266 AZOFWATION 



mgm. Moreover, Krainsky found Azotobacter to utilise from 100 

 to 200 gm. of sugar in the assimilation of 1 gm. nitrogen when 

 grown in solution, but when grown on sand it required only 11 to 

 30 gm. for the same fixation. 



They utilize their energy more economically in the presence of a 

 liberal supply of phosphorus than when the quantity of available 

 phosphorus is limited. This accounts, in a measure, for the high 

 fixation noted in most Utah soils. 



Manure. It has been known for a long time that humus exerts 

 a highly favorable effect on nitrogen-fixation. The great question, 

 however, has been as to the manner of action. Humus, being such 

 a complex, variable substance, varies greatly in action, depending 

 upon its source. Remy considered that some of the products from 

 humus are favorable sources of organic matter for Azotobacter. 

 Definite and valuable information is furnished by the work of 

 Lohnis and Green. They worked with mixed cultures of A. chrco- 

 coccum, A. beijerinckii, A. vinelandii, and A. vitrium in Beijerinck's 

 mannite solution with various forms of organic matter. 



Nitrogen fixed in 

 100 cc. of solution 



Material. after 3 weeks. 



mgm. 



Fresh straw 10.0 



Fresh stable manure 9.8 



Fresh peat .... 9 ! 3 



Green manure 8.0 



Beijerinck's mannite solution 5.6 



After humification, these substances were even more readily 

 assimilated and the nitrogen-fixation was greater than when the 

 unhumified substance was used. 



The same year Hanzawa published results which show that stable 

 manure even up to 3 per cent, greatly stimulated bacterial activities. 

 Green manure humus was found by him to be injurious. From this 

 it is certain that humus can act as a source of energy and usually 

 stimulates bacteria, but the extent is governed largely by its com- 

 position and by the quantity of available combined nitrogen which 

 is being supplied with it to the organism. In addition to this, corn 

 roots, cornstalks, oak leaves, lupine, alfalfa, maple leaves, and pine 

 needles may all serve as a useful source of energy to the nitrogen- 

 fixing organisms. Apparently, the tissues from the non-legume 

 give a greater gain than do those from the legumes. Fulmer has 

 recently confirmed these results. 



The influence of stable manure upon the nitrogen-fixing powers 

 of the soil under field conditions is seen from the following table in 

 which the quantity of nitrogen fixed in the unmanured soil has been 

 taken as 100 per cent. 



