Agronomic and Environmental Evaluation of Soil Phosphorus With Continuous Draw-Down.

Research advances indicate that residual phosphorus (P) in soils is mainly in absorbed forms that can desorb and be available to crop uptake and/or losses in water discharge. A study was conducted to evaluate the agronomic and water quality consequences of continuous P draw-down in a clay loam soil under corn-soybean rotation from 2008 to 2012. Both surface runoff and tile drainage water discharges were year-round continuously monitored for flow volumes and sampled for P determination.  Corn grain yields with P draw-down were largely identical to those with continuous P addition. However, soybean yields with P draw down were 6-8% lower than those with continuous P addition, indicating that soybeans with small and shallow rooting systems may need a booster of fertilizer P to compromise the temporal shortage of soil available P, especially in early spring. Soil test P (Olsen P) remained unchanged for the draw-down treatment over the 4-year period, while for the continuous P addition treatment it increased, with 5 kg P ha^-1 addition of fertilizer P required to increase each mg P kg^-1. Water extractable soil P decreased with continuous P draw-down, but increased with continuous P addition, both at the same numerical value, 0.48 mg P kg^-1 year^-1.  Soil P losses in all forms, including dissolved reactive P and particulate P, in both surface runoff and tile drainage decreased with cropping for the P draw-down, while they increased for the continuous P addition. It appeared that water extractable P was more sensitive that Olsen P to indicate soil P losses. Soil P draw-down can be recommended as a BMP for management of P in high P soils, but supplemental P application may be required for soybeans to ensure the maximized production.