We conducted a field experiment to investigate the effects of white clover living mulch on potassium (K) nutrition and the yield of silage corn. We used a randomized block design with a 2 × 3 factorial arrangement of cropping system and K application treatments. The cropping system treatments were: (1) a white clover living mulch (LM) that had been established 10 months earlier, (2) conventional cultivation with no cover crop (CC). The K application treatments were: (1) no K application (K−), (2) K application at sowing of white clover (Kclover), (3) K application at sowing of corn (Kcorn). In the LM treatments, white clover was sown in August 2006. Corn was sown in June 2007. Before the corn was sown, the white clover shoots in the LM treatments were clipped. The three conventional cultivation treatments were then tilled and corn was sown in all treatments using a minimum tillage seeder. In the LM K− treatment, we observed symptoms of K deficiency in corn leaves on 23 July. At the silking stage of corn, the K and N contents of ear leaves in the LM treatments were significantly lower than those in the conventional cultivation treatments. In the LM K− treatment the K content of ear leaves was <1% and the yield of corn shoots was less than that recorded in the other treatments. Multiple regression analyses revealed that K was the nutrient most strongly related to corn yield. These results suggested that white clover LM declined the K nutrient status of corn and as a result decreased the yield of corn.