Azotobacter numbers on broadbalk field, rothamsted

Summary

Azotobacter chroococcum, an aerobic nitrogen-fixer, was counted during 1955–1958, and in 1962, in soil samples taken from nine plots from Broadbalk field at Rothamsted. In the fallow sections of the plots, and in sections carrying the fourth successive wheat crop, Azotobacter were few, especially in the plots given ammonium sulphate every year. Azotobacter numbers increased after fallowing. In cropped sections, Azotobacter were fewest in the plot with nitrogen only. With nitrogen and phosphate but no potash they were fewer than in the plots without nitrogen. The most Azotobacter were in the plots with nitrogen, phosphate and potash. These trends are correlated with yield, but Azotobacter were too few (ranging from <100 to a maximum of 9400 per gram soil) to fix enough nitrogen to affect crop growth.