Potassium (K)² availability varies widely in soils of the North Central Region of the United States and the adjoining areas of Canada. Soils in the western part of the region generally contain adequate amounts of plant-available K, but in other parts of the region, soils vary from those with abundant supplies of available K to those that are very deficient. Present techniques for predicting crop requirements for K fertilizer on different soils based on the determination of all or a portion of the exchangeable K in the plow layer are often inadequate, even when applied within restricted soil areas. In view of the wide range of K availability in different soils of the region, more effective methods of assessing the K status of the soils must be developed if efficient use of K fertilizers is to be accomplished. Therefore, the major objective of this study was to investigate the relationship between crop yield response from K fertilizer or uptake of soil K by plants in the field and different laboratory indexes of "plant-available K."