[Chap. XLIII BACTERIA OF THE SOIL 533 



Nitrification. Ammonia in turn becomes the compound transformed 

 in nitrate formation. Only a few species of soil bacteria activate this 

 process; they may be classified into two groups. The first group oxi- 

 dizes ammonia to nitrous acid. 



Ammonia + Oxygen > Nitrous acid + Water 



2NH3 + 3O2 > 2HNO2 + 2H2O 



The nitrous acid may form salts with basic ions in the soil. 



Nitrous + Potassium > Potassium + Carbon + Water 



acid carbonate nitrite dioxide 



2HNO2+ K2CO3 > 2KNO2 + CO2 + H2O 



The formation of nitrous acid and nitrites from ammonia occurs in 

 soils in America and Australia primarily in the presence of species of 

 Nitrosococciis; in the soils of Europe, Asia and Africa species of Nitro- 

 somonas are the nitrite formers. 



The last stage in nitrification is the oxidation of nitrous acid and 

 nitrites to nitric acid and nitrates. 



Nitrous acid + Oxygen > Nitric acid 



2HNO2 + O2 > 2HNO3 



The nitric acid may form salts with basic ions in the soil. 



Nitric + Potassium > Potassium + Carbon + Water 



acid carbonate nitrate dioxide 



2HNO3+ K2CO3 > 2KN0, + CO2 + H2O 



Nitrous acid and nitrites seldom accumulate in soils, for as rapidly as 

 they are formed other nitrifying bacteria of the genus Nitrobacter 

 oxidize them to nitrates. 



Thus the process of nitrification includes the oxidation of ammonia 

 to nitrites, and the further oxidation of nitrites to nitrates. Each step in 

 the process from proteins to ammonia, from ammonia to nitrites, nitrites 

 to nitrates, is dependent upon the presence of appropriate bacteria. 

 Nitrification is an oxidation process and the energy released is used by 

 the bacteria in the synthesis of sugar from carbon dioxide and water, 

 and also in the further elaboration of sugar and certain inorganic salts 

 into proteins and other cell compounds of the bacteria. These micro- 

 organisms, then, are autophytes. 



The conditions necessary for natural nitrification include moderate 

 moisture, wami temperatures of the soil, good aeration, and fairly 



