Assessment of the Responsiveness of Soybean Varieties to Bradyrhizobium Japonicum Inoculation in Ogbomoso, Southwest Nigeria

Soybean varieties may respond differently to rhizobium inoculation, for im- proved yield and nitrogen fixation. Therefore, responses of ten soybean varieties to rhizobium were evaluated in 2018 at the Ladoke Akintola University of Tech- nology, Ogbomoso, Nigeria, to determine if rhizobium will improve the perfor- mance of the varieties. The soybean varieties (main plot) and rhizobium (+ or -; subplot) were arranged as split-plot in Completely Randomized Design with five replications. Data were collected on plant height, root, shoot, pod and seed dry weights and nitrogen fixation. Data were subjected to Analysis of Variance, and the treatment means were compared using the Least Significance Difference at 5% probability level. Generally, the non-inoculated plants were taller than inocu- lated ones. The soybean varieties exhibited pronounced variation in plant height without rhizobium inoculation. Variety TGX 2004-10F was the tallest while vari- ety TGX 1485-1D was the shortest. However, with rhizobium inoculation variety TGX 2010-12F was the tallest throughout the sampling period while variety TGX 1485-1D was still the shortest. Rhizobium inoculation had no significant effect on the shoot and root weights. On the average, variety TGX 2004-10F had the highest shoot (28.8 g/pot), and root (6.2 g/pot) weights and variety TGX 1485-1D produced the lowest shoot weight (9.5 g/pot). Varieties TGX 2008-2F and TGX 2008-4F had the lowest root weight (2.5 g/pot). Variety TGX 2010-12F was the most responsive to rhizobium in terms of the shoot (33.7% increase) and root (44.1% increase) weights. In comparison, variety TGX 2008-4F had the most negative response (-57.2% decrease) and (-100% decrease) in shoot and root weights, respectively. Rhizobium inoculation increased seed weight of almost all the varieties (9.4% - 35.4%). Rhizobium inoculation significantly enhanced nitro- gen fixed. However, the varieties exhibited differences in N fixed in response to rhizobium inoculation (range -28.3% to 48.7%). In conclusion, Bradyrhizobium japonicum inoculation has the potential to improve shoot, root, seed yield and N-fixed of selected soybean varieties while other varieties may show negative response