627 



The Assimilation of Nitrogen hy certain Nitrogen-fixing Bacteria 



in the Soil. 



By "W. B. BoTTOMLEY, M.A., Professor of Botany in King's College, London. 



(Communicated by Prof. J. Reynolds Green, F.R.S. Received June 20, — 



Read June 30, 1910.) 



In a communication on " Some Effects of Nitrogen-fixing Bacteria on the 

 Growth of Non-Leguminous Plants,"* it was pointed out that Azotohacter 

 and Pseudomo7ias 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 Azotohacter and Pseudomonas obtained 

 from ordinary soil and leguminous nodules respectively, pure cultures of 

 these organisms were obtained, Azotohacter chrooeoccum 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 O'o 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 : — 



Control 0-53 milligramme N in 100 c.c. per 



unit of carbohydrate. 

 Azotohacter alone 219 milligrammes N in 100 c.c. per 



unit of carbohydrate. 

 Pseudomonas alone 2'30 milligrammes N in 100 c.c. per 



unit of carbohydrate. 

 Pseudomonas + Azotohacter ... 4-51 milligrammes N in 100 c.c. per 



unit of carbohydrate. 



Gerlach,t Lipman,t and others have described experiments showing that 

 pure cultures of Azotohacter 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 



* 'Proc. Roy. Soc.,' B, vol. 81, pp. 287-289. 

 t ' Centralbl. f. Bakt. Abt. II.,' vol. 8, 1902. 

 X ' Eeports New Jersey Expt. St.,' 1904—1907. 

 VOL. LXXXII. — B. 3 A 



