154 ROTATION OF CROPS. 



be remembered, at the same time, that each of these has ren- 

 dered the soil poorer, by a certain quantity of phosphates. By 

 the rotation adopted, we have deferred the period of exhaustion, 

 and have obtained in the crops a greater weight of sugar, starch, 

 &c, but we have not acquired any larger quantity of the con- 

 stituents of the blood, or of the only substances which can be con- 

 sidered as properly the nutritious parts of plants. When the soil 

 is deficient in salts of lime, tobacco, clover, and peas will not 

 flourish ; whilst, under the same conditions, the growth of beet- 

 root or turnips will net be impeded, if the soil, at the same time, 

 contain a proper quantity of alkalies. 



When a soil contains silicates not prone to disintegrate, it may 

 be able, in its natural state, to liberate by the influence of the at- 

 mosphere, in three or four years, only as much silica as suffices, 

 for one crop of wheat. In this case, such a crop can only be 

 grown on it in a three or four years' rotation, assuming that the 

 phosphates necessary for tiie formation of the seeds exist in the 

 soil in sufficient quantity. But we can shorten this period by 

 . working well the soil, and by increasing its surface, so as to make 

 it more accessible to the action of the air and moisture, in order 

 to disintegrate the soil, and to procure a greater provision of solu- 

 ble silicates. The decomposition of the silicates may also be ac- 

 celerated by the use of burnt lime ; hir. it is certain that, although 

 all these means may enable us to ensure rich crops for a certain 

 period, they induce, at the same, time, an earlier exhaustion of 

 the soil, and impair its natural .state of fertility. 



If the proportion of alkali and of silica liberated from the soil 

 in the course of three or four years be sufficient only for one crop 

 of wheat, we cannot in the interval, without injury to this crop, 

 cultivate on the same soil any other plant ; because the alkali 

 necessary for the growth of the latter cannot be applied to the use 

 of the wheat. 



By examining the known proportion of alkali and of silica 

 liberated by the disintegration of the silicates in their conversion 

 into clay, and by the weathering of the latter itself,* we find that, 



•One equivalent of silica is liberated for every equivalent of potash 

 separated from the constituents of an equivalent of felspar. In the straw 



