54 THE CHEMISTRY OF THE FARM. 



adapted for light sandy soils, whicli need humus to increase 

 their retentive power. 



Having glanced at the general advantages to be derived 

 from alternating green crops with cereals, we will consider 

 next the characteristics of different crops which specially 

 fit them to succeed or prepare for each other. 



Distinctive Characteristics of Crops. — Differences in 

 their periods of growth occasion a marked distinction in 

 the relation of different crops to soil nitrogen. Thus the 

 fact that the active grow^th of the cereals commences in 

 spring, and concludes at their time of blooming towards 

 the end of June, places these crops at a disadvantage 

 as to the supply of nitrates from the soil. The autumn 

 and winter rains have frequently washed out the greater 

 part of the nitrates contained in the soil before the 

 growth of the cereal crop commences, and nitrification in 

 the soil has not long recommenced its activity in summer 

 time 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 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. 



A root crop sown in early summer, on the other 

 hand, has at its disposal all the nitrates that w^ould 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 in cereal cultivation is thus assimilated and retained 

 by a root crop, and such crops are found to stand in less 



