582 



PRINCIPLES OF SOIL MICROBIOLOGY 



of lime to bring them to the same optimum reaction. Azotobacter is 

 capable of existing in many soils which contain none or only mere traces 

 of CaC0 3 and also in some soils reacting acid by the ordinary test. 97 

 A reaction equivalent to about pH 6.0 is found to be, in most cases, the 

 limiting acid reaction for Azotobacter. The optimum reaction for nitro- 

 gen fixation by Azotobacter in pure culture seems to be closely associated 

 with the optimum reaction for growth. Different strains of Azotobacter 

 may vary in their sensitiveness to the limiting acid reactions, the mini- 

 mum for growth having been reported to be in some cases pH 6.6 to 



6.8. 98 The optimum reaction for the development of Azotobacter is pH 

 7.0 to 7.8, while the limiting alkaline reaction was reported to be pH 



8.8. 99 Different species of Azotobacter may vary, however, also in 

 their behavior to the optimum reaction. 100 



The fixation of nitrogen in soils of a greater acidity than the limiting 

 reaction for Azotobacter (pH less than 6.0) is due to the activities of 

 Bac. amylobacter (CI. pastorianum) , which has its optimum at pH 6.9 

 to 7.3, but can still grow at an acidity greater than pH 5.7. 101 As a 

 matter of fact growth of this organism can be obtained at pH 5.0. 

 Other nitrogen-fixing forms, like Bad. aerogenes and Radiobacter , can also 

 grow at a higher acidity than Azotobacter. The nitrogen fixed under 

 these conditions is much less than that fixed in soils supporting an Azo- 

 tobacter flora. Figures 41 and 42 show the correlation between the 

 hydrogen-ion concentration and nitrogen fixation by Azotobacter. 102 



Calcium carbonate stimulates nitrogen fixation and is never toxic to 



97 Gainey, P. L. Soil reaction and the growth of Azotobacter. Science, N. S., 

 48: 139-140. 1922; Jour. Agr. Res., 14: 265-271. 1918. 



98 Fred, E. B., and Davenport, A. Influence of reaction on nitrogen-assimilat- 

 ing bacteria. Jour. Agr. Res., 14: 317-336. 1918. 



99 Johnson, H. W., and Lipman, C. B. The effect of reaction on the fixation of 

 nitrogen by Azotobacter. Univ. Cal. Publ. Agr. Sci., 4: 397-405. 1922. 



100 Yamagato and Itano, 1923 (p. 117). 



101 Dorner, 1924 (p. 165). 



102 Gainey, P. L., and Batchelor, H. W. The influence of hydrogen-ion con- 

 centration on the growth and fixation of nitrogen by cultures of Azotobacter. 

 Jour. Agr. Res., 24: 759-768. 1923. 



