Relationships Between Soil Test Phosphorus, Soluble Phosphorus, and Phosphorus Saturation in Delaware Soils

TLDR

Methods to identify agricultural soils that contribute to nonpoint‑source phosphorus pollution of surface waters are increasingly important, especially in high‑animal‑density cropland areas. The study aimed to determine how agronomic soil test phosphorus (STP = Mehlich 1) relates to other soil P tests that predict erosion, runoff, and leaching potential. The authors compared STP with soluble P, strip P, and soil P saturation across 127 soils, and assessed phosphorus saturation using Langmuir isotherms, oxalate extractions, and a single‑point P sorption index. Findings revealed that STP correlated strongly with soluble, strip, and oxalate‑extractable P; strip P better predicted soluble P; the strip P/ox ratio increased as sorption capacity decreased; soluble P, STP, and desorbable P rose when DPS exceeded 30%; and soils with STP > 50 mg kg⁻¹ had higher ratios of soluble, strip, and oxalate‑extractable P to total P.

Abstract

Methods to identify agricultural soils that contribute to nonpoint‐source pollution of surface waters by P are of increasing importance, particularly in areas with high animal densities (animal units per hectare of cropland). Our objective was to determine the relationship between agronomic soil test P (STP = Mehlich 1) and other soil P tests proposed to measure the potential for P loss by erosion, runoff, and leaching. We compared STP with soluble P, P in the “fast desorbing pool” (strip P), and soil P saturation for 127 soils (122 from Delaware and five from the Netherlands). Soil test P was significantly correlated with total , soluble , strip , and oxalate‐extractable Strip P was a better predictor of soluble P than The ratio of strip P/P ox (the percentage of reversibly sorbed P in the fast desorbing pool) increased as P sorption capacity, estimated from oxalate‐extractable Al and Fe (Al ox + Fe ox ), decreased. We also determined the degree of P saturation (DPS) using three methods: Langmuir P sorption isotherms; oxalate extractions of P, Al, and Fe; and STP plus a single‐point P sorption index (PSI). Soluble P, STP, and desorbable P increased for DPS values >30%, similar to upper DPS limits in the Netherlands and Belgium. Soils rated agronomically excessive in STP (>50 mg kg −1 ) had higher ratios of soluble P, strip P, and P ox to total P than those in agronomically optimum or lower categories.