ROTATION OF CROPS. 55 



need of nitrogenous manure than cereals. By consuming 

 the roots on the land the nitrates collected by the crop 

 are returned to the soil in the form of animal manure, 

 and the land thus prepared to carry a cereal crop. 

 Similar remarks might be made respecting other green 

 crops whose active growth extends into tbe autumn.* 



Another important difference between crops lies in 

 their range of roots. Deeply rooted crops, as red clover, 

 sainfoin, rape, and mangel, and among the cereals wheat, 

 and rye, are to a considerable extent subsoil feeders, and 

 have a greater power of obtaining ash constituents from 

 the soil than shallow-rooted crops, as white clover, 

 potatos, turnips, and barley. In accordance with this 

 we find that superphosphate is a very effective manure 

 for the last three crops, but is much less required by such 

 crops as mangel or wheat. By growing deeply-rooted 

 crops as part of a rotation the subsoil is made to contri- 

 bute to the general fertility. Shallow-rooted crops, on 

 the other hand, have generally a special faculty for 

 appropriating food accumulated at the surface, and are 

 often of great use in this respect, as when barley is made 

 to follow turnips fed off on the land. 



Yery little is definitely known as to the capacity of 

 different crops for assimilating different forms of plant 

 food, but there can be no doubt that this also is one of 

 the distinctions between various crops, and one reason of 

 the economy of a rotation. The most plainly marked 

 distinction as to mode of feeding is afforded by the be- 

 haviour of various crops towards silica. Graminaceous 

 crops, as the cereals and grasses, are apparently capable 



* The -writer is indebted to Mr. Lawes for the important ideas contained 

 in the two preceding paragraphs. 



