Elements of Agricultural Chemistry
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Title: Elements of Agricultural Chemistry
Author: Thomas Anderson
Release date: March 28, 2008 [eBook #24931]
Language: English
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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.
CHAPTER II.
THE PROXIMATE CONSTITUENTS OF PLANTS.
The substances absorbed by the plant, which are of simple composition,
and contain only two elements, are elaborated within it, and converted
into the many complicated compounds of which its mass is composed. Some
of these, as, for example, the colouring matters of madder and indigo,
the narcotic principle of the poppy, &c., are confined to a single
species, or small group of plants, while others are found in all plants,
and form the main bulk of their tissues. The latter are the only
substances which claim notice in a treatise like the present. They have
been divided into three great classes, of widely different properties,
composition, and functions.
_1st. The Saccharine and Amylaceous Constituents._--These substances are
compounds of carbon, hydrogen, and oxygen, and all possess a certain
degree of similarity in composition, the quantities of hydrogen and
oxygen they contain being always in the proportion required to form
water, so that they may be considered as compounds of carbon and water;
not that it can be asserted that they actually do contain water, as
such, for of that there is no evidence, but only that its elements are
present in the proportion to form it.
_Cellulose._--This substance forms the fundamental part of all plants.
It is the principal constituent of woody fibre, and is found in a state
of purity in the fibre of cotton and flax, and in the pith of plants;
but in wood it is generally contaminated with another substance, which
has received the name incrusting matter, because it is deposited in and
around the cells of which the plant is in part composed. Cellulose is
insoluble in all menstrua, but, when boiled for a long time with
sulphuric acid, is converted into a substance called dextrine. Cellulose
consists of--
From pith of Elder-tree. Spongioles of roots.
Carbon 43.37 43.00
Hydrogen 6.04 6.18
Oxygen 50.59 50.82
------- -------
100.00 100.00
It is represented chemically by the formula, C_{24}H_{21}O_{21}, which
shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and
21 of oxygen.
_Incrusting matter._--Large quantities of this substance enter into the
composition of all plants. Of its chemical nature little is known, as it
cannot be obtained separate from cellulose, but it is analogous to that
substance in its composition, and probably contains hydrogen and oxygen
in the proportion to form water.
_Starch._--Starch is one of the most abundant constituents of plants,
and is found in most seeds, as those of the cereals and the leguminous
plants; in the tubers of the potatoe, the bulbs of tulips, &c. &c. It is
obtained by placing a quantity of wheat flour in a bag, and kneading it
under a gentle stream of water. When the water is allowed to stand, it
deposits the starch as a fine white powder, which, when examined by the
microscope, is found to be composed of minute grains, formed of
concentric layers deposited on one another. These grains vary
considerably in size and structure in different plants; but in the same
plant they are generally so much alike as to admit of their recognition
by a practised observer. They were formerly believed to be composed of
an external coating of a substance insoluble in water, and containing in
their interior a soluble kernel; but this opinion has been refuted, and
distinct evidence been brought to show that the exterior and interior of
the globules are identical in chemical properties. Starch is insoluble
in cold water, but by boiling, it dissolves, forming a thick paste. By
long continued boiling with water containing a small quantity of acid,
it is completely dissolved and converted into dextrine, and eventually
into sugar. The same change is produced by the action of fermenting
substances, such as the extract of malt; when heated in the dry state to
a temperature of about 390 Fahr., it becomes soluble in cold water. It
is distinguished by giving a brilliant blue compound with iodine. Starch
contains--
Carbon 44.47
Hydrogen 6.28
Oxygen 49.25
------
100.00
and its composition is represented by the formula C_{12}H_{10}O_{10}, so
that it differs but little from cellulose in composition, although its
chemical functions in the plant are extremely different. It is connected
with some of the most important changes which occur in the growing
plants, and by a series of remarkable transformations is converted into
sugar and other important compounds.
_Lichen Starch_ is found in most species of lichens, and is
distinguished from common starch by producing a green colour with
iodine. Its composition is the same as that of ordinary starch.
_Inuline._--The species of starch to which this name is given is
characterised by its dissolving in boiling water, and giving a white
pulverulent deposit in cooling. It is found in the tuber of the dahlia,
in the dandelion, and some other plants. Its composition is identical
with that of cellulose, and its formula is C_{24}H_{21}O_{21}.
_Gum_ is excreted from various plants as a thick fluid, which dries up
into transparent masses. Its composition is identical with that of
starch. It dissolves readily in cold water, and is converted into sugar
by long continued boiling with acids. Its properties are best marked in
gum arabic, which is obtained from various species of acacia; that from
other plants differs to some extent, although its chemical composition
is the same.
_Dextrine._--When starch is exposed to a heat of about 400 deg., or when
treated with sulphuric acid, or with a substance extracted from malt
called _diastase_, it is converted into dextrine. It may also be
obtained from cellulose by a similar treatment. The dextrine so obtained
has the same composition as the starch from which it is produced, but
its properties more nearly resemble those of gum. It plays a very
important part in the process of germination, and may be converted into
sugar on the one hand, and apparently also into starch on the other.
_Sugar._--Under this name are included four or five distinct substances,
of which the most important are, cane sugar, grape sugar, and the
uncrystallisable sugar found in many plants.
_Cane Sugar._--This variety of sugar, as its name implies, is found most
abundantly in the sugar cane, but it occurs also in the maple,
beet-root, and various species of palms, from all of which it is
extracted on the large scale. It is extremely soluble in water, and can
be obtained in large transparent prismatic crystals, as in common
sugar-candy. It swells up, and is converted into a brown substance
called caramel, when heated, and by contact with fermenting substances,
yields alcohol and carbonic acid. It contains--
Carbon 42.22
Hydrogen 6.60
Oxygen 51.18
------
100.00
and its chemical formula is C_{12}H_{11}O_{11}.
_Grape Sugar_ is met with in the grape, and most other fruits, as well
as in honey. It is produced artificially when starch is boiled for a
long time with sulphuric acid, or treated with a large quantity of
diastase. It is less soluble in water than cane sugar, and crystallises
in small round grains. Its composition, when dried at 284 deg., is--
Carbon 40.00
Hydrogen 6.66
Oxygen 53.34
------
100.00
and its formula is C_{12}H_{12}O_{12}; but when crystallised it contains
two equivalents of water, and is then represented by the formula
C_{12}H_{12}O_{12} + 2H_{2}O.
The uncrystallisable sugar of plants is closely allied to grape sugar,
and, so far as at present known, has the same composition, although,
from the difficulty of obtaining it quite free from crystallised sugar,
this is still uncertain.
_Mucilage_ is the name applied to the substance existing in linseed,
and in many other seeds, and which communicates to them the property of
swelling up and becoming gelatinous when treated with water. It is found
in a state of considerable purity in gum tragacanth and some other gums.
Its composition is not known with absolute certainty, but it is either
C_{24}H_{19}O_{19}, or C_{12}H_{10}O_{10}; and in the latter case it
must be identical with starch and gum.
It will be observed that all the substances belonging to this class are
very closely related in chemical composition, some of them, as starch
and gum, though easily distinguished by their properties, being
identical in constitution, while others only differ in the quantity of
water, or of its elements which they contain. In fact, they may all be
considered as compounds of carbon and water, and their relations are,
perhaps, more distinctly seen when their formulae are written so as to
show this, as is done in the following table, in the second column of
which those containing twelve equivalents of carbon are doubled, so as
to make them comparable with cellulose:--
Water.
Grape sugar, C_{12}H_{12}O_{12} C_{24}H_{24}O_{24} C_{24} + 24
Cane sugar, C_{12}H_{11}O_{11} C_{24}H_{22}O_{22} C_{24} + 22
Cellulose, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Inuline, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Starch, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Dextrine, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Gum, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Mucilage, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
The relation between these substances being so close, it is not
difficult to understand how one may be converted into another by the
addition or subtraction of water. Thus, cellulose has only to absorb an
equivalent of water to become grape sugar, or to lose an equivalent in
order to be converted into starch, and we shall afterwards see that
such changes do actually occur in the plant during the process of
germination.
_Pectine and Pectic Acid._--These substances are met with in many fruits
and roots, as, for instance, in the apple, the carrot, and the turnip.
They differ from the starch group in containing more oxygen than is
required to form water along with their hydrogen; but their exact
composition is still uncertain, and they undergo numerous changes during
the ripening of the fruit.
_2d. Oily or Fatty Matters._--The oily constituents of plants form a
rather extensive group of substances all closely allied, but
distinguished by minor differences in properties and constitution. Some
of them are very widely distributed throughout the vegetable kingdom,
but others are almost peculiar to individual plants. They are all
compounds of carbon, hydrogen, and oxygen, and are at once distinguished
from the preceding class, by containing much less oxygen than is
required to form water with their hydrogen. The principal constituents
of the fatty matters and oils of plants are three substances, called
stearine, margarine, and oleine, the two former solids, the latter a
fluid; and they rarely, if ever, occur alone, but are mixed together in
variable proportions, and the fluidity of the oils is due principally to
the quantity of the last which they contain. If olive oil be exposed to
cold, it is seen to become partially solid; and if it be then pressed, a
fluid flows out, and a crystalline substance remains; the former is
oleine, though not absolutely pure, and the latter margarine. The
perfect separation of these substances involves a variety of troublesome
chemical processes; and when it has been effected, it is found that each
of them is a compound of a peculiar acid, with another substance having
a sweet taste, and which has received the name of glycerine, or the
sweet principle of oil. Glycerine, as it exists in the fats, appears to
be a compound of C_{3}H_{2}O, and its properties are the same from
whatever source it is obtained. The acids separated from it are known by
the names of margaric, stearic, and oleic acids.
_Margaric Acid_ is best obtained pure by boiling olive oil with an
alkali until it is saponified, and decomposing the soap with an acid,
expressing the margaric acid, which separates, and crystallising it from
alcohol. It is a white crystalline fusible solid, insoluble in water,
but soluble in alcohol and in solutions of the alkalies. Its composition
is--
Carbon 75.56
Hydrogen 12.59
Oxygen 11.85
------
100.00
and its formula C_{34}H_{34}O_{4}.
_Stearic Acid._--Although this acid exists in many plants, it is most
conveniently extracted from lard. It is a crystalline solid less fusible
than margaric acid, but closely resembling it in its other properties.
Its formula is C_{36}H_{36}O_{4}.
_Oleic Acid._--Under this name two different substances appear to be
included. It has been applied generally to the fluid acids of all oils,
while it would appear that the drying and non-drying oils actually
contain substances of different composition. The acid extracted from
olive oil appears to have the formula C_{36}H_{34}O_{4}, while that from
linseed oil is C_{46}H_{38}O_{6}, but this is still doubtful.
Other fatty acids have been detected in palm oil, cocoa-nut oil, &c.
&c., which so closely resemble margaric and stearic acids as to be
easily confounded with them. Though presenting many points of interest,
it is unnecessary to describe them in detail here.
_Wax_ is a substance closely allied to the oils. It consists of two
substances, cerine and myricine, which are separated from one another by
boiling alcohol, in which the former is more soluble. They are extremely
complex in composition, the former consisting principally of an acid
similar to the fatty acids, called cerotic acid, and containing
C_{54}H_{54}O_{4}. The latter has the formula C_{92}H_{92}O_{4}. The wax
found in the leaves of the lilac and other plants appears to consist of
myricine, while that extracted from the sugar-cane is said to be
different, and to have the formula C_{48}H_{50}O_{2}. It is probable
that other plants contain different sorts of wax, but their
investigation is still so incomplete, that nothing definite can be said
regarding them. Wax and fats appear to be produced in the plant from
starch and sugar; at least it is unquestionable that the bee is capable
of producing the former from sugar, and we shall afterwards see that a
similar change is most probably produced in the plant. The fatty matters
contained in animals are identical with those of plants.
_3d. Nitrogenous or Albuminous Constituents of Plants and Animals._--The
nitrogenous constituents of plants and animals are so closely allied,
both in properties and composition, that they may be most advantageously
considered together.
_Albumen._--Vegetable albumen is found dissolved in the juices of most
plants, and is abundant in that of the potato, the turnip, and wheat. In
these juices it exists in a soluble state, but when its solution is
heated to about 150 deg., it coagulates into a flocky insoluble substance.
It is also thrown down by acids and alcohol. Coagulated albumen is
soluble in alkalies and in nitric acid. Animal albumen exists in the
white of eggs, the serum of blood, and the juice of flesh; and from all
these sources is scarcely distinguishable in its properties from
vegetable albumen.
It is a substance of very complicated composition, and chemists are not
agreed as to the formula by which its constitution is to be expressed, a
difficulty which occurs also with most of the other nitrogenous
compounds. The results of the analyses of albumen from different sources
are however quite identical, as may be seen from those subjoined--
From From From From
Wheat. Potatoes. Blood. White of Egg.
Carbon 53.7 53.1 53.4 53.0
Hydrogen 7.1 7.2 7.0 7.1
Nitrogen 15.6 ... 15.5 15.6
Oxygen } { ... 22.1 22.9
Sulphur } 23.6 { 0.97 1.6 1.1
Phosphorus } { ... 0.4 0.3
----- ----- -----
100.0 100.0 100.0
Closely allied to vegetable albumen is the substance known by the name
of _glutin_, which is obtained by boiling the gluten of wheat with
alcohol. It appears to be a sort of coagulated albumen, with which its
composition completely agrees.
_Vegetable Fibrine._--If a quantity of wheat flour be tied up in a piece
of cloth, and kneaded for some time under water, the starch it contains
is gradually washed out, and there remains a quantity of a glutinous
substance called gluten. When this is boiled with alcohol, the _glutin_
above referred to is extracted, and vegetable fibrine is left. It
dissolves in dilute potash, and on the addition of acetic acid is
deposited in a pure state. Treated with hydrochloric acid, diluted with
ten times its weight of water, it swells up into a jelly-like mass.
When boiled or preserved for a long time under water, it cannot be
distinguished from coagulated albumen.
_Animal Fibrine_ exists in the blood and the muscles, and agrees in all
its characters and composition with vegetable fibrine, as is shown by
the subjoined analyses--
Wheat Flour. Blood. Flesh.
Carbon 53.1 52.5 53.3
Hydrogen 7.0 6.9 7.1
Nitrogen 15.6 15.5 15.3
Oxygen 23.2 24.0 23.1
Sulphur 1.1 1.1 1.2
----- ----- -----
100.0 100.0 100.0
_Caseine._--Vegetable caseine exists abundantly in most plants,
especially in the seeds, and remains in the juice after albumen has been
precipitated by heat, from which it may be separated in flocks by the
addition of an acid. It has been obtained for chemical examination,
principally from peas and beans, and from the almond and oats. When
prepared from the pea it has been called _legumine,_ from almonds
_emulsine_, and from oats _avenine_; but they are all three identical in
their properties, although formerly believed to be different, and
distinguished by these names. Vegetable caseine is best obtained by
treating peas or beans with hot water, and straining the fluid. On
standing, the starch held in suspension is deposited, and the caseine is
retained in solution in the alkaline fluid; by the addition of an acid
it is precipitated as a thick curd. Caseine is insoluble in water, but
dissolves readily in alkalies; its solution is not coagulated by heat,
but, on evaporation, becomes covered with a thin pellicle, which is
renewed as often as it is removed.
_Animal Caseine_ is the principal constituent of milk, and is obtained
by the cautious addition of an acid to skimmed milk, by which it is
precipitated as a thick white curd. It is also obtained by the use of
rennet, and the process of curding milk is simply the coagulation of its
caseine. It is soluble in alkalies, and precipitated from its solution
by acids, and in all other respects agrees with vegetable caseine.
The composition of animal caseine has been well ascertained, but
considerable doubt still exists as to that of vegetable caseine, owing
to the difficulty of obtaining it absolutely pure. The analyses of
different chemists give rather discordant results, but we have given
those which appear most trustworthy--
From Peas.
Carbon 50.6 50.7
Hydrogen 6.8 6.6
Nitrogen 16.5 15.8
Oxygen 25.6 23.8
Sulphur 0.5 0.8
Phosphorus ... 2.3
----- -----
100.0 100.0
Other results differ considerably from these, and some observers have
even obtained as much as eighteen per cent of nitrogen and fifty-three
of carbon.
The composition of animal caseine differs from this principally in the
amount of carbon. Its composition is--
Carbon 53.6
Hydrogen 7.1
Nitrogen 15.8
Oxygen 22.5
Sulphur 1.0
-----
100.0
The most cursory examination of these analytical numbers is sufficient
to show that a very close relation subsists between the different
substances just described. Indeed, with the exception of vegetable
caseine, they may be said all to present the same composition; and, as
already mentioned, there are analyses of it which would class it
completely with the others. While, however, the quantities of carbon,
hydrogen, nitrogen, and oxygen are the same, differences exist in the
sulphur and phosphorus they contain, and which, though very small in
quantity, are indubitably essential to them. Much importance has been
attributed to these constituents by various chemists, and especially by
Mulder, who has endeavoured to make out that all the albuminous
substances are compounds of a substance to which he has given the name
of _proteine_, with different quantities of sulphur and phosphorus. The
composition of proteine, according to his newest experiments, is--
Carbon 54.0
Hydrogen 7.1
Nitrogen 16.0
Oxygen 21.4
Sulphur 1.5
-----
100.0
and is exactly the same from whatever albuminous compound it is
obtained. Although the importance of proteine is probably not so great
as Mulder supposed, it affords an important illustration of the close
similarity of the different substances from which it is obtained, the
more especially as there is every reason to believe that the different
albuminous compounds are capable of changing into one another, just as
starch and sugar are mutually convertible; and the possibility of this
change throws much light on many of the phenomena of nutrition in plants
and animals. Indeed, it would seem probable that these compounds are
formed from their elements by plants only, and are merely assimilated by
animals to produce the nitrogenous constituents they contain.
_Diastase_ is the name applied to a substance existing in malt, and
obtained by macerating that substance with cold water, and adding a
quantity of alcohol to the fluid, when the diastase is immediately
precipitated in white flocks. It is produced during the malting process,
and is not found in the unmalted barley. Its chemical composition is
unknown, but it is nitrogenous, and is believed to be produced by the
decomposition of gluten. If a very small quantity of diastase be mixed
with starch suspended in hot water, the starch is found gradually to
dissolve, and to pass first into the state of dextrine, then into that
of sugar. The change thus effected takes place also in a precisely
similar manner in the plant, diastase being produced during the process
of germination of all seeds and tubers, for the purpose of effecting
this change, and to fulfil other functions less understood, but no doubt
equally important. Diastase is found in the seeds only during the period
when the starch they contain is passing into sugar; as soon as that
change has taken place, its function is ended, and it disappears.
CHAPTER III.
THE CHANGES WHICH TAKE PLACE IN THE FOOD OF PLANTS DURING THEIR GROWTH.
The simple compounds which the plant absorbs from the atmosphere and
soil are elaborated within its system, and converted into the various
complex substances of which its tissues are composed, by a series of
changes, the details of which are still in some respects imperfectly
known, although their general nature is sufficiently well understood.
They may be best rendered intelligible by reference, in the first
instance, to the changes occurring during germination, when the young
plant is nourished by a supply of food stored up in the seed, in
sufficient quantity to maintain its existence until the organs by which
it is afterwards to draw its nutriment from the air and soil are
sufficiently developed to serve that purpose.
_Changes occurring during Germination._--When a seed is placed in the
soil under favourable circumstances, it becomes the seat of an important
and remarkable series of chemical changes, which result in the
production of the young plant. Experiment and observation have shown
that heat, moisture, and air, are necessary to the production of these
changes, and though probably not absolutely essential, the absence of
light is favourable in the early stages. The temperature required for
germination varies greatly in different seeds, some germinating readily
at a few degrees above the freezing point, and others requiring a
tolerably high temperature. The rapidity with which it takes place
appears to increase with the temperature; but this is true only within
very narrow limits, for beyond a certain point heat is injurious, and
when it exceeds 120 deg. or 130 deg. Fahrenheit, entirely prevents the process.
The presence of oxygen is also essential, for it has been shown that if
seeds are placed in a soil exposed to an atmosphere deprived of that
element, or if they be buried so deep that the air does not reach them,
they may lie without change for an unlimited period; but so soon as they
are exposed to the air, germination immediately commences. Illustrations
of this fact are frequently observed where earth from a considerable
depth has been thrown up to the surface, when it often becomes covered
with plants not usually seen in the neighbourhood, which have sprung
from buried seeds. When all the necessary conditions for germination are
fulfilled, the seed absorbs moisture, swells up, and sends out a shoot
which rises to the surface, and a radicle which descends--the one
destined to develop the leaves, the other the roots, by which the plant
is afterwards to derive its nutriment from the air and the soil. But
until these organs are properly developed, the plant is dependent on the
matters contained in the seed itself. These substances are mostly
insoluble, but are brought into solution by the atmospheric oxygen
acting upon the gluten, and converting it into a soluble substance
called diastase, which in its turn reacts upon the starch, converting it
first into dextrine, and then into cellulose, and the latter is finally
deposited in the form of organised cells, and produces the first little
shoot of the plant. At the first moment of germination, the oxygen
absorbed appears simply to oxidize the constituents of the seed, but
this condition exists only for a very limited period, and is soon
followed by the evolution of carbonic acid, water being at the same time
formed from the organic constituents of the seed, which gradually
diminishes in weight. The amount of this diminution is different with
different plants, but always considerable. Boussingault found that the
loss of dry substance in the pea amounted in 26 days to 52 per cent, and
in wheat to 57 per cent in 51 days. Against this, of course, is to be
put the weight of the young plant produced; but this is never sufficient
to counterbalance the diminished weight of the seed, for Saussure found
that a horse bean and the plant produced from it weighed, after 16 days,
less by 29 per cent than the seed before germination. The same
phenomenon is observed in the process of malting, which is in fact the
artificial germination of barley, the malt produced always weighing
considerably less than the grain from which it was obtained. It was
believed by Saussure, and the older investigators, that the carbonic
acid evolved was entirely produced from starch and sugar; and as these
substances may be viewed as compounds of carbon and water, the change
was very simply explained by supposing that the carbon was oxidised and
converted into carbonic acid and its water eliminated. But this
hypothesis is incapable of explaining all the phenomena observed; for
woody fibre, which is one of the chief constituents of the young plant,
contains more carbon than the starch and sugar from which it must have
been produced, and we are, therefore, forced to admit that the action
must be more complicated. There is every reason to believe that the
nitrogenous constituents of the seed are most abundantly oxidized, for
they are remarkably prone to change; but the action of the air is not
confined to them, and it appears most probable that all the substances
take part in the decomposition, and the process of germination may, in
some respects, be compared to decay or putrefaction, which, like it, is
attended by the absorption of oxygen and evolution of carbonic acid; but
while in the latter case the residual substances remain in a useless
state, in the former they at once become part of a new organism.
_Changes occurring during the After-growth of the Plant._--When the
plant has developed its roots and leaves, and exhausted the store of
materials laid up for it in the seed, it begins to derive its
subsistence from the surrounding air, and to absorb carbonic acid,
water, ammonia, and nitric acid, and to decompose and convert them into
the different constituents of its tissues. These changes take place
slowly at first, and more rapidly as the organs fitted for the
elaboration of its food are developed. The roots and the leaves are
equally active in performing this duty, the former absorbing the mineral
matters along with the carbonic acid, ammonia, nitric acid, and moisture
in the soil, or the manure added to it; the latter gathering the gaseous
substances existing in the air. Each of these undergoes a series of
changes claiming our consideration.
_Decomposition of Carbonic Acid._--Carbonic acid, which appears to be
absorbed with equal readiness by the roots, leaves, and stems, undergoes
immediate decomposition, its carbon being retained, and its oxygen, in
whole or in part, evolved into the air. This decomposition occurs only
under the action of the sun's rays, and has been found to be
proportionate to the amount of light to which the plant is exposed. It
takes place only in the green parts of plants, for though the roots
absorb carbonic acid, they cannot decompose it, or evolve oxygen; and
the coloured parts, the flowers, fruits, etc., have an entirely opposite
effect, absorbing oxygen and giving off carbonic acid. The absorption of
carbonic acid and escape of oxygen has been proved by numerous direct
experiments by Saussure and others, in which both atmospheric air and
artificial mixtures containing an increased quantity of carbonic acid
have been employed. Saussure allowed seven plants of periwinkle (_Vinca
minor_) to vegetate in an atmosphere containing 7.5 per cent of carbonic
acid for six days, during each of which the apparatus was exposed for
six hours to the sun's rays. The air was analysed both before and after
the experiment, and the results obtained were--
Volume Carbonic
of the air. Nitrogen. Oxygen. Acid.
Before the experiment, 5746 4199 1116 431
After " 5746 4338 1408 0
---- ---- ---- ----
Difference, 0 +139 +292 -431
In this experiment the whole of the carbonic acid, amounting to 431
volumes, was absorbed, but only 292 volumes of oxygen were given off.
Had the carbonic acid been entirely decomposed, and all its oxygen
eliminated, its volume would have been equal to that of the acid, or
431, so that in this instance 139 volumes of the oxygen of the carbonic
acid have been retained to form part of the tissues of the plant. On the
other hand, the nitrogen is found to be increased after the experiment.
It might be supposed that the nitrogen evolved had been derived from the
decomposition of the nitrogenous constituents of the plant, but this
cannot be the true explanation, because in this particular case it
greatly exceeded the whole nitrogen contained in the plants
experimented on. Its source is not well understood, but Boussingault
supposes it to have existed in the interstices of the plant, and to have
escaped during the course of the experiment. Saussure found that the
oak, the horse-chesnut, and other plants, absorb oxygen and give off
carbonic acid in less volumes than the oxygen, while the house-leek and
the cactus absorb oxygen without evolving carbonic acid. The absorption
and decomposition of carbonic acid takes place only during the day, and
matters are entirely reversed during the night, when oxygen is absorbed
and carbonic acid eliminated from all parts of the plants.
Although the action occurring during the night is the reverse of that
which takes place during the day, it is in no degree to be attributed to
a re-oxidation of the carbon which had been deposited in the tissues of
the plant. It appears, on the contrary, to be a purely mechanical, and
not a chemical process. During the night the sap continues to circulate
through the vessels of the plant, and moisture, carrying with it
carbonic acid in solution, is absorbed by the roots; but when it reaches
the leaves, where the sun's light would have caused its decomposition
during the day, it is again exhaled unchanged. The oxygen absorbed
during the night must, however, take part in some chemical processes,
for if it were merely mechanical, the absorption would not be confined
to that gas alone, but would be participated in by the other
constituents of the air. Moreover, the amount of absorption varies
greatly in different plants--being scarcely appreciable in some, and
very abundant in others. Plants containing volatile oils, which are
readily converted into resins by the action of oxygen, or those
containing tannin or other readily oxidizable substances, take up the
largest quantity. This is remarkably illustrated by an experiment in
which the leaves of the Agave americana, after twenty-four hours'
exposure in the dark, were found to have absorbed only 0.3 of their
volume of oxygen, while those of the fir, in which volatile oil is
abundant, had taken up twice, and those of the oak, containing tannin,
eighteen times as much oxygen.
In the flowers, both by day and night, there is a constant absorption of
oxygen, and evolution of carbonic acid. In fact, an active oxidation is
going on, attended by the evolution of heat, which, in the _Arum
maculatum_ and some other plants, is so great as to raise the
temperature of the flower 10 deg. or 12 deg. above that of the surrounding air.
_Decomposition of Water in the Plant._--In addition to the function
which water performs in the plant, as the solvent of the different
substances which form its nutriment, and hence as the medium through
which they pass into its organs, it serves also as a direct food,
undergoing decomposition, and yielding hydrogen to the organic
substances. Its constituents, along with those of the carbonic acid
absorbed, undergo a variety of transformations, and form the principal
part of the non-nitrogenous constituents. It has been already observed
that starch, sugar, and the other allied substances, may be considered
as compounds of carbon with water; and they might be supposed to owe
their origin to the carbonic acid losing the whole of its oxygen, and
direct combination then ensuing between the residual carbon and a
certain proportion of water; but this would imply that the latter
substance undergoes no decomposition, and though undoubtedly the
simplest view of the case, it is by no means the most probable. It is
much more likely that the carbonic acid is only partially decomposed,
half its oxygen being separated, and replaced by hydrogen, produced by
the decomposition of a certain quantity of water into its elements.
Thus, for instance, sugar may be produced from twelve equivalents of
carbonic acid and twelve equivalents of water, twenty-four equivalents
of oxygen being eliminated, as thus represented:
12 equivalents of carbonic acid, C_{12}O_{12}O_{12}
12 " water, H_{12}O_{12}
1 " sugar, and 24 of ox. C_{12}H_{12}O_{12} + O_{24}
It must not be supposed that we are in a condition to assert that sugar
is really produced in the manner here shown, the illustration being
given merely for the purpose of pointing out how it may be supposed to
occur, and on a similar principle it is possible to explain the
formation of most other vegetable compounds; and this subject has been
very fully discussed by the late Dr. Gregory, in his "Handbook of
Organic Chemistry." That water must be decomposed, is evident from the
fact, established by analysis, that the hydrogen of the plant generally
exceeds the quantity required to form water with its oxygen, so that
this excess at least must be produced by the decomposition of water. The
hydrogen of the volatile oils, many of which contain no oxygen, and that
of the fats, which contain only a small quantity, must manifestly be
obtained in a similar manner.
_Decomposition of Ammonia._--The nitrogenous or albuminous compounds of
vegetables must necessarily obtain their nitrogen from the decomposition
either of ammonia or nitric acid, experiment having distinctly shown
that they are incapable of absorbing it in the free state from the
atmosphere. It has been clearly ascertained that the albuminous
substances do not contain ammonia, and it is hence apparent that a
complete decomposition of that substance must take place in the plant.
No doubt carbonic acid and water take part with it in these changes,
which must be of a very complex character, and in the present state of
our knowledge it seems hopeless to attempt any explanation of them.
_Decomposition of Nitric Acid._--Chemists are not entirely at one as to
whether nitric acid is directly absorbed by the plant, or is first
converted into ammonia. But there are certain facts connected with the
chemistry of the soil, to be afterwards referred to, which seem to us to
leave no doubt that it may be directly absorbed; and in that case it
must be decomposed, its oxygen being eliminated, and the nitrogen taking
part with carbon and hydrogen in the formation of the organic compounds.
It must be clearly understood that while such changes as those described
manifestly must take place, the explanations of them which have been
attempted by various chemists are not to be accepted as determinately
established _facts_; they are at present no more than hypothetical views
which have been expressed chiefly with the intention of presenting some
definite idea to the mind, and are unsupported by absolute proof; they
are only inferences drawn from the general bearings of known facts, and
not facts themselves. Although, therefore, they are to be received with
caution, they have advantages in so far as they present the matter to us
in a somewhat more tangible form than the vague general statements which
are all that could otherwise be made.
CHAPTER IV.
THE INORGANIC CONSTITUENTS OF PLANTS.
When treating of the general constituents of plants, it has been already
stated that the older chemists and vegetable physiologists, misled by
the small quantity of ash found in them, entertained the opinion that
mineral matters were purely fortuitous components of vegetables, and
were present merely because they had been dissolved and absorbed along
with the humus, which was then supposed to enter the roots in solution,
and to form the chief food of the plant. This supposition, which could
only be sustained at a time when analysis was imperfect, has been long
since disproved and abandoned, and it has been distinctly shown by
repeated experiment that not only are these inorganic substances
necessary to the plant, but that every one of them, however small its
quantity, must be present if it is to grow luxuriantly and arrive at a
healthy maturity. The experiments of Prince Salm Horstmar, before
alluded to, have established beyond a doubt, that while a seed may
germinate, and even grow, to a certain extent, in absence of one or more
of the constituents of its ash, it remains sickly and stunted, and is
incapable of producing either flower or seed.
Of late years the analysis of the ash of different plants has formed the
subject of a large number of laborious investigations, by which our
knowledge of this subject has been greatly extended. From these it
appears that the quantity of ash contained in each plant or part of a
plant is tolerably uniform, differing only within comparatively narrow
limits, and that there is a special proportion belonging to each
individual organ of the plant. This fact may be best rendered obvious by
the subjoined table, showing the quantity of ash contained in a hundred
parts of the different substances dried at 212 deg.. Most of these numbers
are the mean of several experiments:--
_Table showing the quantity of inorganic matters in 100 parts of
different plants dried at 212 deg.._
SEEDS.
Wheat 1.97
Barley 2.48
Oats (with husk) 3.80
Oats (without husk) 2.06
Rye 2.00
Millet 3.60
Rice 0.37
Maize 1.20
Peas 2.88
Beans 3.22
Kidney Beans 4.09
Lentils 2.51
Tares 2.60
Buckwheat 2.13
Linseed 4.40
Hemp seed 5.60
Rape seed 4.35
Indian Rape-seed[A] 4.06
Sunflower 3.26
Cotton seed 5.93
Guinea Corn 1.99
Gold of Pleasure 4.10
White Mustard 4.15
Black Mustard 4.31
Poppy 6.56
Niger seed (_Guizotia oleifera_) 7.00
Earth nut 3.88
Sweet Almond 4.90
Horse-chesnut 2.81
Grape 2.76
Clover 6.19
Turnip 3.98
Carrot 10.03
Sainfoin 5.27
Italian Ryegrass 6.91
Mangold-Wurzel 6.58
STRAWS AND STEMS.
Wheat 4.54
Barley 4.99
Oat 7.24
Winter Rye 5.15
Summer Rye 5.78
Millet 8.32
Maize 3.60
Pea 4.81
Bean 6.59
Tares 6.00
Lentil 5.38
Buckwheat 4.50
Hops 4.42
Flax straw 4.25
Hemp 4.14
Gold of Pleasure 6.05
Rape 4.41
Potato 14.90
Jerusalem Artichoke 4.40
ENTIRE PLANT.
Potato 17.70
Spurry 10.06
Red Clover 8.79
White Clover 8.72
Yellow Clover 8.56
Crimson Clover (_T. incarnatum_) 10.81
Cow Grass (_T. medium_) 11.31
Sainfoin 6.51
Ryegrass 6.42
Meadow Foxtail (_Alopecurus pratensis_) 7.81
Sweet-scented Vernal Grass (_Anthoxanthum odoratum_) 6.32
Downy Oat Grass (_Avena pubescens_) 5.22
Bromus erectus 5.21
Bromus mollis 5.82
Cynosurus cristatus 6.38
Dactylis glomeratus 5.31
Festuca duriuscula 5.42
Holcus lanatus 6.37
Hordeum pratense 5.67
Lolium perenne 7.54
Poa annua 2.83
Poa pratensis 5.94
Poa trivialis 8.33
Phleum pratense 5.29
Plantago lanceolata 8.68
Poterium Sanguisorba 7.97
Yarrow 13.45
Rape Kale 8.00
Cow Cabbage 10.00
Asparagus 6.40
Parsley 1.10
Furze 3.11
Chamomile (_Anthemis arvensis_) 9.66
Wild Chamomile (_Matricaria Chamomilla_) 9.10
Corn Cockle (_Agrostemma Githago_) 13.20
Corn Blue Bottle (_Centaurea Cyanus_) 7.32
Foxglove 10.89
Hemlock (_Conium maculatum_) 12.80
Sweet Rush (_Acorus Calamus_) 6.90
Common Reed (_Arundo Phragmites_) 1.44
Celandine (_Chelidonium majus_) 6.85
Equisetum fluviatile 23.60
Equisetum hyemale 11.80
" arvense 13.80
" linosum 15.50
Fucus nodosus 19.03
Fucus vesiculosus 27.63
Laminaria digitata 39.68
LEAVES.
Turnip 9.37
Beet 20.30
Kohl-rabi 18.54
Carrot 10.95
Jerusalem Artichoke 28.30
Hemp 22.00
Hop 17.25
Tobacco 22.62
Spinach 19.76
Chicory 15.67
Poplar 23.00
Red Beech 6.00
White Beech 10.51
Oak 9.80
Elm 16.33
Horse-chesnut 9.08
Maple 28.05
Ash 14.76
Fir 2.31
Acacia 18.20
Olive 6.45
Orange 13.73
Potato 15.10
Tussac Grass 7.15
ROOTS AND TUBERS.
Potato 4.16
Jerusalem Artichoke 5.38
Turnip 13.64
Beet 8.27
Kohl-rabi 6.08
Rutabaga 7.34
Carrot 5.80
Belgian White Carrot 6.22
Mangold-Wurzel 8.78
Parsnip 5.52
Radish 7.35
Chicory 5.21
Madder 8.33
WOODS.
Beech 0.38
Apple 1.29
Cherry 0.28
Birch 1.00
Oak 2.50
Walnut 1.57
Lime 5.00
Horse-chesnut 1.05
Olive 0.58
Mahogany 0.81
Vine 2.57
Larch 0.32
Fir 0.14
Scotch Fir 0.17
Filbert 0.50
Chesnut 3.50
Poplar 0.80
Hazel 0.50
Orange 2.74
Vine 2.57
BARKS.
Beech 6.62
Cherry 10.37
Fir 1.79
Oak 6.00
Horse-chesnut 7.85
Filbert 6.20
Cork 1.12
FRUITS.
Plum 0.40
Cherry 0.43
Strawberry 0.41
Pear 0.41
Apple 0.27
Chesnut 0.99
Cucumber 0.63
Vegetable Marrow 5.10
On examining this table it may be observed that, notwithstanding the
very great variety in the proportion of ash in different plants, some
general relations may be traced. A certain similarity may be observed
between those belonging to the same natural family, the seeds of all the
cereal grains, for instance, containing in round numbers two per cent of
inorganic matters. Leguminous seeds (peas and beans) contain about three
per cent, while in rape-seed, linseed, and the other oily seeds, it
reaches four per cent. In the stems and straws less uniformity exists,
but with the exception of a few extreme cases, the quantity of ash in
general approaches pretty closely to five per cent. Still more
diversified results are obtained from the entire plants; but this
diversity is probably much more apparent than real, and must be, in part
at least, dependent on the proportion existing between the stem and
leaves, for the leaves are peculiarly rich in ash, and a leafy plant
must necessarily yield a higher total percentage of ash, although, if
stems and leaves were separately examined, they might not show so
conspicuous a difference.
The leaves surpass all other parts of plants, in the proportion of
inorganic constituents they contain, the table showing that in some
instances, as in the maple and Jerusalem artichoke, they exceed
one-fourth of the whole weight of the dry matter. In other leaves, and
more especially in those of the coniferae, the proportion is much
smaller. Taking the average of all the analyses hitherto made, it
appears that leaves contain about thirteen per cent of ash, but the
variations on either side are so large that little value is to be
attached to it except as an indication of the general abundance of
mineral matters.
In roots and tubers the variations are less, and all, except the potato
and the turnip, contain about seven per cent of ash.
The smallest proportion of mineral matter is found in wood. In one case
only does the proportion reach five per cent, while the average scarcely
exceeds one, and in the fir the quantity amounts to no more than one
six-hundredth of the dry matter. In the bark the quantity is much
larger, and may be stated at seven per cent.
The general proportion of ash found in different parts of plants is
given in round numbers in the subjoined table:--
Wood 1
Seeds 3
Stems and straws 5
Roots and tubers 7
Bark 7
Leaves 13
The differences in the quantity of ash contained in different parts of
plants are obviously intended to serve a useful purpose, and it is
interesting to observe that the wood which is destined to remain for a
long period, sometimes for several centuries, a part of the plant,
contains the smallest proportion, and it is not improbable that what it
does contain is really due, not to the actual woody matter itself, but
to the sap which permeates its vessels. By this arrangement but a small
proportion of these important mineral matters, which the soil supplies
in very limited quantity, is locked up within the plant, and those which
are absorbed, after circulating through it, and fulfilling their
allotted functions, are accumulated in the leaves, and annually returned
to the soil.
The different proportions of mineral matters contained in the individual
organs of plants is most strikingly illustrated when parallel
experiments are made on the same species; but the number of instances in
which a sufficiently extensive series of analyses has been made to show
this, is comparatively limited, and is confined to the oat, the
orange-tree, and the horse chesnut--each of which has formed the subject
of a very elaborate investigation. The following table gives the results
obtained on the oat:--
+-------------+-----------+---------+-----------+-------+-------+------+
| | Hopetoun | Hopetoun| Potato | Black | Sandy | |
| | Oats, | Oats, | Oats, | Oats, | Oats, | Mean.|
| | North- | Fife- | North- | Edin- | Fife- | |
| | umberland.| shire. | umberland.| burgh.| shire.| |
+-------------+-----------+---------+-----------+-------+-------+------+
| Grain | 2.14 | 1.81 | 2.22 | 2.11 | 1.76 | 2.00|
| Husk | 6.47 | 6.03 | 6.99 | 8.24 | 6.03 | 6.75|
| Chaff | 16.53 | 17.23 | 15.59 | 19.19 | 18.97 | 16.06|
| Leaves | 8.44 | 7.19 | 14.59 | 10.29 | 15.92 | 10.88|
| Upper part | | | | | | |
| of straw | 4.95 | 5.44 | 9.22 | 8.25 | 11.0 | 7.77|
| Middle part | | | | | | |
| of straw | 6.11 | 5.23 | 7.41 | 6.53 | 9.01 | 6.66|
| Lower part | | | | | | |
| of straw | 5.33 | 5.18 | 9.76 | 7.11 | 7.30 | 6.93|
+-------------+-----------+---------+-----------+-------+-------+------+
The specimens of oats on which these analyses were made were from
different districts of country, grown on soils of different quality, and
were, further, of different varieties; and yet they show, on the whole,
a remarkable similarity in the proportion of ash in each part, and
indicate that there is a normal quantity belonging to it. Such a series
of analyses also affords the most convincing proof that the inorganic
matters cannot be fortuitous, and merely absorbed from the soil along
with their organic food, as the old chemists supposed, because, in that
case, they ought to be uniformly distributed throughout the entire
plant, and not accumulated in particular proportions in each individual
organ.
Not only does the proportion of ash vary in the different parts of a
plant, but even in the same part it is greatly influenced by its period
of growth. The laws which regulate these variations are very imperfectly
known, but in general it is observed that during the period of active
growth the quantity of ash is largest. Thus, it has been found that in
early spring the wood of the young shoots of the horse-chesnut contains
9.9 per cent of ash. In autumn this has diminished to 3.4, and the last
year's twigs contain only 1.1 per cent, while in the old wood the
quantity does not exceed 0.5. Saussure has also observed that the
quantity of ash diminishes in certain plants when the seed has ripened.
Thus, he found that the percentages of ash, before flowering, and after
seeding, were as follows:--
Before flowering. With ripe seed.
Sunflower 14.7 9.3
Wheat 7.9 3.3
Maize 12.2 4.6
On the other hand, the quantity of ash in the leaves of trees increases
considerably in autumn, as shown by this table:--
PER-CENTAGE OF ASH IN
May. September.
Oak leaves 5.3 5.5
Poplar 6.6 9.3
Hazel 6.1 7.0
Horse-chesnut 7.2 8.6
In general, the proportion of ash appears to increase as the plant
reaches maturity, and this is particularly seen in the oat, of which
very complete analyses have been made at different periods of its
growth:--
_Proportion of Ash in different parts of the Oat at different periods of
its growth._
+--------------+---------+---------+--------+------------+
| | | | | Grain |
| Date. | Stalks. | Leaves. | Chaff. | with husk. |
+--------------+---------+---------+--------+------------+
| 2d July | 7.83 | 11.35 | ... | 4.91 |
| 9th July | 7.80 | 12.20 | ... | 4.36 |
| 16th July | 7.94 | 12.61 | 6.00 | 3.38 |
| 23d July | 7.99 | 16.45 | 9.11 | 3.62 |
| 30th July | 7.45 | 16.44 | 12.28 | 4.22 |
| 5th August | 7.63 | 16.05 | 13.75 | 4.31 |
| 13th August | 6.62 | 20.47 | 18.68 | 4.07 |
| 20th August | 6.66 | 21.14 | 21.07 | 3.64 |
| 27th August | 7.71 | 22.13 | 22.46 | 3.51 |
| 3d September | 8.35 | 20.90 | 27.47 | 3.65 |
+--------------+---------+---------+--------+------------+
The increase is here principally confined to the leaves and chaff, while
the stalks, which owe their strength to a considerable extent to the
inorganic matters they contain, are equally supplied at all periods of
their growth. In the grain only is there a diminution, but this is
apparent and not real, and is due to the fact that the determination of
the quantity of ash, as made on the grain with its husk, and the former,
which contains only a small quantity of mineral matters, increases much
more rapidly in weight than the latter, when it approaches the period of
ripening, and it is accordingly during the last three weeks of its
growth that this diminution becomes apparent.
The nature of the soil has also a very important influence on the
proportion of mineral matters, and of this an interesting illustration
is given in the following table, which shows the quantities found in the
grain and straw of the same variety of the pea grown on fourteen
different soils:--
+----+-------+--------+
| | Seed. | Straw. |
+----+-------+--------+
| 1 | 2.30 | |
| 2 | 3.25 | 3.43 |
| 3 | 4.27 | 3.62 |
| 4 | 3.40 | 3.39 |
| 5 | 2.99 | 3.90 |
| 6 | 3.19 | 6.80 |
| 7 | 2.53 | 3.90 |
| 8 | 2.27 | 6.59 |
| 9 | 2.69 | 3.49 |
| 10 | 1.61 | 3.91 |
| 11 | 3.11 | 5.28 |
| 12 | 3.34 | 7.57 |
| 13 | 2.78 | 3.76 |
| 14 | 3.01 | 3.38 |
+----+-------+--------+
Although those differences are very large, especially in the straw, and
must be attributed to the soil, it has hitherto been found impossible to
ascertain the nature of the relation subsisting between it and the crops
it yields; indeed, it must obviously be dependent on very complicated
questions, which cannot at present be solved, for it may be observed
that the increase in the grain does not occur simultaneously with that
in the straw, and in several cases a large proportion of ash in the
former is associated with an unusually small amount in the latter. _A
priori_, it might be expected that those soils which are especially rich
in the more important constituents of the ash should yield a produce
containing more than the average quantity, but this is very far from
being an invariable occurrence, and not unfrequently the very reverse
is the case. In some instances the variations may be traced to the soil,
as in the following analyses of the fruit of the horse-chesnut, grown on
an ordinary forest soil, and on a rich soil, produced by the
disintegration of porphyritic rock, in which the latter yields a much
larger quantity of ash:--
Kernel of seed. Green husk. Brown husk.
Forest soil 2.26 4.53 1.70
Porphyry soil 3.36 7.29 2.20
In the majority of instances we fail to establish any connection between
the nature of the soil and the plants it yields, chiefly because we are
still very deficient in analyses of those grown on uncultivated soils;
and on cultivated land it is impossible to draw conclusions, because the
nature of the manure exerts an influence quite as great, if not greater,
than that of the soil itself.
The relative proportion in which the different mineral matters enter
into the composition of the ash varies within very wide limits, as will
be apparent from the following table, containing a selection of the best
analyses of our common cultivated and a few uncultivated plants.
_Table of the Composition of the Ash of different Plants in 100 Parts._
_Note._--Alumina and oxide of manganese occur so rarely, that separate
columns have not been introduced for them, but their quantity is stated
in notes at the end of the table.
+---------------------+--------+------+-----------+---------+------+----------+
| | Potash.| Soda.| Chloride | Chloride| Lime.| Magnesia.|
| | | | of | of | | |
| | | | Potassium.| Sodium. | | |
+---------------------+--------+------+-----------+---------+------+----------+
|Wheat, grain | 30.02 | 3.82 | ... | ... | 1.15| 13.39 |
| straw | 17.98 | 2.47 | ... | ... | 7.42| 1.94 |
| chaff | 9.14 | 1.79 | ... | ... | 1.88| 1.27 |
|Barley, grain | 21.14 | ... | 5.65 | 1.01 | 1.65| 7.26 |
| straw | 11.22 | ... | ... | 2.14 | 5.79| 2.70 |
|Oats, grain[B] | 20.63 | ... | 1.03 | ... | 10.28| 7.82 |
| straw | 19.46 | 1.93 | 2.71 | 4.27 | 7.01| 3.79 |
| chaff[C] | 6.33 | 3.93 | ... | 0.24 | 1.95| 0.38 |
|Rye, grain | 33.83 | 0.39 | ... | ... | 2.61| 12.81 |
| straw | 17.20 | ... | 0.30 | 0.60 | 9.10| 2.40 |
|Maize, grain | 28.37 | 1.74 | ... | trace | 0.57| 13.60 |
| stalks and leaves | 35.26 | ... | ... | 2.29 | 10.53| 5.52 |
|Rice, grain | 20.21 | 2.49 | ... | ... | 7.18| 4.26 |
|Buckwheat, straw | 31.71 | ... | 7.42 | 4.55 | 15.71| 1.66 |
|Peas (gray), seed | 41.70 | ... | 3.82 | 1.24 | 4.78| 5.78 |
| straw | 21.30 | 4.22 | ... | ... | 37.17| 7.17 |
|Beans (common field),| | | | | | |
| grain | 51.72 | 0.54 | ... | ... | 5.20| 6.90 |
| straw | 32.85 | 2.77 | ... | 11.54 | 19.85| 2.53 |
|Tare, straw | 32.82 | ... | 3.27 | 4.03 | 20.78| 5.31 |
| straw | 31.72 | ... | 7.41 | 4.55 | 15.71| 1.66 |
+---------------------+--------+------+-----------+---------+------+----------+
+-----------------------+-------+------------+-----------+----------+---------+
| | Oxide | Phosphoric | Sulphuric | Carbonic | Silica. |
| | of | Acid. | Acid. | Acid. | |
| | Iron. | | | | |
+-----------------------+-------+------------+-----------+----------+---------+
| Wheat, grain | 0.91 | 46.79 | ... | ... | 3.89 |
| straw | 0.45 | 2.75 | 3.09 | ... | 63.89 |
| chaff | 0.37 | 4.31 | ... | ... | 81.22 |
| Barley, grain | 2.13 | 28.53 | 1.91 | ... | 30.68 |
| straw | 1.36 | 7.20 | 1.09 | ... | 68.50 |
| Oats, grain | 3.85 | 50.44 | ... | ... | 4.40 |
| straw | 1.49 | 5.07 | 3.35 | 1.36 | 49.56 |
| chaff | 1.58 | 1.04 | 9.61 | ... | 72.85 |
| Rye, grain | 1.04 | 39.92 | 0.17 | ... | 9.22 |
| straw | 1.40 | 3.80 | 0.80 | ... | 64.50 |
| Maize, grain | 0.47 | 53.69 | ... | ... | 1.55 |
| stalks and leaves | 2.28 | 8.09 | 5.16 | 2.87 | 27.98 |
| Rice, grain | 2.12 | 62.23 | ... | ... | 1.37 |
| Buckwheat, straw | ... | 10.34 | 4.67 | 20.37 | 3.57 |
| Peas (gray), seed | 0.18 | 36.50 | 4.47 | 0.82 | 0.68 |
| straw | 1.07 | 4.65 | 8.68 | 12.48 | 3.23 |
| Beans (common field), | | | | | |
| grain | ... | 28.72 | 3.05 | 3.42 | 0.42 |
| straw | 0.61 | 0.49 | 1.40 | 25.32 | 2.61 |
| Tare, straw | 0.65 | 10.59 | 2.52 | 18.73 | 1.28 |
| straw | ... | 10.34 | 4.67 | 20.37 | 3.57 |
+-----------------------+-------+------------+-----------+----------+---------+
+---------------------+--------+------+-----------+---------+------+----------+
| | Potash.| Soda.| Chloride | Chloride| Lime.| Magnesia.|
| | | | of | of | | |
| | | | Potassium.| Sodium. | | |
+---------------------+--------+------+-----------+---------+------+----------+
|Flax, seed | 34.17 | 1.69| ... | 0.36 | 8.40| 13.11 |
| straw | 21.53 | 3.68| ... | 9.21 | 21.20| 4.20 |
|Rape, seed[D] | 16.33 | 0.34| ... | 0.96 | 8.30| 8.80 |
| straw[E] | 16.63 | 10.57| ... | 2.53 | 21.51| 2.92 |
|Spurry | 26.12 | 1.14| ... | 8.90 | 14.46| 8.88 |
|Chicory root | 34.64 | ...| 8.92 | 2.98 | ...| ... |
|Red clover | 25.60 | ...| 9.08 | 6.02 | 21.57| 8.47 |
|Cow grass, | | | | | | |
| _Trifolium medium_ | 22.78 | ...| 12.39 | 1.86 | 24.42| 8.86 |
|Yellow clover | 27.48 | ...| 11.72 | 8.16 | 17.26| 8.39 |
|Alsike clover | 29.72 | ...| 6.29 | 1.05 | 26.83| 4.01 |
|Lucerne | 27.56 | ...| 11.64 | 1.91 | 20.60| 5.22 |
|Anthoxanthum odoratum| 32.03 | ...| 7.03 | 4.90 | 9.21| 2.53 |
|Alopecurus pratensis | 37.03 | ...| 9.50 | ... | 3.90| 1.28 |
|Avena pubescens | 31.21 | ...| 4.05 | 5.66 | 4.72| 3.17 |
|Bromus erectus | 20.33 | ...| 10.63 | 1.38 | 10.38| 4.99 |
|Bromus mollis | 30.09 | 0.33| ... | 3.11 | 6.64| 2.60 |
|Cynosurus cristatus | 24.99 | ...| 11.60 | ... | 10.16| 2.43 |
|Dactylis glomerata | 29.52 | ...| 17.86 | 3.09 | 5.82| 2.22 |
|Festuca duriuscula | 31.84 | ...| 8.17 | 0.62 | 10.31| 2.83 |
|Holcus lanatus | 34.83 | ...| 3.91 | 6.66 | 8.31| 3.41 |
|Lolium perenne | 24.67 | ...| 13.80 | 7.25 | 9.64| 2.85 |
|Annual ryegrass | 28.99 | 0.87| ... | 5.11 | 6.82| 2.59 |
|Poa annua | 41.86 | ...| 0.47 | 3.35 | 11.69| 2.44 |
|Poa pratensis | 31.17 | ...| 11.25 | 1.31 | 5.63| 2.71 |
|Poa trivialis | 29.40 | ...| 6.90 | ... | 8.80| 3.22 |
|Phleum pratense | 31.09 | ...| 0.70 | 3.24 | 14.94| 5.30 |
|Plantago lanceolata | 33.26 | ... | 4.53 | 8.80 | 19.01| 3.51 |
|Poterium Sanguisorba | 30.26 | ... | 3.27 | 1.35 | 24.82| 4.21 |
|Achillea Millefolia | 30.37 | ... | 20.49 | 3.63 | 13.40| 3.01 |
|Potato, tuber | 43.18 | 0.09| ... | 7.92 | 1.80| 3.17 |
| stem | 39.53 | 3.95| ... | 20.43 | 14.85| 4.10 |
| leaves | 17.27 | ... | 4.95 | 11.37 | 27.69| 7.78 |
|Jerusalem Artichoke | 55.89 | ... | 4.88 | ... | 3.34| 1.30 |
| stem | 38.40 | 0.69| ... | 4.68 | 20.31| 1.91 |
| leaves | 6.81 | 3.72| ... | 1.82 | 40.15| 1.95 |
|Turnip, seed | 21.91 | 1.23| ... | ... | 17.40| 8.74 |
| bulb | 23.70 | 14.75| ... | 7.05 | 11.82| 3.28 |
| leaves | 11.56 | 12.43| ... | 12.41 | 28.49| 2.62 |
|Mangold Wurzel, root | 21.68 | 3.13| ... | 49.51 | 1.90| 1.79 |
| leaves | 8.34 | 12.21| ... | 37.66 | 8.72| 9.84 |
|Carrot, root | 42.73 | 12.11| ... | ... | 5.64| 2.29 |
| leaves | 17.10 | 4.85| ... | 3.62 | 24.05| 0.89 |
|Kohl-rabi, bulb | 36.27 | 2.84| ... | 11.90 | 10.20| 2.36 |
| leaves | 9.31 | ... | 5.99 | 6.66 | 30.31| 3.62 |
|Cow cabbage, head | 40.86 | 2.43| ... | ... | 15.01| 2.39 |
| stalk | 40.93 | 4.05| ... | 2.08 | 10.61| 3.85 |
|Poppy seed | 9.10 | ... | 7.15 | 1.94 | 35.36| 9.49 |
| leaves | 36.37 | ... | 2.50 | 2.51 | 30.24| 6.47 |
|Mustard seed (white) | 25.78 | 0.33| ... | ... | 19.10| 5.90 |
|Radish root | 21.16 | ... | 1.29 | 7.07 | 8.78| 3.53 |
|Tobacco leaves | 36.37 | ... | 2.50 | 2.51 | 30.24| 6.47 |
|Fucus nodosus[F] | 20.03 | 4.58| ... | 24.33 | 9.60| 6.65 |
|Fucus vesiculosus[G] | 20.75 | 6.09| ... | 24.81 | 8.92| 5.83 |
|Laminaria digitata[H]| 12.16 | ... | 2.30 | 19.34 | 4.62| 10.94 |
+---------------------+--------+------+-----------+---------+------+----------+
+---------------------+-------+------------+-----------+----------+---------+
| | Oxide | Phosphoric | Sulphuric | Carbonic | Silica. |
| | of | Acid. | Acid. | Acid. | |
| | Iron. | | | | |
+---------------------+-------+------------+-----------+----------+---------+
|Flax, seed | 0.50 | 38.54 | 1.56 | 0.22 | 1.45 |
| straw | 5.58 | 7.53 | 3.39 | 15.75 | 7.92 |
|Rape, seed | 1.79 | 31.90 | 5.38 | 5.44 | 19.98 |
| straw | 1.30 | 4.68 | 3.90 | 23.04 | 11.80 |
|Spurry | ... | 10.20 | 1.79 | 27.38 | 1.14 |
|Chicory root | ... | ... | ... | ... | ... |
|Red clover | 1.26 | 4.09 | 2.96 | 18.05 | 1.95 |
|Cow grass, | | | | | |
| _Trifolium medium_ | 1.09 | 4.94 | 2.66 | 20.16 | 1.12|
|Yellow clover | 1.40 | ... | 4.82 | 4.31 | 1.76 |
|Alsike clover | 0.71 | 5.64 | 3.25 | 20.74 | 1.73 |
|Lucerne | 2.23 | 6.47 | 4.80 | 15.94 | 2.63 |
|Anthoxanthum odoratum| 1.18 | 10.09 | 3.39 | 1.26 | 28.35 |
|Alopecurus pratensis | 0.47 | 6.25 | 2.16 | 0.65 | 38.75 |
|Avena pubescens | 0.72 | 10.82 | 3.37 | ... | 36.28 |
|Bromus erectus | 0.26 | 7.53 | 5.46 | 0.55 | 38.48 |
|Bromus mollis | 0.28 | 9.62 | 4.91 | 9.07 | 33.34 |
|Cynosurus cristatus | 0.18 | 7.24 | 3.20 | ... | 40.11 |
|Dactylis glomerata | 0.59 | 8.60 | 3.52 | 2.09 | 26.65 |
|Festuca duriuscula | 0.78 | 12.07 | 3.45 | 1.38 | 28.53 |
|Holcus lanatus | 0.31 | 8.02 | 4.41 | 1.82 | 28.31 |
|Lolium perenne | 0.21 | 8.73 | 5.20 | 0.49 | 27.13 |
|Annual ryegrass | 0.28 | 10.07 | 3.45 | ... | 41.79 |
|Poa annua | 1.57 | 9.11 | 10.18 | 3.29 | 16.03 |
|Poa pratensis | 0.28 | 10.02 | 4.26 | 0.40 | 32.93 |
|Poa trivialis | 0.29 | 9.13 | 4.47 | 0.29 | 37.50 |
|Phleum pratense | 0.27 | 11.29 | 4.86 | 4.02 | 31.09 |
|Plantago lanceolata | 0.90 | 7.08 | 6.11 | 14.40 | 2.37 |
|Poterium Sanguisorba | 0.86 | 7.81 | 4.84 | 21.72 | 0.83 |
|Achillea Millefolia | 0.21 | 7.13 | 2.44 | 9.36 | 9.92 |
|Potato, tuber | 0.44 | 8.61 | 15.24 | 18.29 | 1.94 |
| stem | 1.34 | 6.68 | 6.56 | ... | 2.56 |
| leaves | 4.50 | 13.60 | 6.37 | ... | 6.47 |
|Jerusalem Artichoke | 0.45 | 16.99 | 3.77 | 11.80 | 1.52 |
| stem | 0.88 | 2.97 | 3.23 | 25.40 | 1.51 |
| leaves | 1.14 | 6.61 | 2.21 | 24.31 | 17.25 |
|Turnip, seed | 1.95 | 40.17 | 7.10 | 0.82 | 0.67 |
| bulb | 0.47 | 9.31 | 16.13 | 10.74 | 2.69 |
| leaves | 3.02 | 4.85 | 10.36 | 6.18 | 8.04 |
|Mangold Wurzel, root | 0.52 | 1.65 | 3.14 | 15.23 | 1.40 |
| leaves | 1.46 | 5.89 | 6.54 | 6.92 | 2.35 |
|Carrot, root | 0.51 | 12.31 | 4.26 | 18.00 | 1.11 |
| leaves | 3.43 | 6.21 | 5.08 | 23.15 | 11.61 |
|Kohl-rabi, bulb | 0.38 | 13.45 | 11.43 | 10.24 | 0.83 |
| leaves | 5.50 | 9.43 | 10.63 | 8.97 | 9.57 |
|Cow cabbage, head | 0.77 | 12.53 | 7.27 | 16.68 | 1.66 |
| stalk | 0.41 | 19.57 | 11.11 | 6.33 | 1.04 |
|Poppy seed | 0.41 | 31.38 | 1.92 | ... | 3.24 |
| leaves | 2.14 | 3.28 | 5.09 | ... | 11.40 |
|Mustard seed (white) | 0.39 | 44.97 | 2.19 | ... | 1.31 |
|Radish root | 1.19 | 41.09 | 7.71 | ... | 8.17 |
|Tobacco leaves | 2.18 | 3.24 | 5.09 | ... | 11.40 |
|Fucus nodosus | 0.26 | 1.71 | 21.97 | 6.39 | 0.38 |
|Fucus vesiculosus | 0.35 | 2.14 | 28.01 | 2.20 | 0.67 |
|Laminaria digitata | 0.45 | 1.75 | 7.26 | 15.23 | 1.20 |
+---------------------+-------+------------+-----------+----------+---------+
A simple inspection of this table leads to various interesting
conclusions. It is particularly to be observed that some of the
constituents of the ash are not invariably present, and two at
least--namely, alumina and manganese--are found so rarely as to justify
the inference that they are not indispensable. Of the other substances,
iodine is restricted exclusively to sea-plants, but to them it appears
to be essential. Oxide of iron, which occurs only in small quantities,
has sometimes been considered fortuitous, but it is almost invariably
present, and the experiments of Prince Salm Horstmar leave no doubt that
it is essential to the plant. Its function is unknown, but it is an
important constituent of the blood of herbivorous animals, and may be
present in the plant, less for its own benefit than for that of the
animal of which it is destined to become the food.
Soda appears to be a comparatively unimportant constituent of the ash,
of which it generally forms but a small proportion, although the
instances of its entire absence are rare. In the cruciferous plants
(turnip, rape, etc.) it is found abundantly, and to them it appears
indispensable, but in most other plants it admits of replacement by
potash. It seems probable that where the soil is rich in the latter
substance, plants will select that alkali in preference to soda; but as
they must have a certain quantity of alkali, the latter may supply the
place of the former where it is deficient. Cultivation, probably by
enriching the soil in that element, increases the proportion of potash
found in the ash of plants, as is remarkably seen in the asparagus,
which gave the following quantities of alkalies and chlorine:--
Wild. Cultivated.
Potash 18.8 50.5
Soda 16.2 trace.
Chlorine 16.5 8.3
The soda having almost entirely disappeared in the cultivated plant,
while a corresponding increase had taken place in the quantity of
potash.
Potash is one of the most important elements of the ash of all plants,
rarely forming less than 20, and sometimes more than 50 per cent of its
weight. The latter proportion occurs chiefly in the roots and tubers,
but it is also abundant in all seeds and in the grasses. The straw, and
particularly the chaff of the cereals, and the leaves of most plants,
contain it in smaller quantity, although exceptions to this are not
unfrequent, one of the most curious being the case of poppy-seed, which
contains only about 12 per cent, while the leaves yield upwards of 37
per cent.
The proportion of lime varies within very wide limits, being sometimes
as low as 1, and in other plants reaching 40 per cent of their ash. The
former proportion occurs in the grains of the cerealia, and the latter
in the leaves of some plants, and more especially in the Jerusalem
artichoke. The turnip and some of the leguminous plants also contain it
abundantly.
Magnesia is generally found in small quantity. It is largest in the
grains, amounting in them to about 12 or 13 per cent of the ash, but in
other plants it varies from 2 to 4 per cent. Although small in quantity,
it is an important substance, and apparently cannot be dispensed with;
at least there is no instance known of its entire absence.
_Chlorine_ is by no means an invariable constituent of the ash, although
it is generally present, and sometimes in considerable quantity. It is
most abundant when the proportion of soda is large, and exists in the
ash principally in combination with that base as common salt. The
relation between these two elements may be traced more or less
distinctly throughout the whole table of analyses, and conspicuously in
that of mangold-wurzel, where the common salt amounts to almost exactly
one-half of the whole mineral matter. The analyses of the cultivated and
uncultivated asparagus also show that a diminution in the soda is
accompanied by a reduction in the proportion of chlorine.
_Sulphuric Acid_ is an essential constituent of the ash. But it is to be
observed that it is in some instances entirely, and in all partially, a
product of the combustion to which the plant has been submitted in order
to obtain the ash. It is partly derived from the sulphur contained in
the albuminous compounds, which is oxidised and converted into sulphuric
acid during the process of burning the organic matter, and remains in
the ash. The quantity of sulphuric acid found in the ash is, however, no
criterion of that existing in the plant, for a considerable quantity of
it escapes during burning. The extent to which this occurs in particular
instances is well illustrated by reference to the case of white mustard,
which yields an ash containing only 2.19 of sulphuric acid, equivalent
to 0.9 of sulphur; and if calculated on the seed itself, this will
amount to no more than 0.039 per cent, while experiments made in another
manner prove it to contain about thirty times as much, or more than 1
per cent. For the purpose of determining the total quantity of sulphur
which the plants contain in their natural state, it is necessary to
oxidise them by means of nitric acid; and from such experiments the
following table, showing the _total_ amount of sulphur contained in 100
parts of different plants, dried at 212 deg., has been constructed:--
Poa palustris 0.165
Lolium perenne 0.310
Italian Ryegrass 0.329
Trifolium pratense 0.107
repens 0.099
Lucerne 0.336
Vetch 0.178
Potato tuber 0.082
tops 0.206
Carrot, root 0.092
tops 0.745
Mangold-Wurzel, root 0.058
tops 0.502
Swede, root 0.435
tops 0.458
Rape 0.448
Drumhead Cabbage 0.431
Wheat, grain 0.068
straw 0.245
Barley, grain, 0.053
straw 0.191
Oats, grain 0.103
straw 0.289
Rye, grain 0.051
Beans 0.056
Peas 0.127
Lentils 0.110
Hops 1.063
Gold of Pleasure 0.253
Black Mustard 1.170
White Mustard 1.050
_Phosphoric acid_, which may be looked upon as the most important
mineral constituent of plants, is found to be present in very variable
proportions. The straws, stems, and leaves contain it in comparatively
small quantity, but in the seeds of all plants it is very abundant. In
these of the cereals it constitutes nearly half of their whole mineral
components, and it rarely falls below 30 per cent.
_Carbonic acid_ occurs in very variable quantities in the ash. It is of
comparatively little importance in itself, and is really produced by the
oxidation of part of the carbonaceous matters of the plant; but it has a
special interest, in so far as it shows that part of the bases contained
in the plant must in its natural state have been in union with organic
acids, or combined in some way with the organic constituents of the
plant.
_Silica_ is an invariable constituent of the ash, but in most plants
occurs but in small quantity. The cereals and grasses form an exception
to this rule, for in them it is an abundant and important element. It is
not, however, uniformly distributed through them, but is accumulated to
a large extent in the stem, to the strength and rigidity of which it
greatly contributes. The hard shining layer which coats the exterior of
straw, and which is still more remarkably seen on the surface of the
bamboo, consists chiefly of silica; and in the latter plant this element
is sometimes so largely accumulated, that concretions resembling opal,
and composed entirely of it, are found loose within its joints. The
necessity for a large supply of silica in the stems of other plants does
not exist, and in them it rarely exceeds 5 or 6 per cent, but in some
leaves it is more abundant.
A knowledge of the composition of the ash of plants is of considerable
importance in a practical point of view, and enables us in many
instances to explain why some plants will not grow upon particular soils
on which others flourish. Thus, for instance, a plant which contains a
large quantity of lime, such as the bean or turnip, will not grow in a
soil in which that element is deficient, although wheat or barley, which
require but little lime, may yield excellent crops. Again, if the soil
be deficient in phosphoric acid, those plants only will grow luxuriantly
which require but a small quantity of that element, and hence it follows
that on such a soil plants cultivated for the sake of their stems,
roots, or leaves, in which the quantity of phosphoric acid is small, may
yield a good return; while others, cultivated for the sake of their
seed, in which the great proportion of that constituent of the ash is
accumulated, may yield a very small crop. It is obvious also that even
where a soil contains a proper quantity of all its ingredients, the
repeated cultivation of a plant which removes a large quantity of any
individual element, may, in the course of time, so far reduce the amount
of that substance as to render the soil incapable of any longer
producing that plant, although, if it be replaced by another which
requires but little of the element thus removed, it may again produce an
abundant crop. On this principle also, attempts have been made to
explain the rotation of crops, which has been supposed to depend on the
cultivation in successive years of plants which abstract from the soil
preponderating quantities of different mineral matters. But though this
has unquestionably a certain influence, we shall afterwards see reason
to doubt whether it affords a sufficient explanation of all the observed
phenomena.
It may be observed, on examining the table of the percentage and
position of the ash, that some plants are especially rich in alkalies,
while in others lime or silica preponderate, and it would therefore be
the object of the farmer to employ, in succession, crops containing
these elements in different proportions. In carrying out this view,
attempts have been made to classify different plants under the heads of
silica plants, lime plants, and potash plants; and the following table,
extracted from Liebig's _Agricultural Chemistry_, in which the
constituents of the ash are grouped under the three heads of salts of
potash and soda, lime and magnesia, and silica, gives such a
classification as far as it is at present possible:--
----------------------------------------------------------------------
| | Salts of | Salts of | Silica.
| | Potash and | Lime and |
| | Soda. | Magnesia. |
|----------------------------------|------------|-----------|---------
|Silica { Oat straw with seeds | 34.00 | 4.00 | 62.00
|Plants. { Wheat straw | 22.50 | 7.20 | 61.50
| { Barley straw with seeds| 19.00 | 25.70 | 55.30
| { Rye straw | 18.65 | 16.52 | 63.89
| { Good hay | 6.00 | 34.00 | 60.00
|Lime { Tobacco | 24.34 | 67.44 | 8.30
|Plants { Pea straw | 27.82 | 63.74 | 7.81
| { Potato plant | 4.20 | 59.40 | 36.40
| { Meadow Clover | 39.20 | 56.00 | 4.90
|Potash { Maize straw | 72.45 | 6.50 | 18.00
|Plants. { Turnips | 81.60 | 18.40 | --
| { Beet root | 88.00 | 12.00 | --
| { Potatoes | 85.81 | 14.19 | --
| { Jerusalem Artichoke | 84.30 | 15.70 | --
----------------------------------------------------------------------
The special application of these facts must be reserved till we come to
treat of the rotation of crops.
It is manifest that, as the crops removed from the soil all contain a
greater or less amount of inorganic matters, they must be continually
undergoing diminution, and at length be completely exhausted unless
their quantity is maintained from some external source. In many cases
the supply of these substances is so large that ages may elapse before
this becomes apparent, but where the quantity is small, a system of
reckless cropping may reduce a soil to a state of absolute sterility. A
remarkable illustration of this fact is found in the virgin soils of
America, from which the early settlers reaped almost unheard-of crops,
but, by injudicious cultivation, they were soon exhausted and abandoned,
new tracts being brought in and cultivated only to be in their turn
abandoned. The knowledge of the composition of the ash of plants assists
us in ascertaining how this exhaustion may be avoided, and indicates the
mode in which such soils may be preserved in a fertile state.
FOOTNOTES:
[Footnote A: Apparently a species of Sinapis.]
[Footnote B: Oxide of Manganese, 0.42.]
[Footnote C: Oxide of Manganese, 0.92.]
[Footnote D: Alumina, 1.02.]
[Footnote E: Alumina, 0.63.]
[Footnote F: Iodide of Potassium, 0.44; Sulphuret of Sodium, 3.66.]
[Footnote G: Iodide of Potassium, 0.23.]
[Footnote H: Iodide of Potassium, 1.68.]
CHAPTER V.
THE SOIL--ITS CHEMICAL AND PHYSICAL CHARACTERS.
No department of agricultural chemistry is surrounded with greater
difficulties and uncertainties than that relating to the properties of
the soil. When chemistry began to be applied to agriculture, it was not
unnaturally supposed that the examination of the soil would enable us to
ascertain with certainty the mode in which it might be most
advantageously improved and cultivated, and when, as occasionally
happened, analysis revealed the absence of one or more of the essential
constituents of the plant in a barren soil, it indicated at once the
cause and the cure of the defect. But the expectations naturally formed
from the facts then observed have been as yet very partially fulfilled;
for, as our knowledge has advanced, it has become apparent that it is
only in rare instances that it is possible satisfactorily to connect
together the composition and the properties of a soil, and with each
advancement in the accuracy and minuteness of our analysis the
difficulties have been rather increased than diminished. Although it is
occasionally possible to predicate from its composition that a
particular soil will be incapable of supporting vegetation, it not
unfrequently happens that a fruitful and a barren soil are so similar
that it is impossible to distinguish them from one another, and cases
even occur in which the barren appears superior to the fertile soil. The
cause of this apparently anomalous phenomenon lies in the fact that
analysis, however minute, is unable to disclose all the conditions of
fertility, and that it must be supplemented by an examination of its
physical and other chemical properties, which are not indicated by
ordinary experiments. Of late years very considerable progress has been
made in the investigation of the properties of the soil, and many facts
of great importance have been discovered, but we are still unable to
assert that all the conditions of fertility are yet known, and the
practical application of those recently discovered is still very
imperfectly understood.
It must not be supposed that a careful analysis of a soil is without
value, for very important practical deductions may often be drawn from
it, and when this is not practicable it is not unfrequently due to its
being imperfect or incomplete, for it is so complex that the cases in
which all the necessary details have been eliminated are even now by no
means numerous. In fact, the want of a large number of thorough analyses
of soils of different kinds is a matter of some difficulty, and so soon
as a satisfactory mode of investigation can be determined upon, a full
examination of this subject would be of much importance.
_Origin of Soils._--The constituents of the soil, like those of the
plant, may be divided into the great classes of organic and inorganic.
The origin of the former has been already discussed: they are derived
from the decay of plants which have already grown upon the soil, and
which, in various stages of decomposition, form the numerous class of
substances grouped together under the name of humus. The organic
substances may therefore be considered as in a manner secondary
constituents of the soil, which have been accumulated in it as the
consequence of the growth and decay of successive generations of plants,
while the primeval soil consisted of inorganic substances only.
The inorganic constituents of the soil are obtained as the result of a
succession of chemical changes going on in the rocks which protrude
through the surface of the earth. We have only to examine one of these
rocks to observe that it is constantly undergoing a series of important
changes. Under the influence of air and moisture, aided by the powerful
agency of frost, it is seen to become soft, and gradually to
disintegrate, until it is finally converted into an uniform powder, in
which the structure of the original rock is with difficulty, if at all
distinguishable. The rapidity with which these changes take place is
very variable; in the harder rocks, such as granite and mica slate it is
so slow as to be scarcely perceptible, while in others, such as the
shales of the coal formation, a very few years' exposure is sufficient
for the purpose. These actions, operating through a long series of
years, are the source of the inorganic constituents of all soils.
Geology points to a period at which the earth's surface must have been
altogether devoid of soil, and have consisted entirely of hard
crystalline rocks, such as granite and trap, by the disintegration of
which, slowly proceeding from the creation down to the present time, all
the soils which now cover the surface have been formed. But they have
been produced by a succession of very complicated processes; for these
disintegrated rocks being washed away in the form of fine mud, or at
least of minute particles, and being deposited at the bottom of the
primeval seas, have there hardened into what are called sedimentary
rocks, which being raised above the surface by volcanic action or other
great geological forces, have been again disintegrated to yield
different soils. Thus, then, all soils are directly or indirectly
derived from the crystalline rocks, those overlying them being formed
immediately by their decomposition, while those found above the
sedimentary rocks may be traced back through them to the crystalline
rocks from which they were originally formed.
Such being the case, the composition of different soils must manifestly
depend on that of the crystalline rocks from which they have been
derived. Their number is by no means large, and they all consist of
mixtures in variable proportions of quartz, felspar, mica, hornblende,
augite, and zeolites. With the exception of quartz and augite, these
names are, however, representatives of different classes of minerals.
There are, for instance, several different minerals commonly classified
under the name of felspar, which have been distinguished by
mineralogists by the names of orthoclase, albite, oligoclase, and
labradorite; and there are at least two sorts of mica, two of
hornblende, and many varieties of zeolites.
Quartz consists of pure silica, and when in large masses is one of the
most indestructible rocks. It occurs, however, intermixed with other
minerals in small crystals, or irregular fragments, and forms the entire
mass of pure sand.
The four kinds of felspar which have been already named are compounds of
silica with alumina, and another base which is either potash, soda, or
lime. Their composition is as follows, two examples of each being
given--
----------------------------------------------------------------------------
| Orthoclase. | Albite. | Oligoclase. | Labradorite. |
------------------|--------------|-------------|-------------|--------------|
Silica | 65.72| 65.00| 67.99| 68.23| 62.70| 63.51| 54.66| 54.67|
Alumina | 18.57| 18.64| 19.61| 18.30| 23.80| 23.09| 27.87| 27.89|
Peroxide of iron | traces| 0.83| 0.70| 1.01| 0.62| -- | -- | 0.31|
Oxide of manganese| traces| 0.13| -- | -- | -- | -- | -- | -- |
Lime | 0.34| 1.23| 0.66| 1.26| 4.60| 2.44| 12.01| 10.60|
Magnesia | 0.10| 1.03| -- | 0.51| 0.02| 0.77| -- | 0.18|
Potash | 14.02| 9.12| -- | 2.53| 1.05| 2.19| -- | 0.49|
Soda | 1.25| 3.49| 11.12| 7.99| 8.00| 9.37| 5.46| 5.05|
----------------------------------------------------------------------------|
| 100.00| 99.47|100.08| 99.83|100.79|101.37|100.00| 99.19 |
-----------------------------------------------------------------------------
It is obvious that soils produced by the disintegration of these
minerals must differ materially in quality. Those yielded by orthoclase
must generally abound in potash, while albite and labradorite,
containing little or none of that element, must produce soils in which
it is deficient. The quality of the soil they yield is not however
entirely dependent on the nature of the particular felspar which yields
it, but is also intimately connected with the extent to which the
decomposition has advanced. It is observed that different felspars
undergo decomposition with different degrees of rapidity but after a
certain time they all begin to lose their peculiar lustre, acquire a
dull and earthy appearance, and at length fall into a more or less white
and soft powder. During this change water is absorbed, and, by the
decomposing action of the air, the alkaline silicate is gradually
rendered soluble, and at length entirely washed away, leaving a
substance which, when mixed with water, becomes plastic, and has all the
characters of common clay. The nature of this change will be best seen
by the following analysis of the clay produced during this composition,
which is employed in the manufacture of porcelain under the name of
kaolin, or china clay--
Silica 46.80
Alumina 36.83
Peroxide of iron 3.11
Carbonate of lime 0.55
Potash 0.27
Water 12.44
----
100.00
In this instance the decomposition of the felspar had reached its limit,
a mere trace of potash being left, but if taken at different stages of
the process, variable proportions of that alkali are met with. This
decomposition of felspar is the source of the great deposits of clay
which are so abundantly distributed over the globe, and it takes place
with nearly equal rapidity with potash and soda felspar. It is rarely
complete, and the soils produced from it frequently contain a
considerable proportion of the undecomposed mineral, which continues for
a long period to yield a supply of alkalies to the plants which grow on
them.
Mica is a very widely distributed mineral, and two varieties of it are
distinguished by mineralogists, one of which is characterised by the
large quantity of magnesia it contains. Different specimens are found to
vary very greatly in composition, but the following analyses may
represent their most usual composition:
MICA.
|----------------|
Potash. Magnesia.
Silica 46.36 42.65
Alumina 36.80 12.96
Peroxide of iron 4.53 ---
Protoxide of iron --- 7.11
Oxide of manganese 0.02 1.06
Magnesia --- 25.75
Potash 9.22 6.03
Hydrofluoric acid 0.70 0.62
Water 1.84 3.17
---- ----
99.47 99.35
Mica undergoes decomposition with extreme slowness, as is at once
illustrated by the fact that its shining scales may frequently be met
with entirely unchanged in the soil. Its persistence is dependent on the
small quantity of alkaline constituents which it contains; and for this
reason it is observed that the magnesian micas undergo decomposition
less rapidly than those containing the larger quantity of potash.
Eventually, however, both varieties become converted into clay, their
magnesia and potash passing gradually into soluble forms.
Hornblende and augite are two widely distributed minerals, which are so
similar in composition and properties that they may be considered
together. Of the former two varieties, basaltic and common have been
distinguished, and their composition is given below:--
Hornblende.
|----------------|
Common. Basaltic. Augite.
Silica 41.50 42.24 50.12
Alumina 15.75 13.92 4.20
Protoxide of iron 7.75 14.59 11.60
Oxide of manganese 0.25 0.33 --
Lime 14.09 12.24 20.55
Magnesia 19.40 13.74 13.70
Water 0.50 -- --
---- ---- ----
99.24 97.05 99.67
In these minerals alkalies are entirely absent, and their decomposition
is due to the presence of protoxide of iron, which readily absorbs
oxygen from the air, when the magnesia is separated and a ferruginous
clay left.
The minerals just referred to, constitute the great bulk of the mountain
masses, but they are associated with many others which take part in the
formation of the soil. Of these the most important are the zeolites
which do not occur in large masses but are disseminated through the
other rocks in small quantity. They form a large class of minerals of
which Thomsonite and natrolite may be selected as examples--
Thomsonite. Natrolite.
Silica 38.73 48.68
Alumina 30.84 26.36
Lime 13.43 --
Potash 0.54 0.23
Soda 3.85 16.00
Water 13.09 9.55
---- ----
100.48 100.83
They are chiefly characterized by containing their silica in a soluble
state, and hence may yield that substance to the plants in a condition
particularly favourable for absorption.
It is obvious from what has been stated that all these minerals are
capable, by their decomposition, of yielding soft porous masses having
the physical properties of soils, but most of them would be devoid of
many essential ingredients, while not one of them would yield either
phosphoric acid, sulphuric acid, or chlorine. It has, however, been
recently ascertained that certain of these minerals, or at least the
rocks formed from them, contain minute, but distinctly appreciable
traces of phosphoric acid, although in too small quantity to be detected
by ordinary analysis; and small quantities of chlorine and sulphuric
acid may also in most instances be found.
Still it will be observed that most of these minerals would yield a soil
containing only two or three of those substances, which, as we have
already learned, are essential to the plant. Thus, potash felspar, while
it would give abundance of potash, would be but an inefficient source
of lime and magnesia; and labradorite, which contains abundance of lime,
is altogether deficient in magnesia and potash.
Nature has, however, provided against this difficulty, for she has so
arranged it that these minerals rarely occur alone, the rocks which form
our great mountain masses being composed of intimate mixtures of two or
more of them, and that in such a manner that the deficiencies of the one
compensate those of the other. We shall shortly mention the composition
of these rocks.
Granite is a mixture of quartz, felspar, and mica in variable
proportions, and the quality of the soil it yields depends on whether
the variety of felspar present be orthoclase or albite. When the former
is the constituent, granite yields soils of tolerable fertility,
provided their climatic conditions be favourable; but it frequently
occurs in high and exposed situations which are unfavourable to the
growth of plants. Gneiss is a similar mixture, but characterised by the
predominance of mica, and by its banded structure. Owing to the small
quantity of felspar which it contains, and the abundance of the
difficulty decomposable mica, the soils formed by its disintegration are
generally inferior. Mica slate is also a mixture of quartz, felspar, and
mica, but consisting almost entirely of the latter ingredient, and
consequently presenting an extreme infertility. The position of the
granite, gneiss, and mica slate soils in this country is such that very
few of them are of much value; but in warm climates they not
unfrequently produce abundant crops of grain. Syenite is a rock similar
in composition to granite, but having the mica replaced by hornblende,
which by its decomposition yields supplies of lime and magnesia more
readily than they can be obtained from the less easily disintegrated
mica. For this reason soils produced from the syenitic rocks are
frequently possessed of considerable fertility.
The series of rocks of which greenstone and trap are types, and which
are very widely distributed, differ greatly in composition from those
already mentioned. They are divisible into two great classes, which have
received the names of diorite and dolerite, the former a mixture of
albite and hornblende, the latter of augite and labradorite, sometimes
with considerable quantities of a sort of oligoclase containing both
soda and lime, and of different kinds of zeolitic minerals. Generally
speaking, the soils produced from diorite are superior to those from
dolerite. The albite which the former contains undergoes a rapid
decomposition, and yields abundance of soda along with some potash,
which is seldom altogether wanting, while the hornblende supplies both
lime and magnesia. Dolerite, when composed entirely of augite and
labradorite, produces rather inferior soils; but when it contains
oligoclase and zeolites, and comes under the head of basalt, its
disintegration is the source of soils remarkable for their fertility;
for these latter substances undergoing rapid decomposition furnish the
plants with abundant supplies of alkalies and lime, while the more
slowly decomposing hornblende affords the necessary quantity of
magnesia. In addition to these, the basaltic rocks are found to contain
appreciable quantities of phosphoric acid, so that they are in a
condition to yield to the plant almost all its necessary constituents.
The different rocks now mentioned, with a few others of less general
distribution, constitute the whole of our great mountain masses; and
while their general composition is such as has been stated, they
frequently contain disseminated through them quantities of other
minerals which, though in trifling quantity, nevertheless add their
quota of valuable constituents to the soils. Moreover, the exact
composition of the minerals of which the great masses of rocks are
composed is liable to some variety. Those which we have taken as
illustrations have been selected as typical of the minerals; but it is
not uncommon to find albite containing 2 or 3 per cent of potash,
labradorite with a considerable proportion of soda, and zeolitic
minerals containing several per cent of potash, the presence of which
must of course considerably modify the properties of the soils produced
from them. They are also greatly affected by the mechanical influences
to which the rocks are exposed; and being situated for the most part in
elevated positions, they are no sooner disintegrated than they are
washed down by the rains. A granite, for instance, as the result of
disintegration, has its felspar reduced to an impalpable powder, while
its quartz and mica remain, the former entirely, the latter in great
part, in the crystalline grains which existed originally in the granite.
If such a disintegrated granite remains on the spot, it is easy to see
what its composition must be; but if exposed to the action of running
water, by which it is washed away from its original site, a process of
separation takes place, the heavy grains of quartz are first deposited,
then the lighter mica, and lastly the felspar. Thus there may be
produced from the same granite, soils of very different nature and
composition, from a pure and barren sand to a rich clay formed entirely
of felspathic debris.
The sedimentary or stratified rocks are formed of particles carried down
by water and deposited at the bottom of the primeval seas from which
they have been upheaved in the course of geological changes. The
process of their formation may be watched at the present day at the
mouths of all great rivers, where a delta composed of the suspended
matters carried down by the waters is slowly formed. The nature of these
rocks must therefore depend entirely on that of the country through
which the river flows. If its course runs through a country in which
lime is abundant, calcareous rocks will be deposited, and if it passes
through districts of different geological characters the deposit must
necessarily consist of a mixture of the disintegrated particles of the
different rocks the river has encountered. For this reason it is
impossible to enter upon a detailed account of their composition. It is
to be observed, however, that the particles of which they are composed,
though originally derived from the crystalline rocks, have generally
undergone a complex series of changes, geology teaching that, after
deposition, they may in their turn undergo disintegration and be carried
away by water, to be again deposited. Their composition must therefore
vary not merely according to the nature of the rock from which they have
been formed, but also according to the extent to which the decomposition
has gone, and the successive changes to which they have been exposed.
They may be reduced to the three great classes of clays, including the
different kinds of clay slates, shales, etc., sandstone and limestone.
It must be added also, that many of them contain carbonaceous matters
produced by the decomposition of early races of plants and animals, and
that mixtures of two or more of the different classes are frequent.
The purest clays are produced by the decomposition of felspar, but
almost all the crystalline rocks may produce them by the removal of
their alkalies, iron, lime, etc. Where circumstances have been
favourable, the whole of these substances are removed, and the clay
which remains consists almost entirely of silica and alumina, and yields
a soil which is almost barren, not merely on account of the deficiency
of many of the necessary elements of plants, but because it is so stiff
and impenetrable that the roots find their way into it with difficulty.
It rarely happens, however, that decomposition has advanced so far as to
remove the whole of the alkalies, which is exemplified by the following
analyses of the fire clay of the coal formation, and of transition clay
slate:--
Transition Fire Clay.
Clay Slate.
Silica 60.03 54.77
Alumina 14.91 28.61
Peroxide of iron 8.94 4.92
Lime 2.08 0.58
Magnesia 4.22 1.14
Potash 3.87 1.00
Soda -- 0.24
Carbonic acid } 5.67 8.24
Water }
---- ----
99.72 99.50
The sandstones are derived from the siliceous particles of granite and
other rocks, and consist in many cases of nearly pure silica, in which
case their disintegration produces a barren sand, but they more
frequently contain an admixture of clay and micaceous scales, which
sometimes form a by no means inconsiderable portion of them. Such
sandstones yield soils of better quality, but they are always light and
poor. Where they occur interstratified with clays, still better soils
are produced, the mutual admixture of the disintegrated rocks affording
a substance of intermediate properties, in which the heaviness of the
clay is tempered by the lightness of the sandstone.
Limestone is one of the most widely distributed of the stratified rocks,
and in different localities occurs of very different composition.
Limestones are divided into two classes, common and magnesian; the
former a nearly pure carbonate of lime, the latter a mixture of that
substance with carbonate of magnesia. But while these are the principal
constituents, it is not uncommon to find small quantities of phosphate
and sulphate of lime, which, however trifling their proportions, are not
unimportant in an agricultural point of view. The following analyses
will serve to illustrate the general composition of these two sorts of
limestone as they occur in the early geological formations:--
COMMON. MAGNESIAN.
|-------------------------| |----------------------|
Mid-Lothian. Sutherland. Sutherland. Dumfries.
Silica 2.00 7.42 6.00 2.31
Peroxide of iron } 0.45 0.76 1.57 2.00
and alumina }
Carbonate of lime 93.61 84.11 50.21 58.81
Carbonate of } 1.62 7.45 41.22 36.41
magnesia }
Phosphate of lime 0.56 ... ... ...
Sulphate of lime 0.92 ... ... 0.10
Organic matter 0.20 ... ... ...
Water 0.50 ... 0.69 ...
---- ---- ---- ----
99.86 99.74 99.69 99.63
These limestones are hard and possess to a greater or less extent a
crystalline texture. They are replaced in later geological periods by
others which are much softer, and often purer, of which the oolitic
limestones, so called from their resemblance to the roe of a fish, and
chalk are the most important. Other limestones are also known which
contain an admixture of clay. The soils produced by the disintegration
of limestone and chalk are generally light and porous, but when mixed
with clay, possess a very high degree of fertility, and this is
particularly the case with chalk, which yields some of the most valuable
of all soils. But it is true only of the common limestones, for
experience has shown that those which contain magnesia in large quantity
are often prejudicial to vegetation, and sometimes yield barren or
inferior soils.
Such are the general characters of the three great classes of stratified
rocks; any attempt to particularise the numerous varieties of each would
lead us far beyond the limits of the present work. It is necessary,
however, to remark, that in many instances one variety passes into the
other, or, more correctly speaking, sedimentary rocks occur, which are
mixtures of two or more of the three great classes. In fact, the name
given to each really expresses only the preponderating ingredient, and
many sandstones contain much clay, shales and clay slates abound in
lime, and limestones in sand or clay, so that it may sometimes be a
matter of some difficulty to decide to which class they belong. Such
mixtures usually produce better soils than either of their constituents
separately, and accordingly, in those geological formations in which
they occur, the soils are generally of excellent quality. The same
effect is produced where numerous thin beds of members of the different
classes are interstratified, the disintegrated portions being gradually
intermixed, and valuable soils formed.
The fertility of the soils formed from the stratified rocks is also
increased by the presence of organic remains which afford a supply of
phosphoric acid, and which are sometimes so abundant as to form a by no
means unimportant part of their mass. They do not occur in the oldest
sedimentary rocks, but as we ascend to the more recent geological
epochs, they increase in abundance, until, in the greensands and other
recent formations, whole beds of coprolites and other organic remains
are met with. Great differences are observed in the quality of the soils
yielded by different rocks. In general, those formed by the
disintegration of clay slates are cold, heavy, and very difficult and
expensive to work; those of sandstone light and poor, and of limestone
often poor and thin. These statements must, however, be considered as
very general; for individual cases occur in which some of these
substances may produce good soils, remarkable exceptions being offered
by the lower chalk and some of the shales of the coal formation. Little
is at present known regarding the peculiar nature of many of these
rocks, or their composition; and the cause of the differences in the
fertility of the soil produced from them is a subject worthy of minute
investigation.
_Chemical Composition of the Soil._--Reference has been already made to
the division of the constituents of the soil into the two great classes
of organic and inorganic. And when treating of the sources of the
organic constituents of plants, we entered with some degree of
minuteness into the composition and relations of the different members
of the former class, and expressed the opinion that they did not admit
of being directly absorbed by the plant. But though the parts then
stated lead to the inference that, as a direct source of these
substances, humus is unimportant, it has other functions to perform
which render it an essential constituent of all fertile soils. These
functions are dependent partly on the power which it has of absorbing
and entering into chemical composition with ammonia, and with certain of
the soluble inorganic substances, and partly on the effect which the
carbonic acid produced by its decomposition exerts on the mineral
matters of the soil. In the former way, its effects are strikingly seen
in the manner in which ammonia is absorbed by peat; for it suffices
merely to pour upon some dried peat a small quantity of a dilute
solution of ammonia to find its smell immediately disappear. This
peculiar absorptive power extends also to the fixed alkalies, potash and
soda, as well as to lime and magnesia, and has an important effect in
preventing these substances being washed out of the soil--a property
which, as we shall afterwards see, is possessed also by the clay
contained in greater or less quantity in most soils. On the other hand,
the air and moisture which penetrate the soil cause its decomposition,
and the carbonic acid so produced attacks the undecomposed minerals
existing in it, and liberate the valuable substances they contain.
In considering the composition of a soil, it is important to bear in
mind that it is a substance of great complexity, not mere
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