Studies on the Occurrence of Azotobacter in Some Soil Types

Soil samples from places around Chidambaram, Madras State were examined for their total microbial and Azotobacter populations and for some chemical and physical properties. Azotobacter was present invariably in all the soils and in the three layers; the population, however, decreased with depth. There was no apparent correlation between the total microbial and Azotobacter populations in a given soil. A. chroococcum Beij. and A. indicus Stark, and De were the predominant species present followed by A. agilis Beij. and A. lacticogenus Kauf. and Tours. In acid soils, A. indicus was predominant, whereas A. chroococcum was so in alkaline and neutral soils. The most important factor affecting the Azotobacter population was the hydrogen ion concentration of the soil. The presence of organic matter, CaO and P₂O₅ also seemed to influence the number but not nitrogen and K₂O contents. Isolates of A., chroococcum fixed relatively more nitrogen in the liquid medium than those of A. indicus and one of the isolates of the former fixed almost double the quantity, indicating thereby the prevalence of strain variations in the species.