NITROGEN-FIXATION IN PURE CULTURES 431 



293. Mixed cultures of nitrogen-fixing organisms. - 

 Mixed cultures of the various organisms mentioned 

 fix larger amounts of nitrogen than do the pure cultures 

 of any one of them, while some forms are incapable 

 of fixing nitrogen in pure cultures. Certain algse, par- 

 ticularly the blue-green alga?, aid greatly in promoting 

 growth and nitrogen-fixation by these organisms. 

 This they probably do by producing carbohydrates, 

 which are used by the bacteria as a source of energy 

 for nitrogen-fixation, the bacteria furnishing the alga? 

 with nitrogenous compounds. To what extent the 

 relation is symbiotic is not known at present, but it 

 seems probable that a relation may exist similar to 

 that between leguminous plants and the nitrogen- 

 gathering bacteria in their nodules. 



294. Nitrogen-fixation and denitrification antagonis- 

 tic. Nitrogen-fixation and denitrification are reverse 

 processes. The former is, for most bacteria, favored 

 by an abundant air-supply and a moderately high 

 temperature. Thus, at 75 Fahr., fixation was rapid; 

 at 59 Fahr., it was decreased, and at 44 Fahr, 

 there was none. Denitrification is favored by a some- 

 what limited supply of oxygen. 



There is no reason to believe that the practical 

 importance of nitrogen-fixation without legumes is 

 equal, under the most favorable conditions, to that 

 with legumes. A further knowledge of the organisms 

 effecting fixation and of their habits will doubtless 

 make possible a greater utilization of their powers, 

 to supplement the use of legumes, as a source of com- 

 bined nitrogen in the soil. 



