94 THl5 CHEMISTEY OF THE FAtlM 



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 distinc- 

 tion in the relation of different crops to soil nitrogen. 

 Thus the fact that the active growth 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 crops com- 

 mences, and nitrification in the soil has 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 continual application of nitrogenous 

 manures if fertility is to be maintained. Maize, with 

 its much later period of growth, is better able to supply 

 itself with nitrogen from the soil, and yields in conse- 

 quence a much larger crop per acre. 



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 



