Following the discovery of the nitrogen fixing powers of the symbiotic bacteria in the soil, early investigators found that the power of utilizing the free atmospheric nitrogen was not confined to the symbiotic bacteria alone. They noted increases in soils which had borne no legumes and they found that fallow soils in particular increased appreciably in nitrogen content. These . facts stimulated researches which led to the discovery of many forms of bacteria which are able, when growing alone, to fix nitrogen from the air. The chief of these is now known as the azotobacter group.
"Soil Inoculation with Azotobacter,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 4
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol4/iss45/1