706 



PRINCIPLES OF SOIL MICROBIOLOGY 



amount of nitrate formed in the control soil and in the soil to which 

 clover was added. This shows that about 2 per cent of nitrogen was 

 sufficient to allow the microorganisms to utilize the available energy in 

 the fresh organic matter; the greater the amount of organic matter 

 added, the less is the amount of nitrate found and the more of it is used 

 up by the microorganisms. In soil treated with dried blood (any other 

 source of organic matter containing more than 2 per cent nitrogen could 

 have been used), the amount of nitrate found was greater than in the 



TABLE 74 



Relation of nitrogen content of various plant roots to nitrogen in teachings from soil 



in which they ivere decomposed 



TABLE 75 

 Milligrams of nitrate nitrogen per 100 grams of dry soil 



control. The actual amount of nitrate produced was regulated by the 

 per cent of nitrogen contained in the organic matter. The higher the 

 nitrogen content, all other factors being alike, the greater will be the 

 amount of nitrate formed. 



The nitrogen fixed by the microorganisms in their cells will sooner or 

 later become available, when these cells begin to decompose, as seen in 

 table 75. 100 



100 Hill, H. H. The effect of green manuring on soil nitrates under greenhouse 

 conditions. Virginia Agr. Exp. Sta. Tech. Bui. 6. 1915. 



