Soybean yield response to managed depletion irrigation regimes in a Mid-South silt loam soil

Soybean irrigation experiments were conducted for five years (2013–2017) on silt loam soil in Jackson, TN, a sub-humid region with high and variable precipitation. The purpose of the experiment was to investigate managed depletion irrigation (MDI) regimes in soybean to obtain the highest yield with minimal water applied on high water-holding capacity soil. Another goal was to find the best use of soil matric potential sensors and water balances to achieve MDI goals. Irrigation supplemented rainfall at a rate of 1.27, 2.54, and 3.81 cm wk−1 starting from first bloom (R1), first pod (R3), and first seed (R5) stages and these treatments were compared against a rainfed control. The results lead to the recommendation that delaying irrigation to R5 in most years and R3 in dry years produced yields at the highest significance level. The connection between irrigation rate and yield fluctuates yearly, but generally, higher rates result in greater crop yields, necessitating a high irrigation rate and rainfall for managed depletion after initiation. The water balance method best emulated optimum MDI results when irrigation was delayed until the soil moisture depletion reached the management allowable depletion (MAD) and then was followed by irrigation that maintained soil water at the MAD level. The irrigation trigger points were initially lowered for matric potential sensors to avoid excess watering in early growth, transitioning to higher levels for optimal yield later in the R3 stage. These results may be applicable in other regions with high water holding capacity soil and moderate to high rainfall rates during the growing season.