ROTATION OP CROPS.

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itself; by tlie action of the air, the further decomposi-tion of the silicates and other similar stony compoundsin the soil, is effected, and a fresh supply of potash isprovided.

379. The same end is obtained by a system of rotation.In place of sowing a second crop of the plant whichrequires so much potash, some other plant is taken,which does not require potash, hut which requires othersubstances that the soil contains.

380. In both these cases we restore, by the sloweffects of the air, those substances which we removewith the first crop ; in manuring, we at once add to thesoil that which the plants require.

381. Generally speaking, wheat cannot be grown onthe same land year after year, and accordingly varioussystems of rotation are adopted. It is evident, however,that as soon as we know what are the substances requiredby wheat and other crops, we shall at once know howto restore to the soil those substances which each cropremoves, and thus be enabled to obtain crops of thesame plant, for any number of years in succession, fromthe same soil. The labours of chemists are, however,hardly far enough advanced for this, at present.

382. With respect to the natural sources of salinematters in the soil, it must be observed that althoughtheir nature and quantity are in great measure influencedby the composition of the subsoil and stony substratum,yet it frequently happens that from want of mixturethe soil is deficient in some of those very substances