FIXATION OF ATMOSPHERIC NITROGEN 579 



Some salts may act directly as stimulants to nitrogen-fixing bacteria. 

 The optimum concentration of manganese for nitrogen-fixation by 

 Azotobacter chroococcum in mannite solution was found to be 83 1 to 2 

 nigra, per 100 gram of medium; further increases diminished nitrogen- 

 fixation. One part of manganese per million parts of soil was the opti- 

 mum for Bac. amylobacter. The addition of small amounts of arsenic 

 (10 parts per million) exerted a stimulating effect upon nitrogen-fixa- 

 tion in the soil and upon the more economic utilization of energy by 

 Azotobacter. 84 Aluminum has been looked upon 85 both as a stimulant 

 and as a retarding agent. 



Uranium salts exert a decided stimulating effect upon Azotobacter. 86 

 The maximum amount of nitrogen fixed and maximum growth take place 

 in the yellow or green rays; the least, in violet light. The maximum 

 amount of nitrogen per gram of sugar decomposed is fixed in blue light ; 

 the least, in yellow light. 



Influence of organic matter upon nitrogen fixation. The favorable 

 influence of soluble soil organic matter, or so-called "humus," on nitro- 

 gen-fixation was known for a long time; 87 however, it was recognized that 

 the nature of this action depends on the source of the organic matter. It 

 was at first suggested that some of the constituents of the soil organic 

 matter are used by Azotobacter as a source of energy. 88 Lohnis and 

 Green, 89 however, working with various species of Azotobacter in 

 mannite solution, found that the amount of nitrogen fixed in three 

 weeks in 100 cc. of solution was 5.6 mgm. ; the addition of green manure 

 increased it to 8.0 mgm., fresh stable manure to 9.8 mgm., and fresh 

 straw to 10.0 mgm. When these substances were previously decom- 



83 Olaru, D. A. Role du manganese en agriculture. Son influence sur quel- 

 ques microbes du sol. Paris, Bailleres. 1920. 



84 Greaves, J. E. Stimulating influence of arsenic upon the nitrogen-fixing 

 organisms in the soil. Jour. Agr. Res., 6: 389-416. 1916. 



86 Kaserer, H. Zur Kenntnis des Mineralstoffbedarfs von Azotobacter. Ber. 

 deut. bot. Gesell., 28: 208-262. 1910. 



86 Kayser, E. Influence des radiations lumineuses sur 1' Azotobacter. Compt. 

 Rend. Acad. Sci., 171: 969-971. 1920; 172: 183-185, 491-493, 1133-1134. 1921; 

 Ann. Inst. Nat. Agron. (2), 16: 11-43. 1922. 



87 Krzemieniewski, S. Untersuchungen uber Azotobacter chroococcum Beij. 

 Bull. Int. Acad. Sci. Cracovie, CI, Sci. Math. Nat., No. 9: 929-1051. 1908. 



88 Remy, 1909 (p. 569). 



89 Lohnis, F., and Green, H. H. Uber die Entstehung und die Zersetzung 

 von Humus, sowie uber dessen Einwirkung auf Stickstoff-Assimilation. Centrbl. 

 Bakt. II, 41: 52-60. 1914. 



