1221. The disintegration of soils by the action of the atmosphere has most effect when the soils are partly composed of the fragments of any of the harder rocks, as these fine particles of stone, being acted upon by the atmosphere, are constantly decomposing, and furnishing the soil with potash, soda, and various saline substances. 'The soluble salts formed in this manner,' Professor Solly observes, 'are dissolved by the rains, and in great part washed away from the surface; a portion, however, always remains in the soil, and is absorbed by plants. When a crop of some plant requiring, for example, a large quantity of potash, is raised in such a soil, it often happens that the crop takes away nearly all the soluble potash the soil contains; and, in consequence, it would be impossible to raise a second crop of that plant on the same soil, as there would not be potash enough in it. If, however, the soil is left for some time fallow, if no crop at all is raised on it, the soil has tune to renew itself, by the action of the air, the further decomposition of the silicates, and other similar stony compounds in the soil, is effected, and a fresh supply of potash is provided.' (Solly's Chemistry, 2nd edit., p. 168.)
