468 



Nitrogen Fixation by Aerobic Organisms. 



humus. As these experiments had reference only to Azotohacter an investiga- 

 tion was made on the effect of humus and basic slag (which contains iron) on 

 the fixation of nitrogen by pure cultures of Pseudomonas. 



Three series of four flasks each were arranged. Each flask contained 

 100 c.c. of culture solution containing 1 per cent, dextrin, 0"2 per cent, 

 di-potassium phosphate and O02 per cent, magnesium sulphate. Each flask 

 was inoculated with 1 c.c. of a pure culture of Pseudomonas from the root 

 nodules of Phaseolus vulgaris. Series A received 1 per cent, of chalk ; 

 Series B, 0'03 per cent, of potassium humate ; and Series C, 0-4 per cent, basic 

 slag (containing lime 47 per cent., phosphoric acid 18"88 per cent., and iron 

 6 - 89 per cent.). 



The flasks were inoculated at 26° C. until all the dextrin had disappeared. 

 Mtrogen determinations then gave the following: — 



Series. 



Contents. 



Total 

 nitrogen. 



Gain in N. per grm. 

 carbohydrate. 



Average gain. 



A 



Culture + 1 per cent. 



0-28 









3 



2-26 



1-98 



| 1-95 





4 



2-19 



1-91 



B 



Culture + 1 per cent, 

 chalk + P 03 per cent, 

 potassium humate — 



0-55 







3 



3-67 



3-12 

 3-26 



j 3-19 







3-81 



C 



Culture + 4 per cent, 

 basic slag — 



1 



0-28 











3-39 



3-11 



| 3-04 





4 



3-25 



2-97 











The above table indicates that for cultures of Pseudomonas basic slag is a 

 much more effective neutralising agent than chalk, and that its stimulative 

 effect on nitrogen fixation is very nearly equal to that of humus. 



Erom the two sets of experiments described above it is evident that a 

 simple and effective nutrient medium for both Azotohacter and Pseudomonas 

 is obtained by adding to distilled water 1 per cent, of dextrin, - 2 per cent, of 

 di-potassium phosphate, 0'02 per cent, magnesium sulphate and - 4 per cent, 

 basic slag. 



