Many field trials have been established and evaluated at the ISU Northern Research Farm, Kanawha, Iowa to study potassium (K) fertilization rates and placement methods on corn and soybean grain yield, K uptake, and soil-test K values. However, no research has investigated K recycling to the soil by maturing plants and crop residue until the next crop is planted. The amount and the timing of the K recycled to the soil should have a significant impact on soil-test K values, and should explain a great deal of usually very high soil-test K temporal variability. Therefore, plots of several ongoing field K trials were used to investigate these issues.