Determination of residual reactive phosphate in soil

Abstract

Soil P tests are not reliable for the estimation of soil phosphorus status where reactive phosphate rock (RPR) has been used as a direct application fertiliser. This is because the soil tests used either do not include the residual RPR in the soil, or do not take into account the influence of soil and environmental factors on the rate of RPR dissolution in the soil. An alternative approach is use of a RPR dissolution model that incorporates effects of the fertiliser, soil and environmental characteristics. The model requires an estimate of residual RPR in soil accumulated from previous fertiliser additions. This paper proposes an analytical method for this that does not have the complications of the P fractionation procedures normally used. Extraction of soil with a pH 4 buffer enables differentiation between soil fluorapatite‐P and RPR‐P. In the proposed method this extraction is followed by addition of NaOH to make the suspension alkaline. A parallel sample of the same soil is treated similarly, but without the pH 4 buffer extraction to provide a correction for non‐apatite P extracted. Dissolved RPR‐Ca and exchangeable Ca are complexed with citrate to prevent precipitation of P during the alkaline extraction. Using this method small background levels (0–30 μg P g^‐1 soil), due to slight dissolution of soil fluorapatite, were found for most soils. 100% of the most reactive RPRs were recovered from soil/RPR mixtures.