Dependence of Runoff Phosphorus on Extractable Soil Phosphorus

TLDR

Sustainable management of fertilizer and manure phosphorus to prevent freshwater eutrophication requires identifying soil P levels that exceed crop needs and can enrich runoff, yet existing state thresholds lack sufficient theoretical support. This study examines the relationship between phosphorus concentrations in runoff and in soil. Ten Oklahoma soils (0–10 cm) were packed, incubated with poultry litter (0–20 Mg ha⁻¹), and subjected to five 30‑min rainfalls at 1‑day intervals to generate a range of Mehlich‑3 P contents (7–360 mg kg⁻¹). Dissolved, bioavailable, and particulate P in runoff were strongly correlated (r² > 0.90) with surface soil Mehlich‑3 P, with slopes 2.0–7.2 that increased with sorption maxima, indicating that runoff‑soil P relationships are soil‑specific and that a single linear model linking dissolved/bioavailable P to soil P sorption saturation better estimates potential runoff loss than soil P alone.

Abstract

Abstract 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 m 2 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 ( r 2 > 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 ( r 2 = 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 ( r 2 = 0.86) and bioavailable P ( r 2 = 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.