06 THE CHEMISTRY OE* TSB FARM 



between various crops, and is one reason of the 

 economy of a rotation. A very plainly-marked dis- 

 tinction as to mode of feeding is afforded by the 

 behaviour of various crops towards silica. Gramineous 

 crops, as the cereals and grasses, are apparently capable 

 of assimilating certain of the silicates contained in the 

 soil, while other crops exhibit no such capacity. In 

 such a case it is easy to imagine that an alternation 

 of cereals with crops of a different description may be 

 for the benefit of both, each drawing to some extent 

 upon distinct supplies of food. Again, leguminous 

 crops are clearly able to assimilate nitrogen to a far 

 greater extent than cereals, and from a different source.' 

 If crops of winter beans and winter wheat are grown 

 on similar unmanured land, the bean crop will gene- 

 rally contain twice as much nitrogen as the wheat. 

 The land is not, however, impoverished for wheat 

 by the growth of beans, for wheat after beans will 

 be a far better crop than wheat after wheat, thus 

 affording a striking example of the advantages of a 

 rotation. 



The quantities of plant food required by different 

 crops are given in the table on pp. 72-74; these 

 also furnish reasons for the alternation of crops. It 

 will be seen, for example, that the cereals require but 

 little potash and lime, while root crops, beans, and 

 clover demand a large supply ; it is obvious, therefore, 

 that the resources of the soil are husbanded by grow- 

 ing these two classes of crops in alternation, the 

 greater demand for potash and lime thus falling every 

 alternate year. 



The net result of a judicious alternation of crops, in 

 which the special characteristics of each are turned to 



