EXHAUSTION OF SOILS. U>3 



furnish these constituents to a new vegetation ; when it must 

 become completely exhausted, and be at last quite sterile, even 

 for weeds. 



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 phosphates 

 out 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 

 oarley or of oats.* But if this soil be deficient in lime, the bar- 

 ley will grow upon it very imperfectly. 



It is owing to the deficiency of these salts, so indispensable to 

 the formation of the seeds, that it happens, however abundant may 

 be the quantity of silicates, that in one year we may obtain nine 

 times, in a second thrice, in a third twice as much corn as may 

 grow on the same soil in another year. 



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 quantity of 

 phosphates. If we cultivate on this soil peas or beans, these 

 plants will leave, after the removal of the crop, a quantity of si- 

 lica in a soluble state sufficient for a succeeding crop of wheat ; 

 but they will exhaust the coil pi 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 



* The weight of the ashes of a crop ct the seeds of wheat is to that in a 

 rrop of oats as 34:426, the phosphates contained in them as 26: 10; th« 

 phosphates of the straw not heing included in the calculation. 

 8* 



