A long-term experiment has been conducted since 1979 to study the effects of phosphorus (P) and potassium (K) fertilization on soil-test values and grain yield of corn and soybean grown in rotation. The soil is Kenyon loam, and initially tested High for P (28 ppm, Bray-1 test) and borderline between Optimum and High for K (170 ppm, ammonium-acetate test). Boundary values for the Optimum class are 16 and 20 ppm P and 130 to 170 ppm K (ISU Extension publication PM-1688). Both crops are grown each year by alternating them between adjacent trials with identical design and management. Nine annual treatments are the combinations of 0, 46, or 92 lb P2 O5/acre and 0, 72, or 144 lb K2 O/acre. One other treatment applies 92 lb P2 O5/acre and 144 lb K2 O/acre every other year before corn, and another treatment applies similar rates every other year before soybean. Granulated fertilizers (triple superphosphate and potassium chloride) are broadcast in the fall. Corn residues are chisel plowed in the fall, and all plots are disked or field cultivated in spring. Nitrogen rates of 150 to 180 lb N/acre are applied in spring to all corn plots.