NON-SYMBIOTIC NITROGEN-FIXING BACTERIA 



107 



represented in the soil by the following species: Az. chroococcum, 

 Az. agile, Az. heijerinckii, Az. vinelandii, Az. vitreum, and Az. 

 woodstownii (see Fig. 10). The first species, Az. chroococcum, 

 occurs much more abundantly in soils than any of the other forms. 

 (2) Clostridium — spore-forming, rod-shaped organisms, anaerobic 

 in nature, that is, capable of existing in the complete absence of free 

 oxygen; the nitrogen-fixing species of this organism are generally 

 referred to as CI. pastorianum (or Bacillus amylohacter) , but it is 

 likely that there is a large group of such forms embracing numerous 

 closely related forms, most of which produce butyric acid when 

 utilizing carbohydrate material. 



For the fixation of nitrogen, these organisms require a source of 

 energy, some available forms of phosphorus and potassium, suf- 

 ficient calcium to keep the reaction from becoming acid, and smaller 

 amounts of various other minerals. These nutrients with nitrogen 

 from the atmosphere are used for the growth of the cells; as a 

 result of growth there is an increase in the fixed forms of nitrogen in 

 the environment where the cells develop. The amount of nitrogen 

 fixed is closely related to the amount of energy available (as well as 

 to the available phosphorus when the available energy is not a 

 limiting factor). This is shown in Table 23. The amount of 

 nitrogen fixed is very intimately associated with the extent of cell 

 development, the fixed nitrogen being built up into cell constitu- 

 ents. Fixation of nitrogen is, therefore, an index of the growth 

 of the cells and an increase in the cell substance (see Fig. 50). 

 Cells may remain alive and decompose the available energy source 

 without fixing nitrogen; such a condition may exist if the cells are 

 merely respiring without increasing in numbers. 



TABLE 23 



Influence of Amount of Glucose on Fixation of Nitrogen 



BY AZOTOBACTER (fROM HuNTEr) 



