Dependence of Runoff Phosphorus on Extractable Soil Phosphorus

The sustainable management of fertilizer and manure P to minimize freshwater eutrophication requires identification of soil P levels that exceed crop P requirements and have the potential for P enrichment of runoff. Although several states have established such P levels, insufficient data are available to theoretically justify them. Thus, this study investigates the relationship between the concentrations of P in runoff and in soil. Surface samples (0–10 cm) of 10 Oklahoma soils were packed in 0.15 m² boxes, incubated for 7 d with poultry litter (0–20 Mg ha⁻¹) to obtain a range in Mehlich‐3 P contents (7–360 mg kg⁻¹), and received five 30‐min rainfalls applied at 1‐d intervals. The concentration of dissolved, bioavailable, and particulate P in runoff was related (r² > 0.90; P < 0.1) to the Mehlich‐3 P content of surface soil (0–1 cm), with regression slopes ranging from 2.0 to 7.2, increasing as soil P sorption maxima increased (r² = 0.93). Two soils of 200 mg kg⁻¹ Mehlich‐3 P supported a dissolved P concentration in runoff of 280 μg L⁻¹ (San Saba clay; fine, montmorillonitic, thermic Udic Pellustert) and 1360 μg L⁻¹ (Stigler silt loam; fine, mixed, thermic Aquic Paleudalf). Thus, relationships between runoff and soil P will have to be soil specific for use in management recommendations. A single linear relationship described the dependence of dissolved (r² = 0.86) and bioavailable P (r² = 0.85) on soil P sorption saturation. The added complexity of the P saturation approach may limit its application; however, the approach integrates the effect of soil type with soil P content to better estimate the potential for P loss in runoff than soil P alone.