116 PRINCIPLES OF SOIL MICROBIOLOGY 



bacter missing in thirty-four cases, chiefly in heavy clay soils. Jones 

 and Murdoch 55 found Azotobacter in nine out of seventeen soil types 

 examined and in twenty-two out of twenty-nine soil samples repre- 

 senting nine types. Eighteen was the maximum number of Azotobac- 

 ter cells found per one gram of soil. 



The absence of Azotobacter in the soil is probably due, in the majority 

 of cases, to the soil reaction, since this organism cannot develop in a soil 

 having an acidity greater than pH 6.0. As soon as the reaction of the 

 soil is adjusted by means of lime so that the pH becomes greater than 

 6.0, an Azotobacter flora will develop. 56-58 



Azotobacter is widely represented in the soil by various forms, so 

 that Lohnis and Westermann counted in 1908 as many as twenty-one 

 forms. These were believed, however, to represent only four types. 

 Lipman and Burgess 59 isolated in 1915 a number of forms from Ameri- 

 can and foreign soil including several new species. The most common 

 form, however, was in both cases A. chroococcum. 



As a result of a study of a large number of soils, Lipman and Burgess 59 

 came to the conclusion that, with a proper supply of energy producing 

 materials, all agricultural soils may be made to fix atmospheric nitrogen 

 when inoculated into a properly constituted mannite solution; however, 

 only a fraction of these soils (one-third) contain Azotobacter organisms. 

 Those soils that contain Azotobacter have a more vigorous nitrogen 

 fixing power. As much as 10 mgm. of nitrogen are fixed per gram of 

 mannite in solution and 12.6 mgm. in the soil by pure cultures of 

 Azotobacter, which approaches that obtained from A . vinelandii. The 

 latter is found to be the strongest nitrogen fixing organism, while A. 

 chroococcum the most common. 



66 Jones, D. H., and Murdoch, F. G. Quantitative and qualitative bacterial 

 analysis of soil samples taken in the fall of 1918. Soil Sci. 8 : 259-267. 1919. 



66 Christensen, H. R. Untersuchungen iiber einige neuere Methoden zur 

 Bestimmung der Reaktion und des Kalkbediirfnisses des Erdbodens. Intern. 

 Mitt. Bodenk. 13: H. 3-4. 1923. 



57 Gainey, F. L. A study of the effect of changing the absolute reaction of 

 soil upon their Azotobacter content. Jour. Agr. Res. 24: 289-296, 759-767, 907- 

 938. 1923. 



53 Waksman, S. A. The occurrence of Azotobacter in cranberry soils. 

 Science, 48: 653. 1918. 



69 Lipman, C. B., and Burgess, F. A. Studies on nitrogen fixation and Azoto- 

 bacter forms in soils of foreign countries. Centrbl. Bakt. II, 44: 481-511. 1915. 



