Data secured by various investigators have proven quite conclusively that the aerobic nitrogen-fixing bacteria of the genus Azotobacter are not active or even present in soils more acid than pH 6.0. Many of the soils in Iowa are more strongly acid than this, and while no Azotobacter have been found in them, some nitrogen fixation certainly occurs. It appears, therefore, that some other organisms or groups of organisms must be responsible.
Proceedings of the Iowa Academy of Science
©1931 Iowa Academy of Science, Inc.
Willis, W. Hillman and Walker, R. H.
"Anaerobic Nitrogen Fixation in Some Iowa Soils,"
Proceedings of the Iowa Academy of Science, 38(1), 293-298.
Available at: https://scholarworks.uni.edu/pias/vol38/iss1/106