Some observations on the occurrence of azotobacter in Finnish soils

In the present paper some observations upon the occurence of Azotobacter chroococcum in Finnish soils are reported. Without enrichment Azotobacter could be cultivated on the agar plates only from 5 soils out of the 72 samples tested. The enrichment procedure markedly increased the number of soils that gave positive results in the test. Azotobacter could not be found in soils with a pH value lower than 5.8. On the basis of the results it is concluded that Azotobacter probably is present in a large amount of our less acid soils, although generally, in very low numbers. The phosphorus status of the soil exerted some influence upon the development of inoculated Azotobacter in soil plaques. In soil tests the phosphorus requirement of Azotobacter appeared to be markedly higher than that of the crop plants, in general. It was supposed that in some cases this fact could be explained not only on the basis of the phosphorus need of the bacteria but on the basis of the high sensitivity of the organism to the effects of soil aluminium and iron prevented by phosphates. The cultivation of Azotobacter indicum from the soils failed, but Clostridium species could be found in all the samples tested. The increase in the nitrogen content of the mannitol solution cultures during incubation was highest when the soil contained Azotobacter chroococcum. But also in several cultures of soils from which no Azotobacter could be found by the agar plate method, a considerable increase in nitrogen was observed. This was connected with the formation of a thick membrane of fungi and bacteria on the surface of the liquid. The possibility of nitrogen fixation by the associations of soil microorganisms is discussed.