Stabilization of molybdate reactive phosphorus in 0.5 M sodium bicarbonate extracts of soils

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.