An evaluation of eight soil phosphate extractants on acidic wheat growing soils

In a study using earlier data from 44 wheat field experiments on acidic (pH < 5.6) soils in southern New South Wales, eight soil phosphate extractants (Bray,, Bray,, neutral fluoride, Mehlich, Truog, lactate, Olsen and Colwell) were evaluated and calibrated in terms of responsiveness (â) and response curvature (C) parameters derived from the Mitscherlich equation. All extraction procedures, except Colwell, had a standard shaking time of 30 min and so1ution:soil ratio of 50. The order of efficacy of the tests was different from that obtained on moderately acid to alkaline soils of central and northern New South Wales. Neutral fluoride and acidic sulfate (Truog) replaced lactate as the best extractant, followed by the double acid Mehlich extractant. The Colwell test was more effective on these very acid soils than on the more alkaline soils, being equal to Bray1 and greatly superior to Bray2 and Olsen. The critical values of the fluoride, Mehlich, Truog, lactate and Colwell tests were significantly lower than they were for more alkaline soils in central New South Wales. The changes in extraction procedures could explain the lower critical values of the lactate and Truog tests, but differences in the critical values of the other three tests were inconsistent with changes in extraction procedures. The results support the hypothesis that a different phosphate extractant is required on very acid soils from that required on moderately acid to alkaline soils.