110 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



the atmospheric sources only in the absence of other available 

 sources of supply (see Table 25). Growth of most higher plants in 

 soils is dependent upon a rather extensive and continuous supply 

 of nitrogen in such fixed forms as ammonium salts or nitrate. 

 Since the presence of such nitrogen in soils would prevent fixation 

 processes, it is likely that fixation does not occur very extensively 

 in soils while plants are growing. This does not eliminate the pos- 

 sibilities of fixation in some localized regions of the soils or at 

 certain periods when considerable amounts of plant residues may 

 become introduced into soils. 



TABLE 25 

 Influence of Nitrates on Transformations of Nitrogen by 



AZOTOBACTER (fROM BoNAZZI) 



Associated with the decomposition of 100 parts of available 

 organic matter free from nitrogen there may be fixed about one 

 part of nitrogen. Assuming that an acre of soil received a liberal 

 application of plant residues, such as 2 tons of water-free material, 

 the amount of carbohydrates available for the use of non-symbiotic 

 nitrogen-fixing bacteria would not exceed 800 pounds. Assuming 

 further that all this energy was used by the specific bacteria, only 

 about 8 pounds of nitrogen would be fixed per acre per year if the 

 aerobic organisms were active. Less would be fixed by the anaerobic 

 organisms. Considering the fact that the 4,000 pounds of dry 

 organic matter would contain 20 to 60 pounds of nitrogen, some of 

 which will be liberated through the activities of the numerous 

 soil fungi and bacteria, the evidence that any appreciable amounts 



