EFFECT OF RHIZOBIUM INOCULATION AND PHOSPHORUS FERTILIZER RATES ON YIELD COMPONENTS AND YIELD OF FABA BEAN (Vicia faba L.) ON NITISOLS AT WOLAITA SODO, SOUTHERN ETHIOPIA

Poor soil fertility, lack of indigenous soil populations of rhizobia strains and lack of adequate information on the use of Rhizobium inoculants and phosphorus fertilizer are the major production constraints that limit the potential yield of faba bean. Thus, field experiment was conducted to assess the effect of Rhizobium inoculation and phosphorus fertilizer rates on yield components and yield of faba bean; and to estimate the economically feasible combination of inoculants and phosphorus fertilizer rate that maximizes the productivity of faba bean in the study area during 2016 crop season at Wolaita Sodo, southern Ethiopia. Faba bean variety Degaga was used as the test crop. Factorial combinations of three Rhizobium inoculants (FB-1017, FB-1035 and EAL-110) and control (without Rhizobium) and five phosphorus fertilizer levels (0, 23, 46, 69 and 92 kg P2O5 ha-1) were laid out in a Randomized Complete Block Design (RCBD) with three replications. The result showed that crop stand count at harvest was significantly affected by the main effect of P2O5 rates; while, harvest index was significantly affected by the main effect of both Rhizobium inoculants and P2O5 rates. The highest harvest index (40.11%) was obtained from Rhizobium strain FB-1017, whereas, the highest harvest index (41.49%) was attained from 69 kg ha-1 P2O5 rate. The interaction of Rhizobium inoculation and P2O5 rates significantly influenced the phenological and growth parameters of faba bean. The highest number of days to flowering (55 days) was recorded at FB-1017 and 46 kg P2O5 ha-1, whereas the highest days to physiological maturity (111.7 days), number of primary branches per plant (3.8), total nodules (138.3) and effective nodules (128.7) were recorded from inoculant FB-1017 at P2O5 rate of 69 kg ha-1. The tallest plant height (137.7 cm) was obtained from inoculant FB-1035 at 69 kg P2O5 ha-1. Similarly, the interaction of Rhizobium inoculation and P2O5 rates significantly influenced yield components and yield of faba bean. Inoculant FB-1017 and 69 kg P2O5 ha-1 gave significantly highest number of pods per plant (42.27), hundred seed weight (56.02 g), grain yield (4101 kg ha-1) and above ground dry biomass (9280 kg ha-1). The economic analysis also indicated that the highest net return (40939.80 ETB ha-1) with a marginal rate of return of 174.51% was obtained from inoculation with Rhizobium strain FB-1017 and 69 kg P2O5 ha-1. Based on the results of this study, it can be tentatively concluded that faba bean seed inoculation with Rhizobium strain FB-1017 and application of P2O5 at a rate of 69 kg ha-1 found to be appropriate for faba bean production in the study area.