Rising potassium (K) fertilizer prices in recent years have made it imperative for farmers to apply optimum K rates for alfalfa-corn rotations. However, little is understood about the effect of excess K applied to alfalfa on grain and silage yield of the subsequent corn crop. Furthermore, relatively few studies have investigated the effects of applied K on the N credit from alfalfa to corn. On-farm research was conducted at 10 fields in 2008 to 2010 on medium-K-testing soils to determine optimal rates of K for last-year alfalfa and optimal rates of N for first-year corn. When K was only applied to last-year alfalfa, each 100 kg ha^-1 increase in the index of available K increased corn grain yield by 0.5 Mg ha^-1, decreased stover yield by 0.4 Mg ha^-1, and did not affect silage yields. Regardless of K rate applied to alfalfa, additional K applied to corn increased corn stover and silage yields by 10 and 8%, respectively. This suggests that carryover K was less available than K applied to corn. On medium STK soils going into the last year of alfalfa, applying fertilizer K to first-year corn rather than alfalfa may enhance economic return. There was no response to N at any previous K rate, which suggests that K had no effect on the alfalfa N credit on soils with medium soil-test K levels at the beginning of the corn year.