4A |Ammonium and ash | | | | |
| constituents (mean) | 7.7 | 7.8 | 7.6 | 23.1 | 7.4
1 AA |Sodium nitrate, 275 lb. | 9.7 | 6.8 | 9.0 | 25.5 | 9.8
2AA-4AA|Sodium nitrate and ash | | | | |
| constituents (mean) | 8.3 | 7.4 | 7.5 | 23.2 | 7.5
1C |Rape-cake, 1000 lb. | 10.6 | 13.7 | 7.9 | 32.2 | 16.5
2C-4C |Rape-cake and ash | | | | |
| constituents (mean) | 8.8 | 11.9 | 8.7 | 29.4 | 13.7
7-1 |No manure, 10 years-- | | | | |
| formerly dung | 14.8 | 11.8 | 10.9 | 37.5 | 21.8
7-2 |Farmyard manure, 14 tons| 18.6 | 14.6 | 10.9 | 44.1 | 28.4
-------+------------------------+-------+-------+-------+-------+-------
NOTE V. (p. 134).
EXAMPLES OF INCREASE OF NITROGEN IN ROTHAMSTED SOILS LAID DOWN IN
PASTURE.
--------------------+----------+---------------
| Age of | Nitrogen in
| pasture. | 1st 9 inches.
--------------------+----------+---------------
| Years. | Per cent.
Arable land | - | 0.140
Barn-field pasture | 8 | 0.151
Apple-tree pasture | 18 | 0.174
Dr Gilbert's meadow | 21 | 0.204
Dr Gilbert's meadow | 30 | 0.241
--------------------+----------+---------------
NOTE VI. (p. 141).
In connection with the loss by drainage of nitrogen in the form of
nitrates, it may be mentioned that the water of many of the famous
rivers contains large quantities of nitrates. Thus the water of the
Seine has been found to contain fifteen parts of nitrates per million of
water, and the Rhine eight parts per million. Some idea of what this
amounts to per annum may be obtained by the statement that "the Rhine
discharges daily 220 tons of saltpetre into the ocean, the river Seine
270, and the Nile 1100 tons."--(Storer's Agric. Chem., vol. i. p. 318.)
NOTE VII. (p. 142).
EXAMPLES OF DECREASE OF NITROGEN IN ROTHAMSTED SOILS.
--------------------------------------+----------------
| Nitrogen in
| 1st 9 inches.
--------------------------------------+----------------
| Per cent.
Old pasture | 0.250
Arable land in ordinary culture | 0.140
Wheat unmanured, 38 years | 0.105
Wheat and fallow unmanured, 31 years | 0.096
Barley unmanured, 30 years | 0.093
Turnips unmanured, 25 years | 0.085
--------------------------------------+----------------
MANURING, PRODUCE OF WHEAT, AND ALTERATION IN THE COMPOSITION OF THE
SOIL IN BROADBALK FIELD, ROTHAMSTED, FROM 1865 TO 1881.
-----+----------------------------------+----------------+----------------------
| | Average | Nitrogen per acre
| | produce | in 1st 9 inches
Plot.| Manures per acre, | per acre. | of soil.
| annually applied, +-------+--------+------+-----+---------
| 16 years, 1865-81. | | | | | Gain or
| |Dressed| Total |1865. |1881.| loss in
| |grain. |produce.| | |16 years.
-----+----------------------------------+-------+--------+------+-----+---------
| | bush. | lb. | lb. | lb. | lb.
3 |Unmanured |11-7/8 | 1715 | 2507 |2404 | -103
5_a_|Mixed mineral manure |12-3/4 | 1963 | 2574 |2328 | -246
10_a_|Ammonium salts, 400 lb. |17-7/8 | 2881 | 2548 |2471 | -77
11_1_|Ammonium salts, with | | | | |
| superphosphate |23-1/4 | 3856 | 2693 |2676 | -17
7_a_|Ammonium salts, with mixed mineral| | | | |
| manure | 28 | 4993 | 2829 |2908 | +79
9_a_|Nitrate of soda, 550 lb., and | | | | |
| mixed mineral manure | 36 | 6949 | 2834 |2883 | +49
16_a_|Unmanured* |13-1/2 | 2194 | 2907 |2557 | -350
2 |Farmyard manure, 14 tons |31-1/2 | 5356 | 4329 |4502 | +173
-----+----------------------------------+-------+--------+------+-----+---------
* During 1852-64 received annually ammonium salts, 800 lb., with mixed mineral
manure, and yielded an average product of 39-1/2 bushels of grain and 46-5/8
cwt. of straw.
NOTE VIII. (p. 141).
$1$2
--------------+--------+-----------------+----------------------------------
| | | Nitrogen as nitrates.
| | +-----------------+----------------
| | Amount of | Per million of | Per acre.
|Rainfall| drainage. | water. |
| +--------+--------+--------+--------+--------+-------
| |20-inch |60-inch |20-inch |60-inch |20-inch |60-inch
| |gauge. |gauge. |gauge. |gauge. |gauge. |gauge.
--------------+--------+--------+--------+--------+--------+--------+-------
|Inches. |Inches. |Inches. | | | lb. | lb.
March | 1.70 | 0.85 | 0.94 | 7.3 | 8.9 | 1.41 | 1.89
April | 2.25 | 0.72 | 0.79 | 8.3 | 9.0 | 1.35 | 1.61
May | 2.48 | 0.80 | 0.79 | 8.4 | 9.1 | 1.53 | 1.63
June | 2.59 | 0.78 | 0.78 | 9.2 | 9.1 | 1.62 | 1.60
July | 2.85 | 0.68 | 0.62 | 13.5 | 11.8 | 2.08 | 1.66
August | 2.69 | 0.84 | 0.76 | 15.1 | 13.3 | 2.87 | 2.28
September | 2.70 | 0.97 | 0.82 | 17.7 | 13.4 | 3.86 | 2.50
October | 3.12 | 1.86 | 1.68 | 13.8 | 11.9 | 5.83 | 4.53
November | 3.20 | 2.44 | 2.32 | 11.8 | 11.4 | 6.50 | 5.98
December | 2.34 | 1.88 | 1.88 | 9.5 | 10.6 | 4.06 | 4.51
January | 2.13 | 1.79 | 1.93 | 7.4 | 8.9 | 2.99 | 3.88
February | 2.16 | 1.84 | 1.74 | 7.7 | 9.1 | 3.19 | 3.57
+--------+--------+--------+--------+--------+--------+-------
March-June | 9.02 | 3.15 | 3.30 | 8.3 | 9.0 | 5.91 | 6.73
July-September| 8.24 | 2.49 | 2.20 | 15.6 | 13.0 | 8.81 | 6.44
October-Feb. | 12.95 | 9.81 | 9.55 | 10.2 | 10.4 | 22.57 | 22.47
+--------+--------+--------+--------+--------+--------+-------
Whole year | 30.21 | 15.45 | 15.05 | 10.7 | 10.5 | 37.29 | 35.64
--------------+--------+--------+--------+--------+--------+--------+-------
CHAPTER IV.
NITRIFICATION.
The most important compound of nitrogen for the plant is _nitric acid_.
It is as nitrates that most plants absorb the nitrogen they require to
build up their tissue. In nature the nitrogen, present in the soil as
ammonia and different organic forms, is constantly being converted into
nitric acid. This conversion of nitrogen into nitrates, known as
_nitrification_, is a process of very great importance, and, as has been
already pointed out in the Introductory Chapter, is effected through the
agency of micro-organisms (ferments).[97] The process of nitrification,
as well as the nature of the other changes taking place in the soil
between the various compounds of nitrogen, are as yet but most
imperfectly understood, but much light has been thrown on this most
interesting department of agricultural research during the last few
years; and it cannot be doubted that the increased attention which it is
receiving from different investigators, both on the Continent and in
this country, will be fraught with most important results for practical
agriculture.
_Occurrence of Nitrates in the Soil._
The occurrence of nitre,[98] or potassium nitrate, in soils has been
long known, although it is only within the last few years that we have
obtained any precise knowledge with regard to the mode of its
production. While its amount in most soils, especially in this
country,[99] is very minute, there are certain parts of the world where
nitrates are found in large quantities. The nitrate fields of Chili and
Peru are the chief natural sources of nitrates, and they are referred to
in the chapter on Nitrate of Soda. We have other parts of the world,
however (in China and India), where soils rich in nitre occur, and which
in the past have formed a source of the commercial article.[100]
_Nitre Soils of India._
The most important of these nitre soils are those found in the
North-west of India, in the province of Bengal. In these districts the
soil is of a light porous texture, rich in lime, and situated at a
considerable height above water-level. They are the sites of old
villages, and the nitre is found in the form of an efflorescence on the
surface of different parts of the soil. The occurrence of nitre under
such conditions is due, partly to the natural richness of the soil in
nitrogen, and partly to its artificial enrichment through receiving the
nitrogenous excrements of the inhabitants of the villages and their
cattle. The constant process of evaporation going on in such a warm
climate has the effect of inducing an upward tendency of the soil-water,
the result being a concentration of all the nitre the soil contains in
its surface layer. This goes on until a regular incrustation is formed,
and the soil is covered by a white deposit of nitre. Whenever this
becomes apparent, the surface portion of the soil is scraped off by the
_sorawallah_, or native manufacturer, and collected and treated for the
purpose of recovering, in a pure state, the saltpetre.
_Saltpetre Plantations._
The large demand for saltpetre, larger than could be supplied by these
nitre soils, soon gave rise to the semi-artificial method of production,
formerly so largely practised in Switzerland, France, Germany, Sweden,
and in many other parts of the Continent, by means of the so-called
"nitre beds," "nitraries," or "saltpetre plantations." Previous to the
introduction of this method of manufacture, the demand for saltpetre for
gunpowder had become so great, that every source of nitre was eagerly
sought for. Thus, when it was discovered that the earth from the floors
of byres, stables, and farmyards were particularly rich in nitre, and
when mixed with wood-ashes formed an important source of it, the right
to remove these in France was vested in the Government under the
Saltpetre Laws, which obtained till the French Revolution. This great
scarcity soon led, however, to a careful investigation being made into
the conditions under which potassium nitrate was formed in nitre
soils.[101] These conditions, which included the presence of rich
nitrogenous matter, warmth, free aeration of the soil, and a certain
proportion of moisture, became, in the course of years, more and more
thoroughly understood, and the result was the institution of numerous
"saltpetre plantations." These generally consisted of heaps of mould,
rich in nitrogen, mixed with decomposing animal matter, rubbish of
various kinds, manurial substances, ashes, road-scrapings, and lime
salts.[102] The heap was interlaid with brushwood, and was watered from
time to time with liquid manure from stables, consisting chiefly of
dilute urine. In forming the heap care was taken to keep the mass
porous, so as to admit of the free access of air. The heap was further
protected from the rain by covering it with a roof. In course of time
considerable quantities of nitrates were developed, and the nitre was
occasionally collected by scraping it from the surface, where it became
concentrated just as in the nitre soils. In all cases, however, the
heaps, when considered rich enough in nitre, were treated from time to
time with water which, by subsequent evaporation, yielded the nitre in a
more or less pure condition.[103]
This mode of obtaining nitre is no longer practised to any extent, since
it is now more conveniently obtained from the treatment of nitrate of
soda with potassium chloride.
_Cause of Nitrification._
We have adverted to these nitre plantations as showing how the
conditions most favourable for the development of nitrification were
recognised long before anything was known as to the true nature of the
process. It was only in 1877 that the formation of nitrates in the soil
was proved to be due to the action of micro-organic life,[104] by the
two French chemists, Schloesing and Muentz, who discovered the fact when
carrying out experiments to see if the presence of humic matter was
essential to the purification of sewage by soil. In these experiments
sewage was made to filter slowly through a certain depth of soil (the
time occupied in this filtration being eight days). It was found that
nitrification of the sewage took place. By treating the soil with
chloroform[105] it was found that it no longer possessed the power of
inducing the nitrification of the sewage. When, however, a small portion
of a nitrifying soil was added, the power was regained. From this it was
naturally inferred that nitrification was effected by some kind of
ferment. This conclusion was soon confirmed by subsequent experiments by
Warington at Rothamsted, who showed that the power of nitrification
could be communicated to media, which did not nitrify, by simply
seeding them with a nitrifying substance, and that light was
unfavourable to the process. Since then the question has formed the
subject of a number of researches by Mr Warington at Rothamsted, as well
as by Schloesing and Muentz, Munro, Deherain, P. F. Frankland,
Winogradsky, Gayon and Dupetit, Kellner, Plath, Pichard, Landolt, Leone,
and others. From these researches we have obtained the following
information with regard to the nature of the organisms concerned in this
process, and the conditions most favourable for their development.
_Ferments effecting Nitrification._
The importance of isolating and studying them microscopically was
recognised at an early period in these researches. Messrs Schloesing and
Muentz were the first to attempt this. They reported that they had
successfully accomplished this, and described the organism as consisting
of very small, round, or slightly elongated corpuscles, occurring either
singly or two together. According, however, to the most recent
researches of Warington, Winogradsky, and P. F. Frankland, nitrification
is not effected by a _single_ micro-organism, but by _two_, both of
which have been successfully isolated and studied.[106] The first of
these to be discovered and isolated was the _nitrous_ organism, which
effects the conversion of ammonia into nitrous acid; the second, which
has only been lately isolated by Warington and Winogradsky, effects the
conversion of nitrous acid into nitric acid. Each of these ferments thus
has its distinctive function to perform in this most important process,
the nitric ferment being unable to act on ammonia, as the nitrous
ferment is unable to convert nitrites into nitrates. Both ferments occur
in enormous quantities in the soil, and seem to be influenced, so far as
is at present known, by the same conditions. Their action will thus
proceed together. Nearly all we know as yet on the subject of their
nature is with regard to the nitrous ferment.
_Appearance of Nitrous Organism._
Mr Warington[107] thus describes the appearance of the nitrous organism:
"As found in suspension in a freshly nitrified solution, it consists
largely of nearly spherical corpuscles, varying extremely in size. The
largest of these corpuscles barely reaches a diameter of 1/1000th of a
millimeter; and some are so minute as to be hardly discernible in
photographs, although shown there with a surface one million times
greater than their own. The larger ones are frequently not strictly
circular. These forms are universally present in nitrifying cultures.
The larger organisms are sometimes seen in the act of dividing."
_Nitric Organism._
So far as at present known, the nitric organism is very similar in
appearance to the nitrous organism, so much so that it is difficult to
distinguish the one from the other. As the same conditions influence
their development, the process may be regarded as a whole.
_Difficulty in isolating them._
A great difficulty has been experienced in the attempt to isolate these
micro-organisms for the purpose of studying their nature. This arises
from the fact that they refuse to grow on the ordinary solid cultivating
media used by bacteriologists. Winogradsky, however, has recently
succeeded in cultivating them in _a purely mineral_ medium--viz.,
_silica-jelly_.[108]
_Nitrifying Organisms do not require Organic Matter._
The fact that they can develop in media destitute of organic matter, is
one of very great interest and importance to Vegetable Physiology. It
implies that they can derive their carbon from carbonic acid--a power
which it was believed was possessed by green plants alone among living
structures. For organisms destitute of chlorophyll, the source of their
protoplasmic carbon, it has been hitherto commonly believed, must be
_organic matter_ of some sort. While it would appear that the nitrifying
organisms can, when opportunity affords, feed upon organic matter, yet
it has been proved beyond doubt that they can also freely develop in
media entirely devoid of it, and are capable, under such circumstances,
of deriving their carbon from a purely mineral source.[109] This fact,
which is subversive of what was believed to be a fundamental law of
Vegetable Physiology, is one of the most important of the many important
and interesting facts which these nitrification researches have
elicited.[110]
CONDITIONS FAVOURABLE FOR NITRIFICATION.
We may now proceed to discuss the conditions favourable for
nitrification.
_Presence of Food-constituents._
Among these conditions the first is the presence of certain
food-constituents. To both animal and vegetable life alike a certain
amount of mineral food is absolutely necessary. Among these phosphoric
acid is one of the most important, and in the experiments on
nitrification it has been found that the nitrifying organisms will not
develop in any medium destitute of it. That other mineral
food-constituents are necessary is highly probable, although the
influence of their absence on the development of the process has not
been similarly studied. Probably potash, magnesia, and lime salts are
necessary. In the cultivating solutions used in the experiments on the
subject, the mineral food-constituents added consisted of lime,
magnesia, and potash salts and phosphoric acid.[111]
As we have seen above, the presence of organic matter is not necessary
for the process. In this respect these organisms are differentiated from
all other ferments hitherto discovered.
_Presence of a Salifiable Base._
The presence of a sufficient quantity of a base in the soil with which
the nitric acid may combine, when it is formed, is another necessary
condition.[112] The process only goes on in a slightly alkaline
solution. The substance which acts as this salifiable base is _lime_.
The presence of a sufficient quantity of carbonate of lime in the soil
will thus be seen to be of first-rate importance. This furnishes an
explanation of one of the many benefits conferred by lime on soils. The
activity of nitrification in many soils may be hindered by the absence
of a sufficiency of lime salts, and in such cases most striking results
may follow the application of moderate dressings of chalk. The absence
of the nitrifying organisms in certain soils, such as peaty and forest
soils, may be thus accounted for. In such soils humic acids are present
and the requisite alkalinity is thus awanting.
_Only takes place in slightly Alkaline Solutions._
But while a certain slight amount of alkalinity is necessary, this must
not exceed a certain strength, otherwise the process is retarded. This
is the reason why strong urine solutions do not nitrify. The amount of
carbonate of ammonia generated in them by putrefaction renders the
development of nitrification impossible by rendering the alkalinity of
the solution too great.[113] The practical importance of this fact is
considerable, as it shows the importance of diluting urine very
considerably before applying it as a manure. Similarly, when large
quantities of lime, especially burnt lime, are applied to soils, the
result will be to arrest the action of nitrification for the time. The
presence of alkaline carbonates in the soil, unless in minute
quantities, is apt, therefore, to seriously interfere with the
process.[114]
_Action of Gypsum on Nitrification._
It has been found by Pichard that the action of certain mineral
sulphates is extremely favourable to the process, and among these
_gypsum_. Warington has carried out some experiments on the action of
gypsum in promoting nitrification. The reason of its favourable action
is probably because it neutralises the alkalinity of nitrifying
solutions. It thus permits the process to go on in unfavourable
conditions. Where, therefore, too great alkalinity exists for the
maximum development of nitrification, the best specific will be found to
be gypsum.[115] The practical value of gypsum as an adjunct to certain
manurial substances, where nitrification is desired to be promoted as
rapidly as possible, such as sewage and farmyard manure, will thus at
once become apparent. So far as there is a proper degree of alkalinity
maintained, the presence of large quantities of saline matter does not
seem to interfere with the process.
_Presence of Oxygen._
The nitrification bacteria belong, it would seem, to the aerobic[116]
class of ferment--_i.e._, they cannot develop without a free supply of
oxygen. Exclusion of the air is sufficient to kill them, and in those
portions of the soil where access of air is not freely permitted,
nitrification will be found to be correspondingly feeble. Thus it has
been found in experiments with different portions of soils, that but
little signs of nitrification occur in the lower soil layers. According
to experiments by Schloesing on a moist soil, in atmospheres
respectively containing no oxygen and varying quantities of it, the
action of oxygen in promoting nitrification was strikingly demonstrated.
In an atmosphere of pure nitrogen, entirely devoid of oxygen, the
process no longer took place, but the nitrates already present in the
soil were reduced and free nitrogen was evolved. In an atmosphere, on
the other hand, containing 1.5 per cent of oxygen, a considerable amount
of nitrification took place; while in the presence of 6 per cent,
nitrification took place to double the extent. An addition of 10 to 15
per cent again doubled the quantity. When the amount of moisture added
was increased, the effect of larger percentages of oxygen was found to
be less marked. The reason of this is that the oxygen probably acts as
dissolved oxygen; the addition of water meaning at the same time an
addition of available oxygen. This condition exemplifies the value of
tillage operations. The more thoroughly a soil is tilled the more
thoroughly will the aeration of its particles take place, and
consequently the more favourable will this necessary condition of
nitrification be rendered. The benefits conferred on clayey soils by
tillage will in this respect be especially great.
_Temperature._
Another of the conditions determining the rate at which nitrification
takes place, and one which is most important, is _Temperature_.
According to Schloesing and Muentz the temperature at which maximum
development takes place is 37 deg. C.[117] (99 deg. F.), at which temperature it
is ten times as active as at 14 deg. C. (57 deg. F.) Below 5 deg. C. (40 deg. F.) the
action is extremely feeble. It is clearly appreciable at 12 deg. C. (54 deg.
F.), and from there up to 37 deg. C. (99 deg. F.) it rapidly increases. From 37 deg.
C. (99 deg. F.) to 55 deg. C. (131 deg. F.), at which temperature no nitrification
takes place, its activity decreases; at 45 deg. C. (113 deg. F.) it is less
active than at 15 deg. C. (59 deg. F.), and at 50 deg. C. (122 deg. F.) it is very
slight. These results by Schloesing and Muentz have not been exactly
confirmed by Warington. He has found that a considerable amount of
nitrification goes on at a temperature between 3 deg. and 4 deg. C. (37 deg. and 39 deg.
F.), while the highest temperature at which he has found it to take
place is considerably lower than 55 deg. C. (131 deg. F.) Thus he was unable to
start nitrification in a solution maintained at 40 deg. C. (104 deg. F.) It
would thus seem that the nitrifying ferments are able to develop at
lower temperatures than most organisms; and although nitrification
entirely ceases during frost, yet in a climate such as our own there
must be a considerable proportion of the winter during which
nitrification is moderately active.
_Presence of a sufficient quantity of Moisture._
The presence of moisture in a soil is another of the necessary
conditions of nitrification. It has been shown that it is at once
arrested, and indeed destroyed, by desiccation. Other conditions being
equal, and up to a certain extent, the more moisture a soil contains the
more rapid is the process. Too much water, however, is unfavourable, as
it is apt to exclude the free access of air, which, as we have just
shown, is so necessary, as well as to lower the temperature. During a
period of drought the rate at which nitrification takes place will,
therefore, be apt to be seriously diminished.
_Absence of strong Sunlight._
It has been found that the process goes on much more actively in
darkness; indeed Warington has found in his experiments that
nitrification could be arrested by simply exposing the vessel in which
it was going on to the action of sunshine.
_Nitrifying Organisms destroyed by Poisons._
It has already been pointed out that nitrification is arrested by the
action of antiseptics, such as chloroform, bisulphide of carbon, and
carbolic acid. Another substance which has been found to have an
injurious action is ferrous sulphate or "copperas," a substance which is
apt to be present in badly drained soils, or soils in which there is
much actively putrefying organic matter. Maercker has found that in moor
soils containing ferrous sulphate, no nitrates, or mere traces of
nitrates, could be found. A substance such as gas-lime, unless submitted
to the action of the atmosphere for some time, would also have a bad
effect in checking nitrification, owing to the poisonous sulphur
compounds it contains. Common salt, it would seem, also arrests the
process; and this antiseptic property which salt exercises on
nitrification throws a certain amount of light on the nature of its
action when applied, as it is often done, along with artificial
nitrogenous manures.
_Denitrification._
In connection with the process of nitrification, it is of interest to
notice that a process of an opposite nature may also take place in
soils--viz., _denitrification_--a process which consists in reducing the
nitrates to nitrites, nitrous oxide, or free nitrogen. That a reduction
of nitrates takes place in the decomposition of sewage with the
evolution of free nitrogen, was a fact first observed by the late Dr
Angus Smith in 1867; and the reduction of nitrates to nitrites, and
nitric and nitrous oxides in putrefactive changes has been subsequently
noticed by different experimenters, who have further observed that such
reduction takes place in the case of putrefaction going on in the
presence of large quantities of water or where there is much organic
matter.
_Denitrification also effected by Bacteria._
This change was supposed to be of a purely chemical nature, and it has
only been recently discovered that it is effected, like nitrification,
by means of bacteria. It has been surmised by some that the action of
denitrification may be effected by the same organisms that effect
nitrification, and that it depends on merely external conditions which
process goes on. There is no reason, however, to suppose that this is
so, and several of the denitrifying organisms have been identified.
_Conditions favourable for Denitrification._
That it is a process that goes on to any extent in properly cultivated
soils is not to be supposed. The conditions which favour denitrification
are exactly the opposite of those which favour nitrification. It is only
when oxygen is excluded, or, which practically means the same thing,
when large quantities of organic matter are in active putrefaction, and
the supply of oxygen is therefore deficient, that denitrification takes
place. Schloesing, as we have already seen, found that in the case of a
moist soil, kept in an atmosphere devoid of oxygen, a reduction of its
nitrates to free nitrogen took place.
_Takes place in water-logged Soils._
The exclusion of oxygen from a soil may be effected by saturating the
soil with water; and Warington has found in experiments carried out in
an arable soil, by no means rich in organic matter, that complete
reduction of nitrates may be effected in this way. It would thus seem
that the process of denitrification will take place in water-logged
soils, or in the putrefaction of sewage matter in the presence of large
quantities of water. Whether this reduction will result in the
production of nitrites, nitrous oxide, or free nitrogen, depends on
different conditions. This process is one of great importance from an
economic point of view, as it reveals to us a source of loss which may
take place in the fermentation of manures. In the rotting of our
farmyard manure it is possible that the denitrifying organisms may be
more active than we have hitherto suspected, and that a considerable
loss of nitrogen may in this way be effected.
_Distribution of the Nitrifying Organisms in the Soil._
The nitrifying organisms are probably chiefly confined to the soil, and
do not usually occur in rain or in the atmosphere. That, however, they
are found in spots which we might be inclined to think extremely
unlikely, is shown by some recent interesting researches carried out by
Muentz, who discovered that the bare surfaces of felspathic, calcareous,
schistose, and other rocks at the summit of mountains in the Pyrenees,
Alps, and Vosges, yielded large numbers of them, and that they occurred
to a considerable depth in the cracks and fissures of the rocks. The
nitrifying organisms are also found in river-water, in sewage, and
well-waters.
_Depth down at which they occur._
In Warington's earlier experiments, the conclusion he arrived at was
that the occurrence of the nitrifying organisms was almost entirely
limited to the superficial layers of the soil, and that they were seldom
to be met with much below a depth of 18 inches. His subsequent
experiments, however, considerably modified this conclusion, and showed
that nitrification may take place to a depth of at least 6 feet.[118]
But although it may take place at this depth, it probably, as a general
rule, is limited to the surface-soil, as it is only there the conditions
for obtaining circulation of air are sufficiently favourable. A great
deal, of course, will depend on the nature of the soil--_i.e._, as to
its texture. In a clayey subsoil the principal hindrance to
nitrification will be the difficulty of obtaining sufficient aeration.
In clay soils it is probable, therefore, that nearly all the
nitrification goes on in the surface layer; in sandy soils it may take
place to a greater depth.[119]
_Action of Plant-roots in promoting Nitrification._
In this connection the action of plant-roots in permitting a more
abundant access of air to the lower layers of the soil, and thus
promoting nitrification, is worth noticing. This has been observed in
the case of different crops. Thus the action of nitrification has been
found to be more marked in the lower layers of a soil on which a
leguminous crop was growing than on that on which a gramineous. "The
conditions which would favour nitrification in the subsoil are such as
would enable air to penetrate it, as artificial drainage, a dry season,
the growth of a luxuriant crop causing much evaporation of the water in
the soil. Such conditions, by removing the water that fills the pores of
the subsoil, will cause the air to penetrate more or less deeply and
render nitrification possible. Subsoil nitrification will thus be most
active in the drier periods of the year" (Warington).
_Nature of Substances capable of Nitrification._
What kinds of nitrogenous substances are capable of undergoing this
process of nitrification are not yet well known. The question is, of
course, one of great importance, as the rapidity with which a
nitrogenous body nitrifies will be an important factor in determining
its value as a manure. Unfortunately, on this subject we know, as yet,
very little. We are well aware that the nitrogen present in the humic
matter of the soil is readily nitrifiable. In the experiments on
nitrification the nitrogenous bodies used have been chiefly ammonia
salts, so that it is difficult to say whether, in the case of other
nitrogenous substances, micro-organic life of a different sort has not
also been active and has converted the nitrogen into ammonia, and
thereby prepared the way for the process of nitrification.
That various manures, such as bones, horn, wool, and rape-cake are
readily nitrifiable, has been shown by experiment. Laboratory
experiments have also been carried out on such different nitrogenous
substances as ethylamine, thiocyanates, gelatin, urea, asparagin, and
albuminoids of milk. But in all these experiments, how far these bodies
have been directly acted upon by the nitrifying organisms, or how far
they have first undergone a preparatory change in which their nitrogen
has been first converted into ammonia, is impossible to say. It is at
least quite probable that all the organic forms of nitrogen have first
to be converted into ammonia ere they are nitrified.
_Rate at which Nitrification takes place_.
A question which is practically of no little importance is the rate at
which nitrification takes place. From what has been already said as to
the nature of the conditions favourable for the process, it will be at
once seen that this will depend on how far these conditions are present
in the soil. In point of fact the rate at which nitrification takes
place will vary very much in different soils. A greater difference,
however, in the rate at which it takes place, will be found even in the
same soils at different periods of the year. In this country, where the
most favourable temperature for its development is seldom reached, it
never goes on at the same rate as in tropical climates. One of the
causes of the greater fertility of tropical soils is due, doubtless, to
the very much longer duration of the period of nitrification, as well as
to its greater intensity. As, however, temperature is not the only
condition, and the presence of moisture is quite as necessary, it may be
that its development is seriously retarded in many tropical climates by
the extreme dryness of the soil during long periods.
_Takes place chiefly during the Summer Months_.
Although in this climate, as has already been pointed out, nitrification
probably goes on during most of the winter months, owing to the fact
that the temperature of our soils is only occasionally below the
minimum temperature at which the process takes place, yet there can be
little doubt that the great bulk of the soil-nitrates are produced
during a few months in summer. A fair conception of this amount is
afforded by the interesting experiments on the composition of
drainage-waters made at Rothamsted, which we shall have occasion to
refer to immediately. It may be pointed out, however, that it is not
always safe to take the amount of nitrates found in drainage-waters as
an infallible indication of this rate, for this amount will depend to a
certain extent on the amount of rainfall, and would be misleading in the
case of a long period of drought. On the whole, however, it furnishes us
with extremely useful data for the elucidation of this important
problem.
_Process goes on most quickly in Fallow Fields._
It has been shown in the Rothamsted experiments that the process goes on
best in fields lying in bare fallow; and in this fact lies the
explanation of one of the many reasons why the practice of leaving
fields in bare fallow, so common in past times, and still practised in
the case of clay soils in some parts of the country, was so beneficial
to the land thus treated. But despite this fact, the practice of leaving
soils in bare fallow can scarcely be justified from this point of view,
as the loss of nitrates through the action of rain is very great in our
moist climate.
_Laboratory Experiments on Rate of Nitrification._
Several interesting experiments have been carried out with the object of
affording data for estimating the rate at which the process may go on in
our soils under certain conditions. An old experiment, carried out by
Boussingault, illustrates, in a general way, how rapid the process is
under favourable circumstances. A small portion of rich soil was placed
on a slab protected by a glass roof, and was moistened from time to time
with water. The amount of nitrate of potash formed under these
circumstances was estimated from time to time during a period of two
months. During the first month (August) the percentage was increased
from .01 to .18 (equal to about 5 cwt. of nitrate of potash per acre).
The increase during the second month (September) was very much
less,--indeed only about a seventh of the amount.[120] The soil
experimented with was an extremely rich garden soil, and all the
conditions for nitrification were most favourable.
Of recent experiments on the rate of nitrification, the most striking,
perhaps, are those by Schloesing. He mixed sulphate of ammonia with a
quantity of soil fairly rich in organic matter, and containing 19 per
cent of water. During the twelve days of active nitrification no less
than 56 parts of nitrogen per million of soil were nitrified per day.
Taking the soil to a depth of 9 inches, this would be equal to more
than 1 cwt. per acre--an amount of nitrogen equal to that contained in
6 cwt. of commercial nitrate of soda. These experiments are interesting
as showing what is probably the maximum rate of nitrification under the
most favourable circumstances, and where there is an abundant supply of
easily nitrifiable nitrogen. That nitrification ever takes place in our
soils to this extent is not to be for a moment supposed.
Warington, in his Rothamsted experiments, has found that the greatest
rate, working with ordinary arable soil (first 9 inches) from the
Rothamsted farm, was .588 parts per million of air-dried soil per
day--_i.e._, 1.3 lb. per acre (equal to about 8 lb. of nitrate of soda).
Similar soil, when supplied with ammonia salts, showed nearly double
this quantity. Higher results were obtained by Lawes and Gilbert with
rich Manitoba soils, the average rate being .7 parts per million per
day.
The last of these interesting laboratory experiments on the rate of
nitrification we shall refer to, are those by Deherain. He experimented
with soils containing different amounts of nitrogen and moisture. With a
soil containing .16 per cent of nitrogen he obtained, during a period of
90 days, rates of nitrification varying from .71 to 1.09 per million
parts of soil. The maximum quantity was formed when the soil contained
25 per cent of moisture. On a soil considerably richer--viz.,.261 per
cent of nitrogen--a higher rate of nitrification took place--1.48 parts
per million. The highest rate obtained in these experiments showed,
when calculated to pounds per acre, about 5-1/2, taking the soil to a
depth of 9 inches. When the soil was alternately dried and moistened the
process was most rapid.
_Portion of Soil-nitrogen more easily Nitrifiable than the rest._
Lastly, it may be noticed that in the above-cited experiments, and
others of a similar kind, the process goes on most rapidly at first, and
steadily diminishes thereafter. This is due to the fact, that there is
generally a certain quantity of nitrogen in most soils in a more easily
nitrifiable condition than the rest, so that when this becomes oxidised
nitrification proceeds more slowly. It would further seem that the
nitrogen of the subsoil is less easily nitrified than that of the
surface-soil.
_Rate of Nitrification deduced from Field Experiments._
While the above experiments throw much light on the question of the rate
at which nitrification may go on under different circumstances, the
results furnished by actual analyses of soils and their drainage-waters
are of still more practical value; and the Rothamsted experiments
fortunately furnish us with a number of these valuable results.
_Quantity of Nitrates formed in the soils of Fallow Fields._
These researches had to be carried out on soil taken from fields lying
in bare fallow; for no true estimate of the amount of nitrates formed
could have been obtained from _cropped_ fields. In the first 27 inches
of soil of six separate fields, nitrate-nitrogen was found to vary from
36.3 lb. to 59.9 lb. per acre. In four of these fields the largest
proportion was found in the first 9 inches of soil; in the remaining
two, in the second 9 inches; while the third 9 inches in two fields
showed almost as large a proportion as the first 9 inches.[121]
_Position of Nitrates depends on Season._
The position of nitrates in the soil depends largely on the season; for,
as has been already pointed out, their production is almost entirely
limited to the surface-soil, and it is only by being washed down in rain
that they find their way to the lower layers. A wet season, therefore,
has the effect of increasing their percentage in the lower soil-layers.
_Nitrates in Drainage-waters._
As there is a certain proportion of nitrates that finds its way even
below the first 27 inches of soil, the above results do not show their
total production. To accurately estimate this amount we must ascertain
the quantity escaping in drainage-water. Here, again, the Rothamsted
experiments furnish us with valuable data. The amount found in
drainage-waters of course naturally varies very much, and depends
largely on the rainfall; but taking an average of twelve years, this has
been found to amount to between 30 and 40 lb. per acre--an amount not so
very far short of that found in the first 27 inches of the soil itself.
This was from comparatively poor soil, it must be remembered, and a much
larger quantity would undoubtedly be produced in the case of richer
soils. Adding then the results together, we find that in soils like
those at Rothamsted, when in bare fallow, between 80 and 90 lb. of
nitrogen are converted into nitrates in some fourteen months' time--an
amount equal to about 5 cwt. of nitrate of soda. It is a fact of no
little practical significance that nearly one-half of this large
quantity is found in the drainage-water.
_Amount produced at Different Times of the Year._
Some indication of the rate at which nitrification takes place during
the different months of the year is obtained from a study of the results
of the analyses of drainage-waters which we have just referred to. This,
however, it must be remembered, only furnishes us with a very
approximate indication. The month showing the greatest amount of
nitrates in the drainage-water must not necessarily be regarded as that
during which nitrification has been most active, for the amount chiefly
depends on the rainfall. In illustration of this it will be found that
the drainage-water during the autumn and early winter months contains
most nitrates, not because nitrification is most active then, but
because the rainfall is greatest, and a large proportion of the nitrates
formed during the drier summer months is being only then washed from the
soil. The amount of nitrates in drainage-waters steadily diminishes from
autumn through the winter months, and is least in spring. The total
amount of nitrates found in the drainage-water is, therefore, not a safe
guide. What, however, does furnish us with a more reliable indication is
the _percentage_ of nitrates in the drainage-water. Regarding the
results of the analyses of drainage-water (see Appendix) from this point
of view, it will be seen that this is greatest during the month of
September, and least during April.[122]
_Nitrification of Manures._
A subject which has not yet been specially referred to, but which is of
great practical importance, is the nitrification of manurial substances.
It is unfortunate that the amount of research hitherto devoted to this
important question has been slight, and that the knowledge we possess is
therefore very limited.
_Ammonia Salts most easily Nitrifiable._
One fact, however, about which there can be little doubt, is that
nitrogen in the form of ammonia salts is, of all compounds of nitrogen,
the most easily nitrifiable. Indeed, as we have already indicated, it is
highly probable that the conversion of the different forms of organic
nitrogen into ammonia is an intermediate stage in the nitrification of
these bodies. At any rate it seems to be invariably the case that when a
mixture of nitrogen compounds, including ammonia salts, are allowed to
nitrify, the nitrogen in the form of ammonia is the first to become
nitrified.
_Sulphate of Ammonia most easily Nitrifiable Manure._
It follows from this that sulphate of ammonia, the most common of
ammoniacal manures, is one of the most speedily nitrified when applied
to the soil. The rate at which the nitrification of this manure takes
place naturally varies according to the quantity applied, and other
circumstances, such as the nature of the soil and the weather, &c. That,
under favourable circumstances, the conversion of ammonia into nitrates
is very rapid, has been shown by a number of experiments. Deherain has
found that when sulphate of ammonia was mixed with soil at the rate of 2
cwt. per acre, nitrification took place at the rate of 1/100th of its
nitrogen per day.
_Rate of Nitrification of other Manures._
Of other nitrogenous manures, guano, it would seem, comes next to
sulphate of ammonia in the rate at which it becomes nitrified in the
soil; while next to guano stand green manures, dried blood, meat-meal,
&c. As we should expect, such a manure as shoddy is very slowly
nitrified. The rate at which the nitrogen compounds in farmyard manure
become nitrified, when incorporated with the soil, vary very much
according to circumstances. It goes on probably at a greater rate than
the ordinary nitrification of soil-nitrogen. It is a somewhat striking
fact that the effect of adding nitrate of soda to the soil may be at
first to check nitrification. That the addition of common salt, even in
small quantities, has this result, is at any rate certain. The presence
of salt to the extent of one-thousandth of the weight of the soil, has a
prejudicial effect.
_Soils best suited for Nitrification._
To recapitulate, then, nitrification is effected through the agency of
micro-organisms, which are present to a greater or less extent in all
soils. It requires for its favourable development air, warmth, moisture,
absence of strong light, presence of a salifiable base--viz., carbonate
of lime--the presence of certain mineral food-constituents, such as
phosphates, and a certain amount of alkalinity. It consequently takes
place to the least extent in barren sandy soils. Soils rich, light, well
ventilated, uniformly moist, warm, and chalky, are best suited for its
development. Other things being equal, it develops better in a
fine-grained soil than in a coarse-grained soil, because, in the case of
the former, aeration and uniform moistening of the soil are best
secured.
_Absence of Nitrification in Forest-soils._
A point of considerable interest is the practical absence of the process
in forest-soils. The absence, or occurrence in the most minute traces,
of nitrates in forest-soils has been accounted for by the lowness of the
normal temperature of such soils and their extreme dryness. This latter
condition is accounted for by the enormous transpiration of water which
takes place through the trees, especially in summer-time, which is such
as to render the soil almost air-dry. Lastly, it may be accounted for by
the want of mineral food ingredients.
_Important Bearing of Nitrification on Agricultural Practice._
Before concluding this chapter, it may be well to draw attention to the
important bearing which nitrification has on agricultural practice. The
light which our present knowledge--imperfect as it is--of this most
interesting process throws on the theory of the rotation of crops is
very striking, for it shows how the adoption of a skilful rotation may
be made to prevent the loss of enormous quantities of the most valuable
of all our soil-constituents,--the one on the presence of which
fertility may be said most to depend--viz., nitrogen.
_Desirable to have Soil covered with Vegetation._
The constant production of nitrates going on in the soil, the inability
of the soil to retain them, and the consequent risk of their being
removed in drainage, furnish a strong argument in favour of keeping our
soils as constantly covered with vegetation as possible.
_Permanent Pasture most Economical Condition of Soil._
From the point of view of conservation of soil-nitrates, permanent
pasture may be said to be the most economical condition for the soil to
be in. In such a case the nitrates are assimilated as they are formed,
and, by being converted in the plant into organic nitrogen, they are at
once removed from all risk of loss. A consideration, therefore, of the
process of nitrification furnishes many arguments in favour of laying
down land in permanent pasture--a practice which of late years has been
increasingly followed in many parts of the country. As, however, it is
not possible or desirable to carry out this practice beyond certain
limits, the rotation which most nearly conforms to the condition of
keeping the soil covered with vegetation, and most approximates in this
respect to permanent pasture, is most to be recommended.
_Nitrification and Rotation of Crops._
The chief risk of loss of nitrates is in connection with a cereal crop
such as wheat. Where turnips follow wheat, there is a period during
which the soil is left uncovered, and during which most serious loss of
nitrates is apt to ensue. The risk of loss is enhanced by the fact that
the assimilation of nitrates by cereals ceases before the season of
their maximum production in the soil. The soil is then left bare of
vegetation during the autumn, which is the most critical period of all,
and the result must be serious loss. In order to minimise this loss, the
practice of growing catch-crops has been had recourse to. As, however,
this practice will be dealt with elsewhere, nothing further need here be
said.
FOOTNOTES:
[97] As the formation of nitrites is a stage in the process, the term
_nitrification_ includes the formation of nitrites as well as nitrates.
[98] Nitre seems to have been known as early as the thirteenth century.
[99] Lawes and Gilbert, for example, have shown that in the Rothamsted
soils it only amounts to a few parts per million of soil.
[100] See Appendix, Note I., p. 196.
[101] The artificial production of nitre seems to have been first
effected by Glauber in the seventeenth century.
[102] The lime-rubbish from old buildings, especially those parts which
have come in contact with the earth, or plastering from the walls of
damp cellars, barns, stables, &c., have been found to be rich in nitrate
of lime, and, as has been long well known, constitute by themselves a
valuable manure. The formation of the nitrate of lime can be accounted
for by the contact of the lime with nitrogenous matter of different
kinds.
[103] As much of the nitric acid in this solution was present as nitrate
of lime, it was usually treated with a solution of potassium carbonate,
the result being the precipitation of the lime as carbonate, pure
saltpetre being left in solution, according to the following equation--
K_{2}CO_{3} + Ca(NO_{3})_{2} = 2 KNO_{3} + CaCO_{3}.
Under the French mode of manufacture, the process was considered to have
developed satisfactorily when 1000 lb. of earth, at the expiration of
two years, yielded 5 lb. of nitre.
[104] Pasteur had already in 1862 expressed the opinion that
nitrification might probably be in some way connected with ferments. A.
Mueller (see 'Journal of Chemical Society,' 1879, p. 249) was the first
to advance the opinion that nitrification was due to the action of a
ferment. This conclusion he was led to by the observation that while the
ammonia in sewage was converted into nitric acid, no change took place
in solutions of ammonia or urine prepared in the laboratory.
[105] Bisulphide of carbon and phenol (carbolic acid) have also been
experimented with in connection with their antiseptic action on
nitrification. In these experiments the former had a similar effect to
chloroform; the phenol, however, while hindering it did not entirely
suspend it, due probably to the difficulty of bringing the phenol vapour
into thorough contact with the soil-particles.
[106] Winogradsky has named the nitrous organism _nitrosomonas_, and the
nitric organism _nitrobaeter_.
[107] From a series of Lectures delivered by him in connection with
Lawes Agricultural Trust, in the United States.
[108] This silica-jelly consists of dialysed silicic acid, ammonium
sulphate, potassium phosphate, magnesium sulphate, calcium chloride, and
magnesium carbonate.
[109] This fact is all the more striking when we remember that this
decomposition of carbonic acid is best effected in the dark, since light
is prejudicial to nitrification.
[110] See Appendix, Note II., p. 196, and Note III., p. 197.
[111] See Appendix, Note V., p. 198.
[112] This is shown by the fact that nitrification will only continue in
a solution of carbonate of ammonia till one-half the ammonia is
nitrified. It then stops. The base, with which the nitrous acid combines
as it is formed, being at that stage entirely used up, nitrification is
no longer possible. With regard to urine solutions the same is the case.
Nitrification thus will only take place where there is a sufficiency of
base.
[113] See Appendix, Note IV., p. 197.
[114] It would seem that an alkalinity much exceeding four parts of
nitrogen per million is prejudicial to the process.
[115] According to Warington, solutions containing 50 per cent of urine
become nitrifiable when sufficient gypsum is added. The gypsum
neutralises the alkalinity of nitrifying solutions by converting the
alkaline ammonium carbonate into neutral ammonium sulphate, the calcium
carbonate being precipitated.
[116] See Chapter on Farmyard Manure.
[117] As practically illustrating this fact, a solution kept at 10 deg. C.
required ten days, while a solution kept at 30 deg. C. required only eight
days for nitrification.
[118] In sixty-nine trials no failure to produce nitrification by
seeding with soil from a depth, of 2 feet was experienced. Similarly in
eleven trials only one failure took place with soil from a depth of 3
feet. With clay soil from a depth of 6 feet success took place to the
extent of 50 per cent. No nitrification was obtained with clay from a
depth of 8 feet. Entire failure was experienced with chalk subsoil. The
process thus diminishes in activity the lower down we go.
[119] Koch has found that in soils he has examined few organisms were
found at a depth below 3 feet.
[120] See Appendix, Note VI., p. 198.
[121] For full analytical results see Appendix, Note VII., p. 198.
[122] We find the least amount in the month of April. In the water, from
a 20-and 60-inch gauge respectively, the amounts were 1.35 lb. and 1.61
lb. per acre (rainfall 2.25 inches). From then on to November the amount
steadily increases. In the latter month it reaches its maximum--viz.,
6.50 lb. (20-inch gauge) and 5.98 lb. (60-inch gauge) per acre (rainfall
2.30 inches). See Appendix to Chapter III., Note VIII, p. 160.
APPENDIX TO CHAPTER IV.
NOTE I. (p. 162).
OLD THEORIES OF NITRIFICATION.
According to the old theories, nitrification was regarded as a simple
case of the oxidation of nitrogen by the oxygen of the air, or by ozone.
The union of nitrogen and oxygen, however, probably takes place only at
very high temperatures, such as are formed during electric discharges.
It is needless to point out that the union of nitrogen and oxygen in
this way is not likely to occur in soils. According to other theories,
nitrification was effected by means of the oxidation of ammonia.
Ammonia, however, can only be oxidised to nitric acid by means of
certain powerful oxidising agents, such as ozone or hydrogen peroxide.
As, however, these substances are not found in the soil, it is much to
be doubted whether nitric acid is ever formed in the soil in this way.
It is possible, however, as held by some, that ferric oxide is capable
of inducing this conversion. On the whole, however, most evidence points
to the conclusion that all nitric acid produced in the soil is formed
through the agency of micro-organic life.
NOTE II. (p. 170).
The important fact that nitrification can take place in solutions
practically devoid of organic matter, was first shown by Dr J. H. M.
Munro ('Chemical Society Journal,' August 1886, p. 561). It was further
corroborated by Warington and P. F. Frankland. Winogradsky, however,
has carried out the most conclusive experiments on the subject. "He
prepared vessels and solutions, carefully purified from organic matter,
and these solutions he sowed with the nitrifying organism. Finding that
under these conditions the nitrifying organism increased enormously and
displayed its full vigour, he proceeded further to determine the amount
of carbonaceous organic matter formed in solutions after the
introduction of the organism. By making the nitrification intensive, he
was able to obtain considerable quantities of carbon from the nitrified
solutions by the process of wet combustion. In his third memoir he
publishes figures which apparently show a close relation between the
amount of nitrogen oxidised, and the amount of carbon assimilated; the
ratio is about 35:1."--See Bulletin of U.S. Department of Agriculture,
No. 8, containing Lectures on Rothamsted Experiments by R. Warington,
F.R.S., p. 50.
NOTE III. (p. 170).
The oxidising power of the micro-organisms of soil is not confined to
the oxidation of ammonia or of organic matter. Muentz has shown that soil
is capable of oxidising iodides to hypo-iodides and iodates, and
bromides to hypo-bromides and bromates. This is a very important result,
and seems to indicate that nitrification is part of a general oxidising
action, and that we must not assume that nitrites or nitrates are
produced because they are in themselves of advantage to the organism.
NOTE IV. (p. 172).
"When urine in different degrees of dilution was treated with soil, 1
gram of soil being added to 100 c.c. of diluted urine, nitrification
commenced in the 1-per-cent solution in 11 days, in the 5-per-cent
solution in 20 days, in the 10-per-cent solution in 62 days, in the
12-per-cent solution in 90 days. The alkalinity of the last-named
solution when nitrification commenced was equal to 447 mgs. of ammonia
per litre. A solution with an alkalinity of 500 mgs. of ammonia per
litre is apparently unnitrifiable."--American Department of Agriculture
Bulletin, Warington's Lectures on Rothamsted Experiments, p. 51.
NOTE V. (p. 171).
Professor P. F. Frankland in his experiments used the following
solutions:--
grms.
NH_{4}Cl .5 }
H_{3}PO_{4} .1 }
MgSO_{4} .02 > In 1000 c.c. of distilled water.
CaCl_{2} .01 }
CaCO_{3} 5.00 }
NOTE VI. (p. 185).
Experiment by Boussingault on Rate of Nitrification.
Percentage of Nitrate
1857. of Potash. = lb. per acre.
August 5 .01 34
August 17 .06 222
September 2 .18 634
September 17 .22 760
October 2 .21 728
NOTE VII. (p. 188).
NITROGEN AS NITRATES IN ROTHAMSTED SOILS AFTER BARE FALLOW IN LB.
PER ACRE.
------------+-----------+---------------------------+-----------+----------
| Alternate | Four-course rotation. | |
| Wheat +-----------+---------------+ |
Depth of | and | Super- | | Claycroft | Foster's
Soil. | Fallow. | phosphate | Mixed Manure. | Field. | Field.
| | only. | | |
+-----------+-----------+-------+-------+-----------+----------
| 1878. | 1878. | 1878. | 1882. | 1881. | 1881.
------------+-----------+-----------+-------+-------+-----------+----------
| lb. | lb. | lb. | lb. | lb. | lb.
1st 9 ins. | 28.5 | 22.3 | 30.0 | 40.1 | 16.4 | 14.6
2d 9 ins. | 5.2 | 14.0 | 18.8 | 14.3 | 26.5 | 24.6
3d 9 ins. | - | - | - | 5.5 | 15.9 | 17.3
------------+-----------+-----------+-------+-------+-----------+----------
Total | 33.7 | 36.3 | 48.8 | 59.9 | 58.8 | 56.5
------------+-----------+-----------+-------+-------+-----------+----------
CHAPTER V.
THE POSITION OF PHOSPHORIC ACID.
We now come to consider the position of phosphoric acid in agriculture.
The question is, however, very much simpler in its nature than that of
nitrogen, and may be consequently discussed in a much shorter space.
Most soils, as we have already had occasion to point out, are better
supplied with available ash-plant ingredients than available nitrogen
compounds. The quantity of phosphoric acid absorbed by the plant is also
less than that of nitrogen; and lastly, the different chemical compounds
of phosphoric acid occurring in the soil are not nearly so numerous as
those of nitrogen. Phosphoric acid, however, must be regarded as ranking
next to nitrogen in its importance as a soil-constituent.
_Occurrence of Phosphoric Acid in Nature._
That phosphoric acid is of universal occurrence may be assumed from the
fact of the almost universal occurrence of vegetable life on the
earth's surface; for plants are unable to grow without it. While thus of
practically universal occurrence, its amount in most soils is very
trifling. As the only source of it in the soil is from the
disintegration of the different rocks, a short description of its
occurrence in the mineral kingdom may first be given.
_Mineral Sources of Phosphoric Acid._
It was first discovered in the mineral kingdom towards the close of last
century; but we have only of late years ascertained any exact knowledge
of its percentage in the different rocks out of which soils are formed.
This has been shown in many cases to be very trifling. It most
abundantly occurs as _apatite_, a mineral consisting of calcium
phosphate, with small quantities of calcium fluoride or calcium
chloride. This apatite, or phosphorite, is found in certain parts of the
world in large masses; but as a rule, it only occurs in small quantities
in most rocks. It may be stated that the older rocks are, as a general
rule, richer in it than those of more recent formation; and Daubeny has
drawn attention to this fact as furnishing a useful guide in estimating
the probable richness of a soil in phosphoric acid. The older,
therefore, a rock is, the richer it is likely to be in phosphoric acid.
_Apatite and Phosphorite._
Of apatite there are a variety of kinds, which differ in their
appearance as well as in their composition. It occurs chiefly in a
crystalline form, and is found sometimes in regular crystals, but it
also occurs in the amorphous form. In colour it may be white, yellow,
brown, red, green, grey, or blue. Two classes of apatite are found. The
first consists of calcium phosphate along with calcium fluoride; and in
other kinds of apatite the calcium fluoride is replaced by calcium
chloride. Phosphorite is another name for apatite, but is chiefly
applied to impure amorphous apatite. The percentage of phosphate of lime
in different kinds of apatite may be stated at from 70 to 90 per cent.
It occurs in very large quantities in Canada, the Canadian apatite being
very rich in phosphate of lime--80 to 90 per cent. In many parts of the
world it forms portions of mountain-masses, and is quarried, crushed,
and used for artificial manurial purposes. Further details of its
occurrence and chemical composition will be found in the Appendix.[123]
_Coprolites._
In many parts of the world round nodules, largely consisting of
phosphate of lime, have been found, to which the name "coprolites" has
been given, on the assumption that they consisted of fossilised animal
excrements. These coprolites, or osteolites as they have also been
called, vary in the percentage of phosphate of lime they contain.
Sometimes this amounts to 80 per cent, but as a rule it is very much
less. They also in the past have formed an important source of manure,
and will be referred to subsequently.
_Guano._
We have, lastly, phosphoric acid occurring in large quantities in
guano-deposits, chiefly found on the west coast of South America. These
deposits, which have been of enormous importance as a source of
artificial manure, are of animal origin, and will be discussed at
considerable length in a chapter specially devoted to the subject; so
that we need do no more than mention them here.
Phosphoric acid is also found in the form of phosphate of lime in
certain rocks as "layers" and "pockets."
_Universal Occurrence in Common Rocks._
But while it is thus found in considerable quantities in various parts
of the world, and while no anxiety need thus be felt as to its abundance
for artificial manurial purposes, its occurrence in the common rocks,
which, as we have already pointed out, is practically universal, is in
many cases very minute.
Fownes first identified it in the felspathic rocks in 1844; and since
then its percentage in granite, lava, trachyte, basalt, porphyry,
dolomite, gneiss, syenite, dolerite, diorite, and a number of other
rocks, has been determined by numerous investigators. For analyses of
these rocks the reader is referred to the Appendix.[124]
_Occurrence in the Soil._
That no soil is actually without phosphoric acid is highly probable, but
in many soils it is present in the merest traces, and even in fertile
soils it is rarely present in quantities over two-tenths of a per cent;
while half that amount may be taken as an average for most fairly
fertile soils. This would be about 3500 lb. per acre, calculating the
soil to a depth of 9 inches. In exceptional cases it has been found to
the extent of .3 per cent; and in the famous Russian _black earth_ it
has been found to amount to .6 per cent.[125] Like nitrogen, it is found
in greatest amount in the surface portion of the soil, but its amount at
different depths does not vary to the same extent as we have found to be
the case with nitrogen.
_Condition in which Phosphoric Acid is present in the Soil._
Unlike nitrogen, phosphoric acid occurs in the soil almost entirely in
an _insoluble_ form; and when applied to the soil in a soluble form, is
speedily converted into an insoluble condition. Its most commonly
occurring forms are as phosphates of lime, iron, and alumina. These
facts are of importance to remember, as they explain why phosphoric acid
is not found in drainage-water in any quantity. It also shows how little
the risk of loss from drainage is in the application of artificial
phosphatic manure to the soil.
_Occurrence in Plants._
The percentage of phosphoric acid in plants, like other
ash-constituents, is subject to considerable variation, and depends on a
variety of conditions, such as the state of the plant's development,
nature of soil, climate, season, treatment with manures, &c. All these
conditions have a certain influence. The different parts of the plant
have been found to contain it in different quantities. The tendency of
phosphoric acid is to travel up to the higher portions of the plant with
the progress of growth, and to finally accumulate in the seed. As
illustrating this, it may be mentioned that the inner portion of the
stalk of a ripe oat-plant has been found to contain only a seventeenth
of the amount of phosphoric acid found in the same portion of the stalk
of a young oat-plant. Similarly it may be mentioned that, while the ash
of the grain of rye and wheat contains nearly half their weight of
phosphoric acid, the percentage present in the ash of other parts of the
plant amounts only to from 5 to 16 per cent. The percentage of
phosphorus is greater in young plants than in mature plants; it is
greater also in quickly developed plants than in slowly developed
plants.
In the plant, phosphorus is present chiefly in the albuminoids; and its
absorption from the soil takes place in greatest quantity during the
period of maximum growth. In beans and peas an oil containing phosphorus
has been found.
_Occurrence in Animals._
That phosphorus in different forms exists in animal tissue is well
known. It is found both in the brain and in the nerves, as well as in
nearly all the fluids of the animal body. It is, however, in the bones
that it is most abundant, the mineral portion of which is almost
entirely made of phosphate of lime,--a fact which renders bones such a
valuable artificial manure. Altogether, phosphoric acid occurs in the
animal body to the extent of 2.3 per cent. There is a point which we
shall have occasion to draw the student's attention to further on in
discussing the nature of farmyard manure--and that is, that the urine of
the common farm animals is practically devoid of phosphoric acid.
_Sources of Loss of Phosphoric Acid in Agriculture._
As we have already done in the case of nitrogen, we may now attempt to
form some conception of the sources of loss and gain of phosphoric acid
in the soil. The sources of loss may be divided into natural and
artificial. Of natural sources of loss we have only one, and that is
loss by drainage.
_Loss of Phosphoric Acid by Drainage._
We have already seen that the condition in which phosphoric acid is
present in the soil is as insoluble phosphate. In drainage-water it
occurs in mere traces. Minute though the amount seems when stated as
percentage, and small as it appears beside the loss (from the same
source) of nitrogen, it is yet, if considered for large areas,
sufficiently striking. Thus it has been estimated that in the river Elbe
there is carried off by drainage from the fields of Bohemia 2-3/4
million pounds (1200 tons) of phosphoric acid annually. This, it is
true, is a very trifling amount compared with the annual loss of
nitrogen from an equal area; but then it must be remembered, on the
other hand, the sources of gain to the soil of this ingredient are not
so numerous as are those of nitrogen, the only sources of phosphoric
acid being in the manure applied to the soil, and that coming from the
gradual disintegration of phosphatic minerals.
_Artificial Sources of Loss._
The other sources of loss may be classed under the term artificial, and
are connected with agricultural practice. Just as we have seen that in
the case of nitrogen enormous quantities of that substance are
constantly being removed from the soil in those crops which are
consumed off the farm, so, too, enormous quantities of phosphoric acid
are being removed in the same way. As illustrating this fact, it may be
mentioned that Professor Grandeau has recently estimated that in the
entire crops grown in France in one year there are about 298,200 tons of
phosphoric acid; while the amount returned in the dung of farm animals
is only 157,200, or only about one-half of what is removed in the crops,
leaving a deficit of 147,000 tons to be made good by the addition of
artificial phosphatic manures, if the fertility of the soil is to be
maintained. The same authority has calculated that in the bones of the
entire farm animals in France there is no less a quantity than 76,820
tons of phosphoric acid.
As an example of how, in many cases, the amount of phosphoric acid
removed from the farm is very often much greater than that restored, a
case quoted by Crusius may be cited. This was a farm of 670 acres
(Saxon) which had received only farmyard manure, and from which, during
sixteen years, 985.67 cwt. of phosphoric acid had been sold off in the
crops; while only 408.33 cwt. had been restored in the manure, leaving a
loss of 577.34 cwt.
_Phosphoric Acid removed in Milk._
A further source of loss is the phosphoric acid removed in milk. In the
total annual yield of milk from one cow there may be from 11 to 12 lb.
of phosphoric acid.
_Loss in Treatment of Farmyard Manure._
The risks of loss of phosphoric acid in the treatment of farmyard manure
are not so great as in the case of nitrogen. There is, however, a
considerable risk, through want of proper precautions, of the soluble
phosphates being washed away by rain.
_Loss in Sewage._
The loss of phosphoric acid incurred by the present method of sewage
disposal is not so large as the loss of nitrogen, inasmuch as the
quantity of phosphoric acid contained in human excreta is very much
less. Roughly speaking, it may be said to amount to a little less than
one-third of the nitrogen lost in this way.
_Sources of Artificial Gain of Phosphoric Acid._
To balance these losses, we have a practically unlimited supply of
mineral phosphates for application as artificial manure, as well as
large quantities of other manures, many of them already mentioned in
connection with nitrogen, such as bones and guanos of all kinds. Quite
recently, also, a large source of phosphoric acid has been opened up in
the basic slag, a rich phosphatic bye-product obtained in considerable
quantity in steel-works from the basic process of steel manufacture. We
have also large quantities of phosphoric acid in the imported
feeding-stuffs, for statistics regarding which we would refer our
readers to a previous chapter. The question of the actual amount
contained in these sources is not of the same interest as in the case of
nitrogen, and need not therefore detain us. We have sufficiently
indicated the importance of phosphoric acid in agriculture by the
statements above given. All further consideration of phosphoric acid
must therefore be deferred to future chapters.
FOOTNOTES:
[123] See Appendix, Note I., p. 210.
[124] See Appendix, Note II., p. 211.
[125] These results, as indeed all soil percentages, are calculated on
the soil in a dry condition.
APPENDIX TO CHAPTER V
NOTE I. (p. 201).
COMPOSITION OF APATITE (Voelcker).
(_Krageroee, Norway._)
Lime 52.16
Phosphoric acid 41.25
Chlorine 4.10
Fluorine 1.23
Oxide of iron 0.29
Alumina 0.38
Potash and soda 0.17
Water 0.42
------
100.00
======
Apatite is found in considerable quantities in America, Germany, France,
Spain, Hungary, Norway, and Great Britain. According to Rose, apatite is
made up of three molecules of tribasic calcium phosphate (Ca(PO_4)_2),
combined with one molecule of calcium fluoride (Ca F_2) or one molecule
of calcium chloride (CaCl_2) respectively.
The composition of the pure mineral should be--
_Chlorapatite._
Per cent.
Calcium phosphate 89.38
Calcium chloride 10.62
_Fluorapatite._
Calcium phosphate 92.31
Calcium fluoride 7.69
NOTE II. (p. 203).
The following is a list of the commoner rocks in which the percentage of
phosphoric acid has been determined. The results are taken from analyses
by Nesbit, Schramm, Bergemann, Rose, Deherain, Handtke, Petersen,
Nessler, Muth, Fleischmann, Storer, and others:--
Per cent.
Felspar 1.7
Granite 0.09 0.25 0.58 0.68
Lava 1.21 1.8
Trachyte 0.30 0.66
Basalt 0.50 1.11
Porphyry 0.26
Marl 1.45 2.31 3.8
Calcareous stones 0.064 0.176
Dolomite 1.24
Lias chalk 1.39
Gneiss 0.18 0.78 1.51
Syenite 0.10
Dolerite 0.3 1.1 1.2
Diorite 0.5 0.69
CHAPTER VI.
THE POSITION OF POTASH IN AGRICULTURE.
We may, lastly, consider the position of _potash_ in agriculture, the
only ash ingredient of the plant, in addition to phosphoric acid, which
it is as a rule necessary to add as a manure.
_Potash of less Importance than Phosphoric Acid._
It is of far less importance than phosphoric acid, from the fact of its
much more abundant occurrence in the soil, as well as from the fact that
under the ordinary conditions of agriculture, although removed from the
soil in considerable quantities by crops, it finds its way back again in
the farmyard manure; for it has not the same tendency to accumulate in
large quantities in the grain or seed as we have seen to be the case
with phosphoric acid. On this account straw contains a much greater
proportion of potash than phosphoric acid, and hence farmyard manure may
be regarded as fairly rich in potash.
_Occurrence of Potash._
Of all sources of potash the ocean must be regarded as the chief.
Millions and millions of tons are present in a state of solution in the
salt water of the ocean.[126] Like phosphoric acid, its occurrence in
the rocks forming the earth's crust may be said to be practically
universal. Many of the commonly occurring rocks and minerals are
extremely rich in it, and by their disintegration furnish large
quantities to the soil. Some of these rocks contain it in such abundance
that they have been tried as potash manures; and were other more
valuable sources less available than they actually are, such a practice
might well be recommended. A volcanic rock known as _palagonite_, and
that most commonly occurring of all potash minerals--viz., felspar--have
both been experimented with in this way with considerable success.
_Felspar and other Potash Minerals._
That felspar should prove, when finally ground, a valuable source of
potash, is not to be wondered at when we remember that some varieties of
it contain over 16 per cent. It has been calculated that a single cubic
foot of this mineral is sufficient to supply an oak-wood, covering a
surface of 26,910 square feet, with potash for a period of no less than
five years.[127] Some idea of the enormous _potential_ fertility of a
soil containing felspar, so far as potash is concerned, may be obtained
from this statement. It must be remembered, however, that it is only the
orthoclase or potash felspars which contain large quantities of
potash--other felspathic rocks, such as oligoclase and labradorite,
being comparatively poor in it. Another commonly occurring mineral which
is rich in potash is mica, which has been found to contain from 5 to 13
per cent. From this it follows that rocks which have large amounts of
these minerals in their composition--such as granite, for example, which
often contains 5 or 6 per cent of potash--form by their disintegration
soils rich in this ingredient.
_Stassfurt Salts._
But in addition to the sources of potash already mentioned, it exists in
other forms in the earth's surface. Till within recent years it was
obtained for commercial purposes from the ashes of plants, which, as we
shall immediately see, are extremely rich in this ingredient; from salt
water--this source giving rise to the so-called "salt gardens" on the
coast of France; and from nitre soils in different parts of India,
referred to already at considerable length. Large mineral deposits,
however, have been recently discovered in the neighbourhood of Stassfurt
in Germany, and have since their discovery supplied all the potash
required for manurial and other purposes. In these deposits (similar
ones have also been found at Kalusz in the Carpathian Mountains) there
are no less than five different minerals which contain potash. The form
in which it is present is as sulphate or chloride, so that it is readily
available for plants, and is of altogether very much greater value than
the form in which it occurs in the minerals already mentioned--viz., as
an insoluble silicate. Of the Stassfurt potash salts, the best known as
a manure is _kainit_, which contains about 32 per cent of sulphate of
potash. A list of the other potash minerals, with the particulars of
their composition and the percentage of potash they contain, will be
found in the Appendix.[128]
_Occurrence of Saltpetre._
We have already had occasion, in Chapter IV., when discussing the
question of nitrification, to refer to the occurrence of nitrate of
potash in certain soils in India, which have formed a large source of
saltpetre used in commerce in the past.
_Occurrence of Potash in the Soil._
From what has been said regarding the richness in potash of certain
commonly occurring minerals, such as felspar, it is only natural to
infer that most soils must contain large quantities of this substance;
and this is so. The wonder is that potash, when applied as an artificial
manure, should have such a marked effect in increasing the fertility of
the soil, as is often the case. We must remember, however, that although
a soil may contain large quantities of potash, there may be a very small
percentage of the whole in an available form for the plant's needs.
_Potash chiefly in insoluble Condition in Soils._
Potash occurs almost entirely in soils in a very insoluble form--viz.,
combined with silica as a silicate of potash. It is only by the slow
disintegration of potash rocks that the potash they contain is set free
for plant uses. When it is applied as an artificial manure, on the other
hand, it is in a soluble form. In most soils the amount soluble in water
probably lies between .001 and .009 per cent; that soluble in dilute
acid solutions from .1 to .5 per cent; and that insoluble from .2 to 3.5
per cent of the soil. It is highly probable that a certain quantity of
potash in the soil may exist in combination with humic and ulmic acids,
forming insoluble potassium humates and ulmates.
_Potash in Plants._
Of all the ash ingredients of plants, potash is the most abundant, as it
forms on an average about 50 per cent of the total ash of plants--about
90 per cent of the alkalies. The ash of plants, indeed, was for long
the chief source of potash. Certain plants remove very large quantities
from the soil. Of these roots, potatoes, the vine, the tobacco-plant,
and hops may be mentioned as examples. It is present in large quantities
in the grain of cereals, although, as we have already pointed out, not
to the same proportional extent as phosphoric acid. It is found in the
plant's extremities, such as twigs and new leaves, in greatest
abundance.[129]
_Potash in the Animal Tissue._
It is also found in all parts of the animal body. Especially rich in
potash salts are the blood corpuscles, which contain about ten times the
amount contained in the serum. It is found in especial abundance in the
fleece of sheep, which may contain more potash than that in the whole
body of the sheep. Animal urine also contains potash in considerable
quantities.
_Sources of Loss of Potash._
The capacity of the soil to retain soluble potash compounds, while not
equal to its capacity for retaining phosphoric acid, is yet very much in
excess of its capacity for retaining nitrates. The result is, that
potash is only found in comparatively minute traces in drainage
water.[130] Taking the same example as we already cited in illustration
of the loss of phosphoric acid, we find that the amount carried away in
the course of a year in the waters of the Elbe from Bohemia is
97,000,000 lb. (43,300 tons).
_Potash removed in Crops._
The amount of potash removed by the different crops from the soil will
be considered in a subsequent chapter. We need only say here that the
class of crops which remove the largest quantity are the root crops,
especially mangels. The loss is least in the case of the cereals. The
amount of potash contained in the straw of cereals is about three times
the amount of that removed in the grain.
_Potash removed in Milk._
Lastly, we may refer to the potash removed in milk, which, on an
average, may be taken at 10 lb. per annum for each cow.
_Potash Manures._
Of potash manures the chief are the sulphate and the chloride, or, as it
is commercially known, the "muriate." The chief source of potash manures
are the Stassfurt deposits already referred to. Wood-ashes have also
been used in large quantities in the past (chiefly as a potash manure),
and in some parts of the world are still used. A considerable source of
artificial potassic manures is the refuse manufacture of sugar-beet,
such a large industry in Germany. Potash occurs as a constituent of
certain other manures, more valuable for nitrogen and phosphoric acid,
such as guano and dried blood.
FOOTNOTES:
[126] According to Boguslawski and Dittmar, the total amount of potash
calculated as sulphate of potash in salt water equals 1141 x 10^{12}
tons.
[127] See Storer's 'Agricultural Chemistry,' vol. ii. p. 291.
[128] See Appendix, Note I., p. 220.
[129] See Appendix, Note II., p. 220.
[130] According to Way, different samples of drainage waters were found
only to contain from .00003 to .00031 per cent.
APPENDIX TO CHAPTER VI.
NOTE I. (p. 215).
AMOUNT OF POTASH IN DIFFERENT MINERALS.
Felspars--
Percentage of potash.
(_a_) Orthoclase { 9.11 10.28 11.07 12.12 12.47
{ 13.49 14.35 15.21 16.7
(_b_) Oligoclase 0.50
(_c_) Labradorite 0.33
Mica { 5.61 6.20 7.23 8.26 8.95
{ 9.00 10.25 12.40 13.15
Amphibole 0.25 2.96
Pyroxene 0.34 2.48
Leucite 13.60 18.61
Zeolites 0.30 9.35 0.98 4.93
Stassfurt potash salts-- Per cent.
(_a_) Polyhallite, _potassium sulphate_ 28
(_b_) Karnallite (KCl.MgCl_{2}6H_{20}), _potassium chloride_ 24 to 27
(_c_) Sylvin, pure _potassium chloride_.
(_d_) Kainit (K_{2}SO_{4}MgSO_{4}MgCl_{2}6H_{2}O), _potassium
sulphate_ 32
(_e_) Schoenite (K_{2}SO_{4}, MgSO_{4}, 6H_{2}O), pure
_potassium magnesium sulphate_.
NOTE II. (p. 217).
The quantity of potash obtainable from various plants in the manufacture
of potashes on a large scale is illustrated by the following statements.
1000 lb. of the following vegetative products yield the following
quantities of potashes:--
lb.
Old spruce-wood 1/2
Old poplar-wood 3/4
Old oak-wood 1-1/2
Corn-stalks 17-1/2
Bean-stalks 20
Grape-vine twigs 40
(Storer, 'Agricultural Chemistry,' vol. ii. p. 108.)
PART III.
MANURES