FOOD REQUIREMENTS OF THE AZOFIERS 257 



The number of organisms developing and the nitrogen^fixed in 

 the one receiving no nitrate is taken as 100 per cent. 



It is quite evident from these results that although nitrates cause 

 more active multiplication of Azotobacter, it greatly reduces their 

 physiological efficiency. The organisms used by Hills had probably 

 grown for a long time on media poor in nitrogen, and their ability 

 to fix nitrogen was, therefore, high. But would they continue to 

 exert this power if grown on media rich in nitrogen? The evidence 

 points strongly to the conclusion that they would not. It is certain, 

 however, that the nitrates are toxic in comparatively low concentra- 

 tions. Nitrates and ammonium sulphate are rather effective in stimu- 

 lating nitrogen-fixation when the Azotobacter are grown in connection 

 with the cellulose ferments. Even here, however, large quantities 

 decrease this power. In pure cultures ammonium sulphate seriously 

 retards nitrogen-fixation, whereas the nitrogen of humus, even in 

 large quantities, appears to have no serious retarding influence. 

 Nevertheless, a high nitrogen content of soils seems to be unfavor- 

 able to vigorous nitrogen-fixation. 



Whether this would be the case where the nitrate content of the 

 soils is kept low but with the readily decomposable protein nitrogen 

 high, is yet to be answered. Hiltner and Stormer consider that when 

 the nitrogen content of the soil passes beyond a certain limit, the 

 decay bacteria increase rapidly, and in the struggle for existence 

 they are able, with the advantage at their disposal, to suppress the 

 more slowly growing Azotobacter. 



Potassium is essential to the higher plants and cannot be replaced 

 entirely by related elements, yet Gerlach and Vogel early reached 

 the conclusion that potassium and magnesium are not essential to 

 the Azotobacter. Their results were, however, generally considered 

 erroneous, for while as much nitrogen was fixed in twenty days 

 without as with potassium, after forty days there was no further 

 fixation in the solution without potassium, but in its presence the 

 nitrogen gain nearly doubled. It was, therefore, argued that the 

 traces of potassium left in the chemicals and dissolved from the 

 glass during sterilization had been enough to permit development 

 for a time. If these elements are essential, it must be in extremely 

 minute quantities, for Vogel, using the purest chemicals obtainable, 

 was able to prepare potassium-free media in which the Azotobacter 

 developed. He did find, however, that potassium favors their 

 development. 



Phosphorus is required by these organisms, large quantities being 

 used for the building of the nucleo-proteins and phospho-proteins in 

 which their bodies are extremely rich. Moreover, it greatly acceler- 

 ates the reaction and economizes the carbohydrates; hence it is 

 rather evident that phosphorus plays a very essential part in Azoto- 

 bacter metabolism. Possibly in the early stages of the process a 

 17 



