Relationships between phosphate sorption and iron oxides in Alfisols from a river terrace sequence of Mediterranean Spain

Abstract

Phosphate sorption isotherms for samples of the A- and upper B-horizons of Alfisols situated on a sequence of terraces of the River Guadalquivir, southern Spain, showed that phosphate sorbed at an equilibrium concentration of 0.3 μg P ml −1 was correlated with several soil properties. Crystalline Fe-oxides (goethite and hematite) appeared to be the most important P-sorbing components of the soil samples. Sorption was highly correlated with percent clay and with dithionite soluble Fe. This last property was as good a property to predict sorption as the specific surface of crystalline Fe-oxides (which was estimated by line profile analysis of the X-ray diffractograms), probably because the range of surface areas for goethite and hematite was relatively narrow in the samples and because of the limited accuracy of the method of line profile analysis. Well drained and imperfectly drained soils differed little in sorption properties. In imperfectly drained, hematite-free soils, chroma was correlated with sorption. This is promising for it would allow rapid field estimation of sorption. The sorption maximum, calculated from the fitted Langmuir isotherm, was about 1.7 μmol P m −2 of Fe-oxides, a value similar to those reported in the literature for natural and synthetic oxides.