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‐P1) 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.

Efficient P fertilization practices are of great concern to Corn Belt farmers because of the agronomic and environmental implications of increasing soil-test P (STP) trends. This study evaluated (1) long-term STP trends for various initial STP levels and annual P application rates, (2) yields of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] as affected by P fertilization and STP. Yield and STP data were collected from three experiments established in Iowa during the 1970s on Webster (fine-loamy, mixed, mesic Typic Endoaquolls)-Nicollet (fine-loamy, mixed, mesic Aquic Hapludolls) soils, Webster-Canisteo (fine-loamy, mixed, calcareous, mesic Typic Endoaqualls) soils, and Kenyon (fine-loamy, mixed, mesic Typic Hapludolls) soils. Crops were grown in rotation each year until 2002. Three initial contrasting STP levels ranging from 17 to 75 mg kg-' (Bray-P1) were created at two 4 sites, and annual treatments of 0, 11, 22, and 33 kg P ha-' were superimposed. At a third site, annual rates of 0, 22, and 45 kg P ha~' were applied annually. Annual P rates required to maintain 16-20 mg kg-' STP were similar at Webster-Nicollet-Canisteo soils (13-17 kg P ha-'), however, >30 kg P ha-' was required to maintain STP levels four times higher. At the Kenyon soil, a .similar Phosphorus rate maintained a higher STP level (28 mg kg-'). Critical STP concentrations identified with linear-plateau and quadratic-plateau models across sites were 18-23 mg kg-' for corn and 11-18 mg kg-' for soybean. Results indicate that 10-15 years of cropping without P fertilization were required on high testing soils before yield response to P was observed.

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