BACTERIA OF THE SOIL 245 



carbon from carbonates and building this up into complex 

 compounds. 



The organisms belonging to this genus of bacteria may 

 be isolated in the laboratory by placing a small amount of 

 soil in a solution containing ammonium sulphate, carbonates 

 and salts (not nitrogenous), such as phosphates, necessary 

 for the growth of bacteria. A clouding of the medium will 

 eventually develop and a microscopic examination show the 

 characteristic bacteria to be present in abundance. Chem- 

 ical tests will also show that a portion, if not all, of the 

 ammonia has been transformed into nitrite. It may be 

 emphasized that organisms such as these must be among the 

 most primitive of living things. They are capable of living 

 wholly upon inorganic sources of materials and they do not 

 even require sunlight for securing energy for manufacture 

 of their food. They may be grown in pure culture in media 

 such as agar or silica jelly containing ammonium sulphate 

 and suitable salts. 



It may further be emphasized that these microorganisms 

 do not oxidize the ammonia to nitrite in order to make use 

 of the nitrite formed, but rather that they may secure 

 energy from the oxidation. The nitrites then constitute a 

 waste product of the bacterial metabolism. Very rarely is 

 there any tendency for the nitrites to accumulate in 

 appreciable quantities in any soil. The conditions which 

 favor the growth of the Nitrosomonas will also favor the 

 growth of organisms which can transform the nitrites to 

 nitrates. Whenever nitrites are present in abundance in 

 soils, it may be taken as evidence that they have formed, not 

 as a result of the oxidation of ammonia, but as a result of 

 anaerobic denitrification, to be discussed later. Nitrites 

 are not utilized by higher plants as a source of nitrogen. In 

 fact, they are in general injurious to plants, particularly to 

 plant roots. 



Nitrites as rapidly as they form in soil are usually 



