Relationships Between Potassium Fertilization, Removal with Harvest and Concentrations in Soil Pools for Long-Term Corn-Soybean Rotations.

More research is needed to understand large soil-test K (STK) temporal variability and inconsistent relationships between K application rates, removal, and STK. These issues were studied using five 15-year experiments with corn-soybean rotations managed with no-tillage. Treatments were broadcast and planter-band K placement methods and several K application rates. Measurements were grain yield, grain K concentration, K removal with harvest, STK and non-exchangeable K for selected treatments, and soil sampling depths. Potassium fertilization had a small effect on grain K concentration but often increased both grain yield and K removal when STK was less than 150-180 mg K kg^-1 (ammonium acetate test). Grain K concentration was poorly correlated with grain yield or STK, but there was a high correlation between K removal and yield. Average grain K concentration estimates used in Iowa for STK maintenance fertilization (4.2 and 22.3 g K kg^-1 for corn and soybean) were among the highest concentrations observed for each crop. There was a clear vertical stratification of both STK and non-exchangeable K. Non-exchangeable K measurements partially explained large STK variability across K application rates and years.