THE INFLUENCE OF SOIL AND ROOT EXTRACTS ON THE ASSOCIATIVE GROWTH OF SELECTED SOIL BACTERIA

Abstract

These studies are concerned with the growth interrelationships of mixed cultures of five soil organisms in soil extract and root extracts of 2-, 4-, and 8-week-old oats, soybeans, and wheat. Population changes of Agrobacterium radiobacter, Arthrobacter citreus, Azotobacter chroococcum, Bacillus cereus, and a Pseudomonas sp. in pure and mixed culture were followed by plating on selective media. B. cereus and A. chroococcum grew poorly alone or in mixed culture in the extracts. In soil extract, A. citreus predominated over, or was nearly equal in number to, the Gram-negative forms (Pseudomonas and Agrobacterium). In root extracts, Pseudomonas sp. always predominated over A. citreus in mixed culture. A. radiobacter was inhibited in mature root extracts (8-week-old plants) although in pure culture it recovered after a period. An antagonistic effect of Pseudomonas sp. on A. chroococcum plated on nitrogen-free agar medium was found to be related to the kind of agar used.