Produced by Steven Giacomelli, Jeannie Howse, Josephine
Paolucci and the Online Distributed Proofreading Team at
http://www.pgdp.net. (This file was produced from images
produced by Core Historical Literature in Agriculture
(CHLA), Cornell University).
ELEMENTS
OF
AGRICULTURAL CHEMISTRY
BY
THOMAS ANDERSON, M.D.
F.R.S.E., F.C.S.
PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF GLASGOW, AND CHEMIST TO THE
HIGHLAND AND AGRICULTURAL SOCIETY OF SCOTLAND.
EDINBURGH:
ADAM AND CHARLES BLACK.
1860.
ERRATUM.
Page 190, line 11, for "gallon" read "ton."
PRINTED BY R. AND R. CLARK, EDINBURGH.
Transcriber's note: Many of the tables needed to be split to fit space
constraints. Minor typos have been corrected and footnotes moved to the
end of the chapters. A word surrounded by underscores like _this_
signifies the word is italics in the text. For numbers and equations,
underscores before bracketed numbers in equations denote a subscript.
PREFACE.
The object of the present work is to offer to the farmer a concise
outline of the general principles of Agricultural Chemistry. It has no
pretensions to be considered a complete treatise on the subject. On the
contrary, its aim is strictly elementary, and with this view I have
endeavoured, as far as possible, to avoid unnecessary technicalities so
as to make it intelligible to those who are unacquainted with the
details of chemical science, although I have not hesitated to discuss
such points as appeared essential to the proper understanding of any
particular subject.
The rapid progress of agricultural chemistry, and the numerous
researches prosecuted under the auspices of agricultural societies and
private experimenters in this and other countries, render it by no means
an easy task to make a proper selection from the mass of facts which is
being daily accumulated. In doing this, however, I have been guided by a
pretty intimate knowledge of the wants of the farmer, which has induced
me to enlarge on those departments of the subject which bear more
immediately on the every-day practice of agriculture; and for this
reason the composition and properties of soils, the nature of manures,
and the principles by which their application ought to be governed, have
been somewhat minutely treated.
In all cases numerical details have been given as fully as is consistent
with the limits of the work; and it may be right to state that a
considerable number of the analyses contained in it have been made in my
own laboratory, and that even when I have preferred to quote the results
of other chemists, they have not unfrequently been confirmed by my own
experiments.
UNIVERSITY OF GLASGOW,
_1st November 1860._
CONTENTS.
Page
INTRODUCTION 1
CHAPTER I.
THE ORGANIC CONSTITUENTS OF PLANTS.
Carbon ... Carbonic Acid ... Hydrogen ... Nitrogen ... Nitric Acid ...
Ammonia ... Oxygen ... Sources whence obtained ... The Atmosphere ...
The Soil ... Source of the Inorganic Constituents of Plants ... Manner
in which the Constituents of Plants are absorbed 8
CHAPTER II.
THE PROXIMATE CONSTITUENTS OF PLANTS.
The Saccharine and Amylaceous Constituents ... Cellulose ... Incrusting
Matter ... Starch ... Lichen Starch ... Inuline ... Gum ... Dextrine ...
Sugar ... Mucilage ... Pectine and Pectic Acid ... Oily or Fatty Matters
... Margaric, Stearic, and Oleic Acids ... Wax ... Nitrogenous or
Albuminous Constituents of Plants and Animals ... Albumen ... Fibrine
... Casein ... Diastase 40
CHAPTER III.
THE CHANGES WHICH TAKE PLACE IN THE FOOD OF
PLANTS DURING THEIR GROWTH.
Changes occurring during Germination ... Changes during the After-Growth
of the Plant ... Decomposition of Carbonic Acid ... Decomposition of
Water ... Decomposition of Ammonia ... Decomposition of Nitric Acid 54
CHAPTER IV.
THE INORGANIC CONSTITUENTS OF PLANTS.
The Amount of Inorganic Matters in Different Plants ... The Relative
Proportions of Ash in the Different Parts of Plants ... Influence of the
Nature of the Soil on the Proportion of Mineral Matters in the Plant ...
The Composition of the Ashes of Plants ... Classification of Different
Plants 63
CHAPTER V.
THE SOIL--ITS CHEMICAL AND PHYSICAL CHARACTERS.
The Origin of Soils ... Composition of Crystalline and Sedimentary Rocks
... their Disintegration ... Chemical Composition of the Soil ...
Fertile and Barren Soils ... Mechanical Texture of Soils ... Absorbent
Action of Soils ... their Physical Characters ... Relation to Heat and
Moisture ... The Subsoil ... Classification of Soils 83
CHAPTER VI.
THE IMPROVEMENT OF THE SOIL BY MECHANICAL
PROCESSES.
Draining ... Its Advantageous Effects ... Subsoil and Deep Ploughing
... Improving the Soil by Paring and Burning ... Warping
... Mixing of Soils ... Chalking 137
CHAPTER VII.
THE GENERAL PRINCIPLES OF MANURING.
Fundamental Principles upon which Manures are applied ... _Special_ and
_General_ Manures ... Importance of this distinction ... Views regarding
the Theory of Manures ... Remarks on Special Manures ... Action of
Manures on the Chemical and Physical Properties of a Soil ... Remarks on
the Application of Manures 152
CHAPTER VIII.
THE COMPOSITION AND PROPERTIES OF FARM-YARD
AND LIQUID MANURES.
Farm-yard Manure ... Urine ... Composition of ... Dung ... Composition
of ... Farm-yard Manure ... Composition of ... Management of Dung-Heaps
... Box-feeding ... Fermentation and application of Manure ... Liquid
Manure ... Composition and application of ... Sewage Manure ... Its
composition and application 166
CHAPTER IX.
THE COMPOSITION AND PROPERTIES OF VEGETABLE
MANURES.
Rape-Dust, Mustard, Cotton and Castor Cake ... Composition of various
Oil-Cakes ... Malt-Dust, Bran, Chaff, etc. ... Straw and Saw-dust ...
Manuring with Fresh Vegetable Matter ... Green Manuring ... Sea-Weed ...
Composition of various Sea-Weeds ... Leaves ... Peat 195
CHAPTER X.
THE COMPOSITION AND PROPERTIES OF ANIMAL
MANURES.
Guano, different varieties of ... Average composition of ... Division
into Ammoniacal and Phosphatic ... Characters of ... Adulteration of ...
Application of ... Pigeons' Dung ... Urate and Sulphated Urine ...
Night-Soil and Poudrette ... Hair, Skin, Horn, Wool, etc. ... Blood ...
Fish ... "Fish-Guano"--Bones 204
CHAPTER XI.
THE COMPOSITION AND PROPERTIES OF MINERAL
MANURES.
Mineral Manures ... Sulphate and Muriate of Ammonia ... Sulphomuriate
of Ammonia ... Ammoniacal Liquor ... Nitrates of Potash and Soda ...
Muriate and Sulphate of Potash ... Chloride of Sodium, or Common Salt
... Carbonates of Potash and Soda ... Silicates of Potash and Soda ...
Sulphate of Magnesia ... Phosphate of Lime ... Bone-ash ... Coprolites
... Apatite ... Sombrero Guano ... Superphosphates and Dissolved Bones
... Biphosphate of Lime or Soluble Phosphates ... Phospho-Peruvian Guano
... Lime ... Chalk ... Marl ... Application and Action of Lime on Soils
... Sulphate of Lime or Gypsum 226
CHAPTER XII.
THE VALUATION OF MANURES.
The Principle on which Manures are valued ... Its application to
different simple and complex Manures ... Method of Calculation ...
General Remarks 255
CHAPTER XIII.
THE ROTATION OF CROPS.
Its necessity explained ... Quantity of Mineral Matters in the produce
of an Acre of Different Crops ... The Theory of Rotation 266
CHAPTER XIV.
THE FEEDING OF FARM STOCK.
The Principles of Feeding ... The Composition of different Animals in
different stages of Fattening ... The Composition of the Food of Animals
... Milk ... The Principal Varieties of Cattle Food ... General
Observations on Feeding 276
AGRICULTURAL CHEMISTRY.
INTRODUCTION.
That the phenomena of vegetation are dependent on certain chemical
changes occurring in the plant, by which the various elements of its
food are elaborated and converted into vegetable matter, was very early
recognised by chemists; and long before the correct principles of that
science were established, Van Helmont maintained that plants derived
their nourishment from water, while Sir Kenelm Digby, Hook, Bradley, and
others, attributed an equally exclusive influence to air, and enlarged
on the practical importance of the conclusions to be deduced from their
views. These opinions, which were little better than hypotheses, and
founded on very imperfect chemical data, are mentioned by Jethro Tull,
the father of modern agriculture, only to deny their accuracy; and he
contended that the plants absorb and digest the finer particles of the
earth, and attributed the success of the particular system of husbandry
he advocated to the comminution of the soil, by which a larger number
of its particles are rendered sufficiently small to permit their ready
absorption by the roots. Popular opinion at that time was in favour of
the mechanical rather than the chemical explanation of agricultural
facts, and Tull's work had the effect of confirming this opinion, and
turning attention away from the application of chemistry to agriculture.
Indeed, no good results could have followed its study at that time, for
chemistry, especially in those departments bearing more immediately on
agriculture, was much too imperfect, and it was only towards the close
of the last century, when Lavoisier established its true principles,
that it became possible to pursue it with any prospect of success.
Very soon after Lavoisier's system was made known, Lord Dundonald
published his "Treatise on the Intimate Connexion between Chemistry and
Agriculture," in which the important bearings of the recent chemical
discoveries on the practice of agriculture were brought prominently
under the notice of the farmer, and almost at the same time De Saussure
commenced those remarkable researches, which extended over a long series
of years, and laid the foundation of almost all our accurate knowledge
of the chemistry of vegetation. Saussure traced with singular care and
accuracy the whole phenomena of the life of plants, and indicated the
mode in which the facts he established might be taken advantage of in
improving the cultivation of the soil. But neither his researches, nor
Lord Dundonald's more direct appeal to the farmer, excited the attention
they deserved, or produced any immediate effect on the progress of
agriculture. It was not till the year 1812 that the interest of
practical men was fairly awakened by a course of lectures given by Sir
Humphrey Davy, at the instance of Sir John Sinclair, who was at that
time president of the Board of Agriculture. In these lectures, written
with all the clearness and precision which characterised their author's
style, the results of De Saussure's experiments were for the first time
presented to the farmer in a form in which they could be easily
understood by him, the conclusions to which they led were distinctly
indicated, and a number of useful practical suggestions made, many of
which have been adopted into every-day practice, and become so
thoroughly incorporated with it, that their scientific origin has been
altogether forgotten. A lively interest was excited by the publication
of Davy's work, but it soon died out, and the subject lay in almost
complete abeyance for a considerable number of years. Nor could any
other result be well expected, for at that time agriculture was not ripe
for chemistry, nor chemistry ripe for agriculture. The necessities of a
rapidly increasing population had not yet begun to compel the farmer to
use every means adapted to increase the amount of production to its
utmost limit; and though the fundamental principles of chemistry had
been established, its details, especially in that department which
treats of the constituents of plants and animals, were very imperfectly
known. It is not surprising, therefore, that matters should have
remained almost unchanged for the comparatively long period of nearly
thirty years. Indeed, with the exception of the investigation of soils
by Schuebler, and some other inquiries of minor importance, and which, in
this country at least, excited no attention on the part of the
agriculturist, nothing was done until the year 1840, when Liebig
published his treatise on _Chemistry, in its application to Agriculture
and Physiology_.
Saussure's researches formed the main groundwork of Liebig's treatise,
as they had before done for Davy's; but the progress of science had
supplied many new facts which confirmed the opinions of the older
chemists in most respects, and enabled Liebig to generalise with greater
confidence, and illustrate more fully the principles upon which
chemistry ought to be applied to agriculture. Few works have ever
produced a more profound impression. Written in a clear and forcible
style, dealing with scientific truths in a bold and original manner, and
producing a strong impression, as well by its earnestness as by the
importance of its conclusions, it was received by the agricultural
public with the full conviction that the application of its principles
was to be immediately followed by the production of immensely increased
crops, and by a rapid advance in every branch of practical agriculture.
The disappointment of these extravagant expectations, which _chemists_
themselves foresaw, and for which they vainly attempted to prepare the
agriculturist, was followed by an equally rapid reaction; and those who
had embraced Liebig's views, and lauded them as the commencement of a
new era, but who had absurdly expected an instantaneous effect, changed
their opinion, and contemned, as strongly as they had before supported,
the application of chemistry to agriculture.
That this effect should have been produced is not unnatural; for
practical men, having at that time little or no knowledge of chemistry,
were necessarily unable to estimate its true position in relation to
agriculture, and forgetting that this department of science was still in
its early youth, and burthened with all the faults and errors of youth,
they treated it as if it were already perfect in all its parts. Neither
could they distinguish between the fully demonstrated scientific truths,
and the uncertain, though probable conclusions deduced from them; and
when the latter, as occasionally happened, proved to be at variance with
practice, it is not surprising: that this should have produced a feeling
of distrust on the part of persons incapable, from an imperfect, and
still oftener from no knowledge of science, of drawing the line of
demarcation, which Liebig frequently omitted to do, between the positive
fact and the hypothetical inference, which, however probable, is, after
all, merely a suggestion requiring to be substantiated by experiment.
This omission, which the scientific reader can supply for himself,
becomes a source of serious misapprehension in a work addressed to
persons unacquainted with science, who adopt indiscriminately both the
facts and the hypotheses of the author. And this is no doubt the cause
of the vary different estimation in which the work of the Giessen
Professor was held by scientific and practical men.
Liebig's treatise was followed, in the year 1844, by the publication of
Boussingault's _Economic Rurale_, a work winch excited at the time
infinitely less interest than Liebig's, although it is really quite as
important a contribution to scientific agriculture. It is distinguished
by entering more fully into the special details of the application of
chemistry to agriculture, and contains the results of the author's
numerous searches both in the laboratory and the field. Boussingault
possesses the qualification, at present somewhat rare, of combining a
thorough knowledge of practical agriculture with extended scientific
attainments; and his investigations, which have been made with direct
reference to practice, and their results tested in the field, are the
largest and most valuable contribution to the exact data of scientific
agriculture which has yet been made public.
The year 1844 was also distinguished by the foundation of the
Agricultural Chemistry Association of Scotland, an event of no small
importance in the history of scientific agriculture. That association
was instituted through the exertions of a small number of practical
farmers, for the purpose of pursuing investigations in agricultural
chemistry, and affording to its members assistance in all matters
connected with the cultivation of the soil, and has formed the model of
similar establishments in London, Dublin, and Belfast, as well as in
Germany; and it is peculiarly creditable to the intelligence and energy
of the practical farmers of Scotland, that with them commenced a
movement, which has already found imitators in so many quarters, and
conferred such great benefits on agriculture. Within the last ten or
twelve years, and mainly owing to the establishment of agricultural
laboratories, great progress has been made in accumulating facts on
which to found an accurate knowledge of the principles of agricultural
chemistry, and the number of chemists who have devoted themselves to
this subject has considerably increased, though still greatly less than
its exigencies require.
Notwithstanding all that has recently been done, it must not be
forgotten that we have scarcely advanced beyond the threshold, and that
it is only by numerous and frequently repeated experiments that it is
possible to arrive at satisfactory results. Agricultural inquiries are
liable to peculiar fallacies due to the perturbing influence of climate,
season, and many other causes, the individual effects of which can only
be eliminated with difficulty, and much error has been introduced, by
hastily generalising from single experiments, in place of awaiting the
results of repeated trials. Hence it is that the progress of scientific
agriculture must necessarily be slow and gradual, and is not likely to
be marked by any great or startling discoveries. Now that the relations
of science to practice are better understood, the extravagant
expectations at one time entertained have been abandoned, and, as a
necessary consequence, the interest in agricultural chemistry has again
increased, and the conviction daily gains ground that no one who wishes
to farm with success, can afford to be without some knowledge of the
scientific principles of his art.
CHAPTER I.
THE ORGANIC CONSTITUENTS OF PLANTS.
When the water naturally existing in plants is expelled by exposure to
the air or a gentle heat, the residual dry matter is found to be
composed of a considerable number of different substances, which have
been divided into two great classes, called the organic and the
inorganic, or mineral constituents of plants. The former are readily
combustible, and on the application of heat, catch fire, and are
entirely consumed, leaving the inorganic matters in the form of a white
residuum or ash. All plants contain both classes of substances; and
though their relative proportions vary within very wide limits, the
former always greatly exceed the latter, which in many cases form only a
very minute proportion of the whole weight of the plant. Owing to the
great preponderance of the organic or combustible matters, it was at one
time believed that the inorganic substances formed no part of the true
structure of plants, and consisted only of a small portion of the
mineral matters of the soil, which had been absorbed along with their
organic food; but this opinion, which probably was never universally
entertained, is now entirely abandoned, and it is no longer doubted that
both classes of substances are equally essential to their existence.
Although they form so large a proportion of the plant, its organic
constituents are composed of no more than four elements, viz.:--
Carbon.
Hydrogen.
Nitrogen.
Oxygen.
The inorganic constituents are much more numerous, not less than
thirteen substances, which appear to be essential, having been observed.
These are--
Potash.
Soda.
Lime.
Magnesia.
Peroxide of Iron.
Silicic Acid.
Phosphoric Acid.
Sulphuric Acid.
Chlorine.
And more rarely
Manganese.
Iodine.
Bromine.
Fluorine.
Several other substances, among which may be mentioned alumina and
copper, have also been enumerated; but there is every reason to believe
that they are not essential, and the cases in which they have been found
are quite exceptional.
It is to be especially noticed that none of these substances occur in
plants in the free or uncombined state, but always in the form of
compounds of greater or less complexity, and extremely varied both in
their properties and composition.
It would be out of place, in a work like the present, to enter into
complete details of the properties of the elements of which plants are
composed, which belongs strictly to pure chemistry, but it is necessary
to premise a few observations regarding the organic elements, and their
more important compounds.
_Carbon._--When a piece of wood is heated in a close vessel, it is
charred, and converted into charcoal. This charcoal is the most familiar
form of carbon, but it is not absolutely pure, as it necessarily
contains the ash of the wood from which it was made. In its purest form
it occurs in the diamond, which is believed to be produced by the
decomposition of vegetable matters, and it is there crystallized and
remarkably transparent; but when produced by artificial processes,
carbon is always black, more or less porous, and soils the fingers. It
is insoluble in water, burns readily, and is converted into carbonic
acid. Carbon is the largest constituent of plants, and forms, in round
numbers, about 50 per cent of their weight when dry.
_Carbonic Acid._--This, the most important compound of carbon and
oxygen, is best obtained by pouring a strong acid upon chalk or
limestone, when it escapes with effervescence. It is a colourless gas,
extinguishing flame, incapable of supporting respiration, much heavier
than atmospheric air, and slightly soluble in water, which takes up its
own volume of the gas. It is produced abundantly when vegetable matters
are burnt, as also during respiration, fermentation, and many other
processes. It is likewise formed daring the decay of animal and
vegetable matters, and is consequently evolved from dung and compost
heaps.
_Hydrogen_ occurs in nature only in combination. Its principal compound
is water, from which it is separated by the simultaneous action of an
acid, such as sulphuric acid and a metal, in the form of a transparent
gas, lighter than any other substance. It is very combustible, burns
with a pale blue flame, and is converted into water. It is found in all
plants, although in comparatively small quantity, for, when dry, they
rarely contain more than four or five per cent. Its most important
compound is water, of which it forms one-ninth, the other eight-ninths
consisting of oxygen.
_Nitrogen_ exists abundantly in the atmosphere, of which it forms nearly
four-fifths, or, more exactly, 79 per cent. It is there mixed, but not
combined with oxygen; and when the latter gas is removed, by introducing
into a bottle of air some substance for which the former has an
affinity, the nitrogen is left in a state of purity. It is a transparent
gas, which is incombustible and extinguishes flame. It is a singularly
inert substance, and is incapable of directly entering into union with
any other element except oxygen, and with that it combines with the
greatest difficulty, and only by the action of the electric spark--a
peculiarity which has very important bearings on many points we shall
afterwards have to discuss. Nitrogen is found in plants to the extent of
from 1 to 4 per cent.
_Nitric Acid._--This, the most important compound of nitrogen and
oxygen, can be produced by sending a current of electric sparks through
a mixture of its constituents, but in this way it can be obtained only
in extremely small quantity. It is much more abundantly produced when
organic matters are decomposed with free access of air, in which case
the greater proportion of their nitrogen combines with the atmospheric
oxygen. This process, which is known by the name of nitrification, is
greatly promoted by the presence of lime or some other substance, with
which the nitric acid may combine in proportion as it is formed. It
takes place, to a great extent, in the soil in India and other hot
climates; and our chief supplies of saltpetre, or nitrate of potash, are
derived from the soil in these countries, where it has been formed in
this manner. The same change occurs, though to a much smaller extent, in
the soil in temperate climates.
_Ammonia_ is a compound of nitrogen and hydrogen, but it cannot be
formed by the direct union of these gases. It is a product of the
decomposition of organic substances containing nitrogen, and is produced
when they are distilled at a high temperature, or allowed to putrefy out
of contact of the air. In its pure state it is a transparent and
colourless gas, having a peculiar pungent smell, and highly soluble in
water. It is an alkali resembling potash and soda, and, like these
substances, unites with the acids and forms salts, of which the sulphate
and muriate are the most familiar. In these salts it is fixed, and does
not escape from them unless they be mixed with lime, or some other
substance possessing a more powerful affinity for the acid with which it
is united.
_Oxygen_ is one of the most widely distributed of all the elements, and,
owing to its powerful affinities, is the most important agent in almost
all natural changes. It is found in the air, of which it forms 21 per
cent, and in combination with hydrogen, and almost all the other
chemical elements. In the pure state it possesses very remarkable
properties. All substances burn in it with greater brilliancy than they
do in atmospheric air, and its affinity for most of the elements is
extremely powerful. When diluted with nitrogen, it supports the
respiration of animals; but in the pure state it proves fatal after the
lapse of an hour or two. It is found in plants, in quantities varying
from 30 to 36 per cent.
It is worthy of observation, that of the four organic elements, carbon
only is fixed, and the other three are gases; and likewise, when any two
of them unite, their compound is either a gaseous or a volatile
substance. The charring of organic substances, which is one of their
most characteristic properties, and constantly made use of by chemists
as a distinctive reaction, is due to this peculiarity; for when they are
heated, a simpler arrangement of their particles takes place, the
hydrogen, nitrogen, and oxygen unite among themselves, and carry off a
small quantity of carbon, while the remainder is left behind in the form
of charcoal, and is only consumed when access of the external air is
permitted.
Now, in order that a plant may grow, its four organic constituents must
be absorbed by it, and that this absorption may take place, it is
essential that they be presented to it in suitable forms. A seed may be
planted in pure carbon, and supplied with unlimited quantities of
hydrogen, nitrogen, oxygen, and inorganic substances, and it will not
germinate; and a plant, when placed in similar circumstances, shows no
disposition to increase, but rapidly languishes and dies. The obvious
inference from these facts is, that these substances cannot be absorbed
when in the _elementary_ state, but that it is only after they have
entered into certain forms of combination that they acquire the property
of being readily taken up, and assimilated by the organs of the plant.
It was at one time believed that many different compounds of these
elements might be absorbed and elaborated, but later and more accurate
experiments have reduced the number to four--namely, carbonic acid,
water, ammonia, and nitric acid. The first supplies carbon, the second
hydrogen, the two last nitrogen, while all of them, with the exception
of ammonia, may supply the plant with oxygen as well as with that
element of which it is the particular source.
There are only two sources from which these substances can be obtained
by the plant, viz. the atmosphere and the soil, and it is necessary that
we should here consider the mode in which they may be obtained from
each.
_The Atmosphere as a source of the Organic Constituents of
Plants._--Atmospheric air consists of a mixture of nitrogen and oxygen
gases, watery vapour, carbonic acid, ammonia, and nitric acid. The two
first are the largest constituents, and the others, though equally
essential, are present in small, and some of them in extremely minute
quantity. When deprived of moisture and its minor constituents, 100
volumes of air are found to contain 21 of oxygen and 79 of nitrogen.
Although these gases are not chemically combined in the air, but only
mechanically mixed, their proportion is exceedingly uniform, for
analyses completely corresponding with these numbers have been made by
Humboldt, Gay-Lussac, and Dumas at Paris, by Saussure at Geneva, and by
Lewy at Copenhagen; and similar results have also been obtained from air
collected by Gay-Lussac during his ascent in a balloon at the height of
21,430 feet, and by Humboldt on the mountain of Antisano in South
America at a height of 16,640 feet. In short, under all circumstances,
and in all places, the relation subsisting between the oxygen and
nitrogen is constant; and though, no doubt, many local circumstances
exist which may tend to modify their proportions, these are so slow and
partial in their operations, and so counterbalanced by others acting in
an opposite direction, as to retain a uniform proportion between the
main constituents of the atmosphere, and to prevent the undue
accumulation of one or other of them at any one point.
No such uniformity exists in the proportion of the minor constituents.
The variation in the quantity of watery vapour is a familiar fact, the
difference between a dry and moist atmosphere being known to the most
careless observer, and the proportions of the other constituents are
also liable to considerable variations.
_Carbonic Acid._--The proportion of carbonic acid in the air has been
investigated by Saussure. From his experiments, made at the village of
Chambeisy, near Geneva, it appears that the quantity is not constant,
but varies from 3.15 to 5.75 volumes in 10,000; the mean being 4.15.
These variations are dependent on different circumstances. It was found
that the carbonic acid was always more abundant during the night than
during the day--the mean quantity in the former case being 4.32, in the
latter 3.38. The largest quantity found during the night was 5.74,
during the day 5.4. Heavy and continued rain diminishes the quantity of
carbonic acid, by dissolving and carrying it down into the soil.
Saussure found that in the month of July 1827, during the time when nine
millimetres of rain fell, the average quantity of carbonic acid amounted
to 5.18 volumes in 10,000; while in September 1829, when 254 millimetres
fell, it was only 3.57. A moist state of the soil, which is favourable
to the absorption of carbonic acid, also diminishes the quantity
contained in the air, while, on the other hand, continued frosts, by
retaining the atmosphere and soil in a dry state, have an opposite
effect. High winds increase the carbonic acid to a small extent. It was
also found to be greater over the cultivated lands than over the lake of
Geneva; at the tops of mountains than at the level of the sea; in towns
than in the country. The differences observed in all these cases, though
small, are quite distinct, and have been confirmed by subsequent
experimenters.
_Ammonia._--The presence of ammonia in the atmosphere appears to have
been first observed by Saussure, who found that when the sulphate of
alumina is exposed to the air, it is gradually converted into the double
sulphate of alumina and ammonia. Liebig more recently showed that
ammonia can always be detected in rain and snow water, and it could not
be doubted that it had been absorbed from the atmosphere. Experiments
have since been made by different observers with the view of determining
the quantity of atmospheric ammonia, and their results are contained in
the subjoined table, which gives the quantity found in a million parts
of air.
Kemp 3.6800
{ 12 feet above the surface 3.5000
Pierre { 25 feet do. do. 0.5000
Graeger 0.3230
{ By day 0.0980
Fresenius { By night 0.1690
{ { Maximum 0.0317
{ In Paris { Minimum 0.0177
{ { Mean 0.0237
Ville {
{ { Maximum 0.0276
{ Environs { Minimum 0.0165
{ of Paris { Mean 0.0210
Of these results, the earlier ones of Kemp, Pierre, and Graeger are
undoubtedly erroneous, as they were made without those precautions which
subsequent experience has shown to be necessary. Even those of the other
observers must be taken as giving only a very general idea of the
quantity of ammonia in the air, for a proportion so minute as one
fifty-millionth cannot be accurately determined even by the most
delicate experiments. For this reason, more recent experimenters have
endeavoured to arrive at conclusions bearing more immediately upon
agricultural questions, by determining the quantity of ammonia brought
down by the rain. The first observations on this subject were made by
Barral in 1851, and they have been repeated during the years 1855 and
1856 by Mr. Way. In 1853, Boussingault also made numerous experiments
on the quantity of ammonia in the rain falling at different places, as
well as in dew and the moisture of fogs. He found in the imperial
gallon--
Grs.
Rain { Paris 0.2100
{ Liebfrauenberg 0.0350
Dew, Liebfrauenberg { Maximum 0.4340
{ Minimum 0.0714
{ Liebfrauenberg 0.1790
Fog { Paris 9.6000
It thus appears that in Paris the quantity of ammonia in rain-water is
just six times as great as it is in the country, a result, no doubt, due
to the ammonia evolved during the combustion of fuel, and to animal
exhalations, and to the same cause, the large quantity contained in the
moisture of fogs in Paris may also be attributed. Barral and Way have
made determinations of the quantity of ammonia carried down by the rain
in each month of the year, the former using for this purpose the water
collected in the rain-gauges of the Paris Observatory, and representing,
therefore, a town atmosphere; the latter, that from a large rain-gauge
at Rothamsted, at a distance from any town. According to Barral the
ammonia annually deposited on an acre of land amounts to 12.28 lbs., a
quantity considerably exceeding that obtained by Way, whose experiments
being made at a distance from towns, must be considered as representing
more accurately the normal condition of the air. His results for the
years 1855 and 1856 are given below, along with the quantities of nitric
acid found at the same time.
_Nitric Acid._--The presence of nitric acid in the air appears to have
been first observed by Priestley at the end of the last century, but
Liebig, in 1825, showed that it was always to be found after
thunder-storms, although he failed to detect it at other times. In 1851
Barral proved that it is invariably present in rain-water, and stated
the quantity annually carried down to an acre of land at no less than
41.29 lbs. But at the time his experiments were made, the methods of
determining very minute quantities of nitric acid were exceedingly
defective, and Way, by the adoption of an improved process, has shown
that the quantity is very much smaller than Barral supposed, and really
falls short of three pounds. His results for ammonia, as well as nitric
acid, are given in the subjoined table.
---------------------+----------------+----------------+-----------------+
| | Nitric Acid in | Ammonia in | Total Nitrogen |
| | Grains. | Grains. | in Grains. |
| +-------+--------+-------+--------+--------+--------+
| | 1855. | 1856. | 1855. | 1856. | 1855. | 1856. |
+--------------------+-------+--------+-------+--------+--------+--------+
|January | 230 | 1564 | 1244 | 5,005 | 1084 | 4,526 |
|February | 944 | 544 | 2337 | 4,175 | 2169 | 3,579 |
|March | 1102 | 866 | 4513 | 2,108 | 3995 | 1,945 |
|April | 325 | 1063 | 1141 | 8,614 | 1024 | 7,369 |
|May | 1840 | 3024 | 4206 | 18,313 | 3939 | 15,863 |
|June | 3303 | 2046 | 5574 | 4,870 | 5447 | 4,540 |
|July | 2680 | 1191 | 9620 | 2,869 | 8615 | 2,670 |
|August | 3577 | 2125 | 4769 | 4,214 | 4870 | 4,021 |
|September | 732 | 1756 | 3313 | 5,972 | 2917 | 5,373 |
|October | 4480 | 2075 | 7592 | 3,921 | 7414 | 3,767 |
|November | 1007 | 1371 | 3021 | 2,591 | 2749 | 2,489 |
|December | 664 | 2035 | 2438 | 4,070 | 2180 | 3,352 |
+--------------------+-------+--------+-------+--------+--------+--------+
|Total in pounds for}| | | | | | |
|the whole year }| 2.98 | .280 | 7.11 | 9.53 | 6.63 | 8.31 |
+--------------------+-------+--------+----------------+--------+--------+
No attempts have been made to determine the proportion of nitric acid in
air, but its quantity is undoubtedly excessively minute, and materially
smaller than that of ammonia. At least this conclusion seems to be a
fair inference from Way's researches, as well as the recent experiments
of Boussingault on the proportion of nitric acid contained in rain, dew,
and fog, made in a manner exactly similar to those on the ammonia,
already quoted. According to his experiments an imperial gallon
contains--
Grs.
Rain. {Paris 0.0708
{Liebfrauenberg 0.0140
Dew. {Maximum 0.0785
{Minimum 0.0030
Fog. {Paris 0.7092
{Liebfrauenberg 0.0718
Although it thus appears that Barral's results have been only partially
confirmed, enough has been ascertained to show that the quantity of
ammonia and nitric acid in the air is sufficient to produce a material
influence in the growth of plants. The large amount of these substances
contained in the dew is also particularly worthy of notice, and may
serve to some extent to explain its remarkably invigorating effect on
vegetation.
_Carburetted Hydrogen._--Gay-Lussac, Humboldt, and Boussingault have
shown, that when the whole of the moisture and carbonic acid have been
removed from the air, it still contains a small quantity of carbon and
hydrogen; and Saussure has rendered it probable that they exist in a
state of combination as carburetted hydrogen gas. No definite proof of
this position has, however, as yet been adduced, and the function of the
compound is entirely unknown. It is possible that the presence of carbon
and hydrogen may be due to a small quantity of organic matter; but,
whatever be its source, its amount is certainly extremely small.
_Sulphuretted Hydrogen and Phosphuretted Hydrogen._--The proportion of
these substances is almost infinitesimal; but they are pretty general
constituents of the atmosphere, and are apparently derived from the
decomposition of animal and vegetable matters.
The preceding statements lead to the important conclusion, that the
atmosphere is capable of affording an abundant supply of all the organic
elements of plants, because it not only contains nitrogen and oxygen in
the free state, but also in those forms of combination in which they are
most readily absorbed, as well as a large quantity of carbonic acid,
from which their carbon may be derived. At first sight it may indeed
appear that the quantity of the latter compound, and still more that of
ammonia, is so trifling as to be of little practical importance. But a
very simple calculation serves to show that, though relatively small,
they are absolutely large, for the carbonic acid contained in the whole
atmosphere amounts in round numbers to
2,400,000,000,000 tons,
and the ammonia, assuming it not to exceed one part in fifty millions,
must weigh
74,000,000 tons,
quantities amply sufficient to afford an abundant supply of these
elements to the whole vegetation of our globe.
_The Soil as a Source of the Organic Constituents of Plants._--When a
portion of soil is subjected to heat, it is found that it, like the
plant, consists of a combustible and an incombustible part; but while in
the plant the incombustible part or ash is small, and the combustible
large, these proportions are reversed in the soil, which consists
chiefly of inorganic or mineral matters, mixed with a quantity of
combustible or organic substances, rarely exceeding 8 or 10 per cent,
and often falling considerably short of this quantity.
The organic matter exists in the form of a substance called humus, which
must be considered here as a source of the organic constituents of
plants, independently of the general composition of the soil, which will
be afterwards discussed.
The term _humus_ is generic, and applied by chemists to a rather
numerous group of substances, very closely allied in their properties,
several of which are generally present in all fertile soils. They have
been submitted to examination by various chemists, but by none more
accurately than by Mulder and Herman, to whom, indeed, we owe almost all
the precise information we possess on the subject. The organic matters
of the soil may be divided into three great classes; the first
containing those substances which are soluble in water; the second,
those extracted by means of caustic potash; and the third, those
insoluble in all menstrua. When a soil is boiled with a solution of
caustic potash, a deep brown fluid is obtained, from which acids
precipitate a dark brown flocculent substance, consisting of a mixture
of at least three different acids, to which the names of humic, ulmic,
and geic acids have been applied. The fluid from which they have been
precipitated contains two substances, crenic and apocrenic acids, while
the soil still retains what has been called insoluble humus.
The acids above named do not differ greatly in chemical characters, but
they have been subdivided into the humic, geic, and crenic groups, which
present some differences in properties and composition. They are
compounds of carbon, hydrogen, and oxygen, and are characterised by so
powerful an affinity for ammonia that they are with difficulty obtained
free from that substance, and generally exist in the soil in combination
with it. They are all products of the decomposition of vegetable
matters in the soil, and are formed during their decay by a succession
of changes, which may be easily traced by observing the course of events
when a piece of wood or any other vegetable substance is exposed for a
length of time to air and moisture. It is then found gradually to
disintegrate with the evolution of carbonic acid, acquiring first a
brown and finally a black colour. At one particular stage of the process
it is converted into one or other of two substances, called humin and
ulmin, both insoluble in alkalies, and apparently identical with the
insoluble humus of the soil; but when the decomposition is more advanced
the products become soluble in alkalies, and then contain humic, ulmic,
and geic acids, and finally, by a still further progress, crenic and
apocrenic acids are formed as the result of an oxidation occurring at
certain periods of the decay.
The roots and other vegetable debris remaining in the soil undergo a
similar series of changes, and form the humus, which is found only in
the surface soil, that is to say, in the portion which is now or has at
some previous period been occupied by plants, and the quantity of humus
contained in any soil is mainly dependent on the activity of vegetation
on it. Numerous analyses of humus compounds extracted from the soil have
been made, and have served to establish a number of minor differences in
the composition even of those to which the same name has been applied,
due manifestly to the fact that their production is the result of a
gradual decomposition, which renders it impossible to extract from the
soil one pure substance, but only a variable mixture of several, so
similar to one another in properties, that their separation is very
difficult, if not impossible. For this reason great discrepancies exist
in the statements made regarding them by different observers, but this
is a matter of comparatively small importance, as their exact
composition has no very direct bearing on agricultural questions, and it
will suffice to give the names and chemical formulae of those which have
been analysed and described,--
Ulmic acid from long Frisian turf C_{40} H_{18} O_{16}
Humic acid from hard turf C_{40} H_{15} O_{15}
Humic acid from arable soil C_{40} H_{16} O_{16}
Humic acid from a pasture field C_{40} H_{14} O_{14}
Geic acid C_{40} H_{15} O_{17}
Apocrenic acid C_{48} H_{12} O_{24}
Crenic acid C_{24} H_{12} O_{16}
It is only necessary to observe further, that these formulae indicate a
close connection with woody fibre, and the continuous diminution of the
hydrogen and increase of oxygen shows that they must have been produced
by a gradually advancing decay.
The earlier chemists and vegetable physiologists attributed to the humus
of the soil a much more important function than it is now believed to
possess.
It was formerly considered to be the exclusive, or at least the chief
source of the organic constituents of plants, and by absorption through
the roots to yield to them the greater part of their nutriment. But
though this view has still some supporters, among whom Mulder is the
most distinguished, it is now generally admitted that humus is not a
_direct_ source of the organic constituents of plants, and is not
absorbed as such by their roots, although it is so _indirectly_, in as
far as the decomposition which it is constantly undergoing in the soil
yields carbonic acid, which can be absorbed. The older opinion is
refuted by many well-ascertained facts. As regards the exclusive origin
of the carbon of plants from humus, it is easy to see that this at least
cannot be true, for humus, as already stated, is itself derived solely
from the decomposition of vegetable and animal matters; and if the
plants on the earth's surface were to be supported by it alone, the
whole of their substance would have to return to the soil in the same
form, in order to supply the generation which succeeds them. But this is
very far from being the case, for the respiration of animals, the
combustion of fuel, and many other processes, are annually converting a
large quantity of these matters into carbonic acid; and if there were no
other source of carbon but the humus of the soil, the amount of
vegetable life would gradually diminish, and at length become entirely
extinct. Schleiden, who has discussed this subject very fully, has made
an approximative calculation of the total quantity of humus on the
earth's surface, and of the carbon annually converted into carbonic acid
by the respiration of man and animals, the combustion of wood for fuel,
and other minor processes; and he draws the conclusion that, if there
were no other source of carbon except humus, the quantity of that
substance existing in the soil would only support vegetation for a
period of sixty years.
The particular phenomena of vegetation also afford abundant evidence
that humus cannot be the only source of carbon. Thus Boussingault has
shown that on the average of years, the crops cultivated on an acre of
land remove from it about one ton more organic matter than they receive
in the manure applied to them, although there is no corresponding
diminution in the quantity of humus contained in the soil. An instance
which leads still more unequivocally to the same conclusion is given by
Humboldt. He states that an acre of land, planted with bananas, yields
annually about 152,000 pounds weight of fruit, containing about 32,000
pounds, or almost exactly 14 tons of carbon; and as this production goes
on during a period of twenty years, there must be withdrawn in that time
no less than 280 tons of carbon. But the soil on an acre of land weighs,
in round numbers, 1000 tons, and supposing it to contain 4 per cent of
humus, the total weight of carbon in it would amount to little more than
20 tons.
It is obvious from these and many other analogous facts that humus
cannot be the only or even a considerable source of the carbon of
plants, although it is still contended by some chemists that it may be
absorbed to a small extent. But even this is at variance with many
well-known facts. For if humus were absorbed, it might be expected that
vegetation would be most luxuriant on soils containing abundance of that
substance, especially if it existed in a soluble and readily absorbable
form; but so far from this being the case, nothing is more certain than
that peat, in which these conditions are fulfilled, is positively
injurious to most plants. On the other hand, our daily experience
affords innumerable examples of plants growing luxuriantly in soils and
places where no humus exists. The sands of the sea-shore, and the most
barren rocks, have their vegetation, and the red-hot ashes which are
thrown out by active volcanoes are no sooner cool than a crop of plants
springs up on them.
The conclusions to be drawn from these considerations have been further
confirmed by the direct experiments of different observers. Boussingault
sowed peas, weighing 15.60 grains, in a soil composed of a mixture of
sand and clay, which had been heated red-hot, and consequently contained
no humus, and after 99 days' growth, during which they had been watered
with distilled water, he found the crop to weigh 68.72 grains, so that
there had been a fourfold increase. Similar experiments have been made
by Prince Salm Horstmar, on oats and rape sown in a soil deprived of
organic matter by ignition, in which they grew readily, and arrived at
complete maturity. One oat straw attained a height of three feet, and
bore 78 grains; another bore 47; and a third 28--in all 153. These when
dried at 212 deg. weighed 46.302 grains, and the straw 45.6 grains. The most
satisfactory experiments, however, are those of Weigman and Polstorf,
these observers having found that it was possible to obtain a
two-hundred-fold produce of barley in an entirely artificial soil,
provided care was taken to give it the _physical_ characters of a
fertile soil. They prepared a mixture of six parts of sand, two of
chalk, one of white bole, and one of wood charcoal; to which was added a
small quantity of felspar, previously fused with marble and some soluble
salts, so as to imitate as closely as possible the inorganic parts of a
soil, and in it they planted twelve barley pickles. The plants grew
luxuriantly, reaching a height of three feet, and each bearing nine
ears, containing 22 pickles. The grain of the twelve plants weighed 2040
grains.
These experiments show that plants can grow and produce seed when the
most scrupulous care is taken to deprive them of every trace of humus.
But Saussure has gone further, and shown that even when present, humus
is not absorbed. He allowed plants of the common bean and the Polygonum
Persicaria to grow in solutions of humate of potash, and found a very
trifling diminution in the quantity of humic acid present; but the value
of his experiments is invalidated by his having omitted to ascertain
whether the diminution of humic acid which he observed was really due
to absorption by the plant. This omission has been supplied by Weigman
and Polstorf. They grew plants of mint (Mentha undulata) and of
Polygonum Persicaria in solutions of humate of potash, and placed beside
the glass containing the plant, another perfectly similar, and
containing only the solution of humate of potash. The solution, which
contained in every 100 grains, 0.148 grains of solid matter, consisting
of humate of potash, etc. was found to become gradually paler, and at
the end of a month, during which time the plants had increased by 6-1/2
inches, the quantity of solid matter in 100 grains had diminished to
0.132. But the solution contained in the other glass, and in which no
plant had grown, had diminished to 0.136, so that the absorption could
not have amounted to more than 0.004 grains for every 100 grains of
solution employed. This quantity is so small as to be within the limits
of error of experiment, and we are consequently entitled to draw the
conclusion that humus, even under the most favourable circumstances, is
not absorbed by plants.
But though not directly capable of affording nutriment to plants, it
must not, on that account, be supposed that humus is altogether devoid
of importance, for it is constantly undergoing decomposition in the
soil, and thus becomes a source of carbonic acid which can be absorbed,
and, as we shall afterwards more particularly see, it exercises very
important functions in bringing the other constituents of the soil into
readily available forms of combination.
It has been already observed that carbon, hydrogen, nitrogen, and
oxygen, cannot be absorbed by plants when uncombined, but only in the
forms of water, carbonic acid, ammonia, and nitric acid. It is scarcely
necessary to detail the grounds on which this conclusion has been
arrived at in regard to carbon and hydrogen, for practically it is of
little importance whether they can be absorbed or not, as the former is
rarely, the latter never, found uncombined in nature. Neither can there
be any doubt that water and carbonic acid are the only substances from
which these elements can be obtained. Every-day experience convinces us
that water is essential to vegetation; and Saussure, and other
observers, have shown that plants will not grow if they are deprived of
carbonic acid, and that they actually absorb that substance abundantly
from the atmosphere. The evidence for the non-absorption of oxygen lies
chiefly in the fact that plants obtain, in the form of water and
carbonic acid, a larger quantity of that element than they require, and
in place of absorbing, are constantly exhaling it. The form in which
nitrogen may be absorbed has given rise to much difference of opinion.
In the year 1779, Priestley commenced the examination of this subject,
and drew from his experiments the conclusion, that plants absorb the
nitrogen of the air. Saussure shortly afterwards examined the same
subject, and having found, that when grown in a confined space of air,
and watered with pure water, the nitrogen of the plants underwent no
increase, he inferred that they derived their entire supplies of that
element from ammonia, or the soluble nitrogenous constituents of the
soil or manure. Boussingault has since re-examined this question, and by
a most elaborate series of experiments, in which the utmost care was
taken to avoid every source of fallacy, he was led to the conclusion,
that when haricots, oats, lupins, and cresses were grown in calcined
pumice-stone, mixed with the ash of plants, and supplied with air
deprived of ammonia and nitric acid, their nitrogen underwent no
increase. It has been objected to these experiments, that the plants
being confined in a limited bulk of air, were placed in an unnatural
condition, and Ville has recently repeated them with a current of air
passing through the apparatus, and found a slight increase in the
nitrogen, due, as he thinks, to direct absorption. It is much more
probable, however, that it depends on small quantities of ammonia or
nitric acid which had not been completely removed from the air by the
means employed for that purpose, for nothing is more difficult than the
complete abstraction of these substances, and as the gain of nitrogen
was only 0.8 grains, while 60,000 gallons of air, and 13 of water, were
employed in the experiment, which lasted for a considerable time, it is
reasonable to suppose that a sufficient quantity may have remained to
produce this trifling increase.
While these experiments show that plants maintain only a languid
existence when grown in air deprived of ammonia and nitric acid, and
hence, that the direct absorption of nitrogen, if it occur at all, must
do so to a very small extent, the addition of a very minute quantity of
the former substance immediately produces an active vegetation and rapid
increase in size of the plants. Among the most striking proofs of this
are the experiments of Wolff, made by growing barley and vetches in a
soil calcined so as to destroy organic matters, and then mixed with
small quantities of different compounds of ammonia. He found that when
the produce from the calcined soil was represented by 100, that from the
different ammoniacal salts was--
Barley. Vetches.
Muriate of Ammonia 257.2 176.4
Carbonate of Ammonia 123.6 173.8
Sulphate of Ammonia 203.6 125.2
These experiments not only prove that ammonia can be absorbed, but they
also indirectly confirm the statement already made, that humus is not
necessary; for in some instances the produce was higher than that
obtained from the uncalcined soil with the same manures, although it
contained four per cent of humus.
On such experiments Liebig rests his opinion that ammonia is the
exclusive source of the nitrogen of plants, and although he has recently
admitted that it may be replaced by nitric acid, it is obvious that he
considers this a rare and exceptional occurrence. The evidence, however,
for the absorption of nitric acid appears to rest on as good grounds as
that of ammonia, for experience has shown that nitrate of soda acts
powerfully as a manure, and its effect must be due to the nitric acid,
and not to the soda, for the other compounds of that alkali have no such
effect. Wolff has illustrated this point by a series of experiments on
the sunflower, of which we shall quote one. He took two seeds of that
plant, and sowed them on the 10th May, in a soil composed of calcined
sand, mixed with a small quantity of the ash of plants, and added at
intervals during the progress of the experiment, a quantity of nitrate
of potash, amounting in all to 17.13 grains. The plants were watered
with distilled water, containing carbonic acid in solution, and the pot
in which they grew was protected from rain and dew by a glass cover. On
the 19th August one of the plants had attained a height of above 28
inches, and had nine fine leaves and a flower-bud; the other was about
20 inches high, and had ten leaves. On the 22d August, one of the plants
having been accidentally injured, the experiment was terminated. The
plants, which contained 103.16 grains of dry matter, were then carefully
analysed, and the quantity of nitrogen contained in the soil after the
experiment and in the seed was determined.
Grains.
Nitrogen in the dry plants 1.737 }
" remaining in the soil 0.697 } 2.434
" in the nitrate of potash 2.370 }
" in the seeds 0.029 } 2.399
------
Difference 0.035
Hence, the nitrogen contained in the plants must, in this instance, have
been obtained entirely from the nitrate of potash, for the quantity
contained in it and in the seeds is exactly equal to that in the plants
and the soil, the difference of 0.03 grains being so small that it may
be safely attributed to the errors inseparable from such experiments.
For the sake of comparison, an exactly similar experiment was made on
two seeds grown without nitrate of potash, and in this instance, after
an equally long period of growth, the largest plant had only attained a
height of 7.5 inches, and had three small pale and imperfectly developed
leaves. They contained only 0.033 grains of nitrogen, while the seeds
contained 0.032--indicating that, under these circumstances, there was
no increase in the quantity of that element.
But, independently of these experimental results, it may be inferred
from general considerations, that nitric acid must be one of the sources
from which plants derive their nitrogen. It has been already stated,
that the humus contained in the soil consists of the remains of decayed
plants, and there is every reason to suppose that the primeval soil
contained no organic matters, and that the first generation of plants
must have derived the whole of their nitrogen from, the atmosphere. If,
therefore, it be assumed that ammonia is the only source of the
nitrogen of plants, it would follow, that as that substance cannot be
produced by the direct union of its elements, the quantity of ammonia in
the air could only remain undiminished in the event of the whole of the
nitrogen of decaying plants returning into that form. But this is
certainly not the case, for every time a vegetable substance is burned,
part of its nitrogen is liberated in the free state, and in certain
conditions of putrefaction, nitric acid is produced. Now, if ammonia be
the only form in which nitrogen is absorbed, there must be a gradual
diminution of the quantity contained in the air; and further, there must
either be some continuous source of supply by which its quantity is
maintained, or there must be some other substance capable of affording
nitrogen in a form fitted for the maintenance of plant life. As regards
the first alternative, it must be stated that we know of no source other
than the decomposition of plants from which ammonia can be derived, and
we are therefore compelled to adopt the second alternative, and to admit
that there must be some other source of nitrogen, and it cannot be
doubted, from what has been already stated, that it is from nitric acid
only that it can be obtained.
It must be admitted, then, that carbonic acid, ammonia, nitric acid, and
water, are the great organic foods of plants. But while they have
afforded to them an inexhaustible supply of the last, the quantity of
the other three available for food are limited, and insufficient to
sustain their life for a prolonged period. It has been shown by
Chevandrier, that an acre of land under beech wood accumulates annually
about 1650 lb. of carbon. Now, the column of air resting upon an acre of
land contains only about 15,500 lb. of carbon, and the soil may be
estimated to contain 1 per cent., or 22,400 lb. per acre, and the whole
of this carbon would therefore be removed, both from the air and the
soil, in the course of little more than 23 years. But it is a familiar
fact, that plants continue to grow with undiminished luxuriance year
after year in the same soil, and they do so because neither their carbon
nor their nitrogen are permanently absorbed; they are there only for a
period, and when the plant has finished its functions, and dies, they
sooner or later return into their original state. Either the plant
decays, in which case its carbon and nitrogen pass more or less rapidly
into their original state, or it becomes the food of animals, and by the
processes of respiration and secretion, the same change is indirectly
effected. In this way a sort of balance is sustained; the carbon, which
at one moment is absorbed by the plant, passes in the next into the
tissues of the animal, only to be again expired in that state in which
it is fitted to commence again its round of changes.
But while there is thus a continuous circulation of these constituents
through both plants and animals, there are various changes which tend to
liberate in the free state a certain quantity both of the carbon and
nitrogen of plants, and these being thus removed from the sphere of
organic life, there would be a gradual diminution in the amount of
vegetation at the earth's surface, unless this loss were counterbalanced
by some corresponding source of gain. In regard to carbonic acid the
most important source is volcanic action, but the loss of nitrogen,
which is far more important and considerable, is restored by the direct
combination of its elements. The formation of nitric acid during thunder
storms has been long familiar; but it would appear from the recent
experiments of Cloeez, which, should they be confirmed by farther
enquiry, will be of much importance, that this compound is also
produced without electrical action when air is passed over certain
porous substances, saturated with alkaline and earthy compounds.
Fragments of calcined brick and pumice stone were saturated with
solution of carbonate of potash, with carbonates of lime and magnesia
and other mixtures, and a current of air freed from nitric acid and
ammonia passed over them for a long period, at the end of which notable
quantities of nitric acid were detected.
_Source of the Inorganic Constituents of Plants._--The inorganic
constituents of plants being all fixed substances, it is sufficiently
obvious that they can only be obtained from the soil, which, as we shall
afterwards see, contains all of them in greater or less abundance, and
has always been admitted to be the only substance capable of supplying
them. The older chemists and physiologists, however, attributed no
importance to these substances, and from the small quantities in which
they are found in plants, imagined that they were there merely
accidental impurities absorbed from the soil along with the humus, which
was at that time considered to be their organic food. This opinion,
sufficiently disproved by the constant occurrence of the same substances
in nearly the same proportions, in the ash of each individual plant, has
been further refuted by the experiments of Prince Salm Horstmar, who has
established their importance to vegetation, by experiments upon oats
grown on artificial soils, in each of which one inorganic constituent
was omitted. He found that, without silica, the grain vegetated, but
remained small, pale in colour, and so weak as to be incapable of
supporting itself; without lime, it died when it had produced its second
leaf; without potash and soda, it grew only to the height of three
inches; without magnesia, it was weak and incapable of supporting
itself; without phosphoric acid, weak but upright; and without sulphuric
acid, though normal in form, the plant was feeble, and produced no
fruit.
_Manner in which the Constituents of Plants are absorbed._--Having
treated of the sources of the elements of plants, it is necessary to
direct attention to the mode in which they enter their system.
_Water._--The absorption of water by plants takes place in great
abundance, and is connected with many of the most important phenomena of
vegetation. It is principally absorbed by the roots, and passes into the
tissues of the plant, where a part of it is decomposed, and goes to the
formation of certain of its organic compounds; while by far the larger
quantity, in place of remaining in it, is again exhaled by the leaves.
The extent to which this takes place is very large. Hales found that a
sunflower exhaled in twelve hours about 1 lb. 5 oz. of water, but this
quantity was liable to considerable variation, being greater in dry, and
less in wet weather, and much diminished during the night. Saussure made
similar experiments, and observed that the quantity of water exhaled by
a sunflower amounted to about 220 lb. in four months. The exhalation of
plants has recently been examined with great accuracy by Lawes. His
experiments were made by planting single plants of wheat, barley, beans,
peas, and clover, in large glass jars capable of holding about 42 lb. of
soil, and covered with glass plates, furnished with a hole in the centre
for the passage of the stem of the plant. Water was supplied to the soil
at certain intervals, and the jars were carefully weighed. The result of
the experiments, continued during a period of 172 days, is given in the
following table, which shows the total quantity of water exhaled in
grains:--
Wheat 113,527
Barley 120,025
Beans 112,231
Peas 109,082
Clover, cut 28th June 55,093
It further appears, that the exhalation is not uniform, but increases
during the active growth of the plant, and diminishes again when that
period is passed. These variations are shown by the subjoined tables, of
which the first gives the total exhalation, and the second the average
daily loss of water during certain periods.
TABLE I.--_Showing the Number of Grains of Water given off by the Plants
during stated divisional Periods of their Growth._
+-----------+--------+--------+--------+--------+--------+--------+---------+
|Description|9 Days. |31 Days.|27 Days.|34 Days.|30 Days.|14 Days.| 27 Days.|
| +--------+--------+--------+--------+--------+--------+---------+
|of Plant. | From |From |From |From |From |From | From |
| | Mar. 19|Mar. 28 |Apr. 28 |May 25 |June 28 |July 28 | Aug. 11 |
| | to |to |to |to |to |to | to |
| |Mar. 28.|Apr. 28.|May 25. |June 28.|July 28.|Aug. 11.| Sept. 7.|
+-----------+--------+--------+--------+--------+--------+--------+---------+
|Wheat | 129 | 1268 | 4,385 | 40,030 | 46,060 | 15,420 | 6235 |
|Barley | 129 | 1867 | 12,029 | 37,480 | 45,060 | 17,046 | 6414 |
|Beans | 88 | 1854 | 4,846 | 30,110 | 58,950 | 12,626 | 3657 |
|Pease | 101 | 1332 | 2,873 | 36,715 | 62,780 | 5,281 | ... |
|Clover | 400 | 1645 | 2,948 | 50,100 | ... | ... | ... |
+-----------+--------+--------+--------+--------+--------+--------+---------+
TABLE II.--_Showing the average daily Loss of Water (in Grains) by the
Plants, within several stated divisional Periods of their Growth._
+------------+--------+--------+--------+--------+--------+--------+--------+
| Description|9 Days. |31 Days.|27 Days.|34 Days.|30 Days.|14 Days.|27 Days.|
| +--------+--------+--------+--------+--------+--------+--------+
| of Plant. | From | From | From | From | From | From | From |
| |Mar. 19 |Mar. 28 |Apr. 28 |May 25 |June 28 |July 28 |Aug. 11 |
| | to | to | to | to | to | to | to |
| |Mar. 28.|Apr. 28.|May 25. |June 28.|July 28.|Aug. 11.|Sept. 7.|
+------------+--------+--------+--------+--------+--------+--------+--------+
| Wheat | 14.3 | 40.9 | 162.4 | 1177.4 | 1535.3 | 1101.4 | 230.9 |
| Barley | 14.3 | 60.2 | 445.5 | 1102.3 | 1502.0 | 1217.6 | 237.5 |
| Beans | 9.7 | 59.8 | 179.5 | 885.6 | 1965.0 | 901.8 | 135.4 |
| Peas | 11.2 | 42.9 | 106.4 | 1079.8 | 2092.7 | 377.2 | ... |
| Clover | 44.4 | 53.0 | 109.2 | 1473.5 | ... | ... | ... |
+------------+--------+--------+--------+--------+--------+--------+--------+
Similar experiments were made with the same plants in soils to which
certain manures had been added, and with results generally similar.
Calculating from these experiments, we are led to the apparently
anomalous conclusion that the quantity of water exhaled by the plants
growing on an acre of land greatly exceeds the annual fall of rain;
although it is obvious that of all the rain which falls, only a small
proportion can be absorbed by the plants growing on the soil, for a
large quantity is carried off by the rivers, and never reaches their
roots. It has been calculated, for instance, that the Thames carries off
in this way at least one-third of the annual rain that falls in the
district watered by it, and the Rhine nearly four-fifths. Of course this
large exhalation must depend on the repeated absorption of the same
quantity of water, which, after being exhaled, is again deposited on the
soil in the form of dew, and passes repeatedly through the plant. This
constant percolation of water is of immense importance to the plant, as
it forms the channel through which some of its other constituents are
carried to it.
_Carbonic Acid._--While the larger part of the water which a plant
requires is absorbed by its roots, the reverse is the case with carbonic
acid. A certain proportion no doubt is carried up through the roots by
the water, which always contains a quantity of that gas in solution, but
by far the larger proportion is directly absorbed from the air by the
leaves. A simple experiment of Boussingault's illustrates this
absorption very strikingly. He took a large glass globe having three
apertures, through one of which he introduced the branch of a vine, with
twenty leaves on it. With one of the side apertures a tube was
connected, by means of which the air could be drawn slowly through the
globe, and into an apparatus in which its carbonic acid was accurately
determined. He found, in this way, that while the air which entered the
globe contained 0.0004 of carbonic acid, that which escaped contained
only 0.0001, so that three-fourths of the carbonic acid had been
absorbed.
_Ammonia and Nitric Acid._--Little is known regarding the mode in which
these substances enter the plant. It is usually supposed that they are
entirely absorbed by the roots, and no doubt the greater proportion is
taken up in this way, but it is very probable that they may also be
absorbed by the leaves, at least the addition of ammonia to the air in
which plants are grown, materially accelerates vegetation. It is
probable, however, that the rain carries down the ammonia to the roots,
and there is no doubt that that derived from the decomposition of the
nitrogenous matters in the soil is so absorbed.
_Inorganic Constituents._--The inorganic constituents of course are
entirely absorbed by the roots; and it is as a solvent for them that the
large quantity of water continually passing through the plants is so
important. They exist in the soil in particular states of combination,
in which they are scarcely soluble in water. But their solubility is
increased by the presence of carbonic acid contained in the water, and
which causes it to dissolve, to some extent, substances otherwise
insoluble. It is in this way that lime, which occurs in the soil
principally as the insoluble carbonate, is dissolved and absorbed. And
phosphate of lime is also taken up by water containing carbonic acid, or
even common salt in solution. The amount of solubility produced by these
substances is extremely small; but it is sufficient for the purpose of
supplying to the plant as much of its mineral constituents as are
required, for the quantity of water which, as we have already seen,
passes through a plant is very large when compared with the amount of
inorganic matters absorbed. It has been shown by Lawes and Gilbert, that
about 2000 grains of water pass through a plant for every grain of
mineral matter fixed in it, so that there is no difficulty in
understanding how the absorption takes place.
It is worthy of notice, however, that the absorption of the elements of
plants takes place even though they may not be in solution in the soil,
the roots apparently possessing the power of directly acting on and
dissolving insoluble matters; but a distinction must be drawn between
this and the view entertained by Jethro Tull, who supposed that they
might be absorbed in the solid state, provided they were reduced to a
state of sufficient comminution. It is now no longer doubted that,
whatever action the roots may exert, the constituents of the plant must
be in solution before they can pass into it--experiment having
distinctly shown that the spongioles or apertures through which this
absorption takes place are too minute to admit even the smallest solid
particle.