Soil inoculation with Azotobacter

Small quantities of nitrogen increased the nitrogen-fixing power of of Az. chroococcum, Az. vinelandii, and Az. beijerinckii in solution cultures. Sodium nitrate and ammonium sulfate were more stimulating than the organic forms of nitrogen, such as dried blood and cotton meal. The nitrate produced in the fallow soils was greater under the inoculation treatments. The methods of preparing the inoculum had little effect on the results. There was more nitrate in all the soils at the end of the experiment. The ammonia content of the soils increased in the early part of the experiment but decreased at the end. Inoculation increased the ammonia content of the soils. The total number of Azotobacter increased and decreased parrallel with the nitrates, but generally indirectly with the ammonia production. The total nitrogen in the fallow soils was greater at the end of the experiment. Az. chroococcum and Az. vinelandii increased the nitrogen content, but Az. beijerinckii did not. The nitrogen-fixing power of the soil was increased by Az. chroococcum, but not by the other organisms. Soil cultures proved slightly more efficient in increasing the nitrogen-fixing power of the inoculated soils than did the liquid cultures. Inoculation with Az. chroococcum, Az. vinelandii, and Az. beijernckii increased the total nitrogen supply in the Carrington loam when cropped to wheat. Inoculation of the soil with Azotobacter did not increase the yield of wheat, although there was an accumulation of nitrogen. This lack of crop response might have been due to the fact that the soil was already sufficiently supplied with nitrogen and phosphorus or some other element may have been the factor limiting greater crop growth. Soil inoculation with Azotobacter may become a profitable practice under actual field conditions, and the problems connected with the preparation of cultures and their introduction into soils merit further study.