Optimization of deficit irrigation level and phosphorus fertilizer rate for soybean production in Jimma, Southwest Ethiopia

Soybean is a vital oil crop in Ethiopia to attract foreign income. In southwest Ethiopia's irrigated conditions, phosphorus fertilization and water stress limits soybean yield. A three-year field experiment (2018/19–2020/21) assessed the impact of deficit irrigation (DI) and phosphorus rate (P rate) on crop water productivity (WPc), agronomic efficiency of phosphorus (AEP) and soybean yields. It included two DI levels and one control treatment (I50 = 50 % ETc, I75 = 75 % ETc, and I100 = 100 % ETc) in the main plot and five P rates (P0 = 0, P10 = 10, P20 = 20, P30 = 30, and P40 = 40 kg ha−1 P) in the subplot, arranged in a randomized split plot design with three replicates. P rates were optimized for each DI level using a quadratic polynomial regression model, predicting maximum agronomic yield and economically optimal P rate based on yield and cultivation cost. The study's results demonstrated a highly significant difference in soybean yield, WPc, and AEP among various P treatments. Furthermore, the highest WPc (0.63 kg m−3) and AEP (31.67 kg kg−1) were achieved with treatments with 30 kg ha−1 P and 10 kg ha−1 P. Among DI treatments, 75 % ETc reduced water usage by 25 % and increased WPc by 13.3 %. The application of 30 kg ha−1 P improved WPc and increased yield by 91 % and 90 %, respectively. The combined effect of I100×P30 significantly increased plant height (68.23 cm), dry matter (4.4 t ha−1), and yield (2965.49 kg ha−1) by 40.3 %, 104.3 %, and 215.2 %, respectively. The maximum WPc (0.83 kg m−3) and the lowest (0.30 kg m−3) were achieved at I50×P30 and I100×P0, respectively. The lowest AEP (9.36 kg ha−1) was obtained at I50×P40, while AEP (64.49 kg kg−1) significantly improved by 85 % at I100×P10. The maximum agronomic yields ranged from 1842.5 to 2895 kg ha⁻¹, with the corresponding combined treatments of I50×P26 to I100×P32. Economically optimal P rates ranged from 26 kg ha−1 (I50) to 31 kg ha−1 P (I100), giving net returns of $1666.1 to $3028.4 ha−1. Efficient management of DI and P can optimize irrigation, reduce P losses, and mitigate economic and environmental risks. Based on the result of this finding, applying 31 kg ha−1 of P under 100 % ETc and 28 kg ha−1 of P under 75 % ETc is recommended as the optimal P application strategy to achieve higher yield, WPc, AEP, and maximize economic returns in southwest Ethiopia. In addition, the breeder needs to develop soybean varieties that are more resistant to water stress and efficient in P uptake.