Long-term application of potassium fertilizer on maize crops in northeastern China increased potassium levels in plant organs, especially straw, and boosted grain yield. However, increasing the amount of potassium fertilizer did not further increase yield. Soil potassium levels decreased in some treatments, leading to an annual potassium deficit, but one treatment showed a surplus. Applying potassium fertilizer also improved the availability of potassium in the soil. To prevent further decline in soil potassium, returning maize straw to the soil should be considered.