The Relation to Nitrogen 235 



closest attention to those conditions resulting in the high- 

 est nitrification is obvious. 



As a rule nitrate formation in the soil begins rapidly in 

 the spring and with most crops a maximum is reached 

 during the first half of the growing season ; subsequently 

 there is a fall in the nitrate content which may approach 

 a minimum in the late fall, or with the maturity of the crop. 

 Recent studies upon the relation of crops to the nitrate 

 content have developed a number of interesting views which, 

 however, may not be discussed in this place. 



130. Denitrification. Almost the counterpart of nitri- 

 fication is the process exhibited by many micro-organisms 

 of reducing nitrates and nitrites, known as denitrification. 

 From an agricultural standpoint the most serious case is 

 that of the reduction of nitrates and nitrites with the forma- 

 tion of free nitrogen, and the consequent loss to the soil 

 of combined nitrogen. In many cases, however, the reduc- 

 tion is not carried so far as to form free nitrogen. 



A large number of organisms are able to accomplish 

 nitrate reduction, both bacteria and fungi ; but the condi- 

 tions under which denitrification occurs are not usually 

 those developed under the best agricultural practices. 



Aside from the presence of necessary nitrates this reduc- 

 tion requires the presence of considerable soluble organic 

 matter and poor aeration. Many of the organisms which 

 induce denitrification are aerobic forms which in the pres- 

 ence of sufficient oxygen show no tendency toward nitrate 

 reduction. It is apparent that saturation of the soil with 

 water after heavy manuring may actually result in nitro- 

 gen loss and frequently also in the production of in- 

 jurious compounds in the soil. 



