Slow release K sources provide a potential method for supplying mid-season K requirements of cotton without having to apply sidedress or foliar K. Tests on two soil types were used to determine what percentage of K should be supplied as slow-release K and to predict under what conditions slow release K may be beneficial. In this study, the 50% rate of slow release application resulted in optimum yields. Slow release K will only be beneficial in soils with 1) low to medium K levels, 2) high potential for leaching or fixation of soluble K sources, and (3) high yield potential.