Runoff Dissolved and Particulate Phosphorus As Affected By Tillage and Fertilizer Placement Methods in Corn-Soybean Rotations.

Application of excessively high P rates, inappropriate methods of application, and tillage can increase P with surface runoff. A 2-year study was conducted under natural rainfall to evaluate runoff P loss in corn-soybean rotations. Previous management for the site had resulted in 24 plots (30 by 6 m) with approximately optimum soil P (24 mg Bray-1 P kg^-1), 12 with chisel-plow-disk (CH) and 12 with no-till (NT) histories, and three plots with above-optimum P (38 mg kg^-1) and CH histories. Management systems implemented for plots testing optimum were the combinations of CH, NT, and P fertilization broadcast (BrP) or subsurface banded with the planter (BP) with each crop planted each year. The P rates were designed to maintain soil P, and were 50 kg P ha^-1 BrP applied only before corn or 25 kg P ha^-1 BP applied to each crop each year. The other three plots assessed remediation of the high-testing soil by NT, a rye cover crop for both crops, and no P application. All systems were replicated three times. Runoff measurements were volume, dissolved reactive P (DRP), bioavailable P (Fe-oxide coated filter paper method), total P (TP), and total solids. Corn yield was lowest with NT but tillage did not affect soybean yield. The P placement method did not affect yield of any crop. On average for the rotation, soil loss was highest for CH and lowest for NT (5.7 and 0.89 Mg ha^-1). In plots receiving P, the annual mean total P loss was highest for BrP-CH (5.3 kg P ha^-1) and lowest for BP-CH, BrP-NT, and BP-NT (2.2 to 2.5 kg P ha^-1); whereas the annual DRP loss was highest for BrP-CH, BrP-NT, and BP-NT (0.84 to 1.1 kg P ha^-1) and lowest for BP-CH (0.25 kg P ha^-1). The high-testing soil managed with a cover crop, NT, and without P had the lowest DRP and TP loss (0.18 and 1.1 kg P ha^-1). We conclude that when P is applied, various management options greatly reduced TP loss compared to CH with BrP but only CH with BP clearly reduced DRP loss compared with the other management systems. Discontinuing P application to the high-testing soil and implementing NT with a cover crop resulted in the smallest observed DRP and TP losses.