NITROGEN FIXATION 153 



nitrites, to ammonia, and to free nitrogen. The following 

 equations illustrate the reactions: 



2HN0 3 =2HN0 2 +0 2 

 4HN0 2 = 2H2O + 2N 2 +302 

 HNO3 + H2O = NH 3 + 2O2. 



These bacteria can live under aerobic conditions, in which 

 case they use free oxygen for their respiration, but under 

 anaerobic conditions they use the oxygen removed from 

 nitrates. The oxygen, whether from the air or from nitrates, 

 they use in oxidizing organic matter which is necessary for 

 their growth. The denitrifiers occur in manure and on straw 

 to a considerable extent, but are not responsible for loss of 

 nitrogen under ordinary farming conditions. In cases of 

 excessive applications of manure in addition to nitrates, 

 or in greenhouses where soils are very moist and large 

 quantities of organic matter are present, nitrates may be 

 reduced. In any conditions where soils are compact or verv 

 wet so there is no aeration, and where excessive quantities 

 of decomposing organic matter are present in addition to 

 nitrates, there denitrification may occur. It is, however, not 

 a condition that occurs frequently enough to cause anxiety 

 over loss of nitrogen. In any event, where only nitrites and 

 ammonia are the products, loss does not occur, for these 

 compounds may be later oxidized back to nitrates. Only 

 free nitrogen is a total loss. 



130. Nitrogen Fixation. — In the discussion of nitrogen 

 for the use of plants, it has been noted that the source of 

 nitrogen is the organic matter of the soil, that in the decom- 

 position processes most of it is made available but some may 

 be lost to the air; that after entering the plant it is used 

 in tissue building and goes largely to the seed. After the 

 plant dies that part which is left on the field serves as organic 

 nitrogen for bacterial decomposition again. That part of 

 the plant which goes to feed animals returns to the soil 

 sooner or later in the form of manure, or dead animals, or 

 parts of animals. The nitrogen is continually travelling in 

 a circle with some loss. So far no mention has been made 

 of any gain. 



