Influence of Recent Acidification On Available Phosphorus Indices and Sorption In Washington State Soils.

Gradual soil acidification in the Columbia Basin (CB), Washington State, brings into question the validity of the traditional sodium bicarbonate (NaHCO₃) extractable phosphorus (Olsen P, OP) method for making fertility recommendations. Our objectives were to determine how soil pH influences the soil P indices after fertilization and sorption capacity of soils with similar levels of available P at varying soil pH levels. Collected soils represented a range of pH (5.2-8.4) and OP (3-40 mg P kg^-1). Soils were treated with 0, 44, 88, or 132 kg P ha^-1 and incubated at 20^o C for 6 weeks. Subsamples were collected and analyzed for available P with OP, Bray P1 (BP1), Morgan (MMP), Mehlich-III (M3P) extractions. Additionally, P sorption isotherms were performed on untreated soils. Analysis of variance indicated that the slope of the change in extractable P was generally not significant (a = 0.05) for OP, BP1, MMP, and M3P given a P addition for several soils at different pH but of similar initial available P levels. Grouping soils by pH showed differences in slope with OP (0.27-0.34) yielding the least difference and MMP (0.12-0.33) the greatest related to soil pH. Langmuir parameters of b, sorption capacity, and k, binding energy, showed variability for soils grouped by initial available P for several pH levels. Grouped by pH there was little difference in P sorption in relation to soil pH. Continued use of OP on acidified soils of the CB appears to be a viable soil testing strategy.