FIXATION OF ATMOSPHERIC NITROGEN 577 



quantity of this element corresponding to that of calcium. A minimum 

 of 0.38 mgm. K, 0.36 mgm. Ca and 0.35 mgm. Mg was required by Azo- 

 tobacter for every gram of sugar decomposed. When nitrogen- 

 fixation takes place in the soil, the addition of small amounts of potas- 

 sium is without any effect. 75 In higher concentrations, potassium salts, 

 as a rule, become more toxic than sodium salts. The latter do not seem 

 to be indispensable for the growth of Azotobacter, although the addition 

 of three per cent of NaCl does not injure the development of the organ- 

 ism. There are claims in the literature, 76 however, that alkalies, and 

 particularly alkali carbonates, are injurious to nitrogen-fixation; the 

 action of NaCl becomes apparent only when 0.5 to 0.6 part are present 

 in 100 grams of dry soil. Na 2 S0 4 becomes injurious at 0.25 per cent 

 concentration, while 0.4 to 0.5 per cent of Na 2 C0 3 inhibits growth of 

 the nitrogen-fixing organisms completely. 



Phosphorus compounds greatly accelerate the activities of the nitro- 

 gen-fixing organisms, since they play an important role in the metabo- 

 lism of Azotobacter, 77 which requires large quantities of the mineral for 

 the synthesis of its cells. The organism utilizes particularly well those 

 soluble phosphates which do not tend to make the soil reaction acid, 

 as in the case of the di- and tri-sodium and potassium phosphates and 

 di-calcium phosphates. The presence of mono-basic-phosphates which 

 serve as buffers and tend to make the reaction of the medium more acid 

 than the minimum for the growth of Azotobacter is not favorable. 

 The difficultly soluble tri-basic calcium, iron and aluminum salts are 

 available only according to the degree of their solubility. In the pres- 

 ence of an excess of available energy a definite relation is found to 

 exist between the growth of Azotobacter and the phosphorus content 

 of the soil. 78 It was suggested that information can be obtained on 

 the presence of available plant food in the soil by determining the 

 food requirements of bacteria. When a mannite solution free from 

 phosphorus yields a good growth of Azotobacter, after inoculation 

 with soil, it may be assumed that the soil is not deficient in phosphorus 



76 Greaves, J. E., Carter, E. G., and Lund, Y. Influence of salts on azofication 

 in soil. Soil Sci., 13: 481-499. 1922. 



76 Lipman, C. B., and Sharp, L. T. The effects of alkali salts in soils on soil 

 bacteria. III. Nitrogen fixation. Centrbl. Bakt. II, 35: 647-655. 1912. See 

 also Greaves, Carter and Lund, 1922. 



77 Dzierzbicki, A. Beitrage zur Bodenbakteriologie. Bui. Intern. Acad. 

 Sci. Cracovie, B. 1910, 21-66. 



