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Effect of Phosphorus on the Effectiveness of Strains of Rhizobium japonicum¹

ABSTRACT

We examined the relationship between P nutrition of soybean [Glycine max (L.) Merr. cv. ‘Davis’] and N₂ fixation by five strains of Rhizobium japonicum ranging from an ineffective (SM-5) to a highly effective (USDA 110) symbiosis with soybean. Phosphorus at 0, 50, 125, and 400 mg P kg^–1 soil (P₀, P₅₀, P₁₂₅, P₄₀₀) as Ca(H₂PO₄)₂ · H₂O was applied to pots containing 2.7 kg of a P-fixing humoxic tropohumult free of R. japonicum. Treatments consisting of + N and five strains of R. japonicum and the four P treatments were arranged in a complete factorial replicated three times. With 400 mg P kg^–1 soil treatments ranked + N > USDA 110 > USDA 31> USDA 123> USDA 33>SM-5 for shoot dry weight and accumulation of N in the shoot. There were no significant strain effects for shoot dry weight, shoot N or nodule activity at P_o. Strain rankings were the same at the intermediate levels of P but differences were smaller. Although nodule number was somewhat enhanced by P, it bore no relationship to strain effectiveness and was not a factor limiting N₂ fixation at low P. Nodule dry weight and nitrogenase activities significantly increased with P additions. The concentrations of N and P in the shoot also increased with P additions. The concentrations of shoot P and N were higher in plants provided mineral N at the low and intermediate P levels than in the inoculated plants. The results show that P nutrition is important in interpreting the N₂ fixation capability of strains of Rhizobium. Synergism between P and Rhizobium inputs requires that to obtain maximal response to the application of one input necessitates employing the other at maximum levels.

¹ This research was supported by Contract DAN-0613-C-00-2064-00 (NifTAL Project) from the United States Agency for International Development.Conclusions of this paper do not necessarily represent those of the granting agency. Use of a brand name does not constitute an endorsement by the author or granting agency.

² Agronomist and Research Associate, respectively, Univ. of Hawaii NifTAL Project, P.O. Box O, Paia, Hawaii, 96779.

³ Lecturer, Dep of Soil Biochemistry & Soil Science, Faculty of Agriculture, Shambat, Sudan.

Received for publication February 21, 1984. Accepted for publication January 3, 1985.

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