STUDIES ON AZOTOBACTER CHROOCOCCUM BEU. 367 



positively shown whereas it would be assumed that a nitrogen 

 fixation from the atmosphere by the action of non-symbiotic 

 nitrogen fixers should take place at an active rate to judge from 

 laboratory experiments made in selective media and in absence 

 of combined nitrogen. Yet a study of the subject will show 

 that soils are only exceptionally free of nitrates and that these 

 are easily washed away. It is therefore the belief of the present 

 author that Azotobacter rather than serving as an active nitro- 

 gen (free) gatherer, may act to immobihze the nitrate nitrogen, 

 taking the upper hand over the denitrifiers, and, to a con- 

 siderable extent, stopping the mentioned percolation. 



By this it should not be understood that the organism is 

 hereby assumed to be lacking in all power of nitrogen fixation, 

 but only that this function is not to be considered as an all- 

 important phenomenon always active to the full benefit of man 

 and to the detriment of the active organism itself, as it appears 

 that ''all" organisms choose the line of least resistance for obtain- 

 ing and assimilating their food; and microorganisms are not an 

 exception to the rule in spite of the arbitrary classification that 

 is made of them into ''heneficiaV and "non-beneficial." 



That these experiments were made in solution does not detract 

 from the conclusions derived therefrom, since we have seen that 

 an obligate aerobic function such as nitrite formation, when 

 studied by the methods used in this memoir may be advantage- 

 ously compared \vith this function in soils. 



IV. METHODS 



A word is probably necessary on the methods used in the 

 analysis of the cultures. The procedure used for the determi- 

 nation of ammonia, nitric and organic nitrogen on the same 

 sample has been outlined by Davisson elsewhere (1918). The 

 ammonia determinations were done by aeration over 5 grams 

 sodium carbonate and subsequent distillation into standard 

 acid. Subsequent treatment of the material in the aeration 

 flask with 2.5 cc. of concentrated sulphuric acid, to destroy the 

 carbonate, followed by 2 cc. of 50 per cent sodium hydroxid 



