The greater part of the action of lime is unquestionably dependent on its exerting a chemical decomposition on the soil; and it acts equally on both the great divisions of its constituents, the inorganic and the organic. On the former, it operates by decomposing the silicates, which form the main part of the soil, and the alkalies they contain being thus set free, a larger supply becomes available to the plant. On the organic constituents its effects are principally expended in promoting the decomposition which converts their nitrogen into ammonia; and thus a supply of food, which might remain for a long period locked up, is set free in a state in which the plant can at once absorb it. But these chemical decompositions are attended by a corresponding change in the mechanical characters of the soil. Heavy clays are observed to become lighter and more open in their texture; and those which are too rich in organic matter have it rapidly reduced in quantity, and the excessive lightness which it occasions diminished.
The effects of an application of lime are not generally observed immediately, but become apparent in the course of one or two years, when it has had time to exert its chemical influence on the soil; but from that time its effects are seen gradually to diminish and finally to cease entirely. The period within which this occurs necessarily varies with the amount of the application and the nature of the soil, but it may be said generally that lime will last from ten to fifteen years. The cessation of its effects is due to several circumstances, partly of course to the absorption of lime by the plants, partly to its being washed out of the soil by the rains, and partly to its tendency to sink to a lower level, a tendency which most practical men have had opportunities of observing. In the latter case, deep-ploughing often produces a marked effect, and sometimes makes it possible to postpone for a year or two the reapplication of lime. All these circumstances have their influence in bringing its action to an end, but the most important is, that after a time it has exhausted its decomposing effect on the soil, having destroyed all the organic matter, or liberated all the insoluble mineral substances which the quantity added is competent to do, and so the soil passes back to its old state. It does even more, for unless active measures are taken to sustain it by other means, it is found that the fertility of the soil is apt to become less than it was before the use of lime. And that it should be so is manifest, if we consider that the lime added has liberated a quantity of inorganic matter, which, in the natural state of the soil, would have become slowly available to the plant, and that it must have acted chiefly in those very portions which, from having already undergone a partial decomposition, were ready to pass into a state fitted for absorption, and thus as it were must have anticipated the supplies of future years. This effect has been frequently observed by farmers, and is indeed so common, that it has passed into a proverbial saying, that "lime enriches the fathers and impoverishes the sons." But this is true only when the soil is stinted of other manures, for when it is well manured the exhausting effect of lime is not observed; and it must be laid down as a practical rule, that its use necessitates a liberal treatment of the soil in all other respects. But when lime has been once employed it becomes almost necessary to resort to it again; and generally so soon as its effects are exhausted a new quantity is applied, not so large as that which is used when the soil is first limed, but still considerable. When this is done very frequently, however, bad effects ensue; the soil gets into a particular state, in which it is so open that the grain crops become uncertain, and such land is said, in practical language, to be overlimed. The explanation of this state of matters commonly assumed by those unacquainted with chemistry is, that the land has become too full of lime; but a moment's consideration of the very small fraction of the soil which even the largest application of lime forms, will serve to shew that this cannot be the cause. Ten tons of lime per acre amounts to only one per cent of the soil, and as a considerable part of the lime is carried off by drainage in the course of years, it is obvious that even very large and frequently repeated doses are not likely to produce any great accumulation of that substance. In point of fact, analyses of overlimed soils have proved that the lime does not exceed the ordinary quantity found in fertile land. The explanation of the phenomenon is probably to be found in the rapid decomposition of organic matter by the lime, and its escape as carbonic acid, by which the soil is left in that curious porous condition so well known in practice. The cure for overliming is found to be the employment of such means as consolidate the soil, such as eating off with sheep, rolling, or laying down to permanent pasture.
The immediate effect of lime on the vegetation of the land to which it is applied is very striking. It immediately destroys all sorts of moss, makes a tender herbage spring up, and eradicates a number of weeds. It improves the quantity and quality of most crops, and causes them to arrive more rapidly at maturity. The extent to which it produces these effects is dependent on the form in which it is applied. When the lime is used hot, that is, immediately after it has been slaked, they are produced most rapidly and effectually; but if it has been so long exposed to the air as to absorb much of the carbonic acid it lost in burning, and has got into what is commonly called the mild state, it operates more slowly; and when it is applied as chalk, marl, or pounded limestone, its action is still more tardy. Various circumstances, which must depend upon very different considerations, must necessarily influence the farmer in the selection of one or other of these different forms of lime; but on the whole, it will be found that the greatest advantages are on the side of the well-burned and freshly slaked lime. The consideration of all the minutiæ to be attended to, however, would carry us far beyond the limits of this work, and trench to some extent on the subject of practical agriculture.
Various kinds of refuse matters containing lime have been used in agriculture, but they are generally inferior to good lime, and not generally more economical. The most important of these is gas lime, or lime which has been used for purifying coal gas. In going through this process it absorbs carbonic acid from the gas, and consequently passes back, more or less, completely into the form of carbonate of lime. But it also takes up sulphur, which remains in it in the form of sulphuret of calcium. It is well known that all sulphurets are prejudicial to vegetable life, and hence, when fresh gas lime is used, its effects are often injurious rather than beneficial. But if it be exposed for some time to the air, oxygen is absorbed, the sulphur is converted into sulphuric acid, gypsum is produced to the extent of some per cent, and the lime then becomes innocuous. When composted with dry soil, the admission of air into the interior of the lime is facilitated, and this change takes place with greater rapidity. The waste lime from bleach-works, tanneries, and other manufactories, is occasionally used by farmers; but unless obtained at a nominal price, it cannot compete with good quick lime, owing to the large amount of water it contains, and the consequent increase in the cost of carriage.
Sulphate of Lime or Gypsum.—Gypsum has been extensively used as a manure, and is found to exert a very remarkable influence upon clover, and leguminous crops generally. It is employed in quantities varying from two cwt. per acre up to a very large quantity, and almost invariably with good results, in some instances even with the production of double crops. Much speculation has taken place as to the cause of this action which is so specific in its character, and from Sir Humphrey Davy down to the present time, many chemists and agriculturists have considered the matter. Sir Humphrey Davy attributed its action to its supplying sulphur to those plants which, according to him, contain an unusually large quantity of that element. That opinion has been since entertained by others, but it can scarcely be considered as well founded, for the more accurate experiments recently made do not point to any conspicuous differences between the quantities of sulphur contained in these and other plants. It is, moreover, to gypsum alone that these effects are due, and if it were merely as a source of sulphur that it was employed, there are other salts which could be equally, perhaps more advantageously, used; such, for instance, as sulphate of soda. Others have attributed its action to its power of fixing ammonia, but this explanation is certainly untenable, for the soil itself possesses this property very powerfully, and it is inconceivable that the addition of a few hundred weights of gypsum should have any effect in promoting this action. The experiments which have been made with gypsum leave no doubt as to its effect, more especially on leguminous plants, but they do not afford an explanation of its mode of action, for which further inquiries, directed especially to that object, are required.
The application of gypsum to the soil appears to have diminished of late years, and this is probably due to the large consumption of superphosphates, and other manufactured manures, which contain it in abundance. In an ordinary application of these substances, there are contained from one to two hundredweight of gypsum; and it is not likely that when they have been extensively used, much benefit will be derived from a further application of it by itself.