Long-Term Impact of Manure and Fertilizer Application on Phosphorus in Soil and Subsurface Drainage for Corn and Soybean Rotations.

Phosphorus loss through subsurface drainage can be an important mechanism of P delivery to water resources.  This study was conducted during 7 years in moderately to poorly drained Northeast Iowa soils to determine effects of liquid swine manure and fertilizer P application for corn-soybean rotations on soil P build-up and P loss with tile drainage.  Five management systems were fall manure based on N removal for both crops (MNR), fall N-based manure for corn (MNC), spring N-based manure and no-till for corn (MSNC), fall P-based manure for corn (MPC), and fall P fertilizer for both crops (FP).  Swine manure was injected, fertilizer P was spread in fall and incorporated in fall for corn and in spring for soybean.  Profile soil-test P (STP) was measured to 120 cm. The STP for all systems was greatest in the 0-15 cm soil layer, and decreased abruptly below 25 cm being similar for all systems.  The STP buildup by the last year was 95, 70, 35, 25, and 25 mg P kg^-1 for MNR, MNC, MSNC, MPC, and FP, respectively.  Phosphorus losses were highest for MNR (0.0015 kg P ha^-1), intermediate for MNC and MSNC (0.001 kg P ha^-1), and lowest for MPC and FP (0.0005 kg P ha^-1).  We conclude that STP has to be at least four times the optimum levels for corn and soybean in Iowa (16-20 mg P kg^-1) for significant P leaching to occur in these soils.