DISTINCTIVE CHARACTERISTICS OF CEOPS. 69 



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 

 contribute 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 different 

 capacity of different crops for assimilating various forms 

 of plant food, but there can be no doubt that this forms 

 one of the most important distinctions between various 

 crops, and is one reason of the economy of a rotation. 

 A very plainly marked distinction as to mode of feeding 

 is afforded by the behaviour of various crops towards 

 silica. Graminaceous 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 

 generally 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. 



