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FALLOWING.

the soil; and likewise, in how far is it possible toincrease the fertility of the soil by augmenting thenatural quantity of any of its constituents.

375. In practice there are three courses adopted tomaintain the fertility of the soil, namely, fallowing, arotation of crops, and the application of manure.

376. In order to comprehend the effect produced byfallowing, it is necessary to remember the mode in whichsoil is formed. As has already been stated, soils consistprincipally of small fragments of different rocks andstones, which, from a variety of causes, have graduallycrumbled down to powder. The sand or fine particlesof stone in the soil are constantly decomposing (300)and adding to the soil, potash, soda, and very finely-divided earthy matters ; it is from sources of this kindthat most of the saline substances present in the soil arederived.

377. The soluble salts formed in this manner aredissolved by the rains, and in great part washed awayfrom the surface ; a portion, however, always remains inthe soil, and is absorbed by plants. AVhen a crop ofsome plant requiring, for example, a large quantity ofpotash, is raised in such a soil, it often happens that thecrop takes away all the soluble potash the soil contains;and in consequence, it would be impossible to raise asecond crop of that plant on the same soil, as therewould not be potash enough in it.

378. If, however, the soil is left for some time fallow,if no crop at all is raised on it, the soil has time to renew