BACTEBIA OF THE SOIL 249 



has not been adequately worked out. The reaction itself, 

 however, may be illustrated as follows : 



C„H,,0„ + 8HNO2 = 6CO3 + lOH.O + 4N2 



It is probable that in many eases the evolution of 

 elementary nitrogen is due to the simultaneous presence of 

 nitrites and amino acid. In general an amino acid and 

 nitrous acid react with each as follows : 



E • CO ■ COOH + HNO, = R ■ CHOH • COOH + H,0 + N, 



To what extent denitrification can occur in the absence of 

 amino compounds is not well understood. This latter 

 reaction explains the small amount of nitrogen gas fre- 

 quently found accompanying carbon dioxide or hydrogen in 

 fermentation tests of bacteria of the colon typhoid group. 

 Many of these forms apparently can transform nitrates into 

 nitrites but cannot directly attack the nitrite molecule. If 

 amino acids are present, however, there may be an evolution 

 of nitrogen gas. 



Denitrification is negligible in its results in most well- 

 aerated soils. It may prove to be of importance, however, 

 in certain soils containing considerable quantities of nitrate 

 which have become water-logged. The free oxygen soon 

 disappears and the nitrate may be transformed into nitrite 

 by the bacteria present. It is possible that in some cases 

 the amount of nitrites formed in this fashion may be suf- 

 ficient to exert a toxic action upon plant roots. It has been 

 found, for example, that the addition of nitrate as a fer- 

 tilizer to rice is not good practice but that ammonia salts 

 are much to be preferred. Apparently, this is because the 

 roots of the rice grow in a water-logged soil containing little 

 free oxygen. Nitrates may be reduced to nitrites and ren- 

 dered not only unavailable to the plant roots, but actually 

 toxic. Ammonia salts, on the other hand, are not trans- 



