Effects of different irrigation scheduling methods on physiology, yield, and irrigation water productivity of soybean varieties

An adequate irrigation schedule can result in significant water savings and improved crop yields, especially in large areas such as commodity crops. However, insufficient information is available regarding soybean crops in terms of irrigation scheduling methods and their interaction with irrigation water productivity (IWP) for different soybean varieties. The objective of this study was to quantify the effect of climate-, soil-, and plant-based irrigation scheduling methods on the physiology, yield, and IWP of three soybean varieties. A shelter experiment was conducted using a randomized design with split plots of two factors and five replications. Five irrigation methods and three soybean varieties (TMG 7067, 58i60RSF IPRO, and NA 5909) were tested. In CB (climate-based method) the evapotranspiration-water balance was used; in SB1 and SB2 (soil-based methods), irrigation at field capacity was applied when the soil matric potential reach − 20 kPa at depths of 0.1 m and 0.3 m, respectively; while In PB1 and PB2 (plant-based methods), canopy temperature depressions (CTD) of ∼2 °C and ∼4 °C, respectively, were employed to trigger irrigation. The amount of irrigation water applied under all irrigation scheduling methods ranged from 310 (PB1 and PB2) to 786 mm (V1 under SB1). The net photosynthetic rate (A), stomatal conductance (gs), transpiration (E), and leaf water potential (LWP) were higher under SB1 and SB2 than under other irrigation methods. On average, grain yield (GY) was significantly higher in SB1 (3.5 Mg ha-1), than in SB2 (3.0 Mg ha-1), CB (2.5 Mg ha-1), PB1 (2.4 Mg ha-1) and PB2 (2.0 Mg ha-1). However, it was also found that PB1 treatment resulted in significantly increased IWP (0.84 kg m-3) compared to the other irrigation treatments. Overall, the choice of irrigation scheduling methods for soybean crops under tropical conditions should be based on the technology level, water resource availability, and individual farmer goals to maximize GY per unit area (SB1 and SB2) or optimize IWP (PB1). In addition, despite the ease of use of climate- and plant-based methods, all of these methods should be calibrated against soil-based methods under different climatic conditions and for a large number of genotypes.