Corn (Zea mays L.) production has greatly increased in North Dakota since the 1990s; yet, potassium (K) fertilizer recommendations for corn in North Dakota still date from the late 1970s and early 1980s when corn grain yields were comparatively lower and native soil K fertility was sufficient for most crop K requirements. To update K fertilizer recommendations for modern corn production and lower soil K levels, corn grain yield response to K fertilization, various soil K testing methods, and seasonal soil K variation were investigated in a two-year (2015 and 2016) study. The standard soil K testing method of NH4OAC extraction on dried soil remained the best predictor of yield response, although corn on some soils did not respond in accordance with soil test K level. Soil test K was highest in spring and lowest in late summer, indicating that soil test K interpretation should account for seasonal variation.