Long-term corn and soybean response to phosphorus fertilization in Iowa
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−1 (Bray-P1) were created at two sites, and annual treatments of 0, 11, 22, and 33 kg P ha−1 were superimposed. At a third site, annual rates of 0, 22, and 45 kg P ha−1 were applied annually. Annual P rates required to maintain 16-20 mg kg−1 STP were similar at Webster-Nicollet-Canisteo soils (13-17 kg P ha−1), however, >30 kg P ha−1 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−1). Critical STP concentrations identified with linear-plateau and quadratic-plateau models across sites were 18-23 mg kg−1 for corn and 11-18 mg kg−1 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. Abbreviations: AGRON, Agronomy and Agricultural Engineering Research Farm; NERF, Northeast Iowa Research Farm; NIRF, Northern Iowa Research Farm; STP, soil-test P.