159 



Mgrms. N. per unit 

 carbohydrate 

 per 100 cc. 



Control 0.48 



Azotobacter 0.56 



Pseudomonas 0.91 



Azotobacter -|- Pseudomonas . 1.24 



Hence Azotobacter and Pseudomonas fix more nitrogen per 

 unit of carbohydrate when grown together than when grown sepa- 

 rately. Further investigation showed that this increased fixation 

 applied also to Azotobacter and Pseudomonas from ordinary soil 

 and leguminous nodules respectively. Pure cultures grown in a 

 solution consisting of mannite, 0.5 grm.; maltose, 0.5 grm.; potas- 

 sium phosphate, o.i grm.; magnesium sulphate, 0.92 grm. per 100 cc. 

 of water, at 24 C. for ten days, gave the following averages: 



Mgrms. N. 

 per 100 cc. 



Control 0.53 



Azotobacter 2.19 



Pseudomonas 2.30 



Azotobacter -f- Pseudomonas . 4.5 1 



Owing to the different cultural conditions prevailing in soil and 

 culture solutions an attempt was made to acclimatise the pure cul- 

 tures to ordinary soil conditions. About 14 pounds of autoclaved 

 garden soil was well moistened with the mixed culture and incu- 

 bated for twenty-one days at 24 C. A culture solution was then 

 obtained by mixing 5 grms. of this inoculated soil in 100 cc. of 

 water with i grm. of sugar and incubating for twenty-four hours 

 only; 50 cc. of this solution was then applied to pots containing 

 5 ounces of soil each, and incubated at 24 C. for ten days. The 

 nitrogen determinations yielded: 



Mgrms. N. 

 per 100 grms. 



Soil 50 cc. distilled water 324 



Soil 50 cc. autoclaved culture . . . 330 

 Soil 50 cc. living culture 359 



An increase of 35 mgrms. nitrogen per 100 grms. of soil, which 

 represents an increase of about 350 pounds of nitrogen per acre, 

 taking an acre of soil 4 inches deep as weighing i ooo ooo Ibs. 



To further test the effect of the mixed culture (both pure cul- 

 ture and soil culture) under ordinary conditions on different soils, 



