186 SCIENTIFIC AGRICULTURE. 



"If we assume, [says Petzholdt,] that the utility of the rota- 

 tion of crops depends exclusively upon the circumstance that 

 all cultivated plants withdraw from the soil unequal amounts 

 of certain ingredients for their nutrition, all the observed facts 

 are at once satisfactorily explained, and the possibility of deter- 

 mining the rotation of crops, or of avoiding it altogether, if 

 desirable, made evident." 



It is useless to remark, that no plant can vegetate in a soil 

 which does not contain all the elements which it requires for 

 its food. Some species of grass contain, and therefore require 

 for their growth, a large amount of silica : a soil which contains 

 no silica cannot produce them. A soil may contain just enough 

 silica for one crop, but not enough for a second, so that a second 

 could not be produced; but a crop of some other plant requiring 

 much less silica, might be grown upon it as successfully as the 

 grass before. 



" A single crop of wheat may deprive the soil so completely 

 of one of its mineral constituents, that another crop of wheat 

 could not grow upon it; and yet this soil may contain abundant 

 mineral constituents for the production of a good crop of clover 

 or turnips." An analysis of a soil and the ashes of plants 

 desired to be produced upon it, will determine negatively, 

 whether it is eligible to their growth: but the only positive 

 proof is a trial of the crop upon the soil. 



All plants draw certain mineral elements from the soil, but 

 do not all equally exhaust its fertility. All knowledge respect- 

 ing the application of manures, and the adaptation of certain 

 plants to particular soils, is based upon these facts. The 

 necessity for rotation may sometimes be obviated by allowing 

 the land to lie in fallow for a year, after which the crop 

 may be successfully repeated. Manuring may also sometimes 

 answer the same purpose; but as a general rule in practice, 

 however it may be explained in theory, a judicious rotation is 

 beneficial. 



