Soybean Yield and Mineral Concentrations As a Function of Supplemental Irrigation Strategies for a Humid Environment.

Declines in irrigation water supplies and increases in energy costs for pumping emphasize the need for conservation and efficient use of water. Knowing the response of crops to limited water supplies under a specific environment is crucial to be able to implement deficit irrigation strategies for reducing agricultural water use while maintaining current yield levels. The objective of this study was to determine the influence of deficit drip irrigation on yield, growth and mineral concentrations of soybean (Glycine max L.) grown in a humid region of the southeastern USA. A hybrid soybean was grown in 2008 and 2009 in three automated rainout shelters at the University of Georgia Campus in Griffin, USA. The irrigation treatments were 40, 60 and 90% of the irrigation threshold (IT). The Cropping System Model (CSM)–CROPGRO–Soybean was used to determine the amount and timing of irrigation. A delay in flowering date as well as physiological maturity was found for both years as the amount irrigation decreased. When compared to the 90% IT, additional water stress imposed on soybean resulted in a significant reduction in both above–ground biomass and seed yield. The above–ground biomass reduction was 20.8 and 49.5%; and seed yield reduction was 20.7 and 42.6% for the 60% IT and 40% IT irrigation treatments, respectively. Leaf area index and plant height were also reduced as water stress increased. On the other hand, seasonal evapotranspiration (ET) increased with an increase in the amount of irrigation water supplied, ranging from 352.5 mm for 40% IT to 524.6 mm for 90% IT, whereas water use efficiency, defined as the relationship between seed yield and ET, had  less variation among the different treatments, ranging from 2.59 to 3.46 kg ha^-1 mm^-1. Average water–yield response factor (k_y; relative yield decrease and relative ET deficit) was 0.76, indicating soybean is fairly sensitive to seasonal soil water deficit. Deficit irrigation, in most cases, did not significantly affect the mineral concentrations of the leaves, stems, seeds and shells among the different irrigation treatments. Our results indicated that higher above–ground biomass together with seed yield as well as water use efficient could be achieved from higher irrigation water applied. However, the irrigation threshold between 60–90% could be recommended to growers to save water, which is crucial when supplemental water for irrigation is limited or irrigation applications costs are high.