THE NITRIFYING BACTERIA 



273 



nitrogen-fixing bacteria; (3) The denitrifying bacteria; 

 and (4) The sulphur bacteria. 



The Nitrifying Bacteria. — There are two separate 

 and distinct sorts of bacteria concerned in changing am- 

 monia compounds into usable nitrates. The first kind can 

 utilize ammonia and oxidize it to nitrous acid compounds 

 called nitrites. The second set can make no use of am- 

 monia compounds, but must have nitrites which they 



^ Fig. 63. — Soil Bacteria. Fig. 1, Azotobacter, a motile bacterium 

 living in the soil which is able to fix the free nitrogen of the atmos- 

 phere and leave it behind after its death in such a form that it can 

 eventually be used by higher plants. Fig. 2, Tubercle bacteria, which 

 inhabit the soil and invade the roots of clover and other leguminous 

 plants, causing the plant to form tubercles in which the bacteria live. 

 They also fix free nitrogen and thereby enrich the soil. Fig. 3, 

 Sulphur bacteria living in soil water. They are able to utilize the gas, 

 hydrogen sulphide, as a source of energy by oxidizing first the hydrogen 

 to form water and later the sulphur to form compounds of sulphuric 

 acid. 



oxidize to nitrates, being thus dependent on the first 

 sort. 



The Nitrite Bacteria. — There are several different 

 species of soil bacteria which can change ammonia com- 

 pounds into compounds of nitrous acid called nitrites. 

 They all seem to have essentially the same mode of life. 

 This is indicated in a simple way by the formula: 

 NH4OH (ammonia) + 2O2 (oxygen) = HNOo (nitrous 

 acid -f- 2H2O (water). Of course the actual chemical 

 processes are more complicated because the ammonia may 

 occur in the soil in other compounds, e.g., ammonium 

 carbonate (NH4)2C03. The reaction is presumably 

 brought about by the action of an oxidizing enzym 



