A long-term study was established in 1963 and was continued until 2001 at the Northern Research Farm to evaluate the nitrogen (N), phosphorus (P), potassium (K), and lime requirements of continuous corn. The experimental area was on Nicollet soil, pH was 6.1, and both soil P and K were very low. Because one main objective was to study nutrient interactions, 25 treatments evaluated selected combinations of five annual rates of each nutrient using a complex statistical central-composite experimental design.