576 PRINCIPLES OF SOIL MICROBIOLOGY 



absence of such fixed nitrogen and act as a "denitrifying" organism in 

 the presence of nitrates. Except in high concentrations, however, the 

 nitrogen of humus does not seem to exert any injurious influence upon 

 nitrogen-fixation. This is probably due to the fact that its availability 

 is only limited. As a matter of fact, it is even claimed 69 that the soil 

 organic compounds can be used as sources of energy by nitrogen-fixing 

 bacteria. These results, however, need confirmation and elucidation. 



Influence of salts upon nitrogen fixation. In addition to an energy 

 source, the presence of certain minerals in minimum concentrations is 

 important for the activities of the nitrogen-fixing bacteria. Azotobac- 

 ter was found 70 to prefer soils containing calcium salts and races of 

 the organism isolated from those soils were more active than those iso- 

 lated from unlimed soils. Calcium was looked upon as serving a double 

 purpose: (1) directly for metabolism (in the form of soluble phosphates, 

 as the oxide, carbonate, and in the form of salts of organic and inorganic 

 acids) and (2) for the purpose of neutralizing the acids formed in the 

 metabolism (in the form of oxide, hydroxide or carbonate). Certain 

 calcium salts, especially tricalcium phosphate and the chloride, cannot 

 be utilized. 71 Magnesium salts cannot take the place of calcium. 72 

 The primary importance of calcium is due, however, not to its direct 

 nutrient quality but chiefly to its buffering properties, and it may serve 

 to some extent as a catalytic agent. 73 



According to Ashby, 74 magnesium carbonate is very favorable for the 

 development of Azotobacter in solution; its presence discourages also 

 the development of foreign organisms, probably due to their supression 

 by the magnesium. 



Potassium salts favor the development of Azotobacter, the minimum 



69 Lipman, C. B., and Teakle, L. J. H. The fixation of nitrogen by Azotobacter 

 in a displaced solution and in soil residue therefrom. Soil Sci., 19: 99-103. 1925. 



70 Krzemieniewski, 1908 (p. 579). 



71 Christensen, H. R. Uber das Vorkommen und die Verbreitung des Azoto- 

 bacter chroococcum in verschiedenen Boden. Centrbl. Bakt. II, 17: 109-119. 

 1907; 19: 735-6. 1917. 



72 Fischer, H. Ein Beitrag zur Kenntnis der Lebensbedingungen von stick- 

 stoffsammelnden Bakterien. Centrbl. Bakt. II, 14: 33-34. 1905; 15: 235-236. 

 1906. 



73 Vogel, J. Untersuchungen iiber das Kalibedilrfnis von Azotobacter. 

 Centrbl. Bakt. II, 32: 411-442. 1912; Krzemieniewska, H. Zur Ernahrung des 

 Azotobakters. Bui. Intern. Acad. Sci. Cracovie, CI. Sci. Math. Nat. No. 5: 

 445-448. 1908. 



74 Ashby, 1907 (p. 113). 



