Isolation of nitrogen fixing bacteria Filip Claudiu-Denis, Jelea Marian This scientific work shows a better alternative than the use of chemical fertilizers, because they greatly influence soil fertility. There are some alternatives to enhance the soil fertility such as Azotobacter, Rhizobium, etc. Azotobacter sp. are free-living, nitrogen fixing bacteria against to Rhizobium sp. they fix molecular nitrogen from the atmosphere without symbiotic relation with plants, but are some Azotobacter sp. who are associated with plants.