March, 1914.] Soil Bacteria. 275 



the carbon is built into fats, protein, carbohydrates, or directly 

 oxidized and returned to the air. The waste products are sub- 

 jected to bacterial action and \Yhere the action is complete, carbon 

 is converted into carbondioxide again or into carbohydrates. 

 Bacteria are thus the agents which conserve the carbon supply. 



The cellulose of woody tissue of plants is acted upon by many 

 organisms — namely, molds and Streptothrix, which are higher 

 bacteria and look like mycelial threads of mold. The nitrogen 

 absorbing bacteria and denitrifying organisms are also active in 

 cellulose decomposition. Intermediate products of the process 

 are organic acids and under anaerobic conditions, (absence of air) 

 the production of hydrogen and methane. (CH4). 



Nitrogen is present in organic remains in the fomi of complex 

 proteins. By a series of reductions, decomposing bacteria reduce 

 these complex proteins to the fonn of ammonia (NH3) and finally 

 to free nitrogen. The nitrogen waste in animals and birds, in the 

 form of urea and uric acid especially, is reduced likewise to the 

 form of ammonia (NH3). 



Nitrifying Bacteria. 



Within the soil a class of nitrifying bacteria (nitrous and 

 nitric bacteria) convert ammonium salts into nitrates or salts 

 of nitric acid. It is important that a base such as lime be present 

 in the soil, in order to unite with this acid form of nitrogen. These 

 bacteria do not require light to enable them to grew and the 3^ can ob- 

 tain their nitrogen, carbon and other food elements from inorganic 

 salts. Plants, on the other hand, take their carbon from carbon 

 dioxide. Thus these forms of bacteria are absolutely independent 

 forms of life and may have existed before the period of higher green 

 plant life occured upon the surface of the earth. 



The work of these bacteria is to convert nitrogen into the form 

 of nitrates, in which state nitrogen is assimilated by plants. 



Deintrification is the reverse of nitrification. The latter is an 

 oxidation process by which oxygen is added by the acti\aties of 

 bacteria and organic nitrogen converted into nitrates. Denitri- 

 fication is, on the other hand, a reduction process whereby the 

 nitrate is made to part with its oxygen wholly or in part and is 

 •changed to a nitrate, to ammonia, or to nitrogen gas. The re- 

 duction to a nitrate or to ammonia does not remove nitrogen from 

 the soil, as it may again be oxidized to a nitrate. But once re- 

 duced to free nitrogen, it is returned to the air and last to the soil 

 and to the crops. 



The denitrifying bacteria require a certain amount of oxygen 

 for their growth. When oxygen is absent, they take it out of the 

 nitrate (NO3). Thus denitrification is favored b}^ an exclusion of 



