STUDIES ON THE INTERACTION OF PLANTS AND FREE-LIVING NITROGEN-FIXING MICROORGANISMS:I. OCCURRENCE OF AZOTOBACTER IN THE RHIZOSPHERE OF CROP PLANTS

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Abstract
The occurrence of Azotobacter in the rhizosphere of 17 crop plants grown in different types of soil in the field and in the greenhouse has been studied as part of a program of work on the interaction of plants and free-living nitrogen-fixing microorganisms. Azotobacter counts were very low in both rhizosphere and non-rhizosphere soil from the field; they varied with the type and age of the plant and with the type of soil. The organism was usually absent from acidic and poor soils. I n the greenhouse experiments, soil counts of these bacteria were found to be somewhat higher than in the corresponding soil in the field (Granby sandy loam). The root effect was greater with both radish and wheat in the poor soil (Upland sand) than in the fertile Granby soil. Onion exerted no obvious influence on the Azotobacter population in either soil. However, even under optimum conditions of plant growth, numbers of these organisms were very low in comparison with those of other bacteria normally present in soil and in the root zone.

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