62 AGRICULTURE 



I 



their activity, and, as a consequence, nitri- 

 fication goes on more energetically in a soil 

 that is well aerated than in one which is less 

 supplied with atmospheric oxygen. Nitri- 

 fication therefore takes place to a greater 

 extent in the surface soil than in the sub- 

 soil, in a sandy soil than in a clay soil, and in 

 a soil which is frequently stirred by agri- 

 cultural implements than in one which is 

 allowed to become dense and encrusted. 

 Also, like most other living things, the 

 nitrifying organisms can only work well 

 when the temperature is satisfactory. Their 

 activity ceases entirely when the temperature 

 approaches the freezing point, and, similarly, 

 they are put out of action by a temperature 

 that rises above 130 F. It is found by 

 experiment that the temperature which suits 

 them best is in the neighbourhood of 100 F. 

 On account of their dependence upon a com- 

 paratively high soil temperature, the nitrify- 

 ing organisms are practically inactive during 

 winter, performing most of their work during 

 summer and autumn. Crops, therefore, like 

 wheat, which are sowed in autumn, and make 

 a large part of their growth during winter 

 and spring, are often well supplied with 

 available nitrogen, and especially so if they 

 follow a well-worked bare-fallow. 



