The effect of the level of phosphate rock application on its dissolution in soil and on bicarbonate-soluble phosphorus

Abstract

The effect of the level of phosphate rock application on its dissolution in a lateritic podzolic soil was investigated in laboratory incubation studies. Dissolution of phosphate rock (PR) was estimated as the increase in soil exchangeable Ca (Δ Ca). Concomitant increases in bicarbonate-soluble P levels (Δ Bic P) were estimated with the Olsen extractant. Up to 19% of PR dissolved at 200µg P g−1 soil level of PR application whereas a ten-fold increase in the level of application resulted in the percentage dissolution decreasing to 5%. The absolute amount of dissolution however increased with the level of PR application. The amount of PR dissolved was described by the equation ΔP = A(1 − e−CR) where ΔP = P dissolved from PR (µg P g−1 soil) calculated from Δ Ca values, A = maximum dissolution asymptote, C = curvature coefficient and R = level of PR applied expressed asµg P g−1 soil. Values of the A coefficient increased systematically from 28 to 93µg P g−1 soil for incubation periods from 1 to 7 days. Corresponding values of the C coefficient ranged from 0.9 − 2.1 × 10−3. Increases in Δ Bic P arising from the dissolution of PR were also described by an exponential equation and the asymptote values ranged from 4 to about 18µ P g−1 soil. For Δ Bic P the curvature coefficient values increased with incubation period due to increased P retention with increasing contact period between PR and the soil. The form of the yield response curves of plants to PR was predicted on the basis of these data and compared with response curves reported in the literature. The response curves for PR differ in shape from the Mitscherlich response curve for superphosphate and indicate that in some situations PR is unable to support the same maximum yield as soluble P fertilizers due to inadequate dissolutiuon of PR at high rates of application.