Correlation and Calibration of Soil Potassium Availability with Soybean Yield and Trifoliolate Potassium

The ability of soil tests to identify nutrient-deficient soils and recommend fertilizer rates that optimize agronomic yield is essential for profitable soybean [Glycine max (L.) Merr] production. Our objectives were to correlate relative soybean yield to Mehlich-3 and 1 mol L(-1) HNO(3)-extractable soil K and trifoliolate-leaf K concentration at the R1 to R2 development stage and calibrate the K rates for Mehlich-3-extractable soil K. Experiments were established on silt barns at 34 site-years planted with a Maturity Group IV or V cultivar and fertilized at five K rates (0-148 kg K ha(-1)). Mehlich-3-extractable soil K ranged from 46 to 167 mg K kg(-1) and produced relative soybean yields of 59 to 100% when no K was applied. Eleven sites had Mehlich-3-extractable K < 91 mg K kg(-1) and all responded positively to K fertilization. Soybean grown in soil having 91 to 130 mg K kg(-1) responded positively at nine of 15 sites. Mehlich-3 soil K explained 76 to 79% of the variability in relative yields and had critical concentrations of 108 to 114 mg K kg(-1), depending on the model. The linear-plateau model predicted the critical HNO(3)-extractable soil K to be 480 mg K kg(-1). Trifoliolate-leaf K concentration increased significantly, positively, and linearly as Mehlich-3- and HNO(3)-extractable soil K increased, but Mehlich-3 soil K explained only 49 to 53% of the variation in trifoliolate-leaf K. Mehlich-3-extractable K is an excellent predictor of soil K availability for soybean grown on silt barns in eastern Arkansas.