430 THE PRINCIPLES OF SOIL MANAGEMENT 



soil was to be inoculated, and that a large fixation 

 of atmospheric nitrogen would result. A number of 

 tests of this material failed to show that it caused any 

 marked fixation of atmospheric nitrogen. 



A number of other nitrogen-fixing organisms 

 have since been discovered. There are: (1) Several 

 members 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, and others believe to 

 be able to do so, at least in large amounts, only in 

 the presence of certain other organisms. (2) Mem- 

 bers of the Granulobacter group, which are large 

 spore-bearing bacilli of anaerobic habits. (3) B. 

 radiobacter, which appear to be closely related to or 

 identical with the B. radicicola of legume tubercles. 

 The latter has been shown to be able to fix atmospheric 

 nitrogen even when not growing in symbiosis with 

 legumes. 



There are doubtless many other nitrogen-fixing 

 organisms 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, es- 

 pecially carbonates, is necessary for the best per- 

 formance of the nitrogen-fixing function, as is also 

 the presence of a somewhat easily soluble form of 

 phosphorus. They are exceedingly sensitive to an acid 

 condition of the soil. 



