INDIRECT MANURES.
LIME.
We now come to discuss those manures which we may class under the term
_Indirect_, because their value is due, not to their direct action as
suppliers of plant-food--like those manures we have hitherto been
engaged in discussing--but to their indirect action. Of these by far the
most important is lime.
_Antiquity of Lime as a Manure._
Lime is one of the oldest and one of the most popular of all manures. It
is mentioned, and its wonderful action commented on, in the works of
several ancient writers, more especially Pliny. Of late years, perhaps,
its use has become restricted; and, as we shall point out by-and-by, it
is well that it is so.
_Action of Lime not thoroughly understood._
Despite the fact of the long-established and almost universal use of
lime, it can scarcely be said that we as yet clearly understand the
exact nature of its action. Much light, however, has been thrown of late
years on the subject by the great advance which has been made in our
knowledge of agricultural chemistry. Nevertheless, there are many points
connected with the action of lime on the soil which are still obscure.
Perhaps one reason for the conflicting ideas prevalent with regard to
the value of this substance in agriculture is to be found in the fact
that it acts in such a number of different ways, and that the nature of
the changes it gives rise to in the soil is most complicated. The
experience of agriculturists with lime in one part of the country often
seems contradictory to the experience of those in other parts of the
country. Its action on different soils is very dissimilar. For these
reasons, therefore, the discussion of the value of lime as a manure is
by no means an easy one.
_Lime a necessary Plant-food._
Lime, as we have already pointed out in a former chapter, is a necessary
plant-food, and were it present in the soil to a less extent than is
actually the case, would be just as valuable a manure as the different
nitrogenous and phosphatic manures; and in certain circumstances this is
the case. There are soils, though they are by no means of common
occurrence, which actually lack sufficient lime for supporting
plant-growth, and to which its addition directly promotes the growth of
the crop. Poor sandy soils are often of this nature. Another class of
soils are also apt to be lacking in lime--at any rate their surface-soil
is. These are permanent pasture-soils. Originally there may have been an
abundance of lime in the surface portion of the soil; but, as is well
known to every practical farmer, lime has a tendency to sink down in the
soil. This tendency in ordinary arable soils is largely counteracted by
ordinary tillage operations, such as ploughing, &c., by means of which
the lime is again brought to the surface. In permanent pasture-soils,
however, no such counteracting action takes place, hence impoverishment
of the surface-soil in lime eventually results. It is for this
reason--partly at any rate--that permanent pasture benefits in an
especial degree by the application of lime. We say _partly_, for there
are other important reasons. One is, that lime seems to have a striking
effect in improving the quality of pastures by inducing the finer
grasses to predominate. It has also a very favourable action in
promoting the growth of white clover. Another reason for the favourable
effect of lime on pasture-soils is doubtless on account of the action it
has in setting potash free from its compounds. Soils, however, which
directly benefit from the application of lime in the same way as they
benefit from the application of nitrogenous manures, may be safely said
to be rare. In the great majority of soils lime exists, so far as the
demands of plant-life are concerned, in superabundance.
_Lime of abundant Occurrence._
Indeed limestone is one of the most abundant of all rock substances, and
it has been calculated that it forms not less than one-sixth of the
rock-mass of the earth's crust. Nearly all the commonly occurring
minerals contain it, and in the course of their disintegration furnish
it to the soil. Vast tracts of country are composed of nothing but
limestone; and we have examples, even in this country, of so-called
chalk-soils, where it is the most abundant constituent. Nor can it be
classed amongst the insoluble mineral constituents of the soil; for
although insoluble in pure water, it is soluble in water--such as the
soil-water--which contains carbonic acid. This is proved by the fact
that it is the chief dissolved mineral ingredient in all natural waters.
_Lime returned to the Soil in ordinary Agricultural Practice._
It may be further pointed out, as bearing upon the true function of lime
when applied as a manure, that in ordinary agricultural practice nearly
all the lime removed from the soil in crops finds its way back again to
the farm in the straw of the farmyard manure. For these reasons, then,
it is clear that the true function of lime is as an indirect manure.
Let us now proceed to discuss its action. Before doing so, however, it
is important that we should clearly understand the different chemical
forms in which it occurs.
_Different Forms of Lime._
Lime occurs chiefly as carbonate of lime in the forms of limestone,
marble, or chalk, which are all chemically the same. It occurs also as
sulphate of lime or gypsum, as well as in the forms of phosphate and
fluoride. In agriculture it is only used--if we except the phosphate,
which is applied not on account of its lime, but its phosphoric acid--in
the form of the carbonate or _mild_ lime as it is commonly called,
burnt, caustic, or quick lime, and as gypsum. As the value of gypsum as
a manure is of such importance, and depends not entirely on its being a
compound of lime, we shall consider it by itself. Hence we have only to
consider here the action of mild and caustic lime.
_Caustic Lime._
When limestone or mild lime is submitted to a great heat, such as is
practically done on a large scale in lime-kilns, it is converted into
caustic lime or lime proper. Limestone is made up, as we have just
mentioned, of lime and carbonic acid. The latter ingredient is expelled
in the form of a gas, and the lime is left behind. Lime never occurs
naturally as caustic lime, for the simple reason that it is impossible
for it to remain in this state, owing to the great affinity it has both
for water and carbonic acid.
When lime is burnt, and before it is applied to the field, some time is
allowed to elapse in order to permit of its absorbing moisture--or
becoming slaked, as it is technically called. This it does more or less
slowly by absorbing moisture from the air. As, however, the process
would take too long, and as, moreover, the absorption of carbonic acid
gas would also take place at the same time, lime is generally slaked in
another way. This can be done by simply adding water. An objection to
this method is, that the lime is not so uniformly slaked as is
desirable. It becomes gritty. The usual method is to cover it up with
damp earth in heaps, and allow the moisture of the earth to effect the
slaking. When lime absorbs water a new chemical compound is formed,
known as lime hydrate; and so rapidly does the lime unite with water,
that a great deal of heat is evolved in the operation, the temperature
produced being considerably above that of boiling-water. The conversion
of slaked lime into carbonate of lime or mild lime is a slower process.
Sooner or later, however, it takes place, whether the lime is left on
the surface of the soil or buried in it.
A knowledge of these elementary chemical facts is necessary in order
clearly to understand the nature of the action of lime in agriculture.
The respective action of quicklime and mild lime is, on the whole,
similar, although the former is in every case very much more powerful
in its effects than the latter.
_Lime acts both mechanically and chemically._
Lime may be said to act on the soil both mechanically and chemically. It
alters the texture of the soil, and affects its mechanical properties,
such as its absorptive, retentive, and capillary powers with regard to
water. It acts upon its dormant fertility, and decomposes its mineral
substances as well as its organic matter. Lastly, its influence on the
micro-organic life of the soil, which plays such an important part in
the preparation and elaboration of plant-food, is of the highest
importance. We cannot do better, therefore, than discuss its properties
under the headings _mechanical_, _chemical_, and _biological_.
I. MECHANICAL FUNCTIONS OF LIME.
_Action on Soil's Texture._
The effect of lime upon the texture of a soil is among its most striking
properties. Every farmer knows well what a transformation is effected in
the texture of a stiff clay soil by the application of a dressing of
lime. The adhesive property of the soil--its objectionable tendency to
puddle when mixed with water--is greatly lessened, and the soil is
rendered very much more friable when it becomes dry. Several reasons
exist for this change. In the first place, the tendency to puddle in a
clayey soil is due to the fine state of division of the soil-particles.
The way in which lime counteracts this adhesive property is by causing a
coagulation of the fine soil-particles. This flocculation or aggregation
of the fine clay-particles, when mixed with water by lime, is strikingly
demonstrated by adding to some muddy water a little lime-water. The
result will be that the water will speedily be rendered clear, the fine
clay-particles coming together and sinking to the bottom of the vessel.
Even a very small quantity of lime will effect this change. This
property possessed by lime, we may mention, is utilised in the treatment
of sewage. As it is the fine clay-particles that are the chief cause of
the puddling of clay soils, their flocculation does much to destroy this
objectionable property. Another reason why lime renders a clay soil more
friable when dry is, that lime does not undergo any shrinkage in dry
weather. As clay soils shrink very much in drying, the mixture with such
a substance as lime tends to minimise this tendency to cake in hard
lumps. The effect of even a very small addition of lime to a clay soil,
in the way of increasing its friable nature, is very striking, and can
be easily illustrated by taking two portions of clay, into one of which
a small percentage of lime is introduced, and working both into a
plastic mass with water, and then allowing them to dry. It will be found
that while the one is hard and resists disintegration, that portion to
which the lime has been added crumbles away easily to a powder. This
effect which lime has in "lightening" heavy soils has been known to last
for years. The disintegrating effect of quicklime when applied to heavy
soils is also due, it may be added, to the change undergone by the lime
itself from the caustic state to the mild state.
_Lime renders light Soils more cohesive._
Although it may seem somewhat paradoxical, lime, it would appear, in
some cases exercises an effect upon the soil exactly the reverse of what
has just been stated. That lime should act as a binding agent is only
natural when we reflect on the way in which it acts when used as mortar.
It is quite to be understood, therefore, that its action on light
friable soils should be to increase their cohesive powers, and at the
same time to increase the capillary power of the soil to absorb water
from the lower layers. The extent of this action, of course, would
depend on the form in which the lime is applied, and the amount. A
striking example of the binding power of lime is to be found in certain
soils extremely rich in lime, in which what is known as a lime-pan has
been formed at some distance from the surface.
II. CHEMICAL ACTION OF LIME.
But more important probably than even its mechanical action is the
chemical action of lime. It is a most important agent in unlocking the
inert fertility of the soil. This it does by decomposing different
minerals and setting free the potash they contain. The disintegrating
power of lime in this respect depends, of course, on its chemical
condition, the caustic form being much more potent than the other forms.
Its action in decomposing vegetable matter and rendering the inert
nitrogen it contains available for the plant's use, is also one of its
most important properties, and accounts for its beneficial action when
applied to soils, such as peaty soils, rich in organic matter. Again,
its use as a corrective for sour lands has long been practically
recognised. The presence of acidity in a soil is hurtful to vegetable
life. Lime, by neutralising this acidity, removes the sourness of the
land, and does much to restore it to a condition suitable for the growth
of cultivated crops. The generation of sourness in a soil is almost sure
to give rise to certain poisonous compounds. Lime, therefore, in
sweetening a soil, prevents the formation of these poisonous compounds.
Badly drained and sour meadow-lands, as every farmer knows, are
immensely benefited by the application of this useful manure; for not
merely is their sourness removed and their general condition
ameliorated, but many of the coarser and lower forms of plant-life,
which alone flourish on such soils, are killed out, and the more
nutritive grasses are allowed to flourish instead. The action of lime in
promoting the formation of a class of compounds of great importance in
the soil--viz., hydrated silicates--is worthy of notice. According to
the commonly accepted theory, much of the available mineral fertilising
matter of the soil is retained in the form of these hydrated silicates.
Hence lime, by increasing these compounds, not merely adds to the amount
of the available fertility in the soil, but also increases its
absorptive power for food-constituents.
III. BIOLOGICAL ACTION OF LIME.
The last way in which lime acts is what we have termed biological. By
this we mean the important _role_ lime plays in promoting or retarding,
as the case may be, the various kinds of fermentative action which go on
so abundantly in all soils. The presence of carbonate of lime in the
soil is a necessary condition for the process of nitrification. Lime is
the base with which the nitric acid, when it is formed, combines; and as
we have seen, when discussing nitrification, soils of a chalky nature
are among those best suited to promote the natural formation of
nitrates. This is one of the reasons for the beneficial effects produced
by lime when applied to peaty soils. Not merely does it help to
decompose the organic matter so abundant in such soils, but it also
furnishes the base with which the nitric acid may combine when it is
formed. But while the action of lime is to promote fermentation, it must
not be forgotten that there may be cases in which its action is rather
the reverse of this. Fermentation of organic matter goes on when there
is a certain amount of alkalinity present; while, on the other hand, the
presence of acidity seems to retard and check it. Too great an amount of
alkalinity, however, would, in the first instance, retard fermentation
as much as too great acidity. It has been claimed that the addition of
caustic lime to fresh urine may act in this way; and if this were so,
the addition of lime to farmyard manure might, to a certain extent, be
defended. The experiment, however, would be a hazardous one and not to
be recommended, as loss of ammonia would most likely ensue.
_Action of Lime on Nitrogenous Organic Matter._
The action of lime on nitrogenous organic matter is of a very striking
kind, and is by no means very clearly understood. As we have pointed
out, it sometimes acts as an antiseptic or preservative; and this
antiseptic or preservative action has been explained on the assumption
that insoluble albuminates of lime are formed. Its action in such
industries as calico-printing, where it has been used along with casein
for fixing colouring matter; or in sugar-refining, where it is used for
clarifying the sugar by precipitating the albuminous matter in solution
in the saccharine liquor; or lastly, in purifying sewage,--has been
cited in support of this theory. While, however, there may be
circumstances in which lime, especially in its caustic form, acts as an
antiseptic, its general tendency is to promote these fermentative
changes, such as nitrification, so important to plant-life.
An important use of lime in agriculture is in preventing the action of
certain fungoid diseases, such as "rust," "smut," "finger-and-toe," &c.,
as well as in killing, as every horticulturist and farmer knows, slugs,
&c.
_Recapitulation._
We may, in conclusion, sum up in a single paragraph the different ways
in which lime acts. Its action is mechanical, chemical, and biological.
It acts on the texture of the soil, rendering clay soils more friable,
and exerting a certain binding effect on loose soils. It decomposes the
minerals containing potash and other food-constituents, and renders them
available for the plant's needs. It further decomposes organic matter,
and promotes the important process of nitrification. It increases the
power of a soil to fix such valuable food-constituents as ammonia and
potash. It neutralises sourness, and prevents the formation of poisonous
compounds in the soil. It increases the capillary condition of the soil,
prevents fungoid diseases, and promotes the growth of the more nutritive
herbage in pasture-land.