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 (WP_c), agronomic efficiency of phosphorus (AE_P) 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, WP_c, and AE_P among various P treatments. Furthermore, the highest WP_c (0.63 kg m⁻³) and AE_P (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 WP_c by 13.3 %. The application of 30 kg ha⁻¹ P improved WP_c 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 WP_c (0.83 kg m⁻³) and the lowest (0.30 kg m⁻³) were achieved at I50×P30 and I100×P0, respectively. The lowest AE_P (9.36 kg ha⁻¹) was obtained at I50×P40, while AE_P (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, WP_c, AE_P, 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.