464 SOILS: PROPERTIES AND MANAGEMENT 



of the group designated Azotobacter, which are aerobic 

 bacteria, and which some investigators hold to be capable of 

 fixing atmospheric nitrogen when grown in pure cultures, 

 while others believe them to be able to do so, at least in 

 large amounts, only in the presence of certain other 

 organisms; (2) members of the Granulobacter group, 

 which are large spore-bearing bacilli of anaerobic habits; 

 (3) Bacillus radiobacter, which appear to be closely related 

 to or identical with the B. radicicola of legume tubercles. 

 The last-named has been shown to be able to fix atmospheric 

 nitrogen even when not growing in symbiosis with leg- 

 umes. 



There are doubtless many other nitrogen-fixing or- 

 ganisms still to be discovered. 



A peculiarity of these nitrogen-fixing organisms is 

 their use of carbohydrates, which they decompose in 

 the process of nitrogen fixation. They secure more 

 atmospheric nitrogen when in a nitrogen-free medium. 

 The presence of soluble lime or magnesium salts, especially 

 carbonates, is necessary for the best performance of the 

 nitrogen-fixing function, as is also the presence of a some- 

 what easily soluble form of phosphorus. The organisms 

 are exceedingly sensitive to an acid condition of the soil. 



383. 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 algae, particularly 

 the blue-green algae, 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 algae with nitrogenous compounds. 



