CHAPTER Xm 

 DENITRIFICATION 



It has been known for a long time that under certain condi- 

 tions more of the nitrates disappear from the soil than are taken 

 up by the growing plant. As was shown in the preceding chapter, 

 much of this loss can be accounted for in the drainage water. 

 Occasionally there are still other losses. These have been at- 

 tributed to various factors: (i) Elementary nitrogen may be 

 liberated from the protein during the process of decay; (2) the 

 nitrates or ammonia may be reduced with the production of gas- 

 eous nitrogen; (3) The nitrates may be transformed into ni- 

 trites, ammonia, or proteins within the soil. The first two pro- 

 cesses actually deplete the soil of its total nitrogen, and if it 

 occurred to an appreciable extent in normal soils would have con- 

 siderable economic significance. The third process merely changes 

 the form of the nitrogen of the soil without decreasing its total 

 nitrogen content and may be harmful or beneficial, depending 

 upon conditions. If transformed by bacteria into proteins when 

 needed by the growing plant we would have these minute specks 

 of living protoplasm competing with and retarding the growing 

 crop. The nitrogen may become available to later crops. If the 

 nitrates, when the soil is bare, are converted into proteins by bac- 

 teria they are less likely to be leached from the soil. Under this 

 condition the disappearance of nitrates from the soil would be a 

 benefit. On the other hand, where the nitrates are reduced to 

 free nitrogen this is lost completely from the soil. 



The reduction of nitrates to nitrites or ammonia can be re- 

 ferred to as partial denitrification, whereas the liberation of nitro- 

 gen as a gas from either nitrites or nitrates is true denitrification. 



Why This LossF-Early answers given to this question as- 

 sumed that the loss is due to chemical changes. Possibly the or- 



