INTERFERENCE OF COLLOIDAL PARTICLES IN THE DETERMINATION OF ORTHOPHOSPHATE CONCENTRATIONS IN SOIL-WATER EXTRACTS

Accurate determination of orthophosphate (P) in soil solutions is a pr erequisite for rigorously assessing soil P availability for plants as well as the potential risk of P transfer to water bodies. Although P i s usually measured on extracts filtered at 0.45 mu m, little is known on the potential interfences due to the presence of colloidal particle s. This work was conducted to study the influence of colloidal particl es on P concentration by measuring it both by the phosphoantimonylmoly bdenum blue colorimetry and ion chromatography in soil water extracts filtered at 0.45, 0.2, or 0.025 mu m. Results strongly suggest that di rect measurements with colorimetry might lead to an overestimation of the P concentration due to the solubilization of P-containing particle s in the acid medium prevailing in the solution during colorimetric es say. In the present study, the largest overestimations were observed i n soil extracts rich in iron (Fe) and aluminum (Al) that had been filt ered at 0.45 mu m, on the opposite this overestimation remained lower but constant for the three levels of filtration in the studied calcare ous soil. Ion chromatography constantly underestimated P concentration especially when the solutions were rich in Al and Fe. This underestim ation could not be related to the presence of dissolved polyvalent cat ions in the solution and, therefore, might be accounted for by the sor ption of P at the surfaces of colloids. In order to limit these interf erences, it is proposed to measure P concentration by colorimetry afte r 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 sol utions rich in calcium phosphate particles, it might be advisable to d etermine P concentration with ion chromatography.