Full-Season and Double-Crop Soybean Response to Potassium on Mid-Atlantic Coastal Plain and Piedmont Soils.

Quantifying topsoil- and subsoil-K availabilities is necessary to determine optimum fertilizer-K rate for soybean [Glycine max (L.) Merr.] grown on coarse-textured or low cation exchange capacity soils that are prone to K leaching. We characterized full-season and double-crop soybean responses to fertilizer-K by conducting nineteen and twenty-two field trials, respectively with multiple K rates during 2013 to 2015 on Mid-Atlantic Coastal Plain and Piedmont soils. Soil-K concentrations at 0- to 15- and 0- to 30-cm depths were better in explaining relative yield variability for both full-season and double-crop soybean and identifying K deficient sites than soil-K at 0- to 60-cm depth. The critical soil-K concentrations at 0- to 15-and 0- to 30-cm depths ranged from 48.2 to 70.8 mg K kg^-1 and 40.1 to 61.4 mg K kg^-1, respectively for full-season soybean and 40.4 to 75.1 mg K kg^-1 and 36.0 to 65.5 mg K kg^-1 for double-crop soybean. Both plant- and leaf-K concentrations were equally good in predicting full-season soybean relative yield with critical concentrations of 19.2 to 22.2 g plant-K kg^-1 and 18.0 to 20.6 g leaf-K kg^-1. For the double-crop soybean, leaf-K concentration with critical range of 20.4 to 22.9 g K kg^-1 at the R2 stage was better in diagnosing K deficiency and identifying yield responsive sites (75% accurate) than plant-K with critical range of 17.3 to 22.6 g K kg^-1 at the V5 stage (50% accurate). Full-season soybean requires 60% more fertilizer-K, but double-crop soybean required similar fertilizer-K for soils with ≤45 mg K kg^-1 at 0- to 15-cm depth to maximize yield than the current Virginia Cooperative Extension recommendation. However, fertilizer-K rate can be reduced by 11-25% for full-season and 13-38% for double-crop soybean if recommendations are based on soil-K concentrations at 0- to 30- than 0- to 15-cm depths.