68 THE CHEMISTRY OF THE FARM. 



not long recommenced its activity in the summer months 

 when the crop becomes too mature to appropriate fresh 

 supplies of nitrogen. Continuous wheat cropping thus 

 results in a gradual impoverishment of soil nitrogen by 

 autumn and winter drainage, over and above the nitrogen 

 actually removed in the crops, and thus necessitates a 

 considerable application of nitrogenous manure if fertility 

 is to be maintained. Maize, with its later period of growth, 

 is better able to supply itself with nitrogen from the soil. 



A root crop sown in early summer, on the other hand, 

 has at its disposal all the nitrates that would be available 

 for wheat or barley, and in addition the large supply of 

 nitrates formed in the soil during summer and early 

 autumn. A great part of the nitrates which would be 

 lost by drainage during cereal cultivation is thus assimi- 

 lated and retained by a root crop, and such crops are 

 found to stand in less 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 the 

 autumn.* 



Another important difference between crops lies in 

 their range of roots. Deeply-rooted crops, as lucerne^ 

 sainfoin, red clover, rape, and mangel, and among the 

 cereals wheat and rye, are to a considerable extent sub- 

 soil 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 



* The writer is indebted to Sir J. B. Lawes for the important ideas 

 contained in the two preceding paragraphs. 



