THE GENERAL PRINCIPLES OF MANURING.
In their natural condition all soils not absolutely barren are capable
of supporting a certain amount of vegetation, and they continue to do so
for an unlimited period, because the whole of the substances extracted
from them are again restored, either directly by the decay of the
plants, or indirectly by the droppings of the wild animals which have
browzed upon them. Under these circumstances, a soil yields what may be
called its normal produce, which varies within comparatively narrow
limits, according to the nature of the season, temperature, and other
climatic conditions. But the case is completely altered if the crop, in
place of being allowed to decay on the soil, is removed from it, for,
though the air will continue to afford an undiminished supply of those
elements of the food of plants which may be derived from it, the fixed
substances, which can only be obtained from the soil, decrease in
quantity, and are at length entirely exhausted. In this way a gradual
diminution of the fertility of the soil takes place, until, after the
lapse of a period, longer or shorter, according to its natural
resources, it will become entirely incapable of maintaining a crop, and
fall into absolute infertility unless the substances removed from it are
restored from some other source in the form of manure. When this is
done, the fertility of the soil may not only be sustained but greatly
increased, and, in point of fact, all cultivated soils, by the use of
manure, are made to yield a much larger crop than they can do in their
natural condition.
The fundamental principle upon which a manure is employed is that of
adding to the soil an abundant supply of the elements removed from it by
plants in the condition best fitted for absorption by their roots; but
looked at in its broadest point of view, it acts not merely in this way,
but also by promoting the decomposition of the already partially
disintegrated rocks of which the soil is composed, setting free those
substances it already contains, and facilitating their absorption by the
plants.
In considering the practical applications of the broad general principle
just stated, it might be assumed that a manure ought invariably to
contain all the elements of plants in the quantities in which they are
removed by the crops, and that when this has been accurately ascertained
by analysis, it would only be necessary to use the various substances in
the proportions thus indicated. But this, though a very important, and
no doubt in many cases essential condition, is by no means the only
matter which requires to be taken into consideration in the economical
application of manures. And this becomes sufficiently obvious when the
circumstances attending the exhaustion of the soil are minutely
examined. When a soil is cropped during a succession of years with the
same plant, and at length becomes incapable of longer maintaining it,
the exhaustion is rarely, if ever, due to the simultaneous consumption
of all its different constituents, but generally depends upon that of
one individual substance, which, from its having originally existed in
the soil in comparatively small quantity, is removed in a shorter time
than the others. To restore the fertility of a soil in this condition,
it is by no means necessary to supply all the different substances
required by the plant, for it will suffice to add that which has been
entirely removed. On the other hand, if an ordinary soil be supplied
with a manure containing a very small quantity of one of the elements of
plant food, along with abundance of all the others, the amount of
increase which it yields must obviously be measured, not by those which
are abundant, but by that which is deficient; for the crop which grows
luxuriantly so long as it obtains a supply of all its constituents, is
arrested as effectually by the want of one as of all, as has been proved
by the experiments of Prince Salm Horstmar and others, referred to in a
previous chapter; and hence, in order to obtain a good crop, it would be
necessary to use the manure in such abundance as to supply a sufficiency
of the deficient element for that purpose. If this course were
persevered in for a succession of years, the other substances which
would have been used in much more than the quantity required by the
crops, must either have been entirely lost or have accumulated in the
soil. In the latter case it is sufficiently obvious that the soil must
have been gradually acquiring an amount of resources which must remain
dormant until the system of manuring is changed. To render them
available, it is only necessary to add to it a quantity of the
particular substance in which the manure hitherto employed has been
deficient, so as to restore the lost balance, and enable the plant to
make use of those which have been stored up within it. The substance so
used is called a _special_ manure; that containing all the constituents
of the crop is a _general_ manure.
The distinction of these two classes of manures is very important in a
practical point of view, because a special manure is not by itself
capable of maintaining the life of plants, but is only a means of
bringing into use the natural and acquired resources of the soil. In
place of preventing or retarding its exhaustion, it rather accelerates
it by causing the increased crops to consume more abundantly, and within
a shorter period of time, those substances which it contains. On the
other hand, a general manure prevents or diminishes the consumption of
the elements of plant-food contained in the soil, and if added in
sufficient abundance, may cause them to accumulate in it, and even
enable an almost absolutely barren soil to yield a tolerable crop.
General manures must therefore always be the most important and
essential, and no others would be used if it were possible to obtain
them of a composition exactly suited to the requirements of the crop to
be raised. Practically, however, this condition cannot be fulfilled,
because all the substances available for the purpose, and particularly
farm-yard manure, are refuse matters, the exact composition of which is
not under our control, and they do not necessarily contain their
constituents either in the most suitable proportions, or the most
available forms, and consequently when they are used during a succession
of years, certain of their constituents may accumulate in the soil, and
it is under such circumstances that special manures are both necessary
and advantageous.
Several different substances, but more especially farm-yard manure,
fulfil in a very remarkable manner the conditions of a general manure,
and supply abundantly, not merely the mineral, but also the carbonaceous
and nitrogenous matters necessary for building up the organic part of
the plant; and hence its use is governed by principles of comparative
simplicity, and really resolves itself into determining the best mode of
managing it so as effectually to preserve its useful constituents, and,
at the same time, to bring them into those forms of combination in which
they are most available to the plant. But the employment of a special
manure opens up nice questions as to the relative importance of the
different elements of plants which have given rise to much controversy
and difference of opinion.
In treating of the food of plants, it has been already observed that the
fixed or mineral constituents which are contained in their ash, are
necessarily derived exclusively from the soil, but that the carbon,
hydrogen, nitrogen, and oxygen, of which their organic part is composed,
may be obtained either from that source or from the air. The important
distinction which thus exists between these two classes of substances,
has given rise to two different views regarding the theory of manures.
Basing his views on the presence of the organic elements in the air,
Liebig has maintained that it is unnecessary to supply them in the
manure, while others, among whom Messrs. Lawes and Gilbert have taken a
prominent position, hold that, as a rule, fertile soils, cultivated in
the ordinary manner, contain a sufficient supply of mineral matters for
the production of the largest possible crops, but that the quantity of
ammonia and nitric acid which the plants are capable of extracting from
the air is insufficient, and must be supplemented by manures containing
them. A large number of experiments have been made in support of these
views, but the inferences which can be drawn from them are not
absolutely conclusive on either side, and it is necessary to consider
the matter in a general point of view.
Setting out from the proposition already so frequently referred to, that
the plant cannot grow unless it receives a supply of all its elements,
it must be obvious that if, to a soil containing a sufficiency of
mineral matters to raise a given number of crops, a supply of ammonia be
added, its total productive capacity cannot be thus increased; and
though it may yield larger crops than it would have done without that
substance, this can only be accomplished by a proportionate diminution
of their number. In either case, the same quantity of vegetable matter
will be produced, but the time within which it is obtained will be
regulated by the supply of ammonia. That substance differs in no respect
from any other element of plant-food, and used in this way is to all
intents and purposes a special manure, and acts merely by bringing into
play those substances which the soil already contains. Its effect may
not be apparent until after the lapse of a very long period of time, but
it ultimately leads to the exhaustion of the soil. If, on the other
hand, a soil be continuously cropped until it ceases to yield any
produce, it is manifest that the exhaustion must in this instance be
entirely due to the removal of its available mineral nutriment, because
the superincumbent air constantly changed by the winds must continue to
afford the same unvarying supply of the organic elements, and the power
of supporting vegetation would be restored to it, by adding the
necessary inorganic matters. Hence when a soil, which in its natural
condition is capable of yielding a certain amount of vegetable matter,
is rendered barren by the removal of the crop, it may be laid down as an
incontrovertible position, that its infertility is due to the loss of
mineral matters, and that it may be restored to its pristine condition
by the use of them, and of them only.
But the case is materially altered when we come to consider the course
of events in a cultivated soil. The object of agriculture is to cause
the soil, by appropriate treatment, to yield much more than its normal
produce, and the question is, how this can be best and most economically
effected in practice. According to Liebig, it is attained by adding to
the soil a liberal supply of those mineral substances required by the
plant, and that it is unnecessary to use any of the organic elements,
because they are supplied by the air in sufficient quantity to meet the
requirements of the most abundant crops. Other chemists and vegetable
physiologists again hold that though a certain increase may be obtained
in this way, a point is soon reached beyond which mineral matters will
not cause the plant to absorb more ammonia from the air, although a
further increase may be obtained by the addition of nitrogen in that or
some other available form.
It is admitted on both sides, that all the elements of plant food are
equally essential, and the controversy really lies in determining what
practically limits the crop producible on any soil. The point at issue
may be put in a clear point of view by considering the course of events
on a soil altogether devoid of the elements of plants. If a small
quantity of mineral matters be added to such a soil, it immediately
becomes capable of supporting a certain amount of vegetation, deriving
from the air the organic elements necessary for this purpose, and with
every increase of the former, the air will be laid under a larger
contribution of the latter, to support the increased growth, and this
must proceed until the limit of supply from the atmosphere is reached.
At this point a further supply of mineral matters alone must obviously
be incapable of again increasing the crop, and it would thus be
absolutely necessary to conjoin them with a proportionate quantity of
organic substances. Liebig maintains that this limit is never attained
in practice, but that the air affords ammonia and the other organic
elements in excess of the requirements of the largest crop, while
mineral matters are generally though not invariably present in the soil
in insufficient quantity. Messrs. Lawes and Gilbert, on the other hand,
believe that the soil generally contains an excess of mineral matters,
and that a manure which is to bring out their full effect must contain
ammonia, or some other nitrogenous substance fitted to supplement the
deficient supply afforded by the atmosphere. In short, the question at
issue is, whether there is or is not a sufficiency of atmospheric food
to meet the demands of the largest crop which can practically be
produced.
An absolutely conclusive reply to this question is by no means easy. The
experiments by which it is to be resolved are complicated by the fact,
that all soils capable of supporting anything like a crop, contain not
only the mineral, but the organic elements of its food in large and
generally in greatly superabundant quantity, and it is impossible
satisfactorily to ascertain how much is derived from this source, and
how much from the atmosphere. There are in fact no experiments in which
the effects of a purely mineral soil have been ascertained. The
important and carefully performed researches of Messrs. Lawes and
Gilbert were made upon a soil which had been long under cultivation, and
contained decaying vegetable matters in sufficient abundance to supply
nitrogen to many successive crops, and it would be most unreasonable to
assert that the produce they did obtain by means of mineral manures,
drew the whole of its nitrogen from the air. On the contrary, it may be
fairly assumed that the soil did yield a certain quantity of its
nitrogenous compounds, but to what extent this occurs, it is impossible
to determine. This difficulty is encountered more or less in all the
other experiments, and precludes absolute conclusions. The same fallacy
also besets the arguments of Liebig when he holds that the crop,
increased by means of mineral manures alone, must derive the whole of
the additional quantity of nitrogen which it contains from the air, as
appears to be tacitly assumed throughout the whole discussion. So far
from this being the case, it is just as likely that the mineral matters
should cause the plants to take it from the soil, if it is there, as
from the atmosphere.
Taking a general view of the whole question, it is evident that a
certain amount of vegetation may always be produced by means of mineral
manures, and the quantity obtained is generally much beyond the normal
produce of the soil. But it is still open to doubt whether the largest
possible crop can be thus obtained, although the balance of evidence is
against it, and in favour of the addition of ammonia, and other
nitrogenous and organic substances, to the soil. In actual practice
manures containing nitrogen are more important, and more extensively
applied than any others, and the quantity of that element thus used is
very much larger than is generally supposed. Twenty tons of farm-yard
manure, a quantity commonly applied, and often exceeded on well
cultivated land, contain a sufficiency of organic matters to yield about
2-1/2 cwt. of nitrogen. A complete rotation, according to the six-course
shift, contains almost exactly the same quantity of nitrogen, when we
assume average crops throughout the whole, and it is thus made up.[K]
Lbs. of Nitrogen.
1. Turnips (13-1/2 tons) 60
2. { Wheat (28 bushels at 60 lbs.) 29
{ Straw 16
3. Hay (2-1/2 tons) 56
4. { Oats (34 bushels at 40 lbs.) 27
{ Straw 14
5. Potatoes (3 tons) 27
6. Wheat and straw as before 45
----
Total 274
The supply is therefore quite sufficient for the requirements of the
crop; and when it is borne in mind that a considerable quantity of
ammonia and nitric acid is annually carried down by the rain, and that
during a long rotation other substances are very generally used in
addition to farm-yard manure, it is obvious that the crop need not
depend to any extent upon what it derives from the air. What is true of
the nitrogenous matters applies with still greater force to the mineral
constituents of the manure. Twenty tons of farm-yard manure contain 32
cwt. of mineral matters, while the average crops of a six course-shift
contain only 1088 lbs., or less than one-third of this quantity. It is
obvious, therefore, that in well manured land there must be a gradual
increase of all the constituents of plants, but that of the mineral
matters is relatively much greater than that of the nitrogenous. If
therefore from any cause the crop produced on a soil to which farm-yard
manure had been applied were greatly to exceed the average, the amount
of produce, so far as the soil is concerned, would be limited not by
deficiency of mineral, but of nitrogenous food. Hence also when
farm-yard manure is liberally applied, there is a gradual accumulation
of valuable matters, and a progressive improvement of the productive
capacity of the soil.
It is far otherwise, however, if a special manure is employed, because
in that case the crop is thrown upon the resources of the soil itself
for all its constituents except those contained in the substance
employed, and by persisting in its exclusive use exhaustion is the
inevitable result. It would be wrong, however, to infer from this, that
special manures are to be avoided. On the contrary, great benefits are
derived from their judicious employment, and the circumstances under
which they are admissible may be readily gathered from what has already
been said. They are agents which bring into useful activity the dormant
resources of the soil, they restore the proper balance between its
different constituents, and supply the excessive demand of some
particular elements. Thus, for instance, in a soil containing an
abundant supply of mineral matters, a salt of ammonia or nitric acid
increases the crop, by promoting the absorption of the substances
already present. So likewise a soil on which young cattle and milch cows
have been long pastured has its fertility restored by phosphate of lime,
because that substance is removed in the bones and milk in relatively
much larger proportion than any others.
The choice of a special manure is necessarily dependent on a great
variety of circumstances, and is governed partly by the nature of the
soil, and partly by that of the crop. It is obvious that cases may occur
in which any individual element of the plant may be deficient, and ought
to be supplied, but experience has shown that, as a rule, nitrogen and
phosphoric acid are the substances which it is most necessary to furnish
in this way, and which in all but exceptional cases produce a marked
effect on the crop. The other substances, such as potash, soda,
magnesia, etc., occasionally act beneficially, but the results obtained
from them are very uncertain, and frequently entirely negative.
It has been commonly asserted that phosphates are specially adapted to
root crops, and ammonia or nitrates to the cereals, and this statement
is so far true, that the former are used with advantage on the turnip,
while the latter act with great benefit on grain crops and more
especially on oats and barley. The effect of the latter, however, is
more or less apparent in all crops and on all soils, because it promotes
the assimilation of the mineral matters already present. But its
peculiar importance lies in the power which it has of promoting the
rapid development of the young plant, causing it to send its roots out
into the soil, and to spread its leaves into the air, thus enabling it
to take from those two sources, abundance of the useful substances
existing in them. But it ought to be distinctly understood, that the
statement that particular manures are specially suited to particular
crops must be assumed with some reservation, because everything depends
upon the nature of the food contained in the soil. It is well known that
there are many soils in which ammonia acts more favourably on the turnip
than phosphates, and _vice versa_, and the difference is often due to
the previous treatment. In many cases in which ammonia when first used
proved most beneficial, it now begins to lose its effect, and the reason
no doubt is, that by its means the phosphates existing in these soils
have been reduced in amount, while the ammonia has accumulated, so that
a change in the system of manuring becomes necessary. A general manure
may be used year after year in a perfectly routine manner, but where a
special manure is employed, the importance of watching its effects, and
altering it as circumstances indicate, cannot be over-estimated. The
length of time during which special manures have been extensively used
has not been sufficient to bring this prominently before the
agriculturist, but its importance must sooner or later force itself upon
him, and he will then see the necessity for studying the succession of
manures as well as that of crops.
Hitherto we have considered a manure merely as a source from which
plants derive their food, but it exercises a scarcely less important
action on the chemical and physical properties of the soil. Farm-yard
manure, which, as we shall afterwards see, contains a large amount of
decomposing vegetable and animal matters, yields a supply of carbonic
acid, which operates on the mineral constituents, promotes their further
disintegration, and thus liberates their useful elements. It affects
also their physical properties, for it diminishes the tenacity of heavy
clays; each straw as it decomposes forming a channel through which the
roots of plants, air, and moisture can penetrate more readily than
through the stiff clay itself. On the other hand, it diminishes the
porosity of light sandy soils, causes them to retain moisture, and
generally makes their texture more suitable to the plant. Special
manures probably act to some extent chemically on the soil, but the
nature of the changes they produce is as yet imperfectly understood.
Superphosphates which are highly acid in all probability act powerfully
on the mineral substances, and common salt, which, though of little
importance to the plant, occasionally produces very striking effects,
appears to exercise some decomposing action on the soil. It is
difficult, however, to trace the mode in which they operate on a
substance of such complexity as the soil. Lime, as we shall afterwards
see, acts by promoting the decomposition of the vegetable matters on
the soil, and possibly some other substances may have a similar effect.
In the application of manures to the soil there are several
circumstances which must be taken into consideration. It is generally
stated that they ought to be distributed as uniformly as possible, but
this is not always necessary nor even advisable, and certainly is not
acted on in practice. Much must depend upon the nature both of crop and
soil. When the former throws out long and widely penetrating roots, the
more uniformly the manure is distributed the better; but if the rootlets
are short, it is clearly more advisable that it should be deposited at
no great distance from the seed. Practically this is observed in the
case of the potato and turnip, which are short rooted, and where the
manure is generally deposited close to the seed. But this course is
never adopted with the long rooted cereals, the manure being usually
applied to the previous crop, so that the repeated ploughings to which
the soil is subjected in the interval may distribute what remains as
widely and uniformly as possible. In soils which are either excessively
tenacious or light, the accumulation of the manure close to the plants
has also the effect of producing an artificial soil in their immediate
neighbourhood, containing abundance of plant-food, and having physical
properties better fitted for the support of the plant. On the other
hand, when a special manure is used alone, and with the view of
promoting the assimilation of substances already existing in the soil,
the more uniform its distribution the better, because it is essential
that the roots which penetrate through it should find at every point
they reach not only the original soil constituents, but also the
substances used to supplement their deficiencies.
FOOTNOTES:
[Footnote K: The quantities here taken are the averages deduced from the
agricultural statistics taken in Scotland some years since, with the
exception of hay and straw, which are not included in them. I have
therefore assumed a reasonable quantity in these cases.]