Potassium Removal and Post-Harvest Soil Potassium Fractions in Corn-Soybean Rotations as Affected by Fertilization.

Soil-test K (STK) and K removal with harvest are used to determine K fertilization rates for crops. Soil-test K usually estimates exchangeable K by using the ammonium acetate (AA-K) or Mehlich-3 (M3-K) extractants. Previous STK calibration research in Iowa has shown great uncertainty concerning prediction of K sufficiency for crops and fertilization effects on post-harvest STK. Other research has suggested that non-exchangeable K may contribute to the plant-available K pool in the long term. A study based on 20 two-year trials with corn-soybean rotations was conducted in Iowa to study relationships between K fertilization, K removal, and post-harvest soil K (15-cm depth) measured with the AA-K and sodium tetraphenyl-boron (NaTPB-K) procedures. Five K fertilizer rates (0, 28, 56, 112, and 168 kg K ha^-1) were applied before the first crops using conventional plots managed with chisel-plow tillage and four replications. All post-harvest soil samples were analyzed for AAK while plots receiving 0, 56, and 168 kg K ha^-1 were analyzed for NaTPB-K. A grain K removal response was observed for seven first-year crops and seven second-year crops, although the first- and second-year responsive sites did not always coincide. Initial AA-K at these sites was at levels in which fertilization is recommended (<171 mg K kg^-1). The highest K rate applied at least 45% more K than removed by the first crops and at least 15% more than K removed by two crops. Study of initial and post-harvest soil K showed an increase of AAK in 13 sites and of NaTPB-K in 16 sites, which was a frequency lower than expected for either method. However, use of both tests allowed for detection of a soil K increase in 19 of the 20 sites. Consideration of both STK and NaTPB-K significantly improved relationships between K application rate and K removal with post-harvest soil K levels.