IMPORTANCE OF NON-SYMBIOTIC NITROGEN FIXATION 109 



fixation does occur in soils it appears to be a relatively economical 

 process. 



TABLE 24 



Influence of Concentr.\tion of Mannitol on Fixation of Nitrogen 

 BY Azotobacter (from Lipman) 



The energy used by Azotobacter and Clostridium may be 

 obtained from sugars (glucose, galactose, maltose, sucrose), higher 

 alcohols (glycerol, mannitol), starches, dextrins, a number of 

 organic acids, and certain hemicelluloses; the Clostridium uses the 

 latter two sources only to a limited extent. Cellulose, lignin, and 

 some of the hemicelluloses cannot be utilized by any of these 

 nitrogen-fixing bacteria as sources of energy. 



Importance of Non-symbiotic Nitrogen Fixation. — In 

 view of the fact that the natural organic materials added to the 

 soil in the plant residues and in the various organic manures are 

 largely made up of cellulose, lignin, proteins, and hemicelluloses, 

 while the content of soluble carbohydrates is very limited, being 

 not more than 2 to 10 per cent of the total organic matter added 

 to the soil, the amount of energy available for the nitrogen-fixing 

 bacteria is very small. The problem is complicated still further 

 by two facts: (1) The abundant flora of other bacteria and fungi 

 in the soil will also readily utilize the soluble carbohydrates, if 

 available forms of nitrogen are present. (2) In the presence of 

 available forms of fixed or combined nitrogen, such as organic 

 nitrogenous substances, ammonium salts, or nitrates, even the 

 nitrogen-fixing bacteria fail to fix nitrogen. This is not due to the 

 inabihty of the bacteria to develop under such conditions, but 

 rather to the utilization of the nitrogen which already exists in 

 fixed forms to supply their requirements. Nitrogen is used from 



