Estimating Errors Associated with Extracting Phosphorus Using Iron Oxide and Resin Methods

Abstract

AbstractDissolution of particulate P from adhering soil material may cause an error in estimating bioavailable P with Fe oxide‐impregnated filter paper (FeO) strips and anion exchange resin (AER) bags. This error was assessed with two clayey soil samples and agricultural runoff waters high in suspended clay. Iron oxide strips were shaken either unprotected in soil suspension or enclosed in nylon mesh bags. Resin in bags and loose AER beads were used in the AER extractions. Sodium chloride and HCl extractions were compared in the displacement of P retained by the AER. Aluminum and Si served as index elements for the presence of soil material. Overestimation of desorbable P by 2.5 to 2.7 mg kg−1 for each strip was measured, compared with about 30 mg kg1 of FeO strip‐extractable P in the soils. Enclosing the FeO strips in nylon mesh bags reduced soil attachment by about 60%. The error due to dissolution of particulate P was negligible with the AER method. The FeO‐strip procedure was problematic in the water sample study. The highest P yield occurred when FeO strips were protected with nylon mesh, followed by unprotected FeO strips, AER(HCl), AER(NaCl), and loose AER beads. Overnight shaking with an unprotected FeO strip extracted 23 to 65% more P, and four sequential extractions 140 to 196% more P, than the AER(NaCl) method. In analysis of turbid water samples, the risk of overestimating P bioavailability was much lower with the AER(NaCl) method than with the FeO‐strip method.