RESPONSE OF FABA BEAN (Vicia faba L.) TO RHIZOBIUM INOCULATION AND APPLICATION OF MINERAL PHOSPHORUS FERTILIZER IN BORE HIGHLANDS, GUJI ZONE, SOUTHERN ETHIOPIA

Faba bean (Vicia faba L.) is an important legume grown in the highlands of Ethiopia. However, the yield of the crop is limited by a number of edaphic constraints out of which low soil fertility, low pH and reduced N2 fixation due to various biological and environmental factors are the major problem. Therefore, a field experiment was conducted at Bore Agricultural Research Center under rain-fed condition during the 2015 main cropping season to evaluate effects of Rhizobium inoculation and application of mineral phosphorus fertilizer on nodulation, yield and yield components as well as N accumulation and P uptake of faba bean. The treatments consisted of four levels of inoculation (uninoculated, FB-1035, FB-1018 and FB-Murdoch) and five levels of phosphorus application (0, 10, 20, 30 and 40 kg P ha-1) using faba bean variety Gabelcho. The experiment was laid out as a randomized complete block design in a factorial arrangement and replicated three times per treatment. Analysis of the results revealed that the main effect of Rhizobium inoculation as well as phosphorus fertilizer significantly affected a number of phenological and growths parameters and yield attributes. However, the two factors did not interact to influence any of the studied parameters. The highest number of nodules per plant (93.7), nodule dry weight per plant (165.2 g), number of effective nodules per plant (3.28), plant height (126.3 cm), number of pods per plant (14.1), hundred seed weight (84.97 g), aboveground dry biomass yield (10609 kgha-1) and seed yield (3225 kgha-1) were obtained in response to inoculation with FB-1035. Inoculation with FB-Murdoch increased straw nitrogen accumulation, total N accumulation, grain phosphorus uptake, and total phosphorus uptake by 27.3, 24.5, 25.9 and 23.8 respectively over uninoculated control. Phosphorus application also significantly enhanced nodulation attributes, certain phenological and growth parameters, yield and yield traits of faba bean. Accordingly, the highest nodules per plant (91.6), effective nodules (3.3), nodule dry weight per plant (165.6 g), plant height (125 cm), seed yield (3242 kg ha-1), harvest index (0.3), grain nitrogen accumulation (120.53 kg ha-1), grain phosphorus uptake (11.62 kg ha-1) and total phosphorus uptake (18.94 kg ha-1) were obtained in response to phosphorus application at 40 kg ha-1 which is statistically at par with application of 30 kg ha-1. Similarly, application of 30 kg ha-1 resulted in higher number of pods per plant (14.1), aboveground dry biomass yield (10969 kg ha-1), straw nitrogen accumulation (122.8 kg ha-1) and total nitrogen accumulation (239.2 kg ha-1) over the unfertilized control treatments. The results of the economic analysis showed that the combination of Rhizobium strain FB-1035 and application of 30 kg P ha-1 was found to have the highest marginal rate of return for faba bean production in Bore highlands of southern Ethiopia. Therefore it is suggested that using the combination of Rhizobium strain FB-1035 and 30 kg P ha-1 leads to the optimum yield of the crop. Further research work on effect of lime application and other limited nutrient in combination with phosphorus and Rhizobia would be recommended for study area.