152 THE SOIL: ORGANIC MATTER 



Particularly is it difficult to detect traces of nitrous acid 

 because the nitrous and nitric organisms work together, the 

 latter using up nitrous acid as soon as it is formed. 



It is claimed that there is one kind of bacteria which 

 oxidizes ammonia directly to nitric acid, but its identity is 

 not completely established. 



These nitrifying organisms, the nitrous and the nitric, 

 obtain their energy by this oxidation process, and also utilize 

 the ammonia and nitrous acid respectively as food for 

 growth. When this happens, as whenever bacteria use soluble 

 nitrogenous compounds as food, some nitrogen is converted 

 into protein and rendered insoluble and unavailable to plants 

 until acted upon by bacteria, as in the first instance. More- 

 over, these bacteria utilize only inorganic food. From 

 carbon dioxide they manufacture their cellular substances of 

 an organic nature. The synthesis is brought about not by 

 chlorophyl but by the oxidation of ammonia and nitrous 

 acid. Hence organic matter is not essential for these bacteria, 

 and in fact too much soluble organic matter interferes with 

 their growth. This does not happen in ordinary farm soil, 

 but is a serious matter at times in soils very intensively 

 fertilized with manure and sewage, like greenhouse and 

 truck soils. 



It is to be noted that the free acids themselves are the 

 products of these bacteria. In the presence of bases or basic 

 carbonates the acids are neutralized. Since calcium car- 

 bonate is the principal base in the soil, nitric nitrogen occurs 

 in most soils as calcium nitrate, although some of the acid is 

 neutralized by magnesium carbonate and potassium car- 

 bonate. If there is not sufficient basic material to neutralize 

 the acids as they are formed, the bacteria are rendered 

 inactive or are killed by the excess of acid. Other conditions 

 for their growth are much the same as mentioned for bacteria 

 in general (Section 121, a). 



129. Denitrification. — A process just the opposite of nitri- 

 fication is denitrification, which results in a loss of nitrogen . 

 Under anaerobic conditions and in the presence of large 

 quantities of easily decomposed organic matter, there are 

 several species of bacteria which can reduce nitrates to 



