Extensive research has been conducted in Iowa to study the potassium (K) fertilization of corn and soybean, but not in south-central Iowa soils. Soil K deficiency decreases crop yield and economic benefits to growers. Also, research in other locations has shown that a K deficiency can reduce the capacity of corn to respond to nitrogen (N) and can increase the incidence of some soybean diseases. Therefore, a long-term K response study was initiated in 2009 at the McNay Research and Demonstration Farm to study corn and soybean response to K fertilizer.