Long-Term Impacts of Phosphorus and Potassium Fertilization on Soil-Test Values and Grain Yield of Corn and Soybean Grown in Rotation.

Long-term studies are useful for quantifying small responses of soil properties and crop yield to fertilization whose significance become evident only over a long period of time. This study evaluated grain yield responses of corn and soybean as well as changes in soil-test values when P and K fertilizers were applied at various rates during 34 years. Treatments were the factorial combinations of P (0, 22, and 44 kg P ha^-1) and K (0, 67, and 134 kg K ha^-1) annual rates applied from 1979 to 2013 to an initially high-testing Iowa Kenyon loam (Typic Hapludoll) soil. Soil-test P and K of the control plots decreased following an exponential decay trend, for P from 28 to 10 mg P kg^-1 (Bray-1 test) and for K from 230 to 96 mg K kg-1 (ammonium-acetate test). Fertilization with the high rate increased soil-tests with decreasing increments over time. The increases were 45 and 120 mg P kg^-1 for rates of 22 and 44 kg P ha^-1, respectively, and 190 and 393 ppm for rates of 67 and 134 kg K ha^-1. Grain yield was not increased by P or K until about 17 years after the experiment began, and responses have increased recently as soil-test values of the control plots continued declining. There was no significant P by K interaction. Surveys show that approximately 50% of Iowa soils test above optimum levels for corn and soybean due to excess fertilizer or manure application. Therefore, the results of this study provide useful information as of how long fertilization could be withheld without impacting crop yield and probably benefiting water quality.