CHAPTER XXII. 

 DENITRIFICATION. 



IT has been known for a long time that under conditions which 

 were not fully understood, there may and often does result a loss 

 of soil nitrogen. Most of this is due to the loss of nitrates in the 

 drainage water, but occasionally there are losses which cannot thus 

 be accounted for. This has been attributed to various causes, 

 namely: (1) the liberation of elementary nitrogen in the process 

 of decay as the complex protein is broken down into simple products, 

 (2) the reduction of nitrates or nitrites with the production of 

 ammonia or elementary nitrogen, (3) the transforming of nitrates 

 and ammonia into complex proteins through the action of micro- 

 organisms. 



Often the losses from all of these processes have been grouped 

 together and considered as denitrification. This vague usage of the 

 term has led to considerable confusion and often erroneous con- 

 clusions. But the term denitrification in its proper and more 

 limited sense refers only to the complete reduction of nitrates with 

 the evolution of elementary nitrogen. It is, however, often applied 

 in a broader sense to include all deoxidation processes whereby 

 nitrates are partly or wholly reduced. But, as pointed out by Lip- 

 man, for practical agriculture the differences are of some moment. 

 The partial reduction of nitrates to nitrites or to ammonia does not 

 necessarily involve a loss of soil nitrogen, whereas the complete 

 reduction of nitrates, wherever it occurs, must of necessity involve 

 such losses. Hence, there is some justification for referring to the 

 partial reduction of nitrates as denitrification. But it is not justifi- 

 able to classify under the head of denitrification all bacterial activi- 

 ties in the soil which lead to the disappearance of nitrates or even 

 to the diminution in the total store of soil nitrogen. For it has been 

 repeatedly demonstrated that the nitrates may completely disappear 

 without involving any loss of nitrogen. 



Early Theories. We have seen that the early investigators 

 attempted to explain nitrification by purely chemical theories. 

 This was also true with denitrification. Kuhlman, as early as 1846, 

 expressed the belief that nitric nitrogen may be reduced in the soil 

 to ammonia by the fermentation of organic substances. This same 

 idea was brought out twenty-one years later by both Froehde and 

 Angus Smith, and it also appears prominently in the writings of 

 Johnson in 1870, and Davy called attention to the fact that gaseous 

 nitrogen was set free from decomposing organic matter in the soil, 



