Interference of colloidal particles in the determination of orthophosphate concentrations in soil water extracts

Accurate determination of orthophosphate (P) in soil solutions is a prerequisite for rigorously assessing soil P availability for plants as well as the potential risk of P transfer to water bodies. Although P is usually measured on extracts filtered at 0.45 mu m, little is known on the potential interfences due to the presence of colloidal particles. This work was conducted to study the influence of colloidal particles on P concentration by measuring it both by the phosphoantimonylmolybdenum blue colorimetry and ion chromatography in soil water extracts filtered at 0.45, 0.2, or 0.025 mu m. Results strongly suggest that direct measurements with colorimetry might lead to an overestimation of the P concentration due to the solubilization of P-containing particles in the acid medium prevailing in the solution during colorimetric essay. In the present study, the largest overestimations were observed in soil extracts rich in iron (Fe) and aluminum (Al) that had been filtered at 0.45 mu m, on the opposite this overestimation remained lower but constant for the three levels of filtration in the studied calcareous soil. Ion chromatography constantly underestimated P concentration especially when the solutions were rich in Al and Fe. This underestimation could not be related to the presence of dissolved polyvalent cations in the solution and, therefore, might be accounted for by the sorption of P at the surfaces of colloids. In order to limit these interferences, it is proposed to measure P concentration by colorimetry after a filtration of the solution through a 0.2- or 0.025-mu m membrane, instead of a 0.45-mu m filter. In some cases for instance for soil solutions rich in calcium phosphate particles, it might be advisable to determine P concentration with ion chromatography.