Two species of Clostridium (C. americanum and C. aerobicum) have been isolated by Pringsheim as nitrogen fixers. C. aerobicum, as the name indicates, functions in the presence of air. Equally aerobic are some of the later-discovered bacteria, especially of the genus Azotobacter (A. choococcum, A. beijerinckii, A. vinelandii, A. agilis, and A. vitreum). A. choococcum, a notable nitrogen collector, is distributed in most soils. Out of 562 soil samples from Bavaria it was found in 341 (61 per cent). It was absent in soils of high H ion concentration (below pH 5.6) (Niklas, 1925). It thrives best in lime soils at an optimum temperature of about 28°C., while sodium chloride and sulphates are destructive. The former acts toxically even in doses of 0.5 per cent; it can endure sodium sulphate up to 1.25 per cent (André, 1921). (Braun-Blanquet 1932:235)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.