Double‐Crop Soybean Response to Potassium on Mid‐Atlantic Coastal Plain and Piedmont Soils

Core Ideas Soil K at 0‐ to 15 and 0‐ to 30‐cm depths were excellent in predicting double‐crop soybean relative yield. Double‐crop soybean required more fertilizer K to maximize yield than current recommendations. Fertilizer‐K rate can be reduced if recommendations are made based on 0‐ to 30‐cm than 0‐ to 15‐cm depth. Double‐crop yield response to fertilizer K depended on soil K but not on wheat straw management. Straw management may not be an issue in recommending fertilizer K based on soil K at 0‐ to 30‐cm depth. Potassium fertilization research on soybean [Glycine max (L.) Merr.] double cropped with winter wheat (Triticum aestivum L.) is lacking. We characterized double‐crop soybean response to fertilizer K across 22 Coastal Plain and Piedmont sites with and/or without wheat straw removal during 2013 to 2015. Mehlich‐1 soil‐K concentrations at 0‐ to 15‐ and 0‐ to 30‐cm depths were better in explaining relative yield variability (r2 = 0.80), defining critical soil‐K concentrations that ranged from 40 to 75 mg K kg−1 for 0‐ to 15‐cm and 36 to 66 mg K kg−1 for 0‐ to 30‐cm depths, and identifying K‐deficient sites (100% accurate) than soil‐K concentration at 0‐ to 60‐cm depth (r2 = 0.48; 56% accurate). Critical soil‐K concentration at 0‐ to 30‐cm depth did not change with wheat straw management, but slightly increased at 0‐ to 15‐cm depth when straw was removed. The R2 leaf‐K concentration with a critical range of 20 to 23 g K kg−1 was better (75% accurate) in diagnosing K deficiency than V5 plant‐K concentration (50% accurate) with a critical range of 17 to 23 g K kg−1. Double‐crop soybean required 33 to 119% more fertilizer K than Virginia Cooperative Extension recommendations for ≤50 mg soil‐K kg−1 at 0‐ to 15‐cm depth, but 6 to 55% less fertilizer K for similar soil K concentration at 0‐ to 30‐cm depth. Wheat straw management should not be an issue of concern for fertilizer‐K recommendations based on soil‐K concentration at 0‐ to 30‐cm depth for double‐crop soybean production on Coastal Plain and Piedmont soils.