1919] 



ALLEN — AZOTOBACTER CHROOCOCCUM 7 



speculations as to the cause of the benefit derived from the 

 mechanical stirring. 



None of the theories which have been mentioned above will 

 fit all the facts in regard to hastening the growth of Azoto- 

 bacter or of the nitrifying bacteria. These theories have, 

 however, been most suggestive and helpful in furnishing a 

 stimulus to the investigation in this difficult field. It seems to 

 us that the only way to proceed with the formulation of a 

 theory is to keep trying, and to test the theories as they are 

 proposed from as many viewpoints as possible. In this way 

 only is it possible, it seems to us, to avoid the performance of 

 an almost endless amount of empirical work. 



Now, in viewing the results obtained from diverse sources 

 on the stimulated growth of Azotobacter and of the nitrifiers, 

 we find that in all cases growth is very slow in pure cultures 

 in solutions under normal conditions of completely known 

 composition. There appears to be universal agreement on 

 this point. No one doubts, apparently, on the other hand, that 

 the addition of humus to cultures of Azotobacter is quite 

 effective, and that vigorous growth of the nitrifiers may be 

 quite easily produced in all normal soils. 



In trying to find a common factor to account for all the 



various improvements in culture solutions it should be borne 



in mind that Kaserer obtained his beneficial results from a 



"silicophosphate," and that Bottomley believes his "auxi- 



mones" to be a nucleic acid derivative, therefore a carrier of 

 phosphorus. We must bear in mind also that Gerlach and 



Vogel ('03), after studying the mineral requirements of 

 Azotobacter chroococcum, concluded that phosphorus and cal- 

 cium were absolutely indispensable nutrients. After a 67-day 

 incubation period they found the following nitrogen relations 

 per 1,000 cc. of nutrient solution: Gain 



Flask over 



content control 



( nigs. N ) ( mgs. N ) 



Series I (all inorganic nutrients) 45.2 42.5 



Series II (without calcium) 3.1 0.4 



Series III (without potassium) 21.6 18.9 



Series IV (without phosphorus) 2.8 0.1 



