Stabilization of Molybdate Reactive Phosphorus in 0.5 M Sodium Bicarbonate Extracts of Soils

Abstract

Molybdate-reactive phosphorus (MRP) in soil-free 0.5 M sodium bicarbonate (NaHCO₃) extracts of 20 soils held at 22 °C increased 0–25% (median 11%) after 24 h and by 5–120% (median 43%) after 72 h. Addition of 2.5 mL L⁻¹ of chloroform (1) or 0.25–1 g L⁻¹ of thymol (2), phenyl mercury acetate (3), sodium cyanide (4), or sodium azide (5) showed that only (2)–(5) at 1 g L⁻¹ stabilized the MRP for 72 h. Five of the soils were re-extracted with 0.5 M NaHCO₃ containing 1 g L⁻¹ of (2)–(5): all stabilized MRP for 72 h, and (2) increased MRP for three soils (P < 0.05). For 92 additional soils (0.5–200 mg MRP kg⁻¹) extracted with 0.5 M NaHCO₃ ± 1 g of sodium azide L⁻¹, the azide had a negligible effect on MRP extraction, and in its presence, extracted MRP was unchanged after 72 h at 22 °C. We recommend this extractant for wider evaluation.

Keywords:

• Antimicrobials
• Colwell P
• laboratory operations
• Olsen P
• temperature

Acknowledgments

We thank Paul Nicholls for statistical input; Jill Kaldor, Harry Willats, Michael Maguire, Vicki Glover, Samantha Fenton, and Jane Thompson for assistance; and Mark Conyers, George Rayment, and David Harland for comments on the manuscript.