DENITRIFICATION 105 



which contain as a rule great amounts of nitrogen, this gas 

 is set free. The nitrates in the soil may be. destroyed by 

 bacteria. These processes are termed denitrification. 

 There are still other processes by which the niirnuen is not 

 loM from combination, but is changed into forms in which it 

 is not available to the plant. In the absence of air and in 

 the presence of organic matter, many bacteria can use the 

 oxygen contained in nitrates for their respiratory processes, 

 as the ordinary anaerobic bacteria use the oxygen of sugar. 

 This ability to reduce nitrates to nitrites and to am- 

 monia is a very common property of bacteria, and is made 

 use of in the detailed study of organisms. When aerobic 

 conditions are restored in the soil, the ammonia and ni- 

 trites will be reoxidized by the nitrifying bacteria. There 

 is no loss of nitrogen in the process, except such as may 

 occur in a secondary reaction that may take place between 

 the nitrites and ammonia in which the nitrogen of both com- 

 pounds is set free. It is not certain that this secondary 

 react ion is of any importance in the soil, although it may be 

 elsewhere, as in certain methods of sewage disposal. 



A much smaller number of bacteria are able to reduce 

 nitrates to free nitrogen. The conditions necessary for 

 the process are first the presence of nitrate, second a sup- 

 ply of organic matter, and third an absence of free oxygen. 

 The organic matter is essential to furnish the energy needed 

 to decompose the nitrate. It was seen that the nitrifying 

 bacteria obtain energy from the oxidation of ammonia to 

 nitrites and nitrates. If energy is set free in a chemical 

 reaction, energy will need to be absorbed to carry on the 

 reverse operation. In the presence of air these organisms 

 use the free oxygen and leave the nitrate untouched. 



The denitrifying bacteria are found in the soil and in 

 manures, especially in horse manure. It is not believed 

 that the process is of great economic importance, since con- 



