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Field Evaluation of Selected Rhizobium in an Improved Legume Inoculant¹

Increased grain legume production depends on effective symbiotic dinitrogen fixation through successful legume inoculation. Inoculants containing high numbers l0⁷/g of effective Rhizobium must withstand adverse field conditions. Field studies were performed to determine the effects of selected rhizobia in two different inoculant carriers on nodulation and performance of three grain legumes. The soil at the 1979 site (Vertic Haplaquept) contained = l0³ cowpea Rhizobium and = l0² Rhizobium phaseoli per g of dry soil. The soil at the 1980 site (Typic Paleudalf) contained < l0² R. phaseoli per g of dry soil. Selected strains of R. phaseoli and cowpea Rhizobium were incorporated in peat or vegetable oil. Seeds of bean (Phaseolus vulgaris L.), cowpea [Vigna unguiculata (L.) Walp.], and peanut (Arachis hypogaea L.) were inoculated (ca. 10⁸ Rhizobium/seed) and planted. Nodulation, plant growth, and yields were determined in all experiments. Strains of Rhizobium in oil-base inoculants generally formed a greater percentage of nodules on plants than did the same strains applied in peat-base inoculants. Rhizobium phaseoli 971A occupied 93% of bean nodules when applied as oil-base inoculant but only 82% when applied as peat-base inoculant. Fresh weight bean yields of 5,620 kg/ha were obtained with strain 971A (oil-base) compared to 3,370 kg/ha for the peat-base treatment. Similar significant yield increases were obtained for cowpeas and peanuts inoculated with oil-base inoculants. These experiments indicate that an improved inoculant provided high numbers of effective Rhizobium at planting. Through subsequent effective nodulation, oil-base inoculants increased yields and nitrogen fixation by the legumes due to increased nodulation by the superior N₂-fixing strains of Rhizobium.

Key Words: Inoculation • Intrinsic antibiotic resistance • Legume • N₂-fixation • Nodulation • Oil-base inoculant

² Research associate (presently research associate, Dep. of Agronomy, Univ. of Missouri, Columbia, MO 65201) and associate professor of agronomy, Box 5248, Mississippi State University, Mississippi State, MS 39762.

Received for publication January 8, 1982.