Nitrogen Fixation by Aerobic Organisms. 467 



Series 



Contents 



Total 

 nitrogen. 



G-ain in N per grm. 

 carbohydrate. 



Average gain. 



A 



Pseudomonas on 1 gnn. 

 dextrin — 



1 



0-32 









3 



2-72 

 2-88 



2-40 









2-56 



| 2'48 



B 



Pseudomonas on 1 gnn. 

 maltose — 



0-28 







3 



2-96 



2-68 



| 2-54 





4 



2-68 



2-40 



C 



Azotobacter on 1 grm. 

 dextrin — ■ 



0-64 









3 



7-04 

 6-88 



6-40 







4 



6-24 



| 6-32 



D 



Azotobacter on 1 grm. 

 mannite — 



0-32 







3 



7-20 



6-88 







4 



7-36 



7-04 



| 6-96 



E 



Azotobacter + Pseudomonas 

 on 2 grm. dextrin — 

 1 



0-32 







3 



15 -05 



14 -73 



| 14 -89 











V 



Azotobacter + Pseudomonas 

 on 1 grm. maltose + 

 1 grm. mannite — 



1 



0-70 







3 



12 -28 



11 -58 



| 11 -86 





4 



12 -84 



12 -14 









From the above figures it is evident that dextrin is about as efficient as 

 maltose for Pseudomonas, and as mannite for Azotobacter, whilst for a mixed 

 culture of the two organisms it is much more effective than a mixture of 

 maltose and mannite. 



Another difficulty encountered in working with mixed cultures was the 

 neutralisation of the nutrient medium. The neutralisation agents generally 

 employed are calcium carbonate for Pseudomonas, and sodium hydrate for 

 Azotobacter. In some preliminary experiments with various neutralising 

 agents it was found that basic slag gave excellent results with both organisms, 

 not only effectively neutralising the medium but also increasing nitrogen 

 fixation. This tonic effect of basic slag is probably due to the presence of 

 iron and manganese. 



Krzemieniewski* has shown that humus stimulates the fixation of nitrogen 

 by Azotobacter in a remarkable manner. Later it was pointed out by Eemy 

 and Eosingf that this stimulating effect is due to the presence of iron in the 

 * ' Bull. Acad. Sci. Cracovie,' 1908. 

 t 'Centrbl. f. Bakt.,' 1911, vol. 30. 



