106 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



bacteria, a number of other bacteria may be capable of fixing 

 small quantities of nitrogen, especially when these microbes have 

 been recently isolated from the soil. However, neither the 

 amount of nitrogen fixed nor the specificity of their nature can 

 place these organisms on an equivalent basis with the other two 

 groups of bacteria. There are indications that certain highly 

 specialized forms of fungi, such as species of Phoma, which 

 cause the formation of endotrophic mycorrhiza, may fix some 

 nitrogen. Certain of the blue-green algae, as species of Anabena 



1.8 2640 12 



1.5 2200 10 



1.2. J 1760 fc 8 - 



DO .2 c 



E = - 



Z 0.9 ? 1320 i5 6 



fti.o.e 



0.3 



440 



5 6 7 8 9 

 Time (Hours) 



10 11 12 13 



Fig. 50. — Correlation between fixation of nitrogen and growth of bacterial cells 

 (after Meyerhof and Burk). 



and Nostoc, may also be able to fix relatively large amounts of 

 nitrogen under certain conditions, but the grass-green algae have 

 no such capacity. 



The fixation of nitrogen is thus a physiological inheritance of a 

 comparatively small number of microorganisms with the principal 

 representatives included among the bacteria. 



Non-symbiotic Nitrogen-fixing Bacteria. — Those bacteria 

 capable of fixing atmospheric nitrogen which lead a free existence 

 in the soil are found distributed in two groups: (1) Azotobacter — 

 large, coccus-like, non-motile, aerobic organisms. This group is 



