Phosphorus Extraction Methods for Water Treatment Residual–Amended Soils

Abstract

Water treatment residuals (WTR) can adsorb tremendous amounts of phosphorus (P). A soil that had biosolids applied eight times over 16 years at a rate of 6.7 Mg ha⁻¹ y⁻¹ contained 28 mg kg⁻¹ ammonium–bicarbonate diethylenetriaminepentaacetic acid (AB‐DTPA), 57 mg kg⁻¹ Olsen, 95 mg kg⁻¹ Bray‐1, and 53 mg kg⁻¹ Mehlich‐III extractable P. To 10 g of soil, WTRs were added at rates of 0, 0.1, 1, 2, 4, 6, 8, and 10 g, then 20 mL of distilled deionized H₂0 (DI) were added and the mixtures were shaken for 1 week, filtered, and analyzed for soluble (ortho‐P) and total soluble P. The soil–WTR mixtures were dried and P extracted using DI, AB‐DTPA, Olsen, Bray‐1, and Mehlich‐III. Results indicated that all methods except AB‐DTPA showed reduced extractable‐P concentrations with increasing WTR. The AB‐DTPA extractable P increased with increasing WTR rate. The water‐extractable method predicted P reduction best, followed by Bray‐1 and Mehlich‐III, and finally Olsen.

Keywords:

• Water‐treatment residuals (WTR)
• water extractable
• AB‐DTPA
• Olsen
• Mehlich‐III
• Bray‐I
• soluble P
• total soluble P