The root-nodule bacteria as factors in clover establishment in the red basaltic soils of the Lismore district, NSW. II. Survival and success of inocula in laboratory trials.

Abstract

Plant growth, nature of nodulation, and serological identification of cultures isolated from nodules have been used to determine the success achieved with inocula of clover rhizobia. The Lismore krasnozems offer difficulties in the way of successful seed inoculation. Milk and phosphate supplements with the inoculum do not obviate the difficulty, but heavier rates of inoculation give a reasonable degree of nodulation even with untreated soils. The unfavourable nature of these soils is reflected in their relatively low rhizobial population and the failure of added cells to persist or multiply at a rate comparable with other soils and conditions. Raising pH to c. 7 by the addition of Ca(OH)₂ or CaCO₃ markedly improves the soil as a medium for survival and multiplication of clover rhizobia. Even a small rise in pH gives appreciable improvement. Strains of root-nodule bacteria differ markedly in their ability to compete with each other for growth and nodule formation on a host. The relative performance of strains can be affected by such factors as the nature of the soil and the presence and nature of the host. Competition for nodulation need not be related in any simple way to competition for growth. Competition by :"native" strains can be an important factor contributing to success or failure with added inoculum.