CHAPTER VI 



TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



(Continued) 



Nitrate Formation. — Ammonia, the end product of the reac- 

 tions just considered, becomes the raw material for the process of 

 nitrate formation. Very few species of soil organisms are con- 

 cerned in this process. These organisms are conveniently divided 

 into two groups, in each of which a limited number of bacterial 

 species is known. One group oxidizes ammonia to nitrous acid 

 or nitrite, and the other transforms nitrous acid to nitric acid or 

 nitrate. The process of conversion of ammonia to the more 

 highly oxidized inorganic compounds of nitrogen as nitrite and 

 nitrate is frequently referred to as nitrification. 



2NH3 + 3O2 = 2HNO2 + 2H2O + energy (calories) liberated 



2HNO2 + 02 = 2HNO3 + energy (calories) liberated 



None of the bacteria belonging to either group is able to transform 

 ammonia directly to nitrate, but each is confined to merely one 

 stage of the reaction. It seems possible that quite a variety of 

 other microorganisms besides the specific nitrifying bacteria may 

 be concerned in the production of some nitrite or nitrate from 

 ammonia, but information concerning the relationship of these 

 organisms to the oxidation process is indefinite and little more 

 than suggestive. Nitrification by the specific bacteria is propor- 

 tionally rapid and considered to be of major importance in the 

 formation of nitrate in soils. 



The conditions suitable for the formation of nitrite and nitrate 

 by nitrifying bacteria are quite simple and include merely an inor- 

 ganic medium containing salts of ammonia and several nutrient ele- 

 ments, a neutral reaction, and aerobic conditions. If soil is intro- 

 duced into such a medium, active transformation of ammonia 



136 



