FIXATION OF ATMOSPHERIC NITROGEN 403 



pheric nitrogen is apparently possessed by a considerable number of 

 aerobic species. Lipman has demonstrated the fixation of small 

 amounts of nitrogen by Ps. pyocyanea and Lohnis secured similar results 

 with Bad. pneumonia, B. lactis mscosus, B. radiobacter and B. 

 prodigiosus. Gottheil has detected fixation by B. ruminatus and B. 

 simplex; Pillai has described a nitrogen-fixing aerobic bacillus, B. 

 malabarensis; Wester mann studied a similar organism that he named B. 

 danicus; while Beyerinck and van Delden observed, some years earlier, 

 that, certain strains of B. mesentericus could fix relatively large amounts 

 of nitrogen. Similarly Ps. radicicola has been found to possess a slight, 

 but nevertheless an appreciable power to fix elementary nitrogen in 

 culture solutions or in the soil. 



FIG. 132. Azotobacter vinelandi, a non-symbiotic nitrogen-fixing organism. 

 (After Lipman.) 



But while nitrogen fixation among aerobic soil bacteria is not as 

 uncommon as was at one time supposed, this function is so feeble and 

 so variable in most instances, as to be of negative, or, at best, of doubt- 

 ful economic significance. On the other hand, the aerobic, Azotobacter, 

 first described by Beyerinck in 1901, may be regarded not only as pos- 

 sessing a very pronounced ability to fix atmospheric nitrogen, but as 

 playing a role of some moment in maintaining the supply of combined 

 nitrogen in the soil. 



To the two species of Azotobacter, A. chroococcum and A. agilis 

 described by Beyerinck and van Delden, Lipman added A. mnelandii 



