Soil test based Phosphorous Calibration for Soybean [Glycine max (L.) Merrill] Production on Nitisols in Assosa Zone of Benishangul Gumuz Region, Western Ethiopia

Soil test based phosphorous (P) calibration study was conducted on Soybean on Nitisols of farmers’ fields in Assosa Zone. The experiment was arranged in a randomized complete block design with six levels of phosphorous fertilizer (0, 5, 10, 15, 20 and 25 kg ha^-1) with three replications. The results revealed substantial responses of Soybean to phosphorus fertilizer rates on plant height, pod numbers per plant and seed yield. Seed yield increased significantly from 1761 to 2300 kg ha^-1 with increase in the level of P fertilizer from the control to 46 kg P ha^-1. The magnitude of increase in seed yield due to application of 46 kg P ha ^-1 was 30.6 % higher as compared to the control.  Extractable soil P concentrations (Bray II, 0- 20 cm depth) three weeks after planting significantly responded to P fertilizer rate. Correlations of relative yield with soil test phosphorous values showed that the critical soil P concentration and average phosphorous requirement factor (Pf) calculated from soil test phosphorous values of all treatments for the study area were 8.5 and 6.55 mg kg^-1, respectively.