Symbiotic effectiveness and host ranges of indigenous rhizobia nodulating promiscuous soyabean varieties in Zimbabwean soils

Presence of indigenous rhizobia nodulating promiscuous soyabean was determined in 92, mainly sandy soils, from wetter agro-ecological zones of Zimbabwe suited to soyabean production. A total of 129 isolates were obtained from nodules of promiscuous soyabean varieties, Magoye and Hernon 147, and a specific variety, Roan grown in potted soils. Magoye nodulated in 80%, Hernon 147 in 50% and Roan in only 25% of the 92 soils tested. Rhizobia populations ranged from undetectable to 2.4×104 cells g−1 of soil. Twenty-one of these isolates were tested for symbiotic effectiveness on two varieties, promiscuous Magoye and specific Roan. Differences in parameters such as nodule numbers, nodule weights and total N fixed reflected diversity among the indigenous isolates. Three isolates had significantly higher N2-fixing potential in comparison with the commercial strain MAR 1491 on promiscuous Magoye. Host ranges of 34 isolates were evaluated on nine legume species: Arachis hypogaea, Cajanus cajan, Crotalaria juncea, Glycine max, Macroptilium atropurpureum, Phaseolus vulgaris, Sesbania sesbania, Vigna subterranea, Vigna unguiculata. Of these 34 isolates, 33 formed nodules with M. atropurpureum of which 61% were moderately effective to very effective while all nodulated V. unguiculata with 58% being moderately effective to very effective. Twenty-eight isolates nodulated V. subterranea and C. cajan (short season variety) with 76 and 36% of these being moderately to very effective, respectively. None of the isolates formed nodules on Phaseolus vulgaris, Arachis hypogaea or Sesbania sesban. Our results indicate that the ability of even specific varieties of soyabean to nodulate with indigenous isolates in African soils is greater than generally assumed.