Iowa State University (ISU) researchers have conducted much research over time with help from Iowa farmers, crop consultants, and agribusiness to study potassium (K) fertilization of crops and use of soil-test K as a tool to determine crop K needs. Results of this research are reflected on current fertilizer recommendations, which are explained in the ISU Extension publications Pm-1688 and Pm-1310. The research continues to keep pace with changes of production practices, new hybrids and varieties, and new questions. This year Iowa farmers are looking very carefully at fertilization practices because recent sharp increases in fertilizer and fuel prices have not been matched by increases in grain prices. The ISU K recommendations were last updated for the 2003 crop year. The most significant changes were (1) to recommend maintenance of higher soil-test K levels for optimum crop production and (2) to suggest deep K fertilizer placement for grain crops managed with ridge-till and no-till systems. In this presentation we review the K recommendations and share highlights of recent on-farm research. Decisions about fertilization for any economic situation should be made based on knowledge of the probability and size of crop response to fertilization for different soil-test levels, fertilization rates, and prices.