NITRIFICATION 151 



every two or three years) and supplemenl with hay stubble 

 or green manure. 



128. Nitrification. — As has been noted several times, 

 organic matter serves as the sonree of nitrogen for erop 

 plants. But before this organic nitrogen can be used by 

 plants it must undergo a change to nitrates (Section 15), 

 for most of the organic nitrogen is protein in character, and 

 hence insoluble and unavailable to crop plants. 



The process by which organic nitrogen is changed to 

 nitrates, and thereby made available to crops, is bacterial 

 in character, and is generally called nitrification, although 

 it really takes place in three steps, the first called ammoni- 

 fication and the last two nitrification proper. Oxygen is 

 necessary for these changes to take place, hence the impor- 

 tance of thorough aeration to produce nitrates from the 

 nitrogen reserves in the soil. 



(a) Ammonification is brought about by many of the 

 bacteria in the soil and is caused bv the proteolvtic enzymes 

 of the bacteria first breaking down the proteins into simpler 

 compounds, and further decomposing or hydrolyzing them 

 into ammonia among other products. Some of the bacteria 

 producing ammonia use it as a source of nitrogenous food, 

 others leave it merely as a by-product. 



(b) Nitrification Proper is a distinct bacterial oxidation 

 of ammonia to nitrous acid, and of nitrous to nitric acid. 

 There are definitely known two kinds of organisms which 

 oxidize ammonia to nitrous acids, and thev are called nitrous 

 or nitrite bacteria. The equation for this reaction may be 

 expressed as follows: 



2XH 3 + 30-2 = 2HX0 2 + 2H2O. 



There is only one kind of organism oxidizing nitrous acid 

 to nitric acid, called the nitric or nitrate bacteria. The 

 following equation represents the reaction: 



2HXO,+ 02= 2HX0 3 . 



The amount of ammonia or nitrous acid in the soil at any 

 one time is very small because the ammonia is changed very 

 rapidly to nitrous acid and the nitrous acid to nitric acid. 



