212 HOTATION OF CROPS. 



the acre from two hundred and fifty to three hundred and fifty pounds of inorganic matter ; 

 while the wheat plant removes, in its straw, about two hundred pounds, and in its seed from 

 twenty-five to thirty pounds. The largest amount of removed matter in the straw is silica, 

 which exists in the clover plant only in minute quantities. Clover is, however, rich in the 

 alkalies and phosphates; and hence_it follows that a single clover crop will supply a large 

 amount of the valuable and expensive nutriments to two or more crops of the cereals or grains, 

 even from its decayed roots. 



This modified view of the value and use of clover in a rotation seems to me agreeable to 

 observation, and to all the phenomena of growth and nutrition. In weak lands or soils the 

 clover tops should not be removed, while in strong soils it is not of so much consequence to 

 plough in the crop j indeed when the growth is large, and can not be broken up and incorpo- 

 rated with the soil, it has seemed to exert an injurious effect. It has been maintained, and is 

 perhaps now the opinion of many, that the value of clover, as a manure, depends upon its or- 

 ganic matter, and especially upon its nitrogenous compounds. Probably a more rational view 

 is, that its great value depends both upon its organic and inorganic matters ; and facts seem to 

 warrant the conclusion that the latter are equal in importance to the former. We can not 

 stand upon extreme views ; but when it is conceded that wheat does not require large quanti- 

 ties of organic matters, of which carbon is the base, we should certainly be led to adopt a 

 more favorable view of the value of the phosphates and alkalies. I have never subscribed to 

 the opinion that the value of clover, or of any fertilizer, can be determined by the amount of 

 nitrogen it yields, unless indeed it can be shown that the inorganic fertilizers bear a certain 

 constant proportion to it. Those who estimate fertilizers by the amount of azote or nitrogen 

 they furnish govern their rotations accordingly. Clover and grasses, they contend, take azote 

 from the air, and hence there is a real gain — a real addition to the soil ; but it has never 

 been proved that any plant can take, or does take nitrogenous matters directly from the 

 atmosphere, and if nitrogenous matters are derived from the soil, after having been diffused in 

 the atmosphere, it is difficult to see the value of those plants, on the alleged principle. Ab- 

 sorption of azote or of ammonia from the atmosphere is an assumption unsupported by a single 

 fact. 



The only mode by which azotized matters can be furnished, is through the medium of the 

 soil : ammonia from the atmosphere being one of its main sources, and being also soluble in 

 rain water, is furnished through this medium. If it is once established that it is through the 

 roots that the gaseous products are furnished the plant, the hypothesis on which the rotation 

 of plaster, clover and wheat, etc. is founded, loses its importance 5 that is, the rotation will 

 not be followed because of the nitrogenous matters of the clover, but because it furnishes im- 

 portant inorganic matters. If the wheat plant were to stand in the place of the clover plant, 

 it would obtain all the nitrogenous matter it requires, provided the rain brings it down to the 

 earth. It is not, however, probable that the atmosphere is the main source of nitrogen. There 

 is, however, another view which favors the hypothesis I have been disposed to question. When 

 clover is cultivated as a fertilizer, it takes its accustomed nitrogenous compounds from the soil, 



