The assimilation of nitrogen by certain nitrogen-fixing bacteria in the soil

In a communication on “Some Effects of Nitrogen-fixing Bacteria on the Growth of Non-Leguminous Plants,” it was pointed out that Azotobcider and Pseudomonas obtained from the root tubercles of Cycas when grown together fix more nitrogen per unit of carbohydrate than the combined amount of nitrogen when each is grown separately. In order to determine if this is true for a mixed culture of Azotobacter and Pseudomonas obtained from ordinary soil and leguminous nodules respectively, pure cultures of these organisms were obtained, Azotobacter and chroocoecm from garden soil and Pseudomonas radicicola from bean and clover nodules, by the method already described. Erlenmeyer flasks containing a culture solution, consisting of maltose 0·5 gramme, mannite 0·5 gramme, monobasic potassium phosphate 0·1 gramme, magnesium sulphate 0·02 gramme, in 100 c.c. distilled water and rendered neutral by sodium hydrate, were inoculated with 1 c.c. of pure cultures per 100 c.c. of culture solution (the controls being autoclaved to kill the bacteria present), and incubated at 24° C. for 10 days. Nitrogen determinations of the contents of the flasks gave the following averages:- Gerlaeh, Lipman, and others have described experiments showing that pure cultures of Azotobader and Pseudomonas respectively have little or no power to increase the store of soil nitrogen when added directly to the soil. A probable explanation of these negative results may be found in the different conditions for growth and development in ordinary soil and in a culture solution.