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

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Core Ideas Soil K at 0‐ to 15‐ and 0‐ to 30‐cm depths were excellent predictors of full‐season soybean relative yield.Tissue‐K concentration can be used for calibrating fertilizer‐K rate and in‐season K management.Soil sampling to 30‐cm depth would reduce fertilizer amount and cost for soybean on low cation exchange capacity 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 low‐CEC 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 leaf‐K 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 leaf‐K concentration in diagnosing K‐deficient sites. Calibration model confirmed that soybean requires no fertilizer‐K to maximize yield for soil‐K concentrations above the critical ranges at both depths. However, for K‐deficient 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.