FIXATION OF ATMOSPHERIC NITROGEN 563 



due to at least two factors: (1) the abundance of monosaccharides or 

 readily hydrolyzable hemicelluloses in the leguminous plants and fresh 

 materials; (2) the legumes were used fresh and might have exerted, 

 therefore, a more stimulating effect upon the assimilating capacity of 

 the cells, whereas the other substances were all used in a dry condition. 

 Substances especially rich in lignin are not good sources of energy for 

 the activities of the nitrogen fixing organisms. 



The amount of nitrogen fixed depends not oiJy upon the nature of 

 the energy source, but also on the presence of available nitrogen, minerals, 

 reaction and other environmental conditions, as well as upon the specific 

 organisms. 10-12 Some species utilize more readily one source of energy 

 than another. The amount of nitrogen fixed depends upon the energy 

 value of the particular compound as well as the nature of its decomposi- 

 tion. Lipman 13 recorded an increase in the amount of nitrogen fixed with 

 an increase in molecular weight of fatty acids, in the form of sodium salts, 

 namely, acetic, propionic, butyric; the next member of the homolo- 

 gous series (valerianic acid) presented a poor source of carbon; the 

 sodium salts of succinic and citric acids were not utilized at all. Mocke- 

 ridge 14 obtained 6.08 mgm. of nitrogen fixed with butyric acid and 1.47 

 mgm. with formic acid as sources of energy. . There was nearly a cor s':ant 

 ratio between the amount of nitrogen fixed and heat of combustion of the 

 fatty acids; the heat of combustion of butyric acid is 5.96 calories per 

 gram and of formic acid 1.37 calories. Benzene derivatives and most 

 glucosides seem to be unsuitable as sources of energy for Azotobncter. 1 ' 



The amount of nitrogen fixed is usually calculated per unit of carbon 

 source (sugar) consumed. This does not, however, give a true picture 

 of the process. As pointed out above, some organisms (Azotobacter) 

 break down the sugar chiefly to carbon dioxide and water; others (CI. 



10 Lohnis, F., and Pillai, N. K. Tiber Stickstoff-fixierende Bakterien. 

 Centrbl. Bakt. II, 20: 781-800. 1908. A nutrient solution was inoculated with 

 10 gm. of soil and incubated for 10 days. 



11 Krainsky, A. Azotobacter chroococcum und seine Wirkung im Boden. 

 Zhur. Opit. Agron., 9: 6S9. 1908; (Centrbl. Bakt. II, 20: 725-736. 1908). 



12 Hoffmann, C, and Hammer, R. W. Some factors concerned in the fixation 

 of nitrogen by Azotobacter. Wis. Agr. Exp. Sta. Res. Bui. 12. 1910; Centrbl. 

 Bakt. II, 28: 127-139. 1910. Pure cultures used in Ashby's solution containing 

 1 per cent sugar and incubated 30 days. 



13 Lipman, 1903 (p. 115), p. 217; 1904 (p. 112), p. 237. 



14 Mockeridge, F. A. Some organic matter as culture media for Azotobacter. 

 Biochem. Jour., 9: 272-283. 1915; also Ann. Bot., 26: 871-887. 1912. 



"Greaves, J. E. Azofication. Soil Sci., 6: 163-217. 1918. 



