STUDIES ON THE INTERACTION OF PLANTS AND FREE-LIVING NITROGEN-FIXING MICROORGANISMS: II. DEVELOPMENT OF ANTAGONISTS OF AZOTOBACTER IN THE RHIZOSPHERE OF PLANTS AT DIFFERENT STAGES OF GROWTH IN TWO SOILS

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Abstract
This study represents an attempt to correlate the low numbers of Azotobacter in rhizosphere and root-free soils at the Central Experimental Farm, Ottawa, with the incidence of bacterial and actinomycete antagonists of this organism. Wheat, radish, and onion were grown in the greenhouse in two soils varying greatly in fertility and organic matter content, and isolations of bacteria and actinomycetes were made periodically for testing against Azotobacter. It was found that rhizosphere soil contained greater numbers of microorganisms antagonistic to Azotobacter than root-free soil. Of the three crops used wheat exerted the least effect. In all the tests numbers of antagonists were greater in the fertile Granby sandy loam than in the infertile Upland sand. The results correlated well with the Azotobacter populations in these soils as reported in the first paper of this series.