Section Vi.—Use of Lime, Shell-Sand, and Marl.

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.