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 where 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. • (CH 4 ). 
Nitrogen is present in organic remains in the form of complex 
proteins. By a series of reductions, decomposing bacteria reduce 
these complex proteins to the form of ammonia (NH 3 ) 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 (NH 3 ). 
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 they 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 activities 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 (N0 3 ). Thus denitrification is favored by an exclusion of 
