NITRATES AND NITRITES 153 



nitrogen. The ability to reduce nitrates to nitrites is 

 possessed by a large number of different kinds of 

 bacteria : out of 32, forms isolated by Frankland from 

 air and water more than half of them had this 

 property, and of 109 examined by Maassen about 90 

 showed the same power. Many of these, and others 

 equally commonly distributed, are apparently able to 

 carry the reduction further and give rise to ammonia ; 

 the latter, however, may be derived from proteins and 

 other nitrogenous organic compounds often present where 

 bacterial action is going on. It is in cases where the 

 nitrates are decomposed with the formation of the 

 gaseous oxides or free nitrogen that the process becomes 

 of special interest to the agriculturist, since these changes 

 involve a loss of valuable fertilizing material from the 

 soil or from manure heaps. The conditions essential for 

 denitrification are ; 



a. The presence of certain species of bacteria. 



b. A supply of nitrates. 



c. A considerable amount of easily assimilated organic 

 substances. 



d. Total absence or a very limited access of free 

 oxygen. 



These, it will be noticed, are practically opposite con- 

 ditions to those required for nitrification. The exclusion 

 of any one of them checks denitrification. A large 

 number of kinds of bacteria have been isolated which are 

 capable of decomposing nitrates with the evolution of 

 free nitrogen. Many of them are found in water and in 

 the soil. They are also especially abundant upon straw 

 and in the dung of the horse, cow, and other herbivorous 

 animals ; in faeces of man and carnivorous animals they 

 are rarely present. All these organisms are aerobic 



