Northern Research and Demonstration Farm
A 3-yr study was conducted to assess the effects of several potassium (K) fertilization rates on grain yield, K removal by corn and soybean, and soil-test potassium. This study complements long-term experiments at the farm that include fewer K application rates. The Iowa State University (ISU) soil-test K interpretations and fertilization guidelines were updated in 2003 based on research in many Iowa fields. One major change was to increase the soil-test potassium (STK) level suggested for optimum crop production. The category previously deemed Optimum, which ranged from 91 to 130 ppm (ammonium-acetate or Mehlich-3 K tests, 6-in. samples), was changed to Low, and the High category (131 to 170 ppm) was reclassified as Optimum. Fertilization with K is recommended to maintain this level.
Iowa State University
Mallarino, Antonio P.; Clover, Matthew William; and Rueber, David, "Potassium Fertilization Rate Effects on Soil-Test Potassium and Yields of Corn and Soybeans" (2007). Iowa State Research Farm Progress Reports. 937.