Correlation and Calibration of Soil-Test Potassium from Different Soil Depths for Full-Season Soybean on Coarse-Textured Soils

Quantifying soil-K availabilities at deeper depths may be necessary to determine optimum fertilizer-K rate for soybean [ Glycine max (L.) Merr.] grown on low cation exchange capacity (CEC) soils that are prone to K leaching. We characterized full-season soybean response to fertilizer-K across 19 coarse-textured lowCEC sites during 2013 and 2014. Mehlich-1 soil-K concentrations at 0-to 15-and 0-to 30-cm depths better correlated with relative yield and explained 90% of relative yield variation compared to 77% for 0-to 60-cm depth. Critical soil-K concentrations were similar for relative yield, V5 plant-K concentration, and R2 leaf-K concentration, ranging from 48 to 73 mg K kg(-1) for 0-to 15-cm and 41 to 63 mg K kg(-1) for 0-to 30-cm depths. Soil-K concentrations less than this critical range accurately predicted positive yield responses to fertilizer-K 89% of the time for 0-to 15-cm and 80% for 0-to 30-cm depths. Plant-and leafK concentrations were equally good in predicting relative yield with critical concentrations of 19 to 22 g plant K kg-1 and 18 to 21 g leaf K kg(-1). Plant-K concentration was better than leafK concentration in diagnosing K-deficient sites. Calibration model confi rmed that soybean requires no fertilizer-K to maximize yield for soil-K concentrations above the critical ranges at both depths. However, for K-defi cient soils, soil-K concentrations at 0-to 30-cm depth resulted in 7 to 32% less fertilizer-K requirements than 0-to 15-cm depth, indicating the value of deeper sample in recommending fertilizer-K for soybean grown on coarse-textured low-CEC soils.