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

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 P(r=0.57***,significant at the0.001level), soluble P(r=0.71***), strip P(r=0.84***), and oxalate‐extractable P(Pox;r=0.84***) Strip P was a better predictor of soluble P than STP(r2=0.76***) The ratio of strip P/Pox (the percentage of reversibly sorbed P in the fast desorbing pool) increased as P sorption capacity, estimated from oxalate‐extractable Al and Fe (Alox + Feox), 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 Pox to total P than those in agronomically optimum or lower categories.