EVALUATION OF NODULATING AND N2 FIXING CAPACITY OF SIX SOYBEAN (Glycine max (L) Merrill) CULTIVARS WITH AND WITHOUT INOCULATION WITH COWPEA Bradyrhizobium spp

Abstract

The relative responsiveness of six soybean (Glycine max (L.) Merrill) cultivars to inoculation with Bradyrhizobium spp. in terms of nodulation and N₂ fixation was evaluated in a factorial experiment conducted in the greenhouse at the Institute of Agricultural Research Training, Ibadan, using isotope dilution technique. The purpose was to generate empirical evidence to confirm or disprove the assumption that inoculation of soybean with rhizobia in order to enhance N₂ fixation may not be necessary since the crop nodulates in most Nigeria soils.

The soil used for this experiment contained indigenous rhizobia (2.4 x 10³ cell/g soil), and was inoculated with a water suspension of IRC 27 to give an estimated concentration of 2.3 x10⁶ cells/g soil. Four seeds of the soybean cultivars, namely, TGX 1447-3⁰, TGX 1437-1 ⁰, TGX 1448-2% TGX 1447-1⁰, 1019-2^E N and TGX 1455-2^E were sown in pots containing either inoculated or uninoculated soil. The cultivar chippewa was used as a non-nodulating soybean isoline.

Results showed that nodulation and N yield of most of the cultivars were increased by inoculation while the ability of about 50% of the cultivars to fix atmospheric N₂ (% Ndfa) was improved. Lack of improvement in the ability of the others to fix N₂ showed that they did not benefit from inoculation of the soil with the Bradyrhizobium spp. Thus, differential response of the cultivars to inoculation was evident.