Effectiveness of Rhizobium trifolii Populations Associated with Trifolium subterraneum L. in Southwest Oregon Soils

Indigenous Rhizobium trifolii populations were found to nodulate Trifolium subterraneum L. (c.v. Mt. Barker) but were largely ineffective or poorly effective in nitrogen fixation with this host. The rhizobia were isolated from numerous locations in southwest Oregon which contained subclover stands of widely varying quality. Rhizobium populations were greater in soils containing subclover stands while smaller populations were detected both in soils which had previously contained a subclover stand and those soils which had never been planted to subclover. Regression analysis of soil and rhizobia data showed significant correlations between the size of the rhizobial populations, the degree of fineness of texture (% silt + % clay) and the organic matter content while effectiveness was associated with the level of base saturation and the content of exchangeable bases. The requirements for a high quality inoculum in the presence of large ineffective populations and the agronomic implications for increasing subclover production are discussed.