The Azotobacters 



201 



growth of azotobacter even when the land was in good 

 condition. It is not at all impossible that the persistent 

 fertility of limestone soils is due in part to their vigorous 

 flora of azotobacter species and the consequent steady 

 gains of nitrogen through the activities of these organisms. 



Much interesting light is still to be thrown on the 

 relation of azotobacter to other soil bacteria. There is 

 good reason to be- 

 lieve that this re- 

 lation is important, 

 not only from the 

 standpoint of 

 nitrogen -fixation 

 (addition) but also 

 from the stand- 

 point of nitrogen- 

 transformation. 

 Beyerinck thought 

 at first that azoto- 

 bacter have not, 

 by themselves, the 

 power of fixing at- 

 mospheric nitro- 

 gen, and that they become endowed with this power 

 only in the presence of certain other soil bacteria. Sub- 

 sequent investigations by others demonstrated the 

 fallacy of his views, and the writer showed experiment- 

 ally why Beyerinck came to hold such views. 



But, while azotobacter can undoubtedly fix atmos- 

 pheric nitrogen when in pure culture, its power of nitro- 

 gen-fixation is enhanced by other species themselves 



Fig. 31. Azotobacter Vinelandii; X 1,000. One 

 of the non-symbiotic nitrogen-fixing bacteria. 



