Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Old or antiquated spellings have been preserved.
Typographical and punctuation errors have been silently corrected.
_Wiley & Putnam’s New Publications._
LECTURES
ON
AGRICULTURAL
CHEMISTRY AND GEOLOGY;
TO WHICH ARE ADDED,
SUGGESTIONS FOR EXPERIMENTS
IN PRACTICAL AGRICULTURE.
BY
JAS. F. W. JOHNSTON, M.A., F.R.SS. L. & E.
Fellow of the Geological Society, Honorary Member of the Royal
Agricultural Society, &c. &c.; Reader in Chemistry and
Mineralogy in the University of Durham, &c.
These Lectures will be divided into four Parts, of which the First is
now ready; the others are in course of publication, and the whole will
be completed in two volumes.
OUTLINE OF PART I.—“_On the Organic Constituents of Plants._”—Lecture
I. Elementary substances of which plants subsist. II. and III. Compound
substances which minister to the growth of plants. IV. Sources from
which plants immediately derive their elementary constituents. V. How
the food enters into the circulation of plants—general structure of
plants. VI. Into what substances the food is changed in the interior
of plants—substances of which plants chiefly consist. VII. Chemical
changes by which the substances of which plants chiefly consist are
formed from those on which they live. VIII. How the supply of food for
plants is kept up in the general vegetation of the globe.
OUTLINE OF PART II.—“_On the Inorganic Constituents of Plants—the
Origin, Classification, and Chemical Constitution of Soils—General
and Special Relations of Geology to Agriculture—Origin, Constitution,
Analyses, and Methods of Improving Soils in different Districts and
under unlike conditions._—Lecture IX. Kind and proportion of inorganic
matter contained in plants. X. Properties of the inorganic compounds
which exist in vegetable substances, or which promote their growth. XI.
Of the nature, origin, and classification of soils—Structure of the
earth’s crust—Classification and general characters of the stratified
rocks—Agricultural capabilities of the soils derived from them. XII.
Granite and trap rocks, and the soils derived from them—Superficial
accumulations. XIII. On the exact chemical constitution, the analysis,
and the physical properties of soils.
PART III.—Methods of improving the soil by mechanical and by chemical
means—Manures, their nature, composition, and mode of action—theory of
their application in different localities.
PART IV.—The results of vegetation—the nature, constitution, and
nutritive properties of different kinds of produce, and by different
modes of cultivation—the feeding of cattle, the making of cheese, &c.
&c. The constitution and differences of various kinds of wood, and the
circumstances which favour their growth.
CRITICAL NOTICES.
“A valuable and interesting course of lectures.”—_Quarterly Review._
“But it is unnecessary to make large extracts from a book which we
hope and trust will soon be in the hands of nearly all our readers.
Considering it as unquestionably the most important contribution that
has recently been made to popular science, and as destined to exert an
extensively beneficial influence in this country, we shall not fail
to notice the forthcoming portions as soon as they appear from the
press.”—_Silliman’s American Journal of Science. Notice of Part I of
the American reprint._
“We think it no compliment to Professor Johnston to say, that among
our own writers of the present day who have recently been endeavouring
to improve our agriculture by the aid of science, there is probably no
other who has been more eminently successful, or whose efforts have
been more highly appreciated.”—_County Herald._
“Prof. Johnston is one who has himself done so much already for English
agriculture, that to behold him still in hot pursuit of the inquiry
into what _can_ be done, supplies of itself a stimulus to further
exertion on the part of others.”—_Berwick Warder._
ELEMENTS
OF
AGRICULTURAL
CHEMISTRY AND GEOLOGY.
BY
JAS. F. W. JOHNSTON, M.A., F.R.S.,
HONORARY MEMBER OF THE ROYAL ENGLISH AGRICULTURAL
SOCIETY, AND AUTHOR OF “LECTURES ON AGRICULTURAL
CHEMISTRY AND GEOLOGY.”
NEW-YORK:
WILEY AND PUTNAM.
MDCCCXLII.
J. P. WRIGHT, Printer,
18 New Street, N. Y.
INTRODUCTION.
The scientific principles upon which the art of culture depends, have
not hitherto been sufficiently understood or appreciated by practical
men. Into the causes of this I shall not here inquire. I may remark,
however, that if Agriculture is ever to be brought to that comparative
state of perfection to which many other arts have already attained, it
will only be by availing itself, as they have done, of the many aids
which Science offers to it; and that, if the practical man is ever to
realize upon his farm all the advantages which Science is capable of
placing within his reach, it will only be when he has become so far
acquainted with the connection that exists between the art by which he
lives and the sciences, especially of Chemistry and Geology, as to be
prepared to listen with candour to the suggestions they are ready to
make to him, and to attach their proper value to the explanations of
his various processes which they are capable of affording.
The following little Treatise is intended to present a familiar outline
of the subjects of _Agricultural Chemistry and Geology_, as treated of
more at large in my LECTURES, of which the first Part is now before the
public. What in this work has necessarily been taken for granted, or
briefly noticed, is in the LECTURES examined, discussed, or more fully
detailed.
_Durham, 8th April, 1842._
CONTENTS.
CHAPTER I.
PAGE
Distinction between Organic and Inorganic Substances
—The Ash of Plants—Constitution of the Organic
Parts of Plants—Preparation and Properties of
Carbon, Oxygen, Hydrogen, and Nitrogen—Meaning
of Chemical Combination. 13
CHAPTER II.
Form in which these different Substances enter into
Plants—Properties of the Carbonic, Humic, and
Ulmic Acids; of Water, of Ammonia, and of Nitric
Acid—Constitution of the Atmosphere. 25
CHAPTER III.
Structure of Plants—Mode in which their Nourishment
is obtained—Growth and Substance of Plants—
Production of their Substance from the Food they
imbibe—Mutual Transformations of Starch, Sugar,
and Woody Fibre. 38
CHAPTER IV.
Of the Inorganic Constituents of Plants—Their
immediate Source—Their Nature—Quantity of each
in certain common Crops. 49
CHAPTER V.
Of Soils—Their Organic and Inorganic Portions—Saline
Matter in Soils—Examination and Classification of
Soils—Diversities of Soils and Subsoils. 67
CHAPTER VI.
Direct Relations of Geology to Agriculture—Origin
of Soils—Causes of their Diversity—Relation to
the Rocks on which they rest—Constancy in the
Relative Position and Character of the Stratified
Rocks—Relation of this Fact to Practical
Agriculture—General Characters of the Soils upon
these Rocks. 78
CHAPTER VII.
Soils of the Granitic and Trap Rocks—Accumulations
of Transported Sands, Gravels, and Clays—Use
of Geological Maps in reference to
Agriculture—Physical Characters and Chemical
Constitution of Soils—Relation between the Nature
of the Soil and the Kind of Plants that naturally
grow upon it. 103
CHAPTER VIII.
Of the Improvement of the Soil—Mechanical and Chemical
Methods—Draining—Subsoiling—Ploughing, and Mixing
of Soils—Use of Lime, Marl, and Shell-sand—
Manures—Vegetable, Animal, and Mineral Manures. 133
CHAPTER IX.
Animal Manures—Their Relative Value and Mode of
Action—Difference between Animal and Vegetable
Manures—Cause of this Difference—Mineral Manures—
Nitrates of Potash and Soda—Sulphate of Soda,
Gypsum, Chalk, and Quicklime—Chemical Action of
these Manures—Artificial Manures—Burning and
Irrigation of the Soil—Planting and Laying Down
to Grass. 165
CHAPTER X.
The Products of Vegetation—Importance of _Chemical_
quality as well as quantity of Produce—Influence
of different Manures on the quantity and quality
of the Crop—Influence of the Time of Cutting—
Absolute quantity of Food yielded by different Crops
—Principles on which the Feeding of Animals depends
—Theoretical and Experimental Value of different kinds
of Food for Feeding Stock—Concluding Observations. 216
ELEMENTS
OF
AGRICULTURAL CHEMISTRY, &c.
CHAPTER I.
Distinction between Organic and Inorganic
Substances.—The Ash of Plants.—Constitution
of the Organic Parts of Plants.—Preparation
and Properties of Carbon, Hydrogen, and
Nitrogen.—Meaning of Chemical Combination.
The object of the practical farmer is to raise from a given extent of
land the largest quantity of the most valuable produce at the least
cost, and with the least permanent injury to the soil. The sciences
either of chemistry or geology throw light on every step he takes or
ought to take, in order to effect this main object.
SECTION I.—OF THE VEGETABLE AND EARTHY OR THE ORGANIC AND INORGANIC
PARTS OF PLANTS.
In the prosecution of his art, two distinct classes of substances
engage his attention—the _living_ crops he raises, and the _dead_
earth from which they are gathered. If he examine any fragment of
an animal or vegetable, either living or dead, he will observe that
it exhibits pores of various kinds arranged in a certain order—that
it has a species of internal structure—that it has various parts or
_organs_—in short, that it is what physiologists term _organized_. If
he examine, in like manner, a lump of earth or rock, he will perceive
no such structure. To mark this distinction, the parts of animals and
vegetables, either living or dead—whether entire or in a state of
decay, are called _organic_ bodies, while earthy and stony substances
are called _inorganic_ bodies.
Organic substances are also more or less readily burned and dissipated
by heat in the open air; inorganic substances are generally fixed and
permanent in the fire.
But the crops which grow upon it, and the soil in which they are
rooted, contain a portion of both of these classes of substances. In
all fertile soils, there exists from 3 to 10 per cent. of vegetable
or other matter of _organic_ origin; while, on the other hand, all
vegetables, as they are collected for food, leave, when burned, from
one-half to twenty per cent. of _inorganic_ ash.
If we heat a portion of soil to redness in the open air, the organic
matter will burn away, and, in general, the soil, if previously
_dry_, will not be materially diminished in bulk. But if a handful of
wheat, or of wheat straw, or of hay, be burned in the same manner,
the proportion that disappears is so great, that in most cases a
comparatively minute quantity only remains behind. Every one is
familiar with this fact who has seen the small bulk of ash that is left
when weeds, or thorns, or trees, are burned in the field, or when a
hay or corn-stack is accidentally consumed. Yet the ash thus left is a
very appreciable quantity, and the study of its true nature throws much
light, as we shall hereafter see, on the practical management of the
land on which any given crop is to be made to grow.
Thus the quantity of ash left by a ton of wheat straw is sometimes as
much as 360 lbs.; by a ton of oat straw as much as 200 lbs.; while a
ton of the grain of wheat leaves only about 40 lbs.; of the grain of
oats about 90 lbs.; and of oak wood only 4 or 5 lbs. The quantities of
_inorganic_ matter, therefore, though comparatively small, yet, in some
cases, amount to a considerable weight in an entire crop. The nature,
source and uses of this earthy matter will be explained in a subsequent
chapter.
SECTION II.—CONSTITUTION OF THE ORGANIC PART OF PLANTS AND ANIMALS.
The organic part of plants, when in a perfectly dry state, constitutes
therefore from 85 to 99 per cent. of their whole weight. Of those parts
of plants which are cultivated for food, it is only hay and straw, and
a very few others, that contain as much as 10 per cent. of inorganic
matter.
This organic part consists of four substances, known to chemists by the
names of carbon, hydrogen, oxygen, and nitrogen. The first of these,
carbon, is a solid substance, the other three are gases or peculiar
kinds of air.
1. CARBON. When wood is burned in a covered heap, as is done by the
charcoal burners, or is distilled in iron retorts, as in making
wood-vinegar, it is charred and converted into common wood charcoal.
This charcoal is the most usual and best known variety of carbon. It
is black, soils the fingers, and is more or less porous according
to the kind of wood from which it has been formed. Coke obtained by
charring or distilling coal is another variety. It is generally denser
or heavier than the former, though less pure. Black lead is a third
variety, still heavier and more impure. The diamond is the only form in
which carbon occurs in nature in a state of perfect purity.
This latter fact, that the diamond is pure carbon—that it is
essentially the same substance with the finest and purest lamp-black—is
very remarkable; but it is only one of many striking circumstances that
every now and then present themselves before the inquiring chemist.
Charcoal, the diamond, lamp-black, and all the other forms of carbon,
burn away more or less slowly when heated in the air, and are converted
into a kind of gas known by the name of _carbonic acid_. The impure
varieties leave behind them a greater or less proportion of ash.
2. HYDROGEN.—If oil of vitriol (sulphuric acid) be mixed with twice
its bulk of water, and then poured upon iron filings, the mixture will
speedily begin to boil up, and bubbles of gas will rise to the surface
of the liquid in great abundance. These are bubbles of hydrogen gas.
If the experiment be performed in a bottle, the hydrogen which is
produced will gradually drive out the atmospheric air it contained,
and will itself take its place. If a bit of wax taper be tied to the
end of a wire, and when lighted be introduced into the bottle, it will
be instantly extinguished; while the hydrogen will take fire, and burn
at the mouth of the bottle with a pale yellow flame. If the taper be
inserted before the common air is all expelled, the mixture of hydrogen
and common air will burn with an explosion more or less violent,
and may even shatter the bottle and produce serious accidents. This
experiment, therefore, ought to be made with care. It may be safely
made in an open tumbler, covered by a plate or a piece of paper, till a
sufficient quantity of hydrogen is collected, when, on the introduction
of the taper, the light will be extinguished, and the hydrogen will
burn with a less violent explosion.
This gas is also an exceedingly light substance, rising through common
air as wood does through water. Hence, when confined in a bag made
of silk, or other light tissue, it is capable of sustaining heavy
substances in the air, and even of transporting them to great heights.
For this reason it is employed for filling and elevating balloons.
Hydrogen gas is not known to occur anywhere in nature in any sensible
quantity. It is very abundant, as we shall hereafter see, in what by
chemists is called a _state of combination_.
3. OXYGEN.—When strong oil of vitriol is poured upon black oxide of
manganese, and heated in a glass retort: or when red oxide of mercury,
or chlorate of potash, is so heated alone; or when saltpetre, or the
same oxide of manganese, is heated alone in an iron bottle;—in all
these cases a kind of air is given off, which, when collected and
examined by plunging a taper into it, is found to be neither common
air nor hydrogen gas. The taper, when introduced, burns with great
rapidity, and with exceeding brilliancy, and continues to burn till
either the whole of the gas disappears, or the taper is entirely
consumed. If a living animal is introduced, its circulation and its
breathing become quicker—it is speedily thrown into a fever—it lives as
fast as the taper burned—and, after a few hours, dies from excitement
and exhaustion. This gas is not light like hydrogen, but is about
one-ninth part heavier than common air.
In the atmosphere, oxygen exists in the state of gas. It forms about
one-fifth of the bulk of the air we breathe, and is the substance
which, in the air, supports all animal life and the combustion of
all burning bodies. Were it by any cause suddenly removed from the
atmosphere of our globe, every living thing would perish, and all
combustion would become impossible.
4. NITROGEN.—If a saucer be half filled with milk of lime, formed by
mixing slaked quicklime with water, a _very_ small tea-cup containing
a little burning sulphur then placed in the middle, and a common large
tumbler inverted over the whole, the sulphur will burn for a while, and
will then gradually die out. On allowing the whole to remain for some
time, the fumes of the sulphur will be absorbed by the milk of lime,
which will rise a certain way into the tumbler. When the absorption has
ceased, a quantity of air will remain in the upper part of the tumbler.
This air is nitrogen gas.
If the whole be now introduced into a large basin of water, the tumbler
being held in the left hand, the cup and saucer may be removed from
beneath. The saucer may then be inverted and introduced with its under
side into the mouth of the tumbler, which may thus be lifted out of
the water and restored to its upright position, the saucer serving the
purpose of a cover. By carefully removing this cover with the one hand,
a lighted taper may be introduced by the other. It will then be seen
that the taper is extinguished by this air, and that no other effect
follows. Or if a living animal be introduced into it, breathing will
instantly cease, and it will drop without signs of life.
This gas possesses no other remarkable property. It is a very little
lighter than common air, and is known to exist in large quantity in the
atmosphere only. Of the air we breathe it forms nearly four-fifths of
the entire bulk.
These three gases are incapable of being distinguished from common
air, or from each other, by the ordinary senses; but by the aid of the
taper they are readily recognised. Hydrogen extinguishes the taper, but
itself takes fire; nitrogen simply extinguishes it; while in oxygen the
taper burns with extraordinary brilliancy and rapidity.
Of this one solid substance, carbon, and these three gases, hydrogen,
oxygen, and nitrogen, all the organic part of vegetable and animal
substances is made up.
Into these substances, however, they enter in very different
proportions. Nearly one-half the weight of all vegetable productions
which are gathered as food for man or beast—in their dry state—consists
of carbon; the oxygen amounts to rather more than one-third, the
hydrogen to little more than five per cent., while the nitrogen rarely
exceeds two and a half or three per cent. of their weight.
This will appear from the following table, which exhibits the actual
constitution by analysis of some varieties of the more common crops
when perfectly dry.
Carbon. Hydrogen. Oxygen. Nitrogen. Ash.
Hay, 458 50 387 15 90
Potatoes, 441 58 439 12 50
Wheat Straw, 485 52 389½ 3½ 70
Oats, 507 64 367 22 40
These numbers represent the weights of each element in pounds,
contained in 1000 lbs. of the dry hay, potatoes, &c.; but in drying by
a gentle heat, 1000 lbs. of hay from the stack, lost 158 lbs. of water,
of potatoes wiped dry externally 722 lbs.,[1] wheat straw 260 lbs., and
oats 151 lbs.
[1] Both potatoes and turnips contain about four-fifths of their weight
of water, or five tons of either of these roots contain nearly four
tons of water.
SECTION III.—OF THE MEANING OF CHEMICAL COMBINATION.
If the three kinds of air above spoken of be mixed together in a
bottle, no change will take place, and if charcoal in fine powder be
added to them, still no new substance will be produced. If we take the
ash left by a known weight of hay or wheat straw, and mix it with the
proper quantities of the four elementary substances, carbon, hydrogen,
&c., as shewn in the above table, we shall be unable by this means
to form either hay or wheat straw. The elements of which vegetable
substances consist, therefore, are not merely _mixed_ together—they are
united in some closer and more intimate manner. To this more intimate
state of union, the term _chemical combination_ is applied—the elements
are said to be _chemically combined_.
Thus, when charcoal is burned in the air, it slowly disappears, and
forms, as already stated, a kind of air known by the name of carbonic
acid gas, which rises into the atmosphere and disappears. Now, this
carbonic acid is formed by the _union_ of the carbon (charcoal), while
burning, with the oxygen of the atmosphere, and in this new air the two
elements, carbon and oxygen, are _chemically combined_.
Again, if a piece of wood or a bit of straw, in which the elements are
already chemically combined, be burned in the air, these elements are
separated and made to assume new states of combination, in which new
states they escape into the air and become invisible. When a substance
is thus changed by the action of heat, it is said to be _decomposed_,
or if it gradually decay and perish by exposure to the air and
moisture, it undergoes slow _decomposition_.
When, therefore, two or more substances unite together, so as to
form a third possessing properties different from both, they enter
into chemical union—they form a _chemical combination_ or chemical
_compound_. When, on the other hand, one compound body is so changed
as to be converted into two or more substances different from itself,
it is _decomposed_. Carbon, hydrogen, &c., are chemically combined in
the interior of the plant during the formation of wood: wood, again,
is decomposed when by the vinegar-maker it is converted among other
substances into charcoal and wood-vinegar, and the flour of grain when
the brewer or distiller converts it into ardent spirits.
CHAPTER II.
Form in which these different substances enter into
Plants. Properties of the Carbonic, Humic, and
Ulmic Acids—of Water, of Ammonia, and of Nitric
Acid. Constitution of the Atmosphere.
SECTION I.—FORM IN WHICH THE CARBON, ETC. ENTER INTO PLANTS.
It is from their food that plants derive the carbon, hydrogen, oxygen,
and nitrogen, of which their organic part consists. This food enters
partly by the minute pores of their roots, and partly by those which
exist in the green part of the leaf and of the young twig. The roots
bring up food from the soil, the leaves take it in directly from the
air.
Now, as the pores in the roots and leaves are very minute, carbon
(charcoal) cannot enter into either in a _solid_ state; and as it does
not dissolve in water, it cannot, in the state of simple carbon, be any
part of the food of plants. Again, hydrogen gas neither exists in the
air nor usually in the soil—so that, although hydrogen is always found
in the substance of plants, it does not enter them in the state of the
gas above described. Oxygen exists in the air, and is directly absorbed
both by the leaves and by the roots of plants; while nitrogen, though
it forms a large part of the atmosphere, is not supposed to enter
_directly_ into plants in any considerable quantity.
The whole of the carbon and hydrogen, and the greater part of the
oxygen and nitrogen also, enter into plants in a state of _chemical
combination_ with other substances; the carbon chiefly in the state of
_carbonic acid_, and of certain other soluble compounds which exist
in the soil; the hydrogen and oxygen in the form of water: and the
nitrogen in those of ammonia or nitric acid. It will be necessary
therefore briefly to describe these several compounds.
SECTION II.—OF THE CARBONIC, HUMIC, AND ULMIC ACIDS.
1. CARBONIC ACID.—If a few pieces of chalk or limestone be put into
the bottom of a tumbler, and a little spirit of salt (muriatic acid)
be poured upon them, a boiling up or _effervescence_ will take place,
and a gas will be given off, which will gradually collect and fill the
tumbler; and when produced very rapidly, may even be seen to run over
its edges. This gas is carbonic acid. It cannot be distinguished from
common air by the eye; but if a taper be plunged into it, the flame
will immediately be extinguished, while the gas remains unchanged.
This kind of air is so heavy, that it may be poured from one vessel
into another, and its presence recognised by the taper. It has also a
peculiar odour, and is exceedingly suffocating, so that if a living
animal be introduced into it, life immediately ceases. It is absorbed
by water, a pint of water absorbing or dissolving a pint of the gas.
Carbonic acid exists in the atmosphere; it is given off from the lungs
of all living animals while they breathe; it is also produced largely
during the burning of wood, coal, and all other combustible bodies, so
that an unceasing supply of this gas is poured into the air. Decaying
animal and vegetable substances also give off this gas, and hence it is
always present in greater or less abundance in the soil, and especially
in such soils as are rich in vegetable matter. During the fermentation
of malt liquors, or of the expressed juices of different fruits,—the
apple, the pear, the grape, the gooseberry—it is produced, and the
briskness of such fermented liquors is due to the escape of this gas.
From the dung and compost heap it is also given off; and when put into
the ground in a fermenting state, farm-yard manure affords a rich
supply of carbonic acid to the young plant.
Carbonic acid consists of carbon and oxygen only, combined together in
the proportion of 28 of the former to 72 of the latter, or 100 lbs. of
carbonic acid contain 28 lbs. of carbon and 72 lbs. of oxygen.
2. HUMIC AND ULMIC ACIDS.—The soil always contains a portion of
vegetable matter (called _humus_ by some writers), and such matter is
always added to it when it is manured from the farm-yard or the compost
heap. During the decay of this vegetable matter, carbonic acid, as
above stated, is given off in large quantity, but other substances
are also formed at the same time. Among these are the two to which
the names of humic and ulmic acids are respectively given. They both
contain much carbon, are both capable of entering the roots of plants,
and both, no doubt, in favourable circumstances, help to feed the plant.
If the common soda of the shops be dissolved in water, and a portion
of a rich vegetable soil, or a bit of peat, be put into this solution,
and the whole boiled, a brown liquid is obtained. If to this brown
liquid, spirit of salt (muriatic acid) be added till it is sour to the
taste, a brown flocky powder falls to the bottom. This brown substance
is _humic_ acid. But if in this process we use spirit of hartshorn
(liquid ammonia), instead of the soda, _ulmic_ acid is obtained.
These acids exist along with other substances in the rich brown liquor
of the farm-yard, which is so often allowed to run to waste; they are
also produced in greater or less quantity during the decay of the
manure after it is mixed with the soil, and no doubt yield to the plant
a portion of that supply of food which it must necessarily receive from
the soil.
SECTION III.—OF WATER, AMMONIA, AND NITRIC ACID.
1. WATER.—If hydrogen be prepared in a bottle, in the way already
described, and a gas-burner be fixed into its mouth, the hydrogen may
be lighted, and will burn as it escapes into the air. Held over this
flame a cold tumbler will become covered with dew, or with little drops
of water. This water is _produced_ during the burning of the hydrogen;
and as it takes place in pure oxygen gas as well as in the open air,
this water must contain the hydrogen and oxygen which disappear, or
_must consist of hydrogen and oxygen only_.
This is a very interesting fact; and were it not that chemists are now
familiar with many such, it could not fail to appear truly wonderful
that the two gases, oxygen and hydrogen, by their union, should form so
very different a substance as water is from either. It consists of 1
of hydrogen to 8 of oxygen, or every 9 lbs. of water contain 8 lbs. of
oxygen and 1 lb. of hydrogen.
Water is so familiar a substance, that it is unnecessary to dwell upon
its properties. When pure, it has neither colour, taste, nor smell. At
32° of Fahrenheit’s[2] scale (the freezing point), it solidifies into
ice, and at 212° it boils, and is converted into steam. There are two
others of its properties which are especially interesting in connection
with the growth of plants.
[2] This is the scale of the common thermometer used in this country.
_1st_, If sugar or salt be put into water, they disappear or are
_dissolved_. Water has the power of thus dissolving numerous other
substances in greater or less quantity. Hence, when the rain falls
and sinks into the soil, it dissolves some of the soluble substances
it meets in its way, and rarely reaches the roots of plants in a pure
state. So waters that rise up in springs are rarely pure. They always
contain earthy and saline substances in solution, and these they carry
with them, when they are sucked in by the roots of plants.
It has been above stated, that water absorbs (dissolves) its own bulk
of carbonic acid; it dissolves also smaller quantities of the oxygen
and nitrogen of the atmosphere; and hence, when it meets any of these
gases in the soil, it becomes impregnated with them, and conveys them
into the plant, there to serve as a portion of its food.
_2d_, Water is composed of oxygen and hydrogen; by certain chemical
processes it can readily be resolved or decomposed _artificially_ into
these two gases. The same thing takes place _naturally_ in the interior
of the living plant. The roots absorb the water, but if in any part
of the plant hydrogen be required, to make up the substance which it
is the function of that part to produce, a portion of the water is
decomposed and worked up, while the oxygen is set free, or converted to
some other use. So, also, in any case where oxygen is required water is
decomposed, the oxygen made use of, and the hydrogen liberated. Water,
therefore, which abounds in the vessels of all growing plants, if not
directly converted into the substance of the plant, is yet a ready and
ample source from which a supply of either of the elements of which it
consists may at any time be obtained.
It is a beautiful adaptation of the properties of this all-pervading
compound (water), that its elements should be so fixedly bound together
as rarely to separate in external nature, and yet to be at the command
and easy disposal of the vital powers of the humblest order of living
plants.
2. AMMONIA.—If the sal ammoniac of the shops be mixed with quicklime,
a powerful odour is immediately perceived, and an invisible gas is
given off which strongly affects the eyes. This gas is ammonia. Water
dissolves or absorbs it in very large quantity, and this solution forms
the common hartshorn of the shops. The white solid smelling-salts of
the shops are a compound of ammonia with carbonic acid,—a solid formed
by the union of two gases.
The gaseous ammonia consists of nitrogen and hydrogen only, in the
proportion of 14 of the former to 3 of the latter, or 17 lbs. of
ammonia contain 3 lbs. of hydrogen.
The chief natural source of this compound is, in the decay of animal
substances. During the putrefaction of dead animal bodies ammonia is
invariably given off. From the animal substances of the farm-yard it is
evolved, and from all solid and liquid manures of animal origin. It is
also formed in lesser quantity during the decay of vegetable substances
in the soil; and in volcanic countries, it escapes from many of the hot
lavas, and from the crevices in the heated rocks.
It is produced artificially by the distillation of animal substances
(hoofs, horns, &c.), or of coal. Thousands of tons of the ammonia
present in the ammoniacal liquors of the gas-works, which might be
beneficially applied as a manure, are annually carried down by the
rivers, and lost in the sea.
The ammonia which is given off during the putrefaction of animal
substances rises partially into the air, and floats in the atmosphere,
till it is either decomposed by natural causes, or is washed down by
the rains. In our climate, cultivated plants derive a considerable
portion of their nitrogen from ammonia. It is supposed to be one of the
most valuable fertilizing substances contained in farm-yard manure; and
as it is present in greater proportion by far in the liquid than in the
solid contents of the farm-yard, there can be no doubt that much real
wealth is lost, and the means of raising increased crops thrown away in
the quantities of liquid manure which are almost everywhere permitted
to run to waste.
3. NITRIC ACID—is a powerfully corrosive liquid known in the shops by
the familiar name of _aquafortis_. It is prepared by pouring oil of
vitriol (sulphuric acid) upon saltpetre, and distilling the mixture.
The aquafortis of the shops is a mixture of the pure acid with water.
Pure nitric acid consists of nitrogen and oxygen only; the union of
these two gases, so harmless in the air, producing the burning and
corrosive compound which this is known to be.
It never reaches the roots of plants in this free and corrosive state.
It exists in many soils, and is naturally formed in compost heaps,
and in most situations where vegetable matter is undergoing decay
in contact with the air; but it is always in a state of chemical
combination in these cases. With potash, it forms _nitrate of potash_
(saltpetre); with soda, _nitrate of soda_; and with lime, _nitrate
of lime_; and it is generally in one or other of these states of
combination that it reaches the roots of plants.
Nitric acid is also naturally formed, and in some countries probably in
large quantities, by the passage of electricity through the atmosphere.
The air, as has been already stated, contains much oxygen and nitrogen
_mixed_ together, but when an electric spark is passed through a
quantity of air, a certain quantity of the two _unite_ together
chemically, so that every spark that passes forms a small portion of
nitric acid. A flash of lightning is only a large electric spark;
and hence every flash that crosses the air produces along its path a
quantity of this acid. Where thunder-storms are frequent, much nitric
acid must be produced in this way in the air. It is washed down by the
rains, in which it has frequently been detected, and thus reaches the
soil, where it produces one or other of the _nitrates_ above mentioned.
It has been long observed that those parts of India are the most
fertile in which saltpetre exists in the soil in the greatest
abundance. Nitrate of soda, also, in this country, has been found
wonderfully to promote vegetation in many localities; and it is a
matter of frequent remark, that vegetation seems to be refreshed and
invigorated by the fall of a thunder-shower. There is, therefore, no
reason to doubt that nitric acid is really beneficial to the general
vegetation of the globe. And since vegetation is most luxuriant
in those parts of the globe where thunder _or_ lightning are most
abundant, it would appear as if the natural production of this compound
body in the air, to be afterwards brought to the earth by the rains,
were a wise and beneficent contrivance by which the health and vigour
of universal vegetation is intended to be promoted.
It is from this nitric acid, thus universally produced and existing,
that plants appear to derive a large—probably, taking vegetation in
general, the largest—portion of their nitrogen. In all climates they
also derive a portion of this element from ammonia; but less from this
source in tropical than in temperate climates.[3]
[3] For fuller information on this point, see the Author’s “LECTURES
_on Agricultural Chemistry and Geology_,” Part I.
SECTION IV.—OF THE CONSTITUTION OF THE ATMOSPHERE.
The air we breathe, and from which plants also derive a portion of
their nourishment, consists of a mixture of oxygen and nitrogen gases,
with a minute quantity of carbonic acid, and a variable proportion
of watery vapour. Every hundred gallons of dry air contain about 21
gallons of oxygen and 79 of nitrogen. The carbonic acid amounts only to
one gallon in 2500, while the watery vapour in the atmosphere varies
from 1 to 2½ gallons (of steam) in 100 gallons of common air.
The oxygen in the air is necessary to the respiration of animals, and
to the support of combustion (burning of bodies). The nitrogen serves
principally to dilute the strength, so to speak, of the pure oxygen,
in which gas, if unmixed, animals would live and combustibles burn
with too great rapidity. The small quantity of carbonic acid affords
an important part of their food to plants, and the watery vapour in
the air aids in keeping the surfaces of animals and plants in a moist
and pliant state; while, in due season, it descends also in refreshing
showers, or studs the evening leaf with sparkling dew.
There is a beautiful adjustment in the constitution of the atmosphere
to the nature and necessities of living beings. The energy of the pure
oxygen is tempered, yet not too much weakened, by the admixture of
nitrogen. The carbonic acid, which alone is noxious to life, is mixed
in so minute a proportion as to be harmless to animals, while it is
still beneficial to plants; and when the air is overloaded with watery
vapour, it is provided that it shall descend in rain. These rains at
the same time serve another purpose. From the surface of the earth
there are continually ascending vapours and exhalations of a more or
less noxious kind; these the rains wash out from the air, and bring
back to the soil, at once purifying the atmosphere through which they
descend, and refreshing and fertilizing the land on which they fall.
CHAPTER III.
Structure of plants—Mode in which their nourishment
is obtained—Growth and substance of plants—
Production of their substance from the food
they imbibe—Mutual transformations of starch,
sugar, and woody fibre.
From the compound substances, described in the preceding chapter,
plants derive the greater portion of the carbon, hydrogen, oxygen,
and nitrogen, of which their organic part consists. The living
plant possesses the power of absorbing these compound bodies, of
_decomposing_ them in the interior of its several vessels, and of
_recompounding_ their elements in a different way, so as to produce new
substances,—the ordinary products of vegetable life. Let us briefly
consider the general structure of plants, and their mode of growth.
SECTION I.—OF THE STRUCTURE OF PLANTS, AND THE MODE IN WHICH THEIR
NOURISHMENT IS OBTAINED.
A perfect plant consists of three several parts,—a root which throws
out arms and fibres in every direction into the soil,—a trunk which
branches into the air on every side,—and leaves which, from the ends of
the branches and twigs, spread out a more or less extended surface into
the surrounding air. Each of these parts has a peculiar structure and a
special function assigned to it.
The _stem_ of any of our common trees consists of three parts,—the
pith in the centre, the wood surrounding the pith, and the bark which
covers the whole. The pith consists of bundles of minute hollow tubes,
laid horizontally one over the other; the wood and inner bark, of long
tubes bound together in a vertical position, so as to be capable of
carrying liquids up and down between the roots and the leaves. When a
piece of wood is sawn across, the ends of these tubes may be distinctly
seen. The branch is only a prolongation of the stem, and has a similar
structure.
The _root_, immediately on leaving the trunk or stem, has also a
similar structure; but as the root tapers away, the pith gradually
disappears, the bark also thins out, the wood softens, till the white
tendrils, of which its extremities are composed, consist only of a
colourless spongy mass, full of pores, but in which no distinction of
parts can be perceived. In this spongy mass the vessels or tubes which
descend through the stem and root lose themselves, and by them these
spongy extremities are connected with the leaves.
The _leaf_ is an expansion of the twig. The fibres which are seen
to branch out from the base over the inner surface of the leaf are
prolongations of the vessels of the wood. The green exterior portion
of the leaf is, in like manner, a continuation of the bark in a very
thin and porous form. The green of the leaf, though full of pores,
especially on the under part, yet also consists of, or contains, a
collection of tubes or vessels, which stretch along its surface, and
communicate with those of the bark.
Most of these vessels in the living plant are full of sap, and this sap
is in almost continual motion. In spring and autumn the motion is more
rapid, and in winter it is sometimes scarcely perceptible; yet the sap
is supposed to be rarely quite stationary in every part of the tree.
From the spongy part of the root the sap ascends through the vessels
of the _wood_, till it is diffused over the inner surface of the leaf
by the fibres which the wood contains. Hence, by the vessels in the
green of the leaf, it is returned to the bark, and through the vessels
of the inner bark it descends to the root.
Every one understands why the roots send out fibres in every direction
through the soil,—it is in search of water and of _liquid_ food, which
the spongy fibres suck in and send forward with the sap to the upper
parts of the tree. It is to aid these roots in procuring food that, in
the art of culture, such substances are mixed with the soil where these
roots are, as are supposed to be necessary, or at least favourable, to
the growth of the plant.
It is not so obvious that the leaves spread out their broad surfaces
into the air for the same purpose precisely as that for which the roots
diffuse their fibres through the soil. The only difference is, that
while the roots suck in chiefly _liquid_, the leaves inhale almost
solely _gaseous_ food. _In the sunshine, the leaves are continually
absorbing carbonic acid from the air and giving off oxygen gas._ That
is to say, they are continually appropriating carbon from the air.[4]
_When night comes, this process ceases, and they begin to absorb
oxygen and to give off carbonic acid._ But this latter process does
not go on so rapidly as the former, so that, on the whole, plants
when growing gain a large portion of carbon from the air. The actual
quantity, however, varies with the season, with the climate, and with
the kind of tree. The proportion of the whole carbon contained by a
plant, which has been derived from the air, is greatly modified also
by the quality of the soil in which it grows, and by the comparative
abundance of liquid food which happens to be within reach of its roots.
It has been ascertained, however, that in our climate, on an average,
not less than from one-third to three-fourths of the entire quantity of
carbon contained in the crops we reap from land of average fertility,
is really obtained from the air.
[4] Since carbonic acid, as shewn in the previous chapter, consists
only of carbon and oxygen, they retain the carbon and reject the oxygen.
We see then why, in arctic climates, where the sun once risen never
sets again during the entire summer, vegetation should almost rush up
from the frozen soil—the green leaf is ever gaining from the air and
never losing, ever taking in and never giving off carbonic acid, since
no darkness ever interrupts or suspends its labours.
How beautiful, too, does the contrivance of the expanded leaf appear!
The air contains only one gallon of carbonic acid in 2500, and this
proportion has been adjusted to the health and comfort of animals to
whom this gas is hurtful. But to catch this minute quantity, the tree
hangs out thousands of square feet of leaf in perpetual motion, through
an ever-moving air; and thus, by the conjoined labours of millions of
pores, the substance of whole forests of solid wood is slowly extracted
from the fleeting winds. The green stem of the young shoot, and the
green stalks of the grasses, also absorb carbonic acid as the green of
the leaf does, and thus a larger supply is afforded when the growth is
most rapid, or when the short life of the annual plant demands much
nourishment within a limited time.
SECTION II.—OF THE GROWTH AND SUBSTANCE OF PLANTS.
In this way the perfect plant derives its food from the soil and
from the air; but perfect plants arise from seeds; and the study of
the entire life—the career, so to speak—of a plant, presents many
interesting and instructive subjects of consideration.
When a portion of flour is made into dough, and this dough is kneaded
with the hand under a stream of water upon a fine sieve, as long as the
water passes through milky, there will remain on the sieve a glutinous
sticky substance resembling birdlime, while the milky water will
gradually deposit a pure white powder. This powder is _starch_, that
which remains on the sieve is _gluten_. Both of these substances exist,
therefore, in the flour; they both also exist in the grain. The starch
consists of carbon, hydrogen, and oxygen only; the gluten, in addition
to these, contains also nitrogen.
When ground into flour, these substances serve for food to man; in
the unbruised grain they are intended to feed the future plant in its
earliest infancy.
When a seed is committed to the earth, if the warmth and moisture are
favourable, it begins to sprout. It pushes a shoot upwards, it thrusts
a root downwards, but, until the leaf expand, and the root has fairly
entered the soil, the young plant derives no nourishment other than
water, either from the earth or from the air. It lives on the starch
and gluten contained in the seed. But these substances, though capable
of being separated from each other by means of water, as above stated,
yet are neither of them soluble in water. Hence, they cannot, without
undergoing a previous change, be taken up by the sap, and conveyed
along the pores of the young shoot they are destined to feed. But it is
so arranged that, when the seed first shoots, there is produced at the
base of the germ, from a portion of the gluten, a small quantity of a
substance (diastase) which has so powerful an effect upon the starch
as immediately to render it soluble in the sap, which is thus enabled
to take it up and convey it by degrees, just as it is wanted, to the
shoot or to the root.[5] As the sap ascends, it becomes sweet,—the
starch thus dissolved changes into sugar. When the shoot first becomes
tipped with green, the sugar is again changed into the woody fibre, of
which the stem of perfect plants chiefly consists. By the time that the
food contained in the seed is exhausted,—often, as in the potato, long
before,—the plant is able to live by its own exertions, at the expense
of the air and the soil.
[5] In malting barley, it is made to sprout a certain length, and the
growth is then arrested by heating and drying it. Mashed barley, before
sprouting, will not dissolve in water, but when sprouted, the whole of
the starch (the flour) it contains dissolves readily by a gentle heat.
The _diastase_ formed during the germination effects this. By further
heating in the brewer’s wort, this starch is converted into sugar as it
is in the growing plant.
This change of the sugar of the sap into woody fibre is observable
more or less in all plants. When they are shooting fastest the sugar
is most abundant; not, however, in those parts which are growing, but
in those which convey the sap to the growing parts. Thus the sugar of
the ascending sap of the maple and the alder disappears in the leaf
and in the extremities of the twig; thus the sugar-cane _sweetens_
only a certain distance above the ground, up to where the new growth
is proceeding; and thus also the young beet and turnip abound most in
sugar, while in all these plants the sweet principle diminishes as the
year’s growth draws nearer to a close.
In the ripening of the ear also, the sweet taste, at first so
perceptible, gradually diminishes and finally disappears; the sugar of
the sap is here changed into the _starch_ of the grain, which, as above
described, is afterwards destined, when the grain begins to sprout, to
be reconverted into sugar for the nourishment of the rising germ.
In the ripening of fruits a different series of changes presents
itself. The fruit is first tasteless, then becomes sour, and at last
sweet. In this case the acid of the unripe is changed into the sugar of
the ripened fruit.
The substance of plants,—their solid parts that is—consist chiefly of
_woody fibre_, the name given to the fibrous substance, of which wood
evidently consists. It is interesting to inquire how this substance
can be formed from the compounds, carbonic acid and water, of which the
food of plants in great measure consists. Nor is it difficult to find
an answer.
It will be recollected that the leaf drinks in carbonic acid from the
air, and delivers back its oxygen, retaining only its carbon. It is
also known that water abounds in the sap. Hence carbon and water are
thus abundantly present in the pores or vessels of the green leaf. Now,
woody fibre _consists only of carbon and water_ chemically combined
together,—100 lbs. of dry woody fibre consisting of 50 lbs. of carbon
and 50 lbs. of water. It is easy, therefore, to see how, when the
carbon and water meet in the leaf, woody fibre may be produced by their
mutual combination.
If, again, we inquire how this important principle of plants may be
formed from the other substances, which enter by their roots, from the
ulmic acid, for example, the answer is equally ready. This acid also
consists of carbon and water only, 50 lbs. of carbon with 37½ of water
forming ulmic acid, so that when it is introduced into the sap of the
plant, all the materials are present from which the woody fibre may be
produced.
Nor is it more difficult to see how starch may be converted into sugar,
and this again into woody fibre; or how, again, sugar may be converted
into starch in the ear of corn, or woody fibre into sugar during the
ripening of the winter pear after its removal from the tree. _Any one
of these substances may be represented by carbon and water only._ Thus,—
50 lbs. of carbon with 50 of water, make 100 of woody fibre.
50 lbs. 37½ 87½ of ulmic acid.
{ of cane sugar,
50 lbs. 72½ 122½ { of starch, or
{ of gum.
50 lbs. 56 106 of vinegar.
In the interior of the plant, therefore, it is obvious that, whichever
of these substances be present in the sap, the elements are at hand out
of which any of the others may be produced. In what way they really
are produced, the one from the other, and by what circumstances these
transformations are favoured, it would lead into too great detail to
attempt here to explain.[6]
We cannot help admiring to what varied purposes in nature the
same elements are applied, and from how few and simple materials,
substances, the most varied in their properties, are in the living
vegetable daily produced.
[6] For fuller and more precise explanations on these interesting
topics, see the Author’s LECTURES _on Agricultural Chemistry and
Geology_, Part I.
CHAPTER IV.
Of the Inorganic Constitution of Plants—Their
immediate Source—Their Nature—Quantity of each in
certain common Crops.
SECTION I.—SOURCE OF THE EARTHY MATTER OF PLANTS—SUBSTANCES OF WHICH IT
CONSISTS.
When plants are burned, they always leave more or less of ash behind.
This ash varies in quantity in different plants, in different parts of
the same plant, and sometimes in different specimens of the same kind
of plant, especially if grown upon different soils; yet it is never
wholly absent. It seems as necessary to their existence in a state of
perfect health as any of the elements which constitute the organic or
combustible part of their substance. They must obtain it therefore
along with the food on which they live: it is in fact a part of their
natural food, since without it they become unhealthy. We shall speak of
it therefore as the _inorganic food_ of plants.
We have seen that all the elements which are necessary to the
production of the woody fibre, and of the other organic parts of the
plant, may be derived either from the air, from the carbonic acid
and watery vapour taken in by the leaves, or from the soil, through
the medium of the roots. In the air, however, only rare particles of
inorganic or earthy matter are known to float, and these in a solid
form, so as to be unable to enter by the leaves; the earthy matter
which constitutes the ash, therefore, must be all derived from the soil.
The earthy part of the soil, therefore, serves a double use. It is not
merely, as some have supposed, a substratum in which the plant may so
fix and root itself, as to be able to maintain its upright position
against the force of winds and tempests; but it is a storehouse of
food also, from which the roots of the plant may select such earthy
substances as are necessary to, or are fitted to promote, its growth.
The ash of plants consists of a mixture of several, sometimes of as
many as eleven, different earthy substances. These substances are the
following:—
1. _Potash._—The common pearl-ash of the shops is a compound of potash
with carbonic acid; it is a _carbonate of potash_. By dissolving the
pearl-ash in water, and boiling it with quicklime, the carbonic acid is
separated, and potash alone, or caustic potash, as it is often called,
is obtained.
2. _Soda._—The common soda of the shops is a carbonate of soda, and by
boiling it with quicklime, the carbonic acid is separated, as in the
case of pearl-ash.
3. _Lime._—This is familiar to every one as the lime-shells, or
unslaked lime of the limekilns. The unburned limestone is a _carbonate
of lime_; the carbonic acid in this case being separated by the
roasting in the kiln.
4. _Magnesia._—This is the calcined magnesia of the shops. The
uncalcined is a _carbonate of magnesia_, from which heat drives off the
carbonic acid.
5. _Silica._—This is the name given by chemists to the substance of
flint, quartz, and of siliceous sands and sandstones.
6. _Alumina_ is the pure earth of alum, obtained by dissolving alum in
water, and adding liquid ammonia (hartshorn) to the solution. It forms
about two-fifths of the weight of porcelain and pipe-clays, and of some
other very stiff kinds of clay.
7. _Oxide of Iron._—The most familiar form of this substance is the
rust that forms on metallic iron in damp places. It is a compound of
iron with oxygen, hence the name _oxide_.
8. _Oxide of Manganese_ is a brown powder, which consists of oxygen
in combination with a metal resembling iron, to which the name of
manganese is given. It exists in plants, and in soils only in very
small quantity.
9. _Sulphur._—This substance is well known. It generally exists in
the ash in the state of _sulphuric acid_ (oil of vitriol), which is a
compound of sulphur with oxygen. It does not always exist in living
plants, however, in this state.
_Sulphuric acid_ forms with potash a _sulphate of potash_,—with soda,
_sulphate of soda_ (or Glauber’s salts),—with lime, _sulphate of lime_
(gypsum),—with magnesia, _sulphate of magnesia_ (Epsom salts),—with
alumina, _sulphate of alumina_,—and with oxide of iron, _sulphate of
iron_ or green vitriol. When the sulphate of potash is combined with
sulphate of alumina, it forms common alum.
10. _Phosphorus_ is a soft pale yellow substance which readily takes
fire in the air, and gives off, while burning, a dense white smoke. The
white fumes which form this smoke are a compound of phosphorus with
oxygen obtained from the air, and are called _phosphoric acid_. In the
ash of plants the phosphorus is found in the state of phosphoric acid,
though it probably does not all exist in the living plant in that state.
_Phosphoric acid_ forms _phosphates_ with potash, soda, lime, and
magnesia. When bones are burned, a large quantity of a white earth
remains (bone-earth), which is a _phosphate of lime_, consisting of
lime and phosphoric acid. Phosphate of lime is generally present in the
ash of plants; phosphate of magnesia is contained most abundantly in
the ash of wheat and other varieties of grain.
11. _Chlorine._—This is a very suffocating gas, which gives its
peculiar smell to chloride of lime, and is used for bleaching and
disinfecting. It is readily obtained by pouring muriatic acid (spirit
of salt) on the black oxide of manganese of the shops. In combination
with the metallic bases of potash, soda, lime, and magnesia, it forms
the _chlorides_ of potassium, sodium (common salt), calcium and
magnesium,[7] and in one or other of these states it generally enters
into the roots of plants, and exists in their ash.
[7] Potash, soda, lime, and magnesia, are compounds of the metals here
named with oxygen. It is a very striking fact, that the suffocating
gas chlorine, when combined with sodium, a metal which takes fire when
placed upon water, should form the agreeable and necessary condiment,
_common salt_.
Such are the inorganic substances usually found mixed or combined
together in the ash of plants. It has already been observed, that the
quantity of ash left by a given weight of vegetable matter varies
with a great many conditions. This fact deserves a more attentive
consideration.
SECTION II.—OF THE DIFFERENCE IN THE QUANTITY OF ASH.
1. The quantity of ash yielded by _different plants_ is unlike. Thus
1000 lbs. of
Wheat leave 12 lbs.
Oats 26 lbs.
Turnips 8 lbs.
Red Clover 16 lbs.
Rye Grass 17 lbs.
Barley 25 lbs.
Potatoes 8 lbs.
Carrots 7 lbs.
White Clover 17 lbs.
So that the quantity of inorganic food required by different vegetables
is greater or less according to their nature; and if a soil be of such
a kind that it can yield only a small quantity of this inorganic food,
then only those plants will grow well upon it which require the least.
Hence, trees may often grow where arable crops fail to thrive, because
many of them require and contain very little inorganic matter. Thus
while 1000 lbs. of elm wood leave 19 lbs. and of poplar 20 lbs. of
ash, the same weight of the willow leaves only 4½ lbs., of the beech 4
lbs., of the birch 3½ lbs., of different pines less than 3 lbs., and of
the oak only 2 lbs. of ash when burned.
2. The quantity of inorganic matter varies in _different parts of the
same plant_. Thus while 1000 lbs. of the turnip root sliced and dried
in the air leave 70 lbs. of ash, the dried leaves give 130 lbs.; and
while the grain of wheat yields only 12 lbs., wheat straw will yield
60 lbs. of earthy matter. So, though the willow and other _woods_
leave little ash, as above stated, yet the willow leaf leaves 82 lbs.,
the beech leaf 42 lbs., the birch 50 lbs., the different pine leaves
20 lbs. to 30 lbs., and the leaves of the elm as much as 120 lbs. of
incombustible matter when burned in the air.
Most of the inorganic matter, therefore, which is withdrawn from
the soil in a crop of corn is returned to it again, by the skilful
husbandman, in the fermented straw,—in the same way as nature, in
causing the trees periodically to shed their leaves, returns with them
to the soil a very large portion of the soluble inorganic substances
which had been drawn from it by the roots during the season of growth.
Thus an annual top-dressing is given to the land where forests grow;
and that which the roots from spring to autumn are continually sucking
up, and carefully collecting from considerable depths, winter strews
again on the surface, so as, in the lapse of time, to form a soil
which cannot fail to prove fertile,—because it is made up of those
very materials of which the inorganic substance of former races of
vegetables has been entirely composed.
2. The quantity of inorganic matter often differs in _different
specimens of the same plant_. Thus, 1000 lbs. of wheat straw, grown at
different places, gave to four different experimenters 43, 44, 35, and
155 lbs. of ash respectively. Wheat straw, therefore, does not always
leave the same quantity of ash.
To what is this difference owing? Is it to the nature of the soil, or
does it depend upon the _variety_ of wheat experimented upon? It seems
to depend partly upon both. Thus, on the same field, in Ravensworth
dale, Yorkshire, on a rich clay soil abounding in lime, the _Golden
Kent_ and _Flanders Red_ wheats were sown in the spring of 1841. The
former gave an excellent crop, while the latter was a total failure,
the ear containing 20 or 30 grains only of poor wheat. The straw of the
former left 165 lbs. of ash from 1000 lbs., that of the latter only 120
lbs. Something, therefore, depends upon the variety. But as from the
straw of a good wheat crop grown near Durham this last summer on a clay
loam I obtained only 66 lbs. of ash, I am persuaded that the very wide
variations in the quantity of ash left, by different wheat straws, must
be dependent in some considerable degree upon the soil.
The truth, so far as it can as yet be made out, seems to be this—that
every plant must have a certain quantity of inorganic matter to make it
grow in the most healthy manner;—that it is capable of living, growing,
and even ripening seed with very much less than this quantity;—but
that those soils will produce the most perfect plants which can best
supply all their wants,—and that the best seed will be raised in those
districts where the soil, without being too rich or rank, yet can yield
both organic and inorganic food in such proportions as to maintain the
corn plants in their most healthy condition.
SECTION III.—OF THE QUALITY OF THE ASH OF PLANTS.
But much also depends upon the _quality_ as well as upon the _quantity_
of the ash. Plants may leave the same weight of ash when burned, and
yet the nature of the two specimens of ash, the kind of matter of
which they respectively consist, may be very different. The ash of one
may contain much lime, of another much potash, of a third much soda,
while in a fourth much silica may be present. Thus 100 lbs. of the ash
of _bean_ straw contain 53½ lbs. of potash, while that of _barley_
straw contains only 3½ lbs. in the hundred; and, on the other hand, the
same weight of the ash of the latter contains 73½ lbs. of silica, while
in that of the former there are only 7½ lbs.
The quality of the ash seems to vary with the same conditions by which
its quantity is affected. Thus—
1. It varies with the kind of plant. 100 lbs. of the ash of wheat,
barley, and oats, for example, contain, respectively,
Wheat. Barley. Oats.
Potash, 19 12 6
Soda, 20½ 12 5
Lime, 8 4½ 3
Magnesia, 8 8 2½
Alumina, 2 1 ½
Oxide of Iron, 0 trace. 1½
Silica, 34 50 76½
Sulphuric acid, 4 2½ 1½
Phosphoric acid, 3½ 9 3
Chlorine, 1 1 ½
--------- --------- ---------
100 100 100
A comparison of the several numbers opposite to each other in these
three columns, shews how unlike the quantities of the different
substances are, which are contained in an equal weight of the ash of
these three varieties of grain. The ash of wheat contains 19 lbs. of
potash in the 100 lbs., while that of oats contains only 6 lbs. In
wheat are 20½ per cent. of soda, in oats only 5 per cent. Wheat also
contains more sulphuric acid than either of the other grains, while
barley contains a still greater predominance of phosphoric acid.
It is thus evident that a crop of wheat will carry off from the
soil—even suppose the whole _quantity_ of ash left by each the same in
weight—very different quantities of potash, soda, &c. from a crop of
oats. It will take more of these, of sulphuric acid, and of certain
other substances, from the soil. It will, therefore, exhaust the soil
more of _these_ substances—as barley and oats will of others—hence
_one_ reason why a piece of land may suit one of these crops and not
suit the others. That which cannot grow wheat may yet grow oats.
Hence, also, two successive crops of _different_ kinds of grain may
grow where it would greatly injure the soil to take two in succession
of the _same_ kind, especially of either wheat or barley; and hence
we likewise deduce one natural reason for a rotation of crops. The
surface soil may be so far exhausted of one inorganic substance, that
it cannot afford it in sufficient quantity during the present season
to bring a given crop to healthy maturity, and yet may, by natural
processes, be so far supplied again, during the intermediate growth
of certain other crops, as to be prepared in a future season fully to
supply all the wants of the same crop, and to yield a plentiful harvest.
2. The kind of inorganic matter varies with the _part_ of the plant.
Thus the grain and the straw of the corn plants contain very unlike
quantities of the several inorganic constituents, as will appear by
comparing the following with the preceding table:—
Wheat Barley Oat
Straw. Straw. Straw.
Potash, ½ 3½ 15
Soda, ¾ 1 trace.
Lime, 7 10½ 2¾
Magnesia, 1 1½ ½
Alumina, 2¾ 3 trace.
Oxide of Iron, }
Oxide of manganese, } 0 ½ trace.
Silica, 81 73½ 80
Sulphuric acid, 1 2 1½
Phosphoric acid, 5 3 ¼
Chlorine, 1 1½ trace.
------- ------- --------
100 100 100
Not only are the quantities of the several inorganic substances kinds
of straw very unlike—especially the proportions of potash, lime, and
phosphoric acid in each—but these quantities are also very different
from those exhibited by the numbers in the preceding table as contained
in the three varieties of grain. In this difference we see, further,
_one_ reason why the same soil which may be favourable to the growth
of straw may not be equally propitious to the growth of the ear. Wheat
straw contains little either of potash or of soda; the ash of the
grain contains a large proportion; while the ash of the oat-straw,
on the other hand, contains a much larger proportion of potash than
that of its own ear does. It is clear, therefore, that the roots may,
in certain plants and in certain soils, succeed in fully nourishing
the straw while they cannot fully ripen the ear; or contrariwise,
where they feed but a scanty straw, may yet _be able_ to give ample
sustenance to the filling ear.[8]
[8] And occasionally _do_ give; for a plump grain, and even a
well-filled ear, are not unfrequently found where the straw is
unusually deficient.
3. The quality of the ash varies also with the soil in which it grows.
This will be understood from what is stated above. Where the soil is
favourable, the roots can send up into the straw every thing which the
healthy plant requires; when it is poorly supplied with some of those
inorganic constituents which the plant desires, life may be prolonged,
a stunted or unhealthy crop may be raised, in which the kind, and
perhaps the quantity, of ash left in burning will necessarily be
different from that left by the same species of plant grown under more
favouring circumstances. Of this fact there can be no doubt, though
the extent to which such variations may take place without absolutely
killing the plant, has not yet been by any means made out.
4. It varies also with the period of a plant’s growth, or the season at
which it is reaped. Thus, in the young leaf of the turnip and potato,
a greater proportion of the inorganic matter they contain consists of
potash than in the old leaf. The same is true of the stalk of wheat;
and similar differences prevail in almost every kind of plant at
different stages of its growth.
The enlightened agriculturist will perceive that all the facts above
stated have a perceptible connection with the ordinary processes of
practical agriculture, and tend to throw considerable light on some of
the principles by which they ought to be regulated. One illustration of
this is exhibited in the following section.
SECTION IV.—QUANTITY OF INORGANIC MATTER CONTAINED IN AN ORDINARY CROP
OR SERIES OF CROPS.
The importance of the inorganic matter contained in living vegetables,
or in vegetable substances when reaped and dry, will appear more
distinctly if we consider the actual quantity carried off from the soil
in a series of crops.
In a four-years’ course of cropping, in which the crops gathered amount
per acre to—
1st year, _Turnips_, 25 tons of bulbs, and 7 tons of tops.
2d year, _Barley_, 38 bushels of 63 lbs. each,
and 1 ton of straw.
3d year, _Clover and Rye Grass_, 1 ton of each in hay.
4th year, _Wheat_, 25 bushels of 60 lbs., and 1¾ tons of straw.
The quantity of inorganic matter carried off in the four crops,
supposing none of them to be eaten on the land, amounts to—
Potash, 281 lbs.
Soda, 130 ”
Lime, 242 ”
Magnesia, 42 ”
Alumina, 11 ”
Silica, 318 ”
Sulphuric acid, 111 ”
Phosphoric acid, 61 ”
Chlorine, 39 ”
--------
Total, 1240
or, in all, about 11 cwt.—of which gross weight the different
substances form very unlike proportions.
A still clearer idea of these quantities will be obtained by a
consideration of the fact, that if we carry off the entire produce, and
return none of it again in the shape of manure, we must or ought in its
stead, if the land is to be restored to its original condition, add to
each acre every four years:—
Pearl or Potash, 390 lbs. at a cost of _l._3 10 0
Crystallized carbonate of soda, 440 2 5 0
Common salt, 65 0 2 0
Quick (burned) lime, 240 0 1 0
Epsom salts, 250 1 5 0
Alum, 84 0 8 0
Bone-dust, 260 0 16 0
------ ---------
Total, 1729 _l._8 7 0
Several observations suggest themselves from a consideration of the
above statements: _first_, that if this inorganic matter be really
necessary to the plant, the gradual and constant removal of it from the
land ought by-and-by to impoverish the soil of this inorganic food;
_second_, that the more of what grows upon the land we can again return
to it in manure, the less will this deterioration be perceptible;
_third_, that as many of these inorganic substances are readily soluble
in water, the liquid manure of the farm-yard, so often allowed to run
to waste, carries with it to the rivers much of the saline matter
that ought to be returned to the land; and, _lastly_, that the utility
and often indispensable necessity of certain artificial manures is
owing, it may be, in some districts, to the natural poverty of the
land in certain inorganic substances,—but more frequently to a want
of acquaintance with the facts above stated, among practical men, and
to the long continued neglect and waste which has been the natural
consequence.
In certain districts, the soil and subsoil contain within themselves
an almost unfailing supply of some of these inorganic substances, so
that the waste is long in being felt; in others they become sooner
exhausted, and hence call for more care, and, when exhausted, for a
more expensive cultivation, in order to replace them.
One thing is of essential importance to be remembered by the practical
farmer—that the deterioration of land is often an exceedingly slow
process. In the hands of successive generations a field may so
imperceptibly become less valuable, that a century even may elapse
before the change prove such as to make a sensible diminution in the
valued rental. Such slow changes, however, have been seldom recorded;
and hence the practical man is occasionally led to despise the clearest
theoretical principles, because he has not happened to see them
verified in his own limited experience, and to neglect therefore the
suggestions and the wise precautions which these principles lay before
him.
The agricultural history of tracts of land of different qualities,
shewing how they had been cropped and tilled, and the average produce
in grain, hay, straw, and other crops, every five years, during an
entire century, would be invaluable materials both to theoretical and
to practical agriculture.
CHAPTER V.
Of Soils—their Organic and Inorganic Portions—Saline
Matter in Soils—Examination and Classification of
Soils—Diversities of Soils and Subsoils.
Soils consist of two parts,—of an _organic_ part, which can readily be
burnt away when the soil is heated to redness; and of an _inorganic_
part, which is fixed in the fire, and which consists entirely of earthy
and saline substances.
SECTION I.—OF THE ORGANIC PART OF SOILS.
The organic part of soils is derived chiefly from the remains of
vegetables and animals which have lived and died in or upon the soil,
which have been spread over it by rivers and rains, or which have been
added by the hand of man for the purpose of increasing its natural
fertility.
This organic part varies very much in quantity in different soils. In
some, as in peaty soils, it forms from 50 to 70 per cent. of their
whole weight, and even in some rich long cultivated lands it has been
found, in a few rare cases, to amount to as much as 25 per cent. In
general, however, it is present in much smaller proportion, even in our
best arable lands. Oats and rye will grow upon a soil containing only
1½ per cent., barley when 2 to 3 are present, while good wheat soils
generally contain from 4 to 8 per cent. In stiff and very clayey soils
10 to 12 per cent. may occasionally be detected. In very old pasture
lands and in gardens, vegetable matter occasionally accumulates, so as
to overload the upper soil.
To this organic matter in the soil the name of _humus_ has been given
by some writers. It contains or yields to the plant the ulmic and
humic acids described in a previous chapter. It supplies also, by
its decay, in contact with the air which penetrates the soil, much
carbonic acid, which is supposed to enter the roots and minister to the
growth of living vegetables. During the same decay ammonia is likewise
produced,—and in larger quantity, if animal matter be present in
considerable abundance,—which ammonia is found to promote vegetation
in a remarkable manner. Other substances, more or less nutritious,
are also formed from it in the soil. These enter by the roots, and
contribute to nourish the growing plant, though the extent to which
it is fed from this source is dependent, both upon the abundance with
which these substances are supplied, and upon the nature of the plant
itself, and of the climate in which it grows.
Another influence of this organic portion of the soil, whether
naturally formed in it, or added to it as manure, is not to be
neglected. It contains,—as we have seen that all vegetable substances
do,—a considerable quantity of inorganic, that is, of saline and earthy
matter, which is liberated as the organic part decays. Thus living
plants derive from the remains of former races buried beneath the
surface, a portion of that inorganic food which can only be obtained
in the soil,—and which, if not thus directly supplied, must be sought
for by the slow extension of their roots through a greater depth and
breadth of the earth in which they grow. The addition of manure to the
soil, therefore, places within the easy reach of the roots not only
organic but inorganic food also.
SECTION II.—OF THE INORGANIC PART OF SOILS.
The inorganic part of soils,—that which remains behind, when every
thing combustible is burned away by heating it to redness in the open
air,—consists of two portions, one of which is _soluble_ in water, the
other _insoluble_. The soluble consists of _saline_ substances, the
insoluble of _earthy_ substances.
1. _The saline or soluble portion._—In this country the surface soil
of our fields, in general, contains very little soluble matter. If a
quantity of soil be dried in an oven, a pound weight of it taken, and
a pint and a half of pure boiling rain-water poured over it, the whole
well stirred and allowed to settle,—the clear liquid, when poured off
and boiled to dryness, may leave from 2 to 20 grains of saline matter.
This saline matter will consist of common salt, gypsum, sulphate of
soda (Glauber’s salts), sulphate of magnesia (Epsom salts), with
traces of the chlorides of calcium, magnesium, and potassium, and of
the nitrates of potash, soda, and lime.[9] It is from these soluble
substances that the plants derive the greater portion of the saline
ingredients contained in the ash they leave when burned.
[9] See pages 51 and 52, where these substances are described.
Nor must the quantity thus obtained from a soil be considered too small
to yield the whole supply which a crop requires. A single grain of
saline matter in every pound of a soil a foot deep, is equal to 500
lbs. in an acre, which is more than is carried off from the soil in
10 rotations (40 years), where only the wheat and barley are sent to
market, and the straw and green crops are regularly returned to the
land in the manure.[10]
[10] A further portion, it will be recollected, is carried off in the
cattle that are sent to market,—this is here neglected.
In some countries, indeed in some districts of our own country, the
quantity of saline matter in the soil is so great, as in hot seasons to
form a distinct incrustation on the surface. This may often be seen in
the neighbourhood of Durham; and is more especially to be looked for
in districts where the subsoil is sandy and porous, and more or less
full of water. In hot weather the evaporation on the surface causes
the water to ascend from the porous subsoil: and as this water always
brings with it a quantity of saline matter,—which it leaves behind when
it rises in vapour,—it is evident that the longer the dry weather and
the consequent evaporation from the surface continue, the thicker the
incrustations will be, or the greater the accumulation of saline matter
on the surface. Hence, where such a moist and porous subsoil exists in
countries rarely visited by rain, as in the plains of Peru, of Egypt,
or of India, the country is whitened over in the dry season with an
unbroken covering of the different saline substances above mentioned.
When rain falls, the saline matter is dissolved, and descends again to
the subsoil,—in dry weather it reascends. Thus the surface soil of any
field will contain a larger proportion of soluble inorganic matter in
the middle of a hot season than in one of even ordinary rain; and hence
the fine dry weather which, in early summer, hastens the growth of
corn, and later in the season favours its ripening, does so, among its
other modes of action, by bringing up to the roots from beneath a more
ready supply of those saline compounds which the crop requires for its
healthful growth.
2. _The earthy or insoluble portion._—The earthy or insoluble portion
of soils rarely constitutes less than 95 lbs. in a hundred of their
whole weight. It consists chiefly of _silica_ in the form of _sand_,
of _alumina_ in the form of _clay_, and of _lime_ in the form of
_carbonate of lime_. It is rarely free, however, from one or two per
cent. of oxide of iron; and where the soil is of a red colour, this
oxide is present in a still larger quantity. A trace of magnesia also
may be almost always detected, and a minute quantity of phosphate of
lime. The principal ingredients, however, of the earthy part of all
soils are sand, clay, and lime; and soils are named or classified
according to the quantities of each of these three they may happen to
contain.
If an ounce of soil be boiled in a pint of water till it is perfectly
softened and diffused through it, and, after shaking, the heavy parts
be allowed to settle for a few minutes, the sand will subside, while
the clay—which is in finer particles, and is less heavy—will still
remain floating. If the water and clay be now poured into another
vessel, and be allowed to stand till the water has become clear, the
sandy part of the soil will be on the bottom of the one vessel, the
clayey part on that of the other, and they may be dried and weighed
separately.
If 100 grains of dry soil leave no more than 10 of clay, it is called
a _sandy soil_; if from 10 to 40, a _sandy loam_; if from 40 to 70, a
_loamy soil_; if from 70 to 85, a _clay loam_; from 85 to 95, a _strong
clay soil_; and when no sand is separated at all by this process, it is
a pure _agricultural clay_.
The _strong clay soils_ are such as are used for making tiles and
bricks; the pure _agricultural clay_ is such as is commonly employed
for the manufacture of pipes (pipe-clay).
Soils consist of these three substances _mixed_ together. The pure
_clay_ is a chemical _compound_ of silica and alumina, in the
proportion of about 60 of the former to 40 of the latter. Pure clay
soils rarely occur—it being well known to all practical men, that the
strong clays (tile clays) which contain from 5 to 15 per cent. of
sand, are brought into arable cultivation with the greatest possible
difficulty. It will rarely happen, therefore, that arable land will
contain more than 30 to 35 of alumina.
If a soil contain more than 5 per cent. of carbonate of lime, it is
called a _marl_; if more than 20 per cent., it is a _calcareous_ soil.
_Peaty soils_, of course, are those in which the vegetable matter
predominates very much.
To estimate the lime, a quantity of the soil should be burned in the
air, and a weighed portion, 100 or 200 grains, diffused through half a
pint of cold water mixed with half a wine glassful of spirit of salt
(muriatic acid), and allowed to stand for a couple of hours, with
occasional stirring. The water is then poured off, the soil dried,
heated to redness as before, and weighed: the loss is nearly all
lime.[11]
[11] Unless the soil happen to contain a large quantity of magnesia,
which is rarely the case.
The quantity of vegetable or other organic matter is determined by
drying the soil _well_ upon paper in an oven, and then burning a
weighed quantity in the air: the loss is _nearly all_ organic matter.
In stiff clays this loss will comprise a portion of water, which is
not wholly driven off from such soils by drying upon paper in the way
described.
SECTION III.—OF THE DIVERSITIES OF SOILS AND SUBSOILS.
Though the substances of which soils _chiefly_ consist are so few in
number, yet every practical man knows how very diversified they are
in character—how very different in agricultural value. Thus, in some
of our southern counties, we have a white soil, consisting apparently
of nothing else but chalk; in the centre of England a wide plain of
dark red land; in the border counties of Wales, and on many of our
coal-fields, tracts of country almost perfectly black; while yellow,
white, and brown sands give the prevailing character to the soils of
other districts. Such differences as these arise from the different
proportions in which the sand, lime, clay, and the oxide of iron which
colours the soils, have been mixed together.
But how have they been so mixed—differently in different parts of the
country. By what natural agency?—for what end?
Again, the soil on the surface rests on what is usually denominated the
_subsoil_. This, also, is very various in its character and quality.
Sometimes it is a porous sand or gravel, through which water readily
ascends from beneath or sinks in from above; sometimes it is light and
loamy like the soil that rests upon it; sometimes stiff and impervious
to water.
The most ignorant farmer knows how much the value of a piece of land
depends upon the characters of the surface soil,—the intelligent
improver understands best the importance of a favourable subsoil. “When
I came to look at this farm,” said an excellent agriculturist to me,
“it was spring, and damp growing weather: the grass was beautifully
green, the clover shooting up strong and healthy, and the whole farm
had the appearance of being very good land. Had I come in June, when
the heat had drunk up nearly all the moisture which the _sandy subsoil_
had left in the surface, I should not have offered so much rent for
it by ten shillings an acre.” He might have said also, “Had I taken a
spade, and dug down 18 inches in various parts of the farm, I should
have known what to expect in seasons of drought.”
But how come subsoils thus to differ—one from the other—and from the
surface soil that rests upon them? Are there any principles by which
such differences can be accounted for—by which they can be foreseen—by
the aid of which we can tell what kind of soil may be expected in this
or that district—even without visiting the spot—and on what kind of
subsoil it is likely to rest?
Geology explains the cause of all such differences, and supplies us
with principles by which we can predict the general quality of the soil
and subsoil in the several parts of entire kingdoms;—and where the soil
is of inferior quality and yet susceptible of improvement, the same
principles indicate whether the means of improving it are likely, in
any given locality, to be attainable at a reasonable cost.
It will be proper shortly to illustrate these direct relations of
geology to agriculture.
CHAPTER VI.
Direct relations of Geology to Agriculture—Origin
of Soils—Causes of their Diversity—Relation to
the Rocks on which they rest—Constancy in the
relative Position and Character of the Stratified
Rocks—Relation of this fact to Practical
Agriculture—General Character of the Soils upon
these Rocks.
Geology is that branch of knowledge which embodies all ascertained
facts in regard to the nature and internal structure, both physical and
chemical, of the solid parts of our globe. This science has many close
relations with practical agriculture, and especially throws much light
on the nature and origin of soils,—on the cause of their diversity,—on
the kind of materials by the admixture of which they may be permanently
improved,—and on the sources from which these materials may be derived.
SECTION I.—OF THE ORIGIN OF SOILS.
If we dig down through the soil and subsoil to a sufficient depth, we
always come sooner or later to the solid rock. In many places the rock
actually reaches the surface, or rises in cliffs, hills, or ridges,
far above it. The surface (or crust) of our globe, therefore, consists
everywhere of a solid mass of rock, overlaid with a covering, generally
thin, of loose materials. The upper or outer part of these loose
materials forms the soil.
The geologist has travelled over great part of the earth’s surface,
has examined the nature of the rocks, which everywhere repose beneath
the soil, and has found them to be very unlike in character, in
composition, and in hardness—in different countries and districts. In
some places he has met with a sandstone, in other places a limestone,
in others a slate or hardened rock of clay. But a careful comparison
of all the kinds of rock he has observed, has led him to the general
conclusion, _that they are all either sandstones, limestones, or clays
of different degrees of hardness, or a mixture in different proportions
of two or more of these kinds of matter_.
When the loose covering of earth is removed from the surface of any of
these rocks, and it is left exposed, summer and winter, to the action
of the winds and rains and frosts, it may be seen gradually to crumble
away. Such is the case even with many of those which, on account of
their greater hardness, are employed as building-stones, and are kept
generally dry; how much more with such as are less hard, and, beneath
a covering of moist earth, are continually exposed to the action of
water. The natural crumbling of a naked rock thus gradually covers
it with loose materials, in which seeds fix themselves and vegetate,
and which eventually forms a soil. The soil thus produced partakes
necessarily of the character of the rock on which it rests, and to the
crumbling of which it owes its origin. If the rock be a sandstone the
soil is sandy; if a claystone, it is a more or less stiff clay; if a
limestone, it is more or less calcareous; and if the rock consist of
any peculiar mixture of those three substances, a similar mixture is
observed in the earthy matter into which it has crumbled.
Led by this observation, the geologist, after comparing the rocks
of different countries with one another, compared next the soils of
various districts with the rocks on which they immediately rest. The
_general_ result of this comparison has been, that in almost every
country the soils have as close a resemblance to the rocks beneath
them—as the loose earth derived from the crumbling of a rock before
our eyes, bears to the rock of which it lately formed a part. The
conclusion therefore is irresistible, that soils, generally speaking,
have been formed by the crumbling or decay of the solid rocks,—that
there was a time when these rocks were uncovered by any loose
materials,—and that the accumulation of soil has been the slow result
of the natural degradation (wearing away) of the solid crust of the
globe.
SECTION II.—CAUSE OF THE DIVERSITY OF SOILS.
The cause of the diversity of soils in different districts, therefore,
is no longer obscure. If the subjacent rocks in two localities differ,
the soils met with there must differ also, and in an equal degree.
But why, it may be asked, do we find the soil in some countries
uniform, in mineral[12] character and general fertility, over hundreds
or thousands of square miles, while in others it varies from field to
field,—the same farm often presenting many well marked differences both
in mineral character and in agricultural value? The cause of this is to
be found in the mode in which the different rocks are observed to lie,
one upon or by the side of the other.
[12] That is, containing the same general proportions of sand, clay,
lime, &c., or coloured red by similar quantities of oxide of iron.
Geologists distinguish rocks into two classes, the _stratified_ and the
_unstratified_. The former are found lying over each other in separate
beds or _strata_, like the leaves of a book, when laid on its side,
or like the layers of stones in the wall of a building; the latter
form hills, mountains, or sometimes ridges of mountains, consisting
of one more or less solid mass of the same material, in which no
layers or strata are any where distinctly perceptible. Thus, in the
following diagram, (No. 1), A and B represent _unstratified_ masses,
in connection with a series of _stratified_ deposits, 1, 2, 3, lying
over each other in a horizontal position. On A one kind of soil will be
formed, on C another, on B a third, and on D a fourth,—the rocks being
all different from each other.
[Illustration: NO. 1.]
If from A to D be a wide valley of many miles in extent, the undulating
plain at the bottom of the valley, resting in great part on the same
rock (2), will be covered by a similar soil. On B the soil will be
different for a short space; and again at C, and on the first ascent to
A, where the rock (3) rises to the surface. In this case the stratified
rocks lie horizontally; and it is the undulating nature of the country
which, bringing different kinds of rock to the surface, causes a
necessary diversity of soil.
But the degree of _inclination_, which the beds possess, is a more
frequent cause of variation in the characters of the soil in the same
district, and even at shorter distances. This is shewn in the annexed
diagram (No. 2), where A, B, C, D, E, represent the mode in which the
stratified rocks of a district of country not unfrequently occur in
connection with each other.
[Illustration: NO. 2.]
Proceeding from E in the plain, the soil would change when we came upon
the rock D, but would then continue uniform till we reached the layer
C. Each of these layers may stretch over a comparatively level tract of
perhaps hundreds of miles in extent. Again, on climbing the hill-side,
another soil would present itself, which would not change till we
arrived at B. Then, however, we begin to walk over the edges of the
beds, and the soil may vary with every new _stratum_ (or bed) we pass
over, till we gain the ascent to A, where the beds are much thinner,
and where, therefore, still more frequent variations may present
themselves.
Everywhere over the British islands valleys are hollowed out, as in
the former of these diagrams (No. 1), by which the rocks beneath are
exposed, and differences of soil produced,—or the beds are more or less
inclined, as in the latter diagram (No. 2), causing still more frequent
variations of the land to appear. By a reference to these facts, nearly
all the _great_ diversities which the soils of the country present may
be satisfactorily accounted for.
SECTION III.—OF THE CONSTANCY IN THE CHARACTER AND ORDER OF SUCCESSION
OF THE STRATIFIED ROCKS.
Another fact alike important to agriculture and to geology, is the
natural order or mode of arrangement in which the stratified rocks
are observed to occur in the crust of the globe. Thus, if 1, 2, 3, in
diagram No. 1 represent three different kinds of rock, a limestone, for
example, a sandstone, and a hard clay rock (a shale or slate), lying
over each other, in the order here represented; then, in whatever part
of the country nay, in whatever part of the world, these same rocks are
met with, they will always be found in the same relative position. The
bed 2 or 3 will never be observed to lie over the bed 1.
This fact is important to geology, because it enables this science to
arrange all the stratified rocks in a certain invariable order,—which
order indicates their relative age or antiquity,—since that which is
lowest, like the lowest layer of stones in the wall of a building, must
generally have been the first deposited, or must be the oldest. It also
enables the geologist, on observing the kind of rock which forms the
surface in any country, to predict at once, whether certain other rocks
are likely to be met with in that country or not. Thus at C (diagram,
No. 1), where the rock (3) comes to the surface, he knows it would be
in vain, either by sinking or otherwise, to seek for the rock (1), the
natural place of which is far above it; while at D he knows that by
sinking he is likely to find either 2 or 3, if it be worth his while to
seek for them.
To the agriculturist this fact is important, among other reasons,—
1. Because it enables him to predict whether certain kinds of rock,
which might be used with advantage in improving his soil, are likely
to be met with within a reasonable distance or at an accessible depth.
Thus if the bed D (diagram No. 2) be a limestone, the instructed farmer
at E knows that it is not to be found by sinking into his own land,
and, therefore, brings it from D; while, to the farmer upon C, it may
be less expensive to dig down to the bed D in one of his own fields,
than to cart it from a distant spot where it occurs on the surface. Or
if the farmer requires clay, or marl, or sand, to ameliorate his soil,
this knowledge of the constant relative position of beds enables him to
say where these materials are to be got, or where they are to be looked
for, and whether the advantage to be derived is likely to repay the
cost of procuring them.
2. It is observed, that when the soil on the surface of each of a
series of rocks, such as C, or D, or E, in the same diagram, is
uniformly bad, it is almost invariably of better quality at the point
where the two rocks meet. Thus C may be dry, sandy, and barren; D may
be cold, unproductive clay; and E a more or less unfruitful limestone
soil: yet at either extremity of the tract D, where the soil is made up
of an admixture of the decayed portions of the two adjacent rocks, the
land may be of average fertility—the sand of C may adapt the adjacent
clay to the growth of turnips, while the lime of E may cause it to
yield large returns of wheat.[13] Thus, to the tenant in looking out
for a farm, or to the capitalist in seeking an eligible investment, a
knowledge of the mutual relations of geology and agriculture will often
prove of the greatest assistance. Yet how little is such really useful
knowledge diffused among either class of men—how little are either
tenants or proprietors guided by it in their choice of the localities
in which they desire to live!
[13] See page 94.
And yet here and there the agricultural practice of more or less
extended districts, if not really founded upon or directed by, is
yet to be explained only by principles such as those I have above
illustrated. I shall mention only one example. The chalk in Yorkshire,
in Suffolk, and in other southern counties, consists of a vast number
of beds, which, taken all together, form a deposit of very great
thickness. Now, the upper beds of the chalk form poor, thin, dry soils,
producing a scanty herbage, and only under the most skilful culture
yielding profitable crops of corn. The lower beds, on the contrary,
are marly; produce a more stiff, tenacious, and even fertile soil;
and are found in a remarkable degree to enrich the soils of the upper
chalk, when laid on as a top-dressing in autumn, and allowed to crumble
under the action of the winter’s frost. Hence in Yorkshire, Wiltshire,
Hampshire, and Kent, where the lower chalk covers the surface, or is
found at no great depth beneath it, it is dug out of the sides of the
hills, or pits are sunk for it, and it is immediately laid upon the
land with great benefit to the soil. But in parts of Suffolk, where the
soil equally rests upon the upper chalk, there is no other chalk in the
neighbourhood, or to be met with at any reasonable depth, which will
materially improve the land. The farmers find it, from long experience,
to be more economical to bring chalk by sea from Kent to lay on their
lands in Suffolk, than to cover them with any portion of the same
material from their own farms. The following imaginary section will
fully explain the fact here mentioned:—
[Illustration: NO. 3.
Suffolk. Mouth of the Thames. Kent.
]
In this diagram 1 represents the London clay; 2, the plastic clay which
is below it; 3, the upper chalk with flints, rising to the surface in
Suffolk; and 4, the lower chalk, without flints, which is too deep to
be reached in Suffolk, but which rises to the surface in Kent,—where it
is abundant, is easily accessible, and whence it is transmitted across
the estuary of the Thames into Suffolk.
3. The further fact that the several stratified rocks are remarkably
constant in their mineral character, renders this knowledge of
the order of relative superposition still more valuable to the
agriculturist. Thousands of different beds are known to geologists
to occur on various parts of the earth’s surface—each occupying its
own unvarying place in the series. Most of these beds also, when they
crumble or are worn down, produce soils possessed of some peculiarity
by which their general agricultural capabilities are more or less
affected,—and these peculiarities may generally be observed in soils
formed from rocks of the same age—that is, occupying the same place in
the series—in whatever part of the world we find them. Hence if the
agricultural geologist be informed that his friend has bought, or is
in treaty for a farm or an estate, and that it is situated upon such
and such a rock, or geological formation, he can immediately give a
very probable opinion in regard to the agricultural value of the soil,
whether the property be in England, in Australia, or in New Zealand. If
he knows the nature of the climate also, he will be able to estimate
with tolerable correctness how far the soil is likely to repay the
labours of the practical farmer,—nay, even whether it is likely to suit
better for arable land or for pasture, and if for arable, what species
of white crops it may be expected to produce most abundantly.
These facts are so very curious, and illustrate so beautifully
the value of geological knowledge—if not to A and B, the holders
or proprietors of this and that small farm, yet to enlightened
agriculturists,—to scientific agriculture in general,—that I shall
explain this part of the subject more fully in a separate section.
To those who are now embarking in such numbers in quest of new
homes in our numerous colonies, who hope to find, if not a more
willing, at least a more attainable soil in new countries, no kind
of agricultural knowledge can at the outset,—I may say, even through
life,—be so valuable as that to which the rudiments of geology will
lead them. Those who prepare themselves the best for becoming farmers
or proprietors in Canada, in New Zealand, or in wide Australia,
yet leave their native land in general without a particle of that
preliminary _practical_ knowledge, which would qualify them to say,
when they reach the land of their adoption, “On this spot, rather than
that,—in this district, rather than that,—will I purchase my allotment,
because, though both appear equally inviting, yet I know from the
geological structure of the country, that here I shall have the more
permanently productive soil; here I am more within reach of the means
of agricultural improvement; here, in addition to the riches of the
surface, my descendants may hope to derive the means of wealth from
mineral riches beneath.” And this oversight has arisen chiefly from
the value of such knowledge not being understood—often from the very
nature of it being unknown, even to otherwise well instructed practical
men. It is not to men well skilled merely in the details of local
farming, and who are therefore deservedly considered as authorities
and good teachers in regard to local or district practice, that we are
to look for an exposition, often not even for a correct appreciation,
of those general principles on which a universal system of agriculture
must be based—without which principles, indeed, it must ever remain
a mere collection of empirical rules, to be studied and laboriously
mastered in every new district we go to—as the traveller in foreign
lands must acquire a new language every successive frontier he passes.
England, the mistress of so many wide and unpeopled lands, over which
the dwellings of her adventurous sons are hereafter to be scattered, on
which their toil is to be expended, and the glory of their motherland
by their exertions to be perpetuated—England should especially
encourage all such learning, and the sons of English farmers willingly
avail themselves of every opportunity of acquiring it.
SECTION IV.—OF GEOLOGICAL FORMATIONS, AND THE GENERAL CHARACTERS OF THE
SOILS THAT REST UPON THEM.
The thousands of beds or strata of which I have spoken as lying one
over the other in the crust of the globe, have, partly for convenience,
and partly in consequence of certain remarkably distinctive characters
observed among them, been separated by geologists into three great
divisions—the _primary_, which are the lowest and the oldest; the
_secondary_, which lie over them; and the _tertiary_, which are
uppermost, and have been most recently formed. The strata, in these
several divisions, have again been subdivided into groups, called
_formations_. The following table exhibits the names and thicknesses
of these formations, and the mineralogical characters of the rocks of
which they severally consist.
I. TERTIARY STRATA.
1. The _London and Plastic clays_, 500 to 900 feet thick, consist of
stiff, almost impervious, dark-coloured clays,—chiefly in pasture.
The lower beds are mixed with sand, and produce an arable soil, but
extensive heaths and wastes rest upon them in Berkshire, Hampshire, and
Dorset.
II. SECONDARY STRATA.
2. _The Chalk_, about 600 feet in thickness, consists in the upper part
(see diagram, No. 3, p. 88) of a purer chalk with layers of flint; in
the lower, of a marly chalk without flints. The soil of the upper chalk
is chiefly in sheep-walks, that of the lower chalk is very productive
of corn.
3. The _Green Sand_, 500 feet thick, consists of 150 feet of clay, with
about 100 feet of sand above, and 250 feet below it. The upper sand
forms a very productive arable soil, and the clay impervious, wet and
cold lands chiefly in pasture. The lower sand is generally unproductive.
It is an important agricultural remark, that where the clay (plastic
clay) comes in contact with the top of the chalk, an improved soil is
produced, and that where the chalk and the green sand mix, extremely
fertile patches of country present themselves. (See pages 86 and 87.)
4. The _Wealden formation_, nearly 1000 feet thick, consists of 400
feet of sand, covered by 300 of clay, and resting upon 250 of marls and
limestones. The clay forms the poor wet pastures of Sussex and Kent. On
the sands below the clay rest heaths and brushwood; but where the marls
and limestones come to the surface, the land is of better quality, and
is susceptible of profitable arable culture.
5. In the _Upper Oolite_, of 600 feet in thickness, we have a bed of
clay (Kimmeridge clay) 500 feet thick, covered by 100 feet of sandy
limestones. The clay lands are difficult and expensive to work, and are
chiefly in old pasture. The sandy limestone soils above the clay are
also poor, but where they rest immediately upon, and are intermixed
with the clay, excellent arable land is produced.
6. The _Middle Oolite_ of 500 feet consists also of a clay (Oxford
clay) dark-blue, adhesive, and nearly 1000 feet thick, covered by 100
feet of limestones and sandstones. These latter produce good arable
land where the lime happens to abound; the clays form close heavy
compact soils, most difficult and expensive to work. The extensive
pasture lands of Bedford, Huntingdon, Northampton, Lincoln, Wilts,
Oxford, and Gloucester, rest chiefly upon this clay, as do also the
fenny tracts of Lincoln and Cambridge.
7. The _Lower or Bath Oolite_, of 500 feet in thickness, consists of
many beds of limestone and sandstone, with about 200 feet of clay in
the centre of the formation. The soils are very various in quality,
according as the sandstone or limestone predominates. The clays are
chiefly in pasture,—the rest is more or less productive, easily worked,
arable land. In Gloucester, Northampton, Oxford, the east of Leicester,
and in Yorkshire, this formation is found to lie immediately beneath
the surface, and a little patch of it occurs also on the south-eastern
coast of Sutherland.
6. The _Lias_ is an immense deposit of blue clay from 500 to 1000 feet
in thickness, which produces cold, blue, unproductive, clay soils. It
forms a long stripe of land from the mouth of the Tees, in Yorkshire,
to Lyme Regis in Dorset. It is chiefly in old, and often very valuable
pasture.
9. The _New Red Sandstone_, though only 500 feet in thickness, forms
the surface of nearly the whole central plain of England, and stretches
north through Cheshire to Carlisle and Dumfries. It consists of red
sandstones and marls,—the soils on which are easily and cheaply worked,
and form some of the richest and most productive arable lands in the
island. In whatever part of the world the red soils of this formation
have been met with, they have been found to possess in general the same
agricultural capabilities.
10. The _Magnesian Limestone_, from 100 to 500 feet in thickness, forms
a stripe of generally poor thin soil from Durham to Nottingham, capable
of improvement as arable land by high farming, but bearing naturally a
poor pasture, intermingled with sometimes magnificent furze.
11. The _Coal Measures_, from 300 to 3000 feet thick, consist of
beds of sandstones and dark-blue shales (hard clays), intermingled
(_interstratified_) with beds of coal. Where the sands come to the
surface, the soil is thin, poor, hungry, sometimes almost worthless.
The shales, on the other hand, produce stiff, wet, almost unmanageable
clays;—not unworkable, yet expensive to work, and requiring draining,
lime, skill, capital, and a zeal for improvement, to be applied to
them, before they can be made to yield the remunerating crops of corn
they are capable of producing.
12. To the _Millstone Grits_ of 600 feet or upwards in thickness the
same remarks apply. They are often only a repetition of the sandstones
and shales of the coal measures, forming in many cases soils still more
worthless. When the sandstones prevail, large tracts lie naked, or
bear a thin and stunted heath; where the shales abound, the naturally
difficult soils of the coal shales again recur. These rocks are
generally found on the outskirts of our coal-fields.
13. The _Mountain Limestone_, 800 to 1000 feet thick, is a hard blue
limestone rock, separated here and there into distinct beds by layers
of sandstones, of sandy slates, or of blue shales like those of the
coal measures. The soil upon the limestone is generally thin, but
produces a naturally sweet herbage. When the limestone and clay (shale)
adjoin each other, arable land occurs, which is naturally productive of
oats, yet, when the climate is favourable, capable of being converted
into good wheat land. In the north of England a considerable tract of
country is covered by these rocks, but in Ireland they form nearly the
whole of the interior of the island.
14. The _Old Red Sandstone_ varies in thickness from 500 to 10,000
feet. It possesses many of the valuable agricultural qualities of the
_new red_, consisting, like it, of red sandstones and marls, which
crumble down into rich red soils. Such are the soils of Brecknock,
Hereford, and part of Monmouth; of part of Berwick and Roxburgh; of
Haddington and Lanark; of southern Perth; of either shore of the
Moray Firth; and of the county of Sutherland. In Ireland, also, these
rocks abound in Tyrone, Fermanagh, and Monaghan; in Waterford, in
Mayo, and in Tipperary. In all these places, the soils they form are
generally the best in their several neighbourhoods, though here and
there,—where the sandstones are harder, more siliceous and impervious
to water,—tracts, sometimes extensive, of heath and bog occur.
III.—PRIMARY STRATA.
15. _The Upper Silurian system_ is nearly 4000 feet in thickness, and
forms the soils over the lower border counties of Wales. It consists
of sandstones and shales, with occasional limestones; but the soils
formed from these beds take their character from the general abundance
of clay. They are cold, usually unmanageable, _muddy_ clays, with
the remarkably inferior agricultural value of which the traveller is
immediately struck, as he passes westward off the red sandstones of
Hereford on to the upper silurian rocks of Radnor.
16. The _Lower Silurian_ rocks are also nearly 4000 feet in thickness,
and in Wales lie to the west of the upper silurian rocks. They consist
of about 2500 feet of sandstone, on which, when the surface is not
naked, barren heaths alone rest.
Beneath these sandstones lie 1200 feet of sandy and earthy limestones,
from the decay of which, as may be seen on the southern edge of
Caermarthen, fertile arable lands are produced.
17. The _Cambrian System_, of many thousand yards in thickness,
consists in great part of clay slates, more or less hard, which often
weather slowly, and almost always produce either poor and thin soils,
or cold, difficultly manageable clays, expensive to work, and requiring
_high farming_ to bring them into profitable arable cultivation.
Cornwall, western Wales, and the mountains of Cumberland, in England;
the high country which stretches from the Lammermuir hills to
Portpatrick, in Scotland; the mountains of Tipperary, and a large tract
on the extreme south of Ireland,—on its east coast, and far inland
from the bay of Dundalk,—are covered by these slate rocks. Patches of
rich, well cultivated land occur here and there on this formation, with
much also that is improvable; but the greater part of it is usurped by
worthless heath and extensive bogs.
18. The _Mica Slate and Gneiss systems_ are of unknown thickness, and
consist chiefly of hard and slaty rocks, crumbling slowly, forming
poor, thin soils, which rest on an impervious rock, and which, from
the height to which this formation generally rises, are rendered more
unproductive by an unpropitious climate. They form extensive heathy
tracts in Perth and Argyle, and on the north and west of Ireland. Here
and there only, in the valleys or sheltered slopes, and by the margins
of the lakes, spots of bright green meet the eye, and patches of a
willing soil, fertile in corn.
A careful perusal of the preceding sketch of the general agricultural
capabilities of the soils formed from the several classes of
stratified rocks, will have presented to the reader many illustrations
of the facts stated in the preceding section; he will have drawn
for himself—to specify a few examples—the following among other
conclusions.
1. That some formations, like the new red sandstone, yield a soil
almost always productive; others, as the coal measures and millstone
grits, a soil almost always _naturally_ unproductive.
2. That good, or better land at least, than generally prevails in a
district, may be expected where two formations or two different kinds
of rock meet,—as when a limestone and a clay mingle their mutual ruins
for the formation of a common soil.
3. That in almost every country extensive tracts of land on certain
formations will be found laid down to natural grass, _in consequence of
the original difficulty and expense of working_. Such are the Lias, the
Oxford, the Kimmeridge, and the London clays. In raising corn, it is
natural that the lands which are easiest and cheapest worked should be
first subjected to the plough; it is not till implements are improved,
skill increased, capital accumulated, and population presses, that the
heavier lands will be rescued from perennial grass, and made to produce
that greatly increased amount of food for both man and beast, which
they are easily capable of yielding.
The turnip soils of Great Britain are in many districts, it may be,
but indifferently farmed; and the state has reason to complain of
much individual neglect of known and certain methods of increasing
their productiveness; but the next _great_ achievement which British
agriculture has to effect, is to subdue the stubborn clays, and to
convert them into what many of them are yet destined to become, the
richest corn-bearing lands in the kingdom.
CHAPTER VII.
Soils of the Granitic and Trap Rocks.—Accumulations
of transported Sands, Gravels, and Clays.—Use
of Geological Maps in reference to Agriculture.
—Physical characters and Chemical constitution
of Soils.—Relation between the nature of the Soil
and the kind of Plants that naturally grow upon it.
It was stated, in the preceding lecture, (see p. 82,) that rocks are
divided by geologists into the stratified and the _unstratified_.[14]
The stratified rocks cover by far the largest portion of the globe, and
thus form a variety of soils, of which a general description has just
been given. The unstratified rocks are of two kinds—the _granites_
and the _trap_ rocks; and as a considerable portion of the area of our
island is covered by them, it will be proper shortly to consider the
peculiar characters of each, and the differences of the soils produced
from them.
[14] The unstratified are often called _crystalline_ rocks, because
they frequently have a glassy appearance, or contain regular crystals
of certain mineral substances; often also _igneous_ rocks, because they
appear all to have been originally in a melted state, or to have been
produced by fire.
SECTION I.—SOILS OF THE GRANITES AND TRAP ROCKS.
1. The _granites_ consist of a mixture, in different proportions, of
three minerals, known by the names of _quartz_, _felspar_, and _mica_.
The latter, however, is generally present in such small quantity, that
in our general description it may be safely left out of view. Granites,
therefore, consist chiefly of quartz and felspar, in proportions which
vary very much, but the former, on an average, constitutes perhaps from
one-third to one-half of the whole.
_Quartz_ has already been described—(see p. 51)—as the substance of
flint, the silica of the chemist. When the granite decays, this portion
of it forms a more or less coarse siliceous sand.
_Felspar_ is a white, greenish, or flesh-coloured mineral, often more
or less earthy in its appearance, but generally hard and brittle, and
sometimes glassy. It is scratched by, and thus is readily distinguished
from, quartz. When it decays, it forms an exceedingly fine clay.
A remarkable difference appears thus to exist, in chemical
constitution, between these two minerals—a difference which must affect
also the soils produced from them. A granite soil, in addition to the
siliceous sand, will consist chiefly of silica, alumina, and potash;
a hornblende soil, in addition to silica and alumina, of much lime,
magnesia, and oxide of iron—of nearly 2½ cwt. of each of these latter
for every ton of decayed rock. A hornblende soil, therefore, contains
more of those inorganic constituents which the plants require for their
healthy sustenance, and therefore will prove more generally productive
than a soil of decayed felspar. But when the two are mixed, as in the
greenstones, the soil must be still more favourable to vegetable life.
The potash and soda, of which the hornblende is nearly destitute, the
felspar is able abundantly to supply; while, by the hornblende are
yielded lime and magnesia, which are known to exercise a remarkable
influence on the progress of vegetation.
2. The _trap_ rocks, comprising the greenstones and basalts, consist
essentially[15] of felspar and _hornblende_ or _augite_. In contrasting
the trap rocks with the granites, it may be stated _generally_, that
while the granites consist of felspar and _quartz_, the traps consist
of felspar and hornblende (or augite). In the traps, both the felspar
and the hornblende are reduced, by the action of the weather to a more
or less fine powder, affording materials for a soil; in the granites
the felspar is the principal source of all the earthy matter they are
capable of yielding. If we compare together, therefore, the chemical
composition of the two minerals (hornblende and felspar), we shall see
in what respect these two varieties of soil ought to differ. Thus they
consist of
Felspar. Hornblende.
Silica, 65 42
Alumina, 18 14
Potash and soda, 17 trace.
Lime, trace. 12
Magnesia, ” 14
Oxide of iron, ” 14½
Oxide of manganese, ” ½
----- -------
100 97
[15] The reader is referred for more _precise_ information to the
author’s “LECTURES,” pp. 377 to 390.
Thus theory shews, that while granite soils may be eminently
unfruitful, trap soils may be eminently fertile. And such is actually
the result of observation and experience in every part of the globe.
Unproductive granite soils cover nearly the whole of Scotland north
of the Grampians, and large tracts of land in Devon and Cornwall, and
on the east and west of Ireland; while fertile trap soils extend over
thousands of square miles in the lowlands of Scotland, and in the
north of Ireland; and where in Cornwall they occasionally mix with
the granite soils, they are found to redeem them from their natural
barrenness.
While such is the general rule in regard to these two classes of soils,
it happens on some spots that the presence of other minerals in the
granites, or of hornblende or mica in larger quantity than usual, give
rise to a granitic soil of average fertility, as is the case in the
Scilly isles; while, in like manner, the trap rocks are sometimes, as
in parts of the isle of Skye, so peculiar in constitution as to condemn
the land to almost hopeless infertility.
In some districts the decayed traps are dug up, and applied with
advantage, as a top-dressing, to other kinds of land; and as by
admixture with the decayed trap, the granitic soils are known to be
improved in quality, so an admixture of decayed granite with many trap
soils, were it readily accessible, might add to their fertility also.
SECTION II.—OF THE SUPERFICIAL ACCUMULATIONS OF TRANSPORTED MATERIALS
ON DIFFERENT PARTS OF THE EARTH’S SURFACE.
It is necessary to guard the reader against disappointment, when he
proceeds to examine the existing relation between the soils and the
rocks on which they lie, or to infer the quality of the soil from the
known nature of the rock in conformity with what has been above laid
down,—by explaining another class of geological appearances which
present themselves not only in our own country but in almost every
other part of the globe.
The unlearned reader of the preceding section and chapter may say—I
know excellent land resting upon the granites, fine turnip soils on the
Oxford or London clays, tracts of fertile fields on the coal measures,
and poor, gravelly farms on the boasted new red sandstone: I have no
faith in theory—I can have none in theories which are so obviously
contradicted by natural appearances. Such, it is to be feared, is
the hasty mode of reasoning among too many _locally_[16] excellent
practical men, familiar, it may be, with many useful and important
facts, but untaught to look through and beyond isolated facts to the
principles on which they depend.
[16] By _locally_ excellent, I mean those who are the best possible
farmers of their own district and after their own way, but who would
fail in other districts requiring other methods. To the possessor of
agricultural principles the modifications required by difference of
crop, soil, and climate, readily suggest themselves, where the mere
practical man is bewildered, disheartened, and in despair.
Every one who has lived long, on the more exposed shores of our island,
has seen, that when the weather is dry, and the sea winds blow strong,
the sands of the beach are carried inland and spread over the soil,
sometimes to a considerable distance from the coast. In some countries
this sand-drift takes place to a very great extent, and gradually
swallows up large tracts of fertile land.
Again, most people are familiar with the fact, that during periods of
long continued rain, when the rivers are flooded and overflow their
banks, they not unfrequently bear with them loads of sand and gravel,
which they carry far and wide, and strew at intervals over the surface
soil.
So the annual overflowings of the Nile, the Ganges, and the river of
Amazons, gradually deposit accumulations of soil over surfaces of great
extent;—and so also the bottoms of most lakes are covered with thick
beds of sand, gravel, and clay, which have been conveyed into them from
the higher grounds by the rivers through which they are fed.
To these and similar agencies, a large portion of the existing dry land
of the globe has been, and is still exposed. Hence in many places,
the rocks, and the soils naturally derived from them, are buried
beneath accumulated heaps or layers of sand, gravel, and clay, which
have been brought from a greater or less distance, and which have
not unfrequently been derived from rocks of a totally different kind
from those of the district in which they are now found. On these
accumulations of transported materials, a soil is produced which often
has no relation in its characters to the rocks which cover the country,
and the nature of which a familiar acquaintance with these rocks would
not enable us to predict.
To this cause is due that discordance between the first indications
of geology, as to the origin of soils from the rocks on which they
rest, and the actually observed character of those soils in certain
districts—of which discordance mention has been made as likely to
awaken doubt and distrust in the mind of the less instructed student in
regard to the predictions of agricultural geology. There are several
circumstances, however, by which the careful observer is materially
aided in endeavouring to understand what the nature of the soils is
likely to be, and how they ought to be treated, even when the subjacent
rocks are thus overlaid by masses of drifted materials. Thus—
1. It not unfrequently happens, that the materials brought from a
distance are more or less mixed up with the fragments and decayed
matter of the rocks which are native to the spot, so that though
modified in quality, the soil, nevertheless, retains the general
characters of that which is formed on other spots from the decay of
these rocks alone.
2. Where the formation is extensive, or covers a large area, as the
new red sandstones and coal measures do in this country,—the mountain
limestones in Ireland, and the granites in the north of Scotland—the
transported sand, gravel, or clay, strewed over one part of the
formation, has not unfrequently been derived from the rocks of another
part of the _same_ formation, so that, after all, the soils may be said
to be produced from the rocks on which they rest, and may be judged of
from the known constitution of these rocks.
3. Or if not from the rocks of the same formation, they have most
frequently been derived from those of a neighbouring formation—from
rocks which are to be found at _no great distance_, and generally on
higher ground. Thus the ruins of the millstone grit rocks are often
spread over the surface of the coal measures—of these, again, over the
magnesian limestone,—of the latter, over the new red sandstone, and so
on. The effect of this kind of transport upon the soils, is merely to
overlap, as it were, the edges of one formation with the proper soils
of the formations that adjoin it in the particular direction from which
the drifted materials are known to have come.
It appears, therefore, that the occurrence on certain spots, or tracts
of country, of soils that have no apparent relation to the rocks on
which they immediately rest, tends in no way to throw doubt upon, to
discredit or to disprove, the conclusions drawn from the more general
facts and principles of geology. It is still generally true that soils
_are_ derived from the rocks on which they rest. The exceptions are
local, and the difficulties which these local exceptions present,
require only from the agricultural geologist a more careful study of
the structure of each district, before he pronounces a decided opinion
as to the degree of fertility it either naturally possesses, or by
skilful cultivation may be made to attain.
Geological _maps_ point out with more or less precision the extent of
country over which the chalk, the red sandstone, the granites, &c.,
are found immediately beneath the loose materials on the surface; and
these maps are of great value in indicating also the general quality of
the soils over the same districts. It may be true, that here and there
the _natural_ soils are masked or buried by transported materials,
yet the _political economist_ may, nevertheless, with safety estimate
the general agricultural capabilities and resources of a country by
the study of its geological structure—the _capitalist_ judge in what
part of it he is likely to meet with an agreeable investment—and the
_practical farmer_ in what country he may expect to find land that will
best reward his labours—that will admit of the kind of culture to which
he is most accustomed, or, by the application of better methods, will
manifest the greatest agricultural improvement.
SECTION III.—OF THE PHYSICAL CHARACTERS OF SOILS.
The influence of climate on the fertility of a soil is often very
great. This influence depends very much upon what are called the
_physical_ properties of soils.
1. Some soils are heavier and denser than others, sand and marls being
the heaviest, and peaty soils the lightest. In reclaiming peat lands,
it is found to be highly beneficial to increase their density by a
covering of clay, sand, or limestone gravel.
2. Again, some soils absorb the rains that fall, and retain them in
larger quantity and for a longer period than others. Strong clays
absorb and retain nearly three times as much water as sandy soils do,
while peaty soils absorb a still larger proportion. Hence the more
frequent necessity for draining clayey than sandy soils; hence also the
reason why, in peaty lands, the drains must be kept carefully open, in
order that the access of springs and of other water from beneath, may
be as much as possible prevented.
3. When dry weather comes, soils lose water by evaporation with
different degrees of rapidity. In this way a siliceous sand will give
off the same weight of water in the form of vapour, in one-third of the
time necessary to evaporate it from a stiff clay, a peat, or a rich
garden mould, when all are equally exposed to the air. Hence the reason
why plants are so soon burned up in a sandy soil. Not only do such
soils _retain_ less of the rain that falls, but that which is retained
is also more speedily dissipated by evaporation. When rains abound,
however, or in very moist seasons, these same properties of sandy soils
enable them to sustain a luxuriant vegetation, when plants will perish
on clay lands from excess of moisture.
4. In drying under the influence of the sun, soils contract and
diminish in bulk in proportion to the quantity of clay or of peaty
matter they contain. Sand does not at all diminish in bulk in drying,
but peat shrinks in one-fifth, and agricultural clay nearly as much.
The roots are thus compressed, and air is excluded, especially from
the hardened clays, and thus the plant is placed in a condition
unfavourable to its growth. Hence the value of proper admixtures of
sand and clay. By the latter (the clay), a sufficient quantity of
moisture is retained, and for a sufficient length of time; while, by
the former, the roots are preserved from compression, and a free access
of air is permitted.
5. In the hottest and most drying weather, the soil has seasons of
respite from the scorching influence of the sun. During the cooler
season of the night, even when no perceptible dew falls, it has the
power of again extracting from the air a portion of the moisture it
had lost during the day. Perfectly pure sand possesses this power in
the least degree; it absorbs little or no moisture from the air. A
stiff clay, on the other hand, will in a single night absorb sometimes
as much as a 30th part of its own weight, and a dry peat as much as a
12th of its weight; and, generally, the quantity thus drunk in by soils
of various qualities, is dependent upon the proportions of clay and
vegetable matter they severally contain. We cannot fail to perceive
from these facts, how much of the productive capabilities of a soil
is dependent upon the proportions in which its different earthy and
vegetable constituents are mixed together.
6. The temperature of a soil, or the degree of warmth it is capable
of attaining under the influence of the sun’s rays, materially affects
the progress of vegetation. Every gardener knows how much _bottom_
heat forces the growth, especially of young plants; and wherever a
natural warmth exists in the soil, independent of the sun, as in the
neighbourhood of volcanoes, there it exhibits the most exuberant
fertility. One main influence of the sun in spring and summer is
dependent upon its power of thus warming the soil around the young
roots, and thus rendering it propitious to their rapid growth. But the
sun does not warm all soils alike: some become much hotter than others,
though exposed to the same sunshine. When the temperature of the air
in the shade is no higher than 60° to 70°, a _dry_ soil may become so
warm as to raise the thermometer to 90° or 100°. Mrs. Ellis states,
that among the Pyrenees the rocks actually smoke after rain under the
influence of the summer sun, and become so hot, that you cannot sit
down upon them. In _wet_ soils the temperature rises more slowly, and
never attains the same height as in a dry soil by 10° or 15°. Hence it
is strictly correct to say, that wet soils are _cold_; and it is easy
to understand how this coldness is removed by perfect drainage. Dry
sands and clays, and blackish garden mould, become warmed to nearly
an equal degree under the same sun; brownish red soils are heated
somewhat more, and dark-coloured peat the most of all. It is probable,
therefore, that the presence of dark-coloured vegetable matter renders
the soil more absorbent of heat from the sun, while the colour of the
dark-red marls of the new and old red sandstones may, in some degree,
aid the other causes of fertility in the soils which they produce.
Granite generally forms hills and sometimes entire ridges of mountains.
When it decays, the rains and streams wash out and carry down the fine
felspar clay, and leave the (quartz) sand on the sides of the hills.
Hence the soil in the bottoms and flats of granite countries consists
of a cold, stiff, wet, more or less impervious clay, which often bears
only heath, bog, or a poor and unnutritive pasture. The hill sides are
either bare or covered with a thin, sandy, and ungrateful soil, of
which little can be made by the aid even of skill and industry. Yet
the opposite sides of the same mountains often present a remarkable
difference in this respect, those which are most beaten by the rains
having the light clay most thoroughly washed from their surfaces, and
being therefore the most barren.
In reading the above observations, the practical reader can hardly
fail to have been struck with the remarkable similarity in physical
properties between stiff clay and peaty soils. Both retain much of the
water that falls in rain, and both part with it slowly by evaporation.
Both contract much in drying; and both absorb moisture readily from the
air in the absence of the sun. In this similarity of properties, we
see not only why the first steps in improving both kinds of soil must
be very nearly the same; but why, also, a mixture either of clay or of
vegetable matter will equally impart to a sandy soil many of those aids
to, or elements of, fertility—of which they are alike possessed.
SECTION IV.—OF THE CHEMICAL CONSTITUTION OF SOILS.
Soils perform at least three functions, in reference to vegetation.
They serve as a basis in which plants may fix their roots and sustain
themselves in their erect position,—they supply inorganic food to
vegetables at every period of their growth,—and they are the medium in
which many chemical changes take place, that are essential to a right
preparation of the various kinds of food which the soil is destined to
yield to the growing plant.
We have spoken of soils as consisting chiefly of sand, lime, and clay,
with certain saline and organic substances in smaller and variable
proportions. But the study of the ash of plants (see chap. iv.) shews
us, that a fertile soil must of necessity contain an appreciable
quantity of at least eleven different substances, which in most cases
exist in greater or less relative abundance in the ash both of wild and
of cultivated plants.
Two well known geological facts lead to precisely the same conclusion.
We have seen that the soils formed from the unstratified rocks,—the
granites and the traps,—while they each contain certain earthy
substances in proportions peculiar to themselves, yet contain also in
general a _trace_ of most of those different kinds of matter which are
found in the ash of plants. And when to this fact is added the other,
that the stratified rocks appear to be only the long accumulating
fragments and ruins of more ancient unstratified masses—which, under
various agencies, have gradually crumbled to dust, been strewed over
the surface in alternate layers, and afterwards again consolidated,—the
reader will readily grant, that in all rocks, and consequently in all
soils, _traces_ of every one of these substances may generally be
presumed to exist.
Actual _chemical analysis_ confirms these deductions in regard
to the constitution of soils. It shews that, in most soils, the
presence of the several constituents of the ash of plants may be
detected, though in very variable proportions. And following up its
investigations, in regard to the effect of this difference in the
proportion of the generally less abundant constituents of the soil, it
establishes certain other points of the greatest possible importance to
agricultural practice. Thus, it has found, for example,
1. That as a proper adjustment of the proportions of clay and sand
is necessary, in order that a soil may possess the most favourable
_physical_ properties—so that the mere presence of the various kinds
of inorganic food in a soil is not sufficient to make it productive
of a given crop, but that they must be so adjusted in quantity that
the plant shall be able readily and at the proper time to obtain an
adequate supply of each.
2. That when a soil is particularly poor in certain of these
substances, the valuable, cultivated corn crops, grasses, and trees,
refuse to grow upon them in a healthy manner, and to yield remunerating
returns. And,
3. That when certain other substances are present in too great
abundance, the soil is rendered equally unpropitious to the most
important crops.
In these facts the intelligent reader will perceive the foundation of
the varied applications to the soil which are everywhere made under
the direction of a skilful practice, and of the difficulties which, in
so many localities, lie in the way of bringing the land into such a
state as shall fit it readily to supply all the wants of those kinds of
vegetables which it is the special object of artificial culture easily
and abundantly to raise.
Chemical analysis is a difficult art,—one which demands much chemical
knowledge, and skill in chemical practice (manipulation, as it is
called), and calls for both time and perseverance—if valuable,
trustworthy, and _minutely correct_ results are to be obtained. I
believe it is only by aiming after such minutely correct results that
chemical analysis is likely to throw light on the peculiar properties
of those soils which, while they possess much general similarity in
composition and in physical properties, are yet found in practice to
possess very different agricultural capabilities. Many such cases
occur in every country, and they are the kind of difficulties in
regard to which agriculture has a right to say to chemistry—“These are
matters which I hope and expect you will satisfactorily clear up.” But
while agriculture has a right to use such language, she has herself
preliminary duties to perform. She has no right in one breath to deny
the value of chemical theory to agricultural practice, and in another
to ask the sacrifice of time and labour in doing her chemical work.
Chemistry is a wide field, and many zealous lives may be spent in the
prosecution of it without at all entering upon the domain of practical
agriculture. It may be that here and there it may fall in with the
humour or natural bias of some one chemist to apply his knowledge to
this most important art; but hitherto the appreciation of such efforts
has, in general, been so small—the reception of scientific results and
suggestions by the agricultural body so ungracious—that little wonder
can exist that so many have quitted the field in disgust—that the
majority of capable men should studiously avoid it.
Hence it has happened that, in England, the analysis of soils has
rarely been undertaken, except as a matter of professional business,
where so much time was, by a fair calculation, given for so much money,
and an analysis made, of that degree of accuracy only which the time
allotted to it permitted the analyst to attain.
In order, therefore, to illustrate the deductions which, as above
stated, may be drawn from an accurate chemical analysis, I shall
exhibit the constitution of three different soils as determined
by Sprengel, a German chemist, now at the head of the Prussian
Agricultural school, and whose own taste, as well as his professional
function, have long directed his attention, and with much success, to
scientific agriculture.
No. 1 is a very fertile alluvial soil from East Friesland, formerly
overflowed by the sea, but for 60 years cultivated with corn and pulse
crops _without manure_.
No. 2 is a fertile soil near Göttingen, which produces excellent crops
of clover, pulse, rape, potatoes, and turnips, the two last more
especially _when manured with gypsum_.
No. 3 is a very barren soil from Lunenburg.
When washed with water in the manner described in pages 70 to 73, they
gave, respectively, from 1000 parts of soil—
No. 1. No. 2. No. 3.
Soluble saline matter, 18 1 1
Fine earthy and organic matter (clay), 937 839 599
Siliceous sand, 45 160 400
---- ---- ----
1000 1000 1000
The most striking distinction presented by these numbers is the large
quantity of saline matter in No. 1. This soluble matter consisted of
common salt, chloride of potassium, sulphate of potash and gypsum,
with a trace of sulphate of magnesia, sulphate of iron, and phosphate
of soda. The presence of this comparatively large quantity of these
different saline substances,—originally derived, no doubt, in great
part from the sea,—was probably one reason why it could be so long
cropped without manure.
The unfruitful soil is much the lightest of the three, containing
40 per cent. of sand; but this is not enough to account for its
barrenness—many light soils containing a larger proportion of sand, and
yet being sufficiently fertile.
The finer portions separated from the sand, and soluble matter,
consisted in 1000 parts of
No. 1. No. 2. No. 3.
Organic matter, 97 50 40
Silica, 648 833 778
Alumina, 57 51 91
Lime, 59 18 4
Magnesia, 8½ 8 1
Oxide of iron, 61 30 81
Oxide of manganese, 1 3 ½
Potash, 2 trace. trace.
Soda, 4 ” ”
Ammonia, trace. ” ”
Chlorine, 2 ” ”
Sulphuric acid, 2 ¾ ”
Phosphoric acid, 4½ 1¾ ”
Carbonic acid, 40 4½ ”
Loss, 14 — 4½
------ ------ ------
1000 1000 1000
1. The composition of No. 1 illustrates the first of those general
deductions above stated, that a considerable supply of _all_ the
species of inorganic food is necessary to render a soil eminently
fertile. Not only does this soil contain a comparatively large quantity
of soluble saline matter, but it contains also nearly 10 per cent.
of organic matter, and, what in connection with this is of great
importance, 6 per cent. of lime. The potash and soda, and the several
acids, are also present in sufficient abundance.
2. In the second,—a fertile soil, but one which _cannot dispense with
manure_,—there is little soluble saline matter, and in the insoluble
portion we see that there are mere _traces_ of potash, soda, and the
important acids. It contains also 5 per cent. only of organic matter,
and about 2 per cent. of lime, which smaller proportions, together
with the deficiencies above stated, remove this soil from the most
_naturally_ fertile class to that class which is susceptible, in
hands of ordinary skill, of being _brought to_, and _kept in_, a very
productive condition.
3. In the fine part of the third soil, we observe that there are many
more substances deficient than in No. 2. The organic matter amounts
apparently to 4 per cent., and there seems to be nearly half a per
cent. of lime. But it will be recollected, that this soil contains 40
per cent. of sand, so that in every hundred of soil there are only 60
of the fine matter, of which the composition is presented in the table,
or 100 lbs. of the native soil contain only 2½ lbs. of organic matter
and ¼ lb. of lime.
But all these _wants_ would not condemn the soil to hopeless
barrenness, because in favourable circumstances, and where it was worth
the cost, they might all be supplied. But the oxide of iron amounts to
8 per cent. of this fine matter, a proportion of this substance which,
in a soil containing so little organic matter, appears, from practical
experience, to be incompatible with the healthy growth of cultivated
crops. To this soil, therefore, there requires to be added not only
those substances of which it is destitute, but such other substances
also as shall prevent the injurious effect of the large proportion of
oxide of iron.
In these three soils, then, we have examples, _first_, of one which
contains within itself all the elements of fertility; _second_, of a
soil which is destitute, or nearly so, of certain substances,—which,
however, can be readily added by the ordinary manures in general
use,—and to which the elements of gypsum are especially useful, in
aiding it to feed the potato and the turnip; and, _third_, of a soil
not only poor in many of the necessary species of the inorganic food
of plants, but too rich in one which, when present in excess, is
prejudicial to vegetable life.
This illustration, therefore, will aid the general reader in
comprehending how far rigid chemical analysis is fitted to throw light
upon the capabilities of soils, and to _direct_ agricultural practice.
SECTION V.—OF THE RELATION THAT EXISTS BETWEEN THE CHARACTER OF THE
SOIL AND THE KIND OF PLANTS THAT GROW UPON IT.
The importance of this study of the chemical constitution of soils
will, perhaps, be most readily appreciated by a glance at the very
different kinds of vegetables which, under the same circumstances,
different soils naturally produce.
There are none so little skilled in regard to the capabilities of the
soil, as not to be aware that some lands naturally produce abundant
herbage or rich crops, while others refuse to yield a nourishing
pasture, and are deaf to the often repeated solicitations of the
diligent husbandman. There exists, therefore, a universally understood
connection between the kind of soil and the kind of plants that
naturally grow upon it. It is interesting to observe how close this
relation in many cases is.
1. The sands of the sea-shore, and the margins of salt-lakes, are
distinguished by their peculiar tribes of salt-loving plants;—the
drifted sands more remote from the beach produce their own long waving
coarser grass,—while further inland again, other vegetable races appear.
2. Peaty soils laid down to grass, or existing as natural meadows,
produce one woolly soft grass almost exclusively (the _Holcus
lanatus_); when limed, again, these same soils become propitious to
green crops and produce much straw, but refuse to fill the ear.
3. On the margins of water-courses, in which silica abounds, the
mare’s-tail (_Equisetum_) springs up in abundance; while, if the stream
contain much carbonate of lime, the water-cress appears and lines its
sides, and the bottom of its shallow bed, sometimes for many miles from
its source.
4. The Cornish heath (_Erica vagans_) shews itself only above the
serpentine rocks; the red clover and the vetch delight in the presence
of gypsum; and white clover, of alkaline matter in the soil.
5. Then, again, plants seem to alternate with each other on the same
soil. Burn down a forest of pines in Sweden, and one of birch takes
its place _for a while_. The pines after a time again spring up and
ultimately supersede the birch. The same takes place naturally. On the
shores of the Rhine are seen ancient forests of oak from two to four
centuries old,—gradually giving place to a natural growth of beech; and
others where the pine is succeeding to both. In the Palatinate, the
ancient oak woods are followed by natural pines; and in the Jura, the
Tyrol, and Bohemia, the pine alternates with the beech.
These and other similar differences depend upon the chemical
constitution of the soil. The slug may live well, and therefore infest
a field almost deficient in lime; the common land snail will abound at
the roots of the hedges only where lime is plentiful, and can easily
be obtained for the construction of its shell. So it is with plants.
Each grows spontaneously where its wants can be most fully and most
easily supplied. If they cannot move from place to place like the
living animal, yet their seeds can lie dormant, until either the hand
of man or the operation of natural causes produces such a change in the
constitution of the soil as to fit it for ministering to their most
important wants.
And such changes do naturally come over the soil. The oak, after
thriving for long generations on a particular spot, gradually sickens;
its entire race dies out,—and other races succeed it. The operation
of natural causes has gradually removed from the soil that which
favoured the oak, and has introduced or given the predominance to those
substances which favour the beech or the pine.
In the hands of the farmer the land grows sick of this crop,—it becomes
tired of that. These facts are generally indications of a change in
the chemical constitution of the soil. This alteration may proceed
slowly and for many years, and the same crops may still grow upon it
for a succession of rotations. At length the change is too great for
the plant to bear; it sickens, yields an unhealthy crop, and becomes
ultimately extinct.
The plants we raise for food have similar likes and dislikes with those
that are naturally produced. On some kinds of food they thrive,—fed
with others, they sicken or die. The soil must therefore be prepared
for their special growth.
In an artificial rotation of crops, we only follow nature. One crop
extracts from the soil a certain quantity of all the inorganic
constituents of plants; but some of these in much larger proportions
than others. A second crop carries off in preference a larger quantity
of those substances which the former had left; and thus it is clearly
seen, both why an abundant manuring may so alter the constitution of
the soil, as to enable it to grow almost any crop; and why, at the same
time, this soil may in succession yield more abundant crops and in
greater number, if the kinds of plant sown and reaped be so varied as
to extract from the soil, one after the other, the several different
substances which the manure we have originally added is known to
contain.
The management and tilling of the soil, in fact, is a branch of
practical chemistry, which, like the art of dyeing or of lead smelting,
may advance to a certain degree of perfection, without the aid of pure
science; but which can only have its processes explained, and be led on
to shorter,—more simple,—more economical,—and more perfect processes,
by the aid of scientific principles.
CHAPTER VIII.
Of the Improvement of the Soil—Mechanical and Chemical
Methods—Draining—Subsoiling—Ploughing, and Mixing
of Soils—Use of Lime, Marl, and Shell-sand—Manures
—Vegetable, Animal, and Mineral Manures.
The soil is possessed of certain existing and obvious qualities, and
of certain other dormant capabilities; how are these qualities to be
improved,—these dormant capabilities to be awakened?
There are two distinct methods by which these ends may be, in some
measure, attained,—by the use of _mechanical_, and by the application
of _chemical_, means. Mechanical operations produce changes _chiefly_
in the physical properties of the soil,—chemical means alter its
elementary constitution. Ploughing, draining, mixing, &c. belong to
the former class of operations; manuring and irrigation belong to the
latter. It will be proper to consider these methods separately.
SECTION I.—OF MECHANICAL METHODS OF IMPROVING THE SOIL.
1. _Draining._—The first step to be taken, in order to increase the
fertility of nearly all the improveable lands of Great Britain, is
to drain them. So long as they remain wet, they will continue to be
cold. The heat of the sun’s rays, which is intended by nature to warm
the soil, will be expended in evaporating the water from its surface;
and thus the plants will never receive that genial warmth about their
roots which so much favours their rapid growth. Where too much water
is present in the soil also, that food of the plant which the soil
supplies is so much diluted, that either a much greater quantity of
fluid must be taken in by the roots,—much more work done,—or the plant
will be scantily nourished. The presence of so much water in the stem
and leaf keeps down _their_ temperature likewise, when the sunshine
appears; an increased evaporation takes place from their surfaces, a
lower natural heat, in consequence, prevails in the interior of the
plant, and the chemical changes on which its growth depends proceed
with less rapidity.
By the removal of the water, the physical properties of the soil also
are in a remarkable degree improved. Dry pipe-clay can be easily
reduced to a fine powder, but it naturally, and of its own accord,
runs together when water is poured upon it. So it is with clays in
the field. The soil expands, becomes close and adhesive, and excludes
the air from the roots of the growing plant,—the access of which air
appears to be almost an essential element in the healthy growth of the
most important vegetable productions.
Open an outlet for the water below, and as it trickles away, the air
from above will follow it and take its place among the pores of the
soil, carrying to every root the salutary influences it is appointed
to bear with it wherever it penetrates. When freed from water also,
the stiff soil becomes more mellow; and when once stirred up to a
considerable depth, more universally porous,—so that air can make
its way everywhere, and the roots can find their easy way in every
direction. The presence of vegetable matter,—whether existing naturally
in a soil thus physically altered, or artificially added to it,—becomes
of double value. When drenched with water, this vegetable matter
either decomposes very slowly, or produces acid compounds more or less
unwholesome to the plant, and even exerts injurious chemical reactions
upon the earthy and saline constituents of the soil. In the presence
of air, on the contrary, this vegetable matter decomposes rapidly,
produces carbonic acid in large quantity, as well as other compounds
fit for food, and even renders the inorganic constituents of the soil
more fitted to enter the roots, and thus to supply more rapidly what
the several parts of the plant require.
Nor is it only stiff and clayey soils to which draining can with
advantage be applied. It will be obvious to every one, that when
springs rise to the surface in sandy soils, a drain must be made to
carry off the water,—it will also readily occur, that where a sandy
soil rests upon a hard or clayey bottom, drains may also be necessary;
but it is not unfrequently supposed, that when the subsoil is sand or
gravel, that drains can only in special cases be necessary.
Every one, however, is familiar with the fact, that when water is
applied to the bottom of a flower-pot full of soil, it will gradually
find its way to the surface, however light the soil may be. So it is
in sandy soils or subsoils in the open field. If water abound at the
depth of a few feet, or if it so abound at certain seasons of the year,
_that_ water will rise to the surface; and as the sun’s heat dries it
off by evaporation, more water will follow to supply its place. This
attraction from beneath will always go on when the air is dry and warm,
and thus a double evil will ensue—the soil will be kept moist and cold,
and instead of a constant circulation of air downwards, there will be a
constant current of water upwards. Thus will the roots, the under soil,
and the organic matter it contains, be all deprived of the benefits
which the access of the air is fitted to confer. The remedy for these
evils is to be found in an efficient system of drainage.
On this subject I shall add one important practical remark, which will
readily suggest itself to the geologist who has studied the action of
air and water on the various clay beds that occur here and there as
members of the series of stratified rocks. _There are no clays which do
not gradually soften under the united influence of air and of running
water. It is false economy, therefore, to lay down tiles without
soles_—however hard and stiff the clay subsoil may appear to be. In
the course of ten or fifteen years the stiffest clays will soften, so
as to allow the tile to sink; and many very much sooner. The passage
for the water is thus gradually narrowed; and when the tile has sunk a
couple of inches, the whole must be taken up. Thousands of miles of
drains have been thus laid down, both in the low country of Scotland
and in the southern counties of England, which have now become nearly
useless; and yet the system still goes on. It would appear even as if
the farmers and proprietors of each district—unwilling to believe in or
to be benefitted by the experience of others—were determined to prove
the matter in their own case also, before they will consent to adopt
that surer system which, though demanding a slightly greater outlay
at first, will return upon the drainer with no after-calls for either
time or capital. If my reader live in a district where this practice
is now exploded, and if he be inclined to doubt if other counties be
farther behind the advance of knowledge than his own, I would invite
him to spend a week in crossing the county of Durham, where he may find
opportunities not only of satisfying his own doubts, but of scattering
here and there a few words of useful advice among the more intelligent
of our practical farmers.
2. _Subsoiling._—The subsoil plough is an auxiliary to the drain.
Though there are few subsoils through which the water will not at
length make its way, yet there are some so stiff either naturally
or from long consolidation, that the good effect of a well-arranged
line of drains is lessened by the slowness with which they allow the
superfluous rains to pass through them. In such cases, the use of the
subsoil plough is most advantageous in loosening the under layers of
clay, and allowing the water to find a ready escape downwards and to
either side until it reach the drains.
It is well known that if a piece of stiff clay be cut into the shape of
a brick, and then allowed to dry, it will contract and harden—it will
form an air-dried brick, almost impervious to any kind of gas—wet it
again, it will swell and become still more impervious. Cut up _while
wet_, it will only be divided into so many pieces, each of which will
harden when dry, or the whole of which will again attach themselves
and stick together if exposed to pressure. But tear it asunder _when
dry_, and it will fall into many pieces, will more or less crumble, and
will readily admit the air into its inner parts. So it is with a clay
subsoil.
After the land is provided with drains, the subsoil being very
retentive, the subsoil plough is used to open it up—to let out the
water and to let in the air. If this is not done, the stiff under-clay
will contract and bake as it dries, but it will neither sufficiently
admit the air nor open a free passage for the roots. But let this
operation be performed when the clay is still too wet, a good effect
will follow, in the first instance; but after a while, the cut clay
will again cohere, and the former will pronounce subsoiling to be a
useless expense _on his land_. Defer the use of the subsoil plough till
the clay is dry—it will then _tear_ and _break_ instead of _cutting_,
and its openness will remain. Once give the air free access, and it,
after a time, so modifies the drained clay, that it no longer has an
equal tendency to cohere.
Mr. Smith of Deanston very judiciously recommends that the subsoil
plough should never be used till at least a year after the land has
been thoroughly drained. This in many cases will be a sufficient
safeguard—will allow a sufficient time for the clay to dry; in other
cases two years may not be too much. But this precaution has by some
been neglected, and subsoiling being with them a failure, they have
sought, in some supposed chemical or other quality _of their soil_,
for the cause of a want of success which is to be found in their own
neglect of a most necessary precaution. Let not the practical man be
too _hasty_ in desiring to attain those benefits which attend the
adoption of improved modes of culture; let him give every method a fair
trial; _and above all, let him make his trial in the way and with
the precautions recommended by the author of the method_, before he
pronounce its condemnation.
3. _Deep-ploughing_, like subsoiling, aids the effect of the drains,
and so far, and where it goes nearly as deep, more completely effects
the same object. But independent of this, it has other uses and merits,
and where it has been successfully applied, has improved the land by
the operation of other causes.
Subsoiling only lets out the water, and allows access to the air and a
free passage to the roots. Deep-ploughing, in addition to these, brings
new earth to the surface, forms thus a deeper soil, and more or less
alters both its physical qualities and its chemical constitution.
If the plough be made to bring up two inches of clay or sand, it will
stiffen or loosen the soil, as the case may be, or it may affect its
colour or density. It is clear and simple enough, therefore, that by
deep-ploughing the physical properties of the soil may be altered.
But there are certain substances contained in every soil, whether
in pasture or under the plough, which gradually make their way down
towards the subsoil. They sink till they reach at last that point
beyond which the plough does not usually penetrate. Every farmer knows
that lime thus sinks. In peat-soils top-dressed with clay, the clay
thus sinks. In sandy soils also which have been clayed, the clay sinks;
and in all these cases, I believe, the sinking takes place more rapidly
when the land is laid down to grass. Where soils are marled, the marl
sinks; and the rains, in like manner, gradually wash out that which
gives their fertilizing virtue to the under chalk-soils (see page 88),
and render necessary a new application from beneath, to renovate its
productive powers.
If this be the case with earthy substances such as those now mentioned,
which are insoluble in water, it will be readily believed that those
saline ingredients of the soil which are readily soluble will be still
sooner washed out of the upper and conveyed to the under soil. Thus
the subsoil may gradually become rich in those substances of which the
surface-soil has been robbed. Bring up a portion of this subsoil by
deep-ploughing, and you restore to the land a portion of what it has
lost—substances, perhaps, which may render it much more fruitful than
before. Such is an outline of the theory of deep-ploughing, and it is
entirely unexceptionable.
But suppose the land to have originally contained something noxious
to vegetation, which in process of time has been washed down into the
subsoil, then to bring this again to the surface would be materially
to injure the land. This also is true, and a sound discretion must no
doubt be employed, in judging when and where such evil effects are
likely to follow.
Such cases, however, are more rare than many suppose. There are few
subsoils which a full and fair exposure to a winter’s frost will not in
a great degree deprive of all their noxious qualities, and render fit
to ameliorate the general surface of the poorer lands. If the reader
doubt this fact, let him visit Yester, and give a calm consideration to
the efforts produced by the use of deep-ploughing on the home-farm of
the Marquis of Tweeddale.
In many cases the farmer fears, as he does in the county of Durham, to
bring up a single inch of the yellow clay that lies beneath his soil.
In the first inch lodges, among other substances, the iron worn from
his plough, which in some soils, and after a lapse of years, amounts to
a considerable quantity. Till it is exposed to the air, this iron is
hurtful to vegetation, and one of the benefits of a winter’s exposure
of such subsoils to the air, is the effect produced upon the iron it
contains.
It is the want of drainage, however, and of the free access of
air, that most frequently renders subsoils for a time injurious to
vegetation. Let the lands be well drained—let the subsoils be washed
for a few years by the rain-water passing through them,—and there are
few of those which are clayey in their nature that may not ultimately
be brought to the surface, not only with safety, but with advantage to
the soil.
4. _Ploughing._—Other benefits, again, attend upon the ordinary
ploughings, hoeings, and workings of the land. Its parts are more
minutely divided—the air gets access to every particle—it is rendered
lighter, more open, more permeable to the roots. The vegetable matter
it contains decomposes more rapidly by a constant turning of the soil,
so that wherever the fibres of the roots penetrate, they find organic
food provided for them, and an abundant supply of the oxygen of the
atmosphere to aid in preparing it. The production of ammonia and of
nitric acid also (see pages 33 to 36), and the absorption of one or
both from the air, take place to a greater extent, the finer the soil
is pulverised, and the more it has been exposed to the action of the
atmosphere. The general advantage, indeed, to be derived from the
constant working of the soil, may be inferred from the fact, that Tull
reaped twelve successive crops of wheat from the same land by the
repeated use of the plough and the horse-hoe. There are few soils so
stubborn as not to shew themselves grateful in proportion to the amount
of this kind of labour that may be bestowed upon them.
5. _Mixing._—It has been shewn (page 114), that the physical properties
of the soil have an important influence upon its average fertility. The
admixture of pure sand with clay soils produces an alteration which is
often beneficial, and which is wholly physical. The sand merely opens
the pores of the clay, and makes it more permeable to the air.
The admixture of clay with sandy or peaty soils, however, produces both
a physical and a chemical alteration. The clay not only consolidates
and gives body to the sand or peat, but it also mixes with them certain
earthy and saline substances useful or necessary to the plant, which
neither the sand nor peat might originally contain in sufficient
abundance. It thus alters its chemical constitution, and fits it for
nourishing new races of plants.
Such is the case also with admixtures of marl, of shell-sand, and of
lime. They slightly consolidate the sands and open the clays, and
thus improve the mechanical texture of both kinds of soil, but their
main operation is chemical; and the almost universal benefit they
produce depends upon the new chemical element they introduce into the
constitution of the soil.
It is a matter of almost universal remark, that in our climate soils
are fertile—clayey or loamy soils, that is—only when they contain an
appreciable quantity of lime. In whatever way it acts, therefore, the
mixing of lime in any of the forms above mentioned, with a soil in
which little or no lime exists, is one of the surest practical methods
of bringing it nearer in composition to those soils from which the
largest returns of agriculture produce are usually obtained. Some of
the chemical effects of the lime upon the soil will be explained in a
subsequent section. (See page 195.)
SECTION II.—OF THE CHEMICAL METHOD OF IMPROVING THE SOIL BY THE USE OF
MANURES.
None of the above methods of improving the soil are mechanical
only—they all involve some chemical alterations also, which are readily
to be explained by a knowledge of elementary chemical principles.
But the manuring of the land is more strictly a chemical operation,
and may therefore with propriety be separated from those methods of
improving its quality which involve at the same time important and
expensive mechanical operations.
In commencing the tillage of a piece of land, the conscientious farmer
may have three objects in view in regard to it.
1. He may wish to reclaim a waste, or to restore a neglected farm to an
average condition of fertility.
2. Finding the land in this average state, his utmost ambition may be
to keep it in its present condition; or,
3. By _high_ farming he may wish to develope all its capabilities,
and to increase its permanent productiveness in the greatest possible
degree.
The man who aims at the last of these objects is not only the best
tenant and the best citizen, but he is also his own best friend. The
highest farming, skilfully and prudently conducted, is also the most
remunerating.
But whichever of these three ends he aims at, he will be unable to
attain it without a due knowledge of the various manures it may be in
his power to apply to his land—what these manures are, or of what they
consist—the general and special purposes they are each intended to
serve—which are the most effective for this or that crop—how they are
to be obtained in the greatest abundance, and at the least cost—how
their strength may be economized,—and in what state and at what seasons
they may be most beneficially applied to the land. Such are a few of
the questions which the skilful farmer should be ready to ask himself,
and should be able to answer.
By a _manure_ is to be understood whatever is capable of feeding or of
supplying food to the plant. And us plants require earthy and saline
as well as vegetable food, gypsum and nitrate of soda are as properly
called manures as farm-yard dung, bone-dust, or night-soil.
Manures naturally divide themselves into such as are of _vegetable_, of
_animal_, and of _mineral_ origin.
I. OF VEGETABLE MANURES.
There are two purposes which vegetable manure is generally supposed to
serve when added to the soil. It loosens the land, opens its pores,
and makes it lighter; and it also serves to supply organic food to the
roots of the growing plant. It serves, however, a third purpose: it
yields to the roots those saline and earthy matters which it is their
duty to find in the soil, and which exist in decaying plants in a state
more peculiarly fitted to enter readily into the circulating system of
new races.
Decayed vegetable matters, therefore, are in reality mixed manures,
and their value in enriching the land must vary considerably with the
_kind_ of plants and with the _parts_ of those plants of which they
are chiefly made up. This depends upon the remarkable difference which
exists in the _quantity_ and _kind_ of inorganic matter present in
different vegetable substances, as indicated by the ash they leave (see
pages 52 to 62). Thus if 1000 lbs. of the saw-dust of the willow be
fermented, and added to the soil, they will enrich it by the addition
of only 4½ lb. of saline and earthy matter, while 1000 lbs. of the
dry leaves of the same tree fermented, and laid on, will add 82 lbs.
of inorganic matter. Thus, independent of the effect of the vegetable
matter in each, the one will produce a very much greater effect upon
the soil than the other.[17]
[17] It is owing to this large quantity of saline and other inorganic
matter that fermented leaves form too strong a dressing for flower
borders, and that gardeners therefore generally mix them up into a
compost.
There are three states in which vegetable matter is collected by the
husbandman for the purpose of being applied to the land—the _green_
state; the _dry_ state; and that state of imperfect decay in which it
forms _peat_.
1. _Green Manuring._—When grass is mown in the field, and laid in
heaps, it speedily heats, ferments, and rots. But, if turned over
frequently and dried into hay, it may be kept for a great length of
time without undergoing any material alteration. The same is true of
all other vegetable substances—they all rot more readily in the green
state. The reason of this is, that the sap or juice of the green plant
begins very soon to ferment in the interior of the stem and leaves,
and speedily communicates the same condition to the moist fibre of
the plant itself. When once it has been dried, the vegetable matter
of the sap loses this easy tendency to decay, and thus admits of long
preservation.
The same rapid decay of green vegetable matter takes place when it is
buried in the soil. Thus the cleanings and scourings of the ditches and
hedge-sides form a compost of mixed earth and fresh vegetable matter,
which soon becomes capable of enriching the ground. When a green crop
is ploughed into a field, the whole of its surface is converted into
such a compost—the vegetable matter in a short time decays into a
light, black mould, and enriches in a remarkable degree and fertilizes
the soil.
Hence the practice of green manuring has been in use from very early
periods. The second or third crop of _lucerne_ was ploughed in by the
ancient Romans—as it still is by the modern Italians. In Tuscany, the
_white lupin_ is ploughed in, in preference—in Germany, _borage_.
In French Flanders, two crops of _clover_ are cut, and the third is
ploughed in. In Sussex, _turnip_ seed has been sown at the end of
harvest, and after two months again ploughed in, with great benefit to
the land. Turnip leaves and potato tops decay more readily, and more
perfectly, and are more enriching when buried in the green state. It is
a prudent economy, therefore, where circumstances admit of it, to bury
the potato tops on the spot from which the potatoes are raised. Since
the time of the Romans, it has been the custom to bury the cuttings
of the vine stocks at the roots of the vines themselves; and many
vineyards flourish for a succession of years without any other manuring.
Buckwheat, winter tares, clover, and rape, are all occasionally sown
for the purpose of being ploughed in. This should be done _when the
flower has just begun to open_, and if possible at a season when the
warmth of the air and the dryness of the soil are such as to facilitate
decomposition.
That the soil should become richer in vegetable matter by this burial
of a crop than it was before the seed of that crop was sown, and should
also be otherwise benefitted, will be understood by recollecting (see
page 42) that perhaps three-fourths of the whole organic matter we bury
has been derived from the air—that by this process of ploughing in,
the vegetable matter is more equally diffused through the whole soil
than it could ever be by any merely mechanical means;—and that by the
natural decay of this vegetable matter, ammonia and nitric acid are,
to a greater extent (pages 33 and 34), produced in the soil, and its
agricultural capabilities in consequence materially increased.
These considerations, while they explain the effect and illustrate
the value of green manuring, will also satisfy the intelligent
agriculturist that there are methods of improving his land without
the aid either of town or of foreign manures—and that he overlooks
an important natural means of wealth who neglects the green sods
and crops of weeds that flourish by his hedgerows and ditches. Left
to themselves, they ripen their seeds and sow them annually in his
fields—collected in compost heaps they would materially add to his
yearly crops of corn.
_Sea-weeds._—Among green manures, the use of fresh sea-ware deserves
especial mention, from the remarkably fertilizing properties it is
known to possess, as well as from the great extent to which it is
employed on all our coasts. The produce of the isle of Thanet in Kent
is said to have been doubled or tripled by the use of this manure; the
farms on the Lothian coasts are said to be let for 20s. or 30s. more
rent when they have a right of way to the sea, where the weed is thrown
on shore; and in the Western Isles the sea-ware, the shell-marl, and
the peat-ash, are the three great natural fertilizers to which the
agriculture of the district is indebted for the comparative prosperity
to which it has in some of the islands already attained.
Sea-weeds decompose with great ease when collected in heaps or spread
upon the land. During their decay, they yield not only organic food to
the plant but saline matters also, to which much of their efficacy both
on the grass and the corn crops is no doubt to be ascribed.
2. _Manuring with dry Vegetable Matter._—Almost every one knows that
the saw-dust of most common woods decays very slowly—so slowly, that
it is rare to meet with a practical farmer who considers it worth the
trouble of mixing with his composts. This property of slow decay is
possessed in a certain degree by all _dry_ vegetable matter. Heaps
of dry straw alone, or even mixed with earth, will ferment with
comparative difficulty and with great slowness. It is necessary,
therefore, to mix it, as is usually done, with some substance that
ferments more readily, and which will impart its own condition to the
straw. Animal matters of any kind, such as the urine and droppings of
cattle, are of this character; and it is by admixture with these that
the straw which is trodden down in the farm-yard is made to undergo a
more or less rapid fermentation.
The object of this fermentation is twofold—first, to reduce the
particles of the straw to such a minute state of division, that they
may admit of being diffused through the soil; and, second, that the dry
vegetable matter may be so changed by exposure to the air, and other
agencies, as to be fitted to yield both organic and inorganic food to
the roots of the plants it is intended to nourish.
We have seen that this decomposition takes place very speedily, and of
its own accord, when the vegetable matter is green, but that it can
be _induced_ or brought on in the case of dry straw by the agency
of animal matter. The same means will cause the fermentation of any
other vegetable substance which is in a minute state of division.
Even saw-dust made into a compost heap with soil or sods, and watered
regularly and copiously with the liquid manure of the farm-yards, may
be thus converted into a fertilizing vegetable mould.
Differences of opinion have prevailed, and discussions have taken
place, as to the relative efficacy of long and short—or of half
fermented and of fully rotten dung. But if it be added _solely_ for the
purpose of yielding food to the plant, or of preparing food for it, the
case is very simple. The more complete the state of fermentation—if not
carried too far—the more immediate will be the agency of the manure;
hence the propriety of the application of short dung to turnips and
other plants it is desirable to bring rapidly forward; but if the
manure be only half decayed, it will require time in the soil to
complete the decomposition, so that its action will be more gradual and
prolonged.
Though in the latter case the immediate action is not so perceptible,
yet the ultimate benefit to the soil, and to the crops, may be even
greater, supposing them to be such as require no special forcing
at one period of the year. With a view to this slow amelioration,
vegetable matter of any kind may be added with benefit, if in a
sufficient state of division, to the soil. Even saw-dust applied
largely to the land, has been found to improve it, though little at
first, yet more during the second year after it was applied, still more
during the third, and most of all in the fourth season after it was
mixed with the soil. That any dry vegetable matter, therefore, does not
produce an immediate effect, ought not to induce the practical farmer
to despise the application to his land—either alone, or in the form
of a compost—of every thing of the kind he can readily obtain. If his
fields are not already very rich in vegetable matter, both he and they
are likely to be ultimately benefitted by such additions to the soil.
_Rape Dust._—It is from the straw of the corn-bearing plants, or from
the stems and leaves of the grasses, that the largest portion of the
strictly vegetable manures applied to the soil is generally obtained
or prepared. But the seeds of all plants are much more enriching
than the substance of their leaves and stems. These seeds, however,
are in general too valuable for food to admit of their application
as a manure. Still the refuse of some, as that of different kinds
of rape-seed after the oil is expressed, and which is unpalatable to
cattle, is applied with great benefit to the land. Drilled in with
spring wheat, or scattered as a top-dressing in spring at the rate of 5
cwt. to an acre, it gives a largely increased and remunerating return.
It is applied with equal success to the cultivation of potatoes, and
generally it may be substituted for farm-yard manure at the rate of
about 1 cwt. of rape dust for each ton of manure.
_Malt Dust_ consists of the dried sprouts of barley, which, when the
sprouted seed is dried in the process of malting, break off and form
a coarse powder. This is found to be almost equal to rape dust in
fertilizing power.
_Charcoal Powder_ possesses the remarkable property of absorbing
noxious vapours from the air and soil, and unpleasant impurities from
water. It also sucks into its pores much oxygen from the air. Owing
to these and other properties, it is a valuable substance for mixing
with liquid manure, night-soil, farm-yard manure, ammoniacal liquor, or
other rich applications to the soil. It is even capable by itself of
yielding slow supplies of nourishment to living plants, and is said, in
many cases, without any admixture, to have been used with advantage in
practical agriculture. In moist charcoal the seeds of the gardener are
found to sprout with remarkable quickness and certainty.
_Soot_, whether from the burning of wood or of coal, is of vegetable
origin, and consists chiefly of a finely-divided charcoal, possessing
the properties above mentioned. It contains, however, ammonia and
certain other substances in small quantity, to which its well known,
and especially its _immediate_, effects upon vegetation are in part to
be ascribed.
3. _The use of Peat._—In many parts of the world, and in none more
abundantly, perhaps, than in Gt. Britain, is vegetable matter collected
in the form of peat. This ought to supply an inexhaustible store of
organic matter for the amelioration of the adjacent soils. We know that
by draining off the sour and unwholesome water, and afterwards applying
lime and clay, the surface of peat bogs may be gradually converted into
rich corn-bearing lands. It must, therefore, be possible to convert
peat itself by a similar process into a compost fitted to improve the
condition of other soils.
The late Lord Meadowbank, who made many important experiments on
this subject, found, that after being partially dried by exposure to
the air, peat might be readily fermented, and brought into the state
of a rich fertilizing compost by the same means which are adopted
in the ordinary fermenting of straw. He mixed with it a portion of
animal matter, which soon communicated its own fermenting quality to
the surrounding peat, and brought it readily in to a proper heat. He
found that one ton of hot fermenting manure, mixed in alternate layers
with two of half dry peat, and covered by the same, was sufficient to
ferment the whole; and subsequently that the vapours which rise from
naturally fermenting farm-yard manure or animal matters, would alone
produce the same effect upon peat, placed so as readily to receive and
absorb them.
As ammonia is one of the compounds specially given off by putrifying
animal substances, it is not unlikely that a watering with _ammoniacal
liquor_ would materially prepare the peat for undergoing fermentation.
At all events it seems possible to prepare any quantity of valuable
peat compost by mixing the peat with a still less quantity of fermented
manure than was employed by Lord Meadowbank, provided the liquid manure
of the farm-yard be collected in a cistern, and be thrown at intervals
by means of a pump over the prepared heaps.
One important use also to which I think peat may be applied is, after
it is partly dried, to build it into covered heaps, and half burn or
char it till it become readily reducible into a fine powder. In this
state it would be of great value as a mixture to preserve the virtues
of liquid manures of all kinds, of night-soil, and of ammoniacal liquor.
SECTION III.—RELATIVE VALUE OF DIFFERENT VEGETABLE MANURES.
There are two principles on which the relative value of different
vegetable substances, as manures, may be stated to depend—_first_, on
the relative quantity and kind of _inorganic matter_ they contain; and
_second_, on the relative proportions of _nitrogen_ present in each.
1. Valued according to the _quantity_ of inorganic matter they
contain—the worth of the several kinds of straw and hay would be
represented by the following numbers:—
Wheat straw, 70 to 360
Oat straw, 100 to 180
Hay, 100 to 200
Barley straw, 100 to 120
Pea straw, 100
Bean straw, 60 to 80
Rye straw, 50 to 70
Dry potato tops, 100
Dry turnip tops, 260
Rape, cake, 120
that is, a _ton weight_ of each of these substances, when made into
manure—provided nothing is washed out by the rains—will return to the
soil the above quantities of inorganic matter _in pounds_. Generally,
perhaps, these numbers will give the reader an idea of the relative
_permanent_ effect of these different kinds of vegetable matter when
laid upon the soil. But, by a reference to the facts stated in pp. 58
to 64, in regard to the _quality_ of the inorganic matter contained in
plants, he will satisfy himself, that the effect of these manures on
particular crops is not to be judged of solely by the absolute quantity
of earthy and saline matter they contain;—that which the turnip-top,
for example, or the bean-stalk, returns to the soil, may not be exactly
what will best promote the growth of wheat.
2. On the other hand, if the fertilizing value of vegetable substances
is to be calculated by the relative quantities of nitrogen they
severally contain, we should place them in the following order:—the
number opposite to each substance representing that weight of it in
pounds, which would produce the same effect as 100 pounds of farm-yard
manure, consisting of the mixed droppings and litter of cattle.
Equivalent
quantities
in pounds.
Farm-yard manure, 100
Wheat straw, 80 to 170
Oat straw, 150
Barley straw, 180
Buckwheat, 85
Pea straw, 45
Wheat chaff, 50
Green grass, 80
Potato tops, 75
Fresh sea-weed, 80
Rape dust, 8
Fir saw-dust, 250
Oak saw-dust, 180
Coal soot, 30
This table again presents the same substances in a somewhat different
order of value; shewing, for example, not only that such substances
as rape dust and soot should produce a much more remarkable effect
upon vegetation, than the same weight even of farm-yard manure, but
also that certain dry vegetables, such as chaff and pea straw, will
yield, when not unduly fermented, a more enriching manure than barley,
oat, or wheat straw. It agrees, also, with the known effect of green
manuring upon the land, since 80 pounds of meadow-grass ploughed in,
will be equal in virtue to 100 of farm-yard manure.
Some writers ascribe the _entire_ action of these measures to the
nitrogen they contain. This, however, is taking a one-side view of
their real natural operation. The nitrogen, during their decay, is
liberated chiefly in the form of ammonia—an evanescent substance,
producing an immediate effect in hastening or carrying further forward
the growth of the plant, but not remaining permanently in the soil. The
reader, therefore, will form an opinion consistent alike with theory
and with practice, if he conclude—
1. That the _immediate_ effect of a vegetable manure, in hastening the
growth of plants, is dependent, in a great degree, upon the quantity of
nitrogen it contains and gives off during its decay in the soil.
2. That the _permanent_ effect and value of manures is to be estimated
chiefly by the quantity and quality of the inorganic matter they
contain—of the ash they leave when burned.
The effect of the nitrogen may be nearly expended in a single
season—that of the earthy and saline matter may not be exhausted for
several years.
Nor is the carbon of vegetable substances without its important uses
to vegetation. From the statements contained in the earlier chapters
of the present work, it may be inferred that, however much influence
we may allow to the nitrogen and to the earthy matter of plants in
aiding the growth of future races—the soundest view of these important
natural operations is that which considers each element present in
decaying plants to be capable of ministering food to such as are still
alive,—though we may not be able as yet, either to estimate the precise
importance of each element to any particular kind of crop, or exactly
to adjust their relative quantities in our manures, so as to promote
the growth of such a crop in the greatest possible degree.
CHAPTER IX.
Animal Manures—Their relative value and mode of
Action—Difference between Animal and Vegetable
Manures—Cause of this difference—Mineral Manures
—Nitrates of Potash and Soda—Sulphate of Soda,
Gypsum, Chalk, and Quicklime—Chemical action of
these Manures—Artificial Manures—Burning and
Irrigation of the Soil—Planting and laying down
to grass.
The animal substances employed as manure consist chiefly of the flesh,
blood, bones, horns, and hair of animals, of fish—which in some places
are found in sufficient quantity to be laid upon the land—and of the
solid and liquid excrements of animals and birds.
SECTION I.—OF UNDIGESTED ANIMAL MANURES.
Animal substances, in general, act more powerfully as manures than
vegetable substances—it is only the seeds of plants which can at all
compare with them in efficacy.
The _flesh_ of animals is rarely used as a manure, except in the case
of dead horses, or cattle which cannot be used for food. Fish is
chiefly applied in the form of the refuse of the herring and pilchard
fisheries, though occasionally such shoals of sprats, herrings, and
even mackerel, have been caught on our shores, as to make it necessary
to employ them as manure. These recent animal substances are found to
be too _strong_ when applied directly to the land; they are generally,
therefore, made into a compost, with a large quantity of soil. Five
barrels of fish, or fish refuse, made into twenty loads of compost,
will be sufficient for an acre. The refuse of fish oils,—of the fat of
animals that has been melted for the extraction of the tallow—of skins
that have been boiled for the manufacture of glue—horns, hair, wool
(woollen rags), and all similar substances, when made into composts,
exercise, in proportion to their weight, a much greater influence upon
vegetation than any of the more abundant forms of vegetable matter.
Even the bodies of insects are in many parts of the world important
manures of the soil. In warm climates, a handful of soil sometimes
seems almost half made up of the wings and skeletons of dead
insects—the peasant in Hungary and Carinthia occasionally collects as
many as thirty cart-loads of dead marsh flies in a single year;—and in
the richer soils of France and England, where worms and other insects
abound, the presence of their remains in the soil must also aid its
natural productiveness.
_Blood_ is rarely applied to the land directly—though, like the other
parts of animals, it makes an excellent compost. As it comes from
the sugar refineries, however, in which, with lime water and animal
charcoal, it is employed for the refining of sugar, it has obtained
a very extensive employment, especially in the south of France. This
animal black, or _animalized charcoal_, as it is sometimes called,
contains about twenty per cent. of blood, and has risen to such a price
in France, that the sugar refiners actually sell it for more than the
unmixed blood and animal charcoal originally cost them. This has given
rise to the manufacture of artificial mixtures of charcoal, fecal
matters, and blood, which are also sold under the name of animalized
charcoal. The only disadvantage attending these artificial preparations
is, that they are liable to be adulterated, or, for cheapness, prepared
in a less efficient manner.
_Horn_, _hair_, and _wool_, depend for their efficacy precisely on the
same principles as the blood and flesh of animals. They differ chiefly
in this, that they are _dry_, while blood and flesh contain 80 to 90
per cent. of their weight of water. Hence, a ton of horn shavings, of
hair, or of dry woollen rags, ought to enrich the soil as much as ten
tons of blood. In consequence, however, of their dryness, the horn and
wool decompose much more slowly than the blood. Hence, the effect of
soft animal matters is more immediate and apparent, that of hard and
dry substances less visible, but continuing for a much longer period of
time.
_Bones_, again, while they resemble horn in being dry, differ from it
in containing, besides the animal matter, a large quantity of earthy
matter also, and hence they introduce a new agent to aid their effect
upon the soil. Thus, the bones of the cow consist of 100 lbs. of
Phosphate of lime, 55½
Phosphate of magnesia, 3
Soda and common salt, 3½
Carbonate of lime, 3¾
Fluoride of calcium, 1
Gelatine (the substance of _horn_), 33¼
-------
100
While 100 lbs. of bone-dust, therefore, add to the soil as much
_organic_ animal matter as 33 lbs. of horn, or as 300 or 400 lbs.
of blood or flesh, they add, at the same time, much _inorganic_
matter—lime, magnesia, soda, common salt, and phosphoric acid (in
the phosphates),—all of which, as we have seen, must be present in a
fertile soil, since the plants require a certain supply of them all
at every period of their growth. These substances, like the inorganic
matter of plants, may remain in the soil, and may exert a beneficial
action upon vegetation after all the organic or gelatinous matter has
decayed and disappeared.
From what is above stated, therefore, the reader will gather these
general conclusions:
1. That animal substances which, like flesh and blood, contain much
water, decay rapidly, and are fitted to operate _immediately_ and
powerfully upon vegetation, but are only temporary or evanescent in
their action.
2. That when dry, as in horn, hair, and wool, they decompose, and
consequently act more slowly, and continue to manifest an influence, it
may be, for several seasons.
3. That bones, acting like horn, in so far as their animal matter
is concerned, and, like it, for a number of seasons, more or less,
according as they have been more or less finely crushed—may ameliorate
the soil by their earthy matter for a still longer period—permanently
improving the condition and adding to the natural capabilities of the
land.