1. That the oxygen thus absorbed, deprives the terreau of part of its carbon, which it renders soluble and converts into mucilage; and
2. That the carbonic acid, formed in the process, combines with the mucilage, and with it is absorbed by the roots of plants.
If we put a plant and a quantity of slacked lime under the same receiver, the plant will perish; because the lime will take from the atmospheric air all the carbonic acid it contains, and thus starve the plant. Vegetables, placed near heaps of lime in the open air, suffer from the same cause and in the same way; but though lime, in large quantities, destroys vegetation, in small quantities it renders vegetation more vigorous. Its action is of two kinds—mechanical and chymical; the first is a mere division of the soil by an interposition between its parts; the second, the faculty of rendering soluble vegetable matter, and reducing it to the condition of terreau.
The mechanical agency ascribed to lime, belongs also to marle and to ashes, and in an equal degree—but their chymical operation, though similar, is less.[9]
Gypsum is composed of lime and sulphuric acid. Mayer was the first to present to the public a series of experiments upon it, in its relation to agriculture. Many chymists have followed him, and a great variety of opinion yet exists with regard to its mode of operation. Yvart thinks that the action of gypsum is exclusively the effect of the sulphuric acid, which enters into its composition; and founds this opinion upon the fact, that the ashes of turf, which contain sulphate of iron and sulphate of alumina, have the same action upon vegetation as gypsum. Laysterie, observing that plants, whose roots were nearest the surface of the soil, were most acted upon by plaster, concludes, that gypsum takes from the atmosphere the elements of vegetable life and transmits them directly to plants. Bose intimates, that the septic quality of gypsum (which he takes for granted) best explains its action on vegetation; but this opinion is subverted by the experiments of Mr. Davy, who found, that of two parcels of minced veal, the one mixed with gypsum, the other left by itself, and both exposed to the action of the sun, the latter was the first to exhibit symptoms of putrefaction. Mr. Davy's own belief on this subject is, that it makes part of the food of vegetables, is received into the plant and combined with it. The last opinion we shall offer on this head, is that of the celebrated Chaptal. "Of all substances, gypsum is that of whose action we know the least. The prodigious effect it has on the whole race of trefoils, (clover, &c.) cannot be explained by any mechanical agency—the quantity applied being so small—or by any stimulating power—since gypsum, raw or roasted, has nearly the same effect; nor by any absorbent quality, as it only acts when applied to the leaves. If permitted to conjecture its mode of operation, we should say, that its effect being greatest when applied to the wet leaves of vegetables, it may have the faculty of absorbing and giving out water and carbonic acid, little by little, to the growing plant. It may also be considered as an aliment in itself—an idea much supported by Mr. Davy's experiments, which shew, that the ashes of clover yield gypsum, though the clover be raised on soils not naturally containing that substance."
[Alb. Argus.
(To be continued.)