Soybean yield gaps and water productivity in the western U.S. Corn Belt

•We assessed yield gaps in soybean fields using on-farm data collected via surveys and interviews.•Average soybean yield gap ranged from 10 to 30% of yield potential in irrigated fields and ca. 30% of water-limited yield potential in rainfed fields.•Attainable water productivity for soybean in Nebraska is 9.9kg per ha per mm of seasonal water supply.•Aside from water, sowing date set another upper limit to soybean yield in the region.•Other factors explaining on-farm yield variation were tillage, fertilizer, and canopy-applied fungicide.