Characteristics of Rhizobium trifolii Populations Associated with Subclover in Mississippi Soils

Abstract Indigenous Rhizobium trifolii populations that would nodulate Trifolium subterraneum L. (cv. Mt. Barker) were isolated and characterized from pastures with and without subclover in five distinct soil regions in Mississippi. Considerable variation in effectiveness was found among isolates from the different areas and from individual sampling sites within areas, and the combined results indicated that subclover production and soil fertility levels were related and were being better maintained within two of the sampling areas. Pastures without clover yielded small R. trifolii populations that were poorly effective in N fixation on subclover. Significant correlations were found between the Rhizobium population size and soil N levels, soil texture, organic matter content, and exchangeable acidity. Isolate effectiveness was associated with the amount of base saturation, soil acidity, and N levels. These results are discussed in the context of subclover production as related to Rhizobium effectiveness, population size, and soil fertility characteristics.