The use of lime is of the greatest importance in practical
agriculture. It has been employed, in Europe at least, in one or
other of its forms of shells, shell-sand, marl, chalk, limestone, and
quicklime, from the most remote periods.
Native limestone, and all the unburned varieties of chalk, shells, &c.
consist of _carbonate of lime_ (p. 51), more or less pure. When burned
in the kiln, the carbonic acid is driven off, and lime, burned lime, or
quicklime remains.
Quicklime, when exposed to the air, gradually falls into the state
of an exceedingly fine white powder. It will do so more rapidly if
water be thrown upon it, when it also heats much, swells, and becomes
about one-third heavier than before. After being exposed to the air
for some time in this white powdery state, it is found to have again
absorbed from the air a portion of carbonic acid, though a very long
period generally elapses before it is all reconverted into carbonate.
In compost heaps, where much carbonic acid is formed during the
fermentation, the conversion of any quicklime that may be mixed with
them into carbonate of lime, is much more rapid and complete than in
the open air.
Lime, therefore, is laid on the land in two states.
_1st_, In the _mild_ state—that of carbonate—in marls, in chalk, in
shell-sand, &c.
_2d_, In the _caustic_, or quick state, as it comes hot from the kiln,
or after it is simply slaked.
Limes are laid on also in a more or less pure form. Marl contains only
from 5 to 20 per cent. of carbonate of lime, generally in the state
of a very fine powder. Shell-sand consists of a mixture of minute
fragments of shells with from 20 to 50 per cent. of siliceous sand. The
limestones which are burned are also more or less impure, though, when
the impurity is very great, they do not burn well, and are therefore
usually rejected.
Some limestones contain much magnesia, by which their agricultural
qualities are materially affected. These are known by the name of
_magnesian_ limestones. There are few limestones in which a small
quantity of magnesia may not be detected, and this minute proportion is
likely to be beneficial rather than otherwise; but when it is present
to the amount of 10 per cent. or upwards, it appears to have for some
time a poisonous influence upon vegetation, if added in the same large
doses in which other lime may be safely spread upon the land.
The quantity of lime laid on at a single dressing, and the frequency
with which it may be repeated, must depend upon the kind of land, upon
the depth of the soil, and upon the species of culture to which it is
subjected. If land be wet, or badly drained, a larger application is
necessary to produce the same effect, and it must be more frequently
repeated. When the soil is thin, again, a smaller addition will
thoroughly impregnate the whole, than where the plough usually descends
to the depth of 8 or 10 inches. On old pasture lands, where the tender
grasses live in two or three inches of soil only, a feebler dressing,
more frequently repeated, appears to be the more reasonable practice,
though in reclaiming and laying down lands to grass, a heavy first
liming is often indispensable.
In arable culture larger doses are admissible, both because the soil
through which the roots penetrate must necessarily be deeper, and
because the tendency to sink beyond the reach of the roots is generally
counteracted by the frequent turning up of the earth by the plough.
Where vegetable matter abounds, much lime may be usefully added, and on
stiff clay lands after draining, its good effect is most remarkable.
On light land, chiefly because there is neither moisture nor vegetable
matter present in equal quantity, very large applications of lime are
not so usual, and some prefer adding it to such lands in the shape of
composts only.
The largest doses, however, which are applied in practice, alter in
a very immaterial degree the chemical constitution of the soil. We
have seen that the best soils generally contain a natural proportion
of lime, not fixed in quantity, yet scarcely ever wholly wanting. But
an ordinary liming, when well mixed up with a deep soil, will rarely
amount to _one per cent._ of its entire weight. It requires about 300
bushels of burned lime per acre to add one per cent. of lime to a soil
of twelve inches in depth; if only mixed to a depth of six inches, this
quantity would add about two per cent. to the soil.
The most remarkable visible alterations produced by liming are—upon
_pastures_, the greater fineness, closeness, and nutritive character
of the grasses—on _arable lands_, the improvement in the texture and
mellowness of stiff clays, the more productive crops and the earlier
period at which they ripen.
But these effects gradually diminish year by year, till the land
returns again nearly to its original condition. On analyzing the
soil, the lime originally added is found to be in great measure, or
altogether, gone. In this condition the land must either be limed
again, or must be left to produce sickly and un-remunerating crops.
This removal arises from two causes. The rain-water that descends upon
the land holds in solution carbonic acid which it has absorbed from
the air. But water charged with carbonic acid is capable of dissolving
carbonate of lime, and thus year after year the rains slowly remove
as they sink to the drains, or run over the surface, a portion of the
lime which the soil contains. Acid substances are also formed naturally
in the land, by which another portion of the lime is rendered easily
soluble in water, and, therefore, readily removable by every shower
that falls.
The _chemical_ effects of lime upon the soil are chiefly the following:—
1. When laid upon the land in the _caustic_ state, the first action of
the lime is to combine immediately with every portion of acid matter
it may contain, and thus to sweeten the soil. Some of the compounds it
thus forms being soluble in water, either enter into the roots and feed
the plant,—supplying it at once with lime and with organic matter,—or
are washed out by the springs and rains, while other compounds, which
are insoluble, remain more permanently in the soil.
2. Another portion decomposes certain saline compounds of iron,
manganese, and alumina, which naturally form themselves in the soil,
and thus renders them unhurtful to vegetation. A similar action is
exerted upon certain compounds of potash and soda, and of ammonia,—if
any such are present,—by which these substances are set and placed
within the reach of the plant.
3. Its presence in the caustic state further disposes the organic
matter of the soil to undergo more rapid decomposition—it being
observed, that where lime is present in readiness to combine with the
substances produced during the decay of organic matter, that decay,
if other circumstances be favourable, will proceed with much greater
rapidity. The reader will not fail to recollect, that during this
decay many compounds are formed which are of importance in promoting
vegetation.
4. Further, quicklime has the advantage of being soluble in cold water,
and thus the complete diffusion of it through the soil is aided by the
power of water to carry it in solution in every direction.
5. When it has absorbed carbonic acid, and become reconverted into
carbonate, the original caustic lime has no _chemical_ virtue over
chalk, rich shell-sand or marl, or crushed limestone. It has, however,
the important _mechanical_ advantage of being in the form of a far
finer powder, than any to which we could reduce the limestone by
art—in consequence of which it can be more uniformly diffused through
the soil, and placed within the reach of every root, and of almost
every particle, of vegetable matter that is undergoing decay. I shall
mention only three of the important purposes which, in this state of
_carbonate_, lime serves upon the land.
1. It directly affords food to the plant, which, as we have seen,
languishes where lime is not attainable. It serves also to convey other
food to the roots in a state in which it can be made available to
vegetable growth.
2. It neutralizes (removes the _sourness_) of all acid substances as
they are formed in the soil, and thus keeps the land in a condition to
nourish the tenderest plants. This is one of the important agencies
of shell-sand when laid on undrained grass lands—and this effect it
produces in common with wood-ashes, and many similar substances.
3. During the decay of organic matter in the soil, it aids and
promotes the production of nitric acid,—so influential, as I believe,
in the general vegetation of the globe (see page 35). With this acid
it combines and forms _nitrate of lime_—a substance very soluble in
water—entering readily, therefore, into the roots of plants, and
producing upon their growth effects precisely similar to those of the
now well known _nitrate of soda_. The success of frequent ploughings,
harrowings, hoeings, and other modes of stirring the land, is partly
owing to the facilities which these operations afford to the production
of this and other natural nitrates.