THE “RHIZOSPHERE EFFECT” OF MANGELS ON CERTAIN GROUPS OF SOIL MICROORGANISMS

Abstract

Mangel roots exerted a striking selective action on numbers of bacteria, actinomyces, fungi, ammonifying and denitrifying bacteria, and protozoa in both manured and unfertilized soils. Roots in the former supported greater numbers at the period of most vigorous growth, after which a decline occurred. More slowly growing plants (in the unfertilized soil) manifested their full selective power only when approaching maturity. Soil treatment apparently exerted its effect indirectly by influencing the rate of plant growth. A "rhizosphere effect" on algae was noted in the latter part of the growing season and on nitrifying bacteria during the earlier period. By means of a dilution-plate technique it was demonstrated that Azotobactcr were more numerous in the manured soil and on the roots growing in it than in unfertilized soil. No "rhizosphere effect" was apparent. Aerobic cellulose-decomposing bacteria were preferentially stimulated by mangel roots and especially so by those grow-ing in unfertilized soil. Gas-producing clostridia (closely related to if not identical with Clostridium pasteurianum) were very abundant on roots in manured soil throughout the growing season and on roots in untreated soil as they reached maturity. Anaerobic bacteria in general were consistently stimulated in the rhizosphere of plants in both fertilized and unfertilized soil and were always present in greater numbers on the roots in the latter.