Soil‐Test Phosphorus and Crop Grain Yield Responses to Long‐Term Phosphorus Fertilization for Corn‐Soybean Rotations

Farmers and nutrient management regulatory agencies are requesting better knowledge of P fertilization impacts on soil‐test P (STP) and crop yield. This study evaluated STP and grain yield of corn ( Zea mays L.) and soybean Glycine max (L.) Merr. as affected by long‐term P fertilization in three trials evaluated from the 1970s until 2002 near Boone, Kanawha, and Nashua in central, northern, and northeast Iowa. Soils were Aquic Hapludolls and Typic Endoaquolls at Boone, Typic Endoaquolls at Kanawha, and Typic Hapludolls at Nashua. At Boone and Kanawha, treatments were the combinations of three initial STP levels (17–96 mg P kg −1 , Bray‐P 1 ) and four annual rates (0–33 kg P ha −1 ). At Nashua, initial STP was 28 mg P kg −1 and treatments were 0, 22, and 44 kg P ha −1 Ten to twenty years of cropping were needed on soils testing 43 to 96 mg P kg −1 to observe yield response to P. Annual P rates that maintained near optimum STP (16–20 mg P kg −1 ) were 17, 14, and 13 kg P ha −1 yr −1 at Boone, Kanawha, and Nashua, respectively. Phosphorus required to increase STP 1 mg P kg −1 yr −1 were 23, 28, and 17 kg P ha −1 yr −1 at Boone, Kanawha, and Nashua, respectively. Critical STP concentrations (CC) identified across sites and years were 15 to 21 mg P kg −1 for corn and 12 to 18 mg P kg −1 for soybean. Observed grain yield and STP responses are useful to develop effective P management plans for corn–soybean rotations under approximately similar conditions to those in this study.