Effect of Bradyrhizobium japonicum Population on Nodulation, Nitrogen Fixation and Dry Matter Yield on Soybean (Glycine max L Merrill) in Some Ghanaian Soils

Abstract

In as much as the noted ability of the promiscuous TGx soybean varieties developed by the International Instituteof Tropical Agriculture in Ibadan to nodulate profusely with indigenous bradyrhizobia confers advantages wherelack of fertilizers and suitable inoculants act as disincentives to growing the crop, the ability of these nodules tofix nitrogen should be even more important. Selection for promiscuous nodulators in different soils musttherefore go hand in hand with their nitrogen fixing abilities. The symbiotic performance of promiscuoussoybeans depends upon the population size and survival of bradyrhizobia on the field. A pot experiment wasconducted in the greenhouse within the Geography Department in University of Ghana-Legon using soilscollected from ten farmer’s field located in one Agro Ecological Zone (Coastal savanna) to determine nodulation,nitrogen fixation and dry matter yield by indigenous bradyrhizobia in five Ghanaian soils using threepromiscuous soybean varieties and one American type, Bragg, a non promiscuous variety. Nodulation by thethree promiscuous soybean varieties (Bengbie [TGx 536-2D], TGx 1835-10E, and TGx 1830-20E) rangedbetween 21 to 25 and the non-promiscuous variety (Bragg) recorded 13. Hake soil series which contains thehighest bradyrhizobia cell count (6.0 x 103 cellgsoil-1 also produced the highest number of nodules 22. Hakeagain produced the highest nodule dry weight, (176.0mg), shoot dry weight (3.85g), Total nitrogen (N)(106.0mgN plant-1), Total N fixed (88.9mgN plant-1) and %N fixed (69%). TGx 1830-20E also maintain itssuperiority in nodule dry weight (220.0mg plant-1) production, Total N (105.3mgN plant-1), Total N fixed(88.5mgN plant-1) and % N fixed (83.43). Bragg performed poorly in terms of nodule dry weight (67.0mg plant-1), Total N (83.8mg plant-1), Total N fixed (67.0mg plant-1) and %N fixed (78.05). The Most Probable Number(MPN) counts on sand grown soybean indicated 6000, 5300, 4700, 3600 and 3200 native B. japonicum cells g-1in Hake, Goi, Simpa, Agawtaw and Prampram soils respectively. The nodulation, N content and dry matter yieldresults showed that Ghanaian soils contain effective bradyrhizobia capable of nodulating soybean and N2fixation rendering inoculation unnecessary.Keywords: Agrecological zone. Effective bradyrhizobia. Nitrogen fixation. Nodulation Promiscuous soybean