ROTATION OF CROPS. 279 



" This state of sterility will take place earlier for one kind of 

 " plant than for another, according to the unequal quantity of the 

 " different ingredients of the soil. If the soil is poor in phos- 

 *' phates, but rich in silicates, it will be exhausted sooner by the 

 '' cultivation of wheat than by that of oats or of barley, because a 

 " greater quantity of phosphates is removed in the seeds and straw 

 " of one crop of wheat than would be removed in three or four 

 " crops of barley or of oats. But if this soil be deficient in lime, 

 " the barley will grow upon it very imperfectly." 



" In a soil rich in alkaline silicates, but containing only a limited 

 " supply of phosphates, the period of its exhaustion for these salts 

 " will be delayed if we alternate with the wheat plants, which 

 *' we cut before they have come to seed ; or, what is the same 

 " thing, with plants that remove from the soil only a small quan- 

 " tity of phosphates. If we cultivate on this soil peas or beans, 

 " these plants will leave, after the removal of the crop, a quantity 

 " of silica in a soluble state sufficient for a succeeding crop of 

 " wheat ; but they will exhaust the soil of phosphates quite as 

 " much as wheat itself, because the seeds of both require for their 

 " maturity nearly an equal quantity of these salts. 



"We are enabled to delay the period of exhaustion of a soil of 

 " phosphates by adopting a rotation, in which potatoes, tobacco, 

 " or clover, are made to alternate with a white crop. The seeds 

 " of the plants now named are small, and contain proportionally 

 " only minute quantities of phosphates ; their roots and leaves, 

 " also, do not require much of these salts for their maturity. But 

 " it must be remembered, at the same time, that each of these has 

 " rendered the soil poorer, by a certain quantity of phosphates. 

 " By the rotation adopted, we have deferred the period of cxhaus- 

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

 " starch, etc., but we have not acquired any larger quantity of the 

 " constituents of the blood, or of the only substances which can be 

 " considered as properly the nutritious parts of plants. When the 

 " soil is deficient in salts of lime, tobacco, clover, and peas will 

 " not flourish ; while under the same conditions the growth of 



