COMPARISON OF DIFFERENT MODELS FOR PHOSPHATE SORPTION AS A FUNCTION OF THE IRON AND ALUMINUM-OXIDES OF SOILS

Phosphate sorption on topsoil and subsoil samples from different soils located in the eastern part of Germany was studied. Two models were fitted to sorption data obtained after 4 and 40 d of gentle shaking. The models differ with respect to the fractions of iron and aluminium (hydr)oxides that are considered and whether the phosphate initially sorbed in the soil is taken into account. Oxalate-extractable P, (P(ox)), appears to be a major part of the total soil P. The total P sorption measured, F, was predominantly related to the amounts of amorphous iron (Fe(ox)) and aluminium (Al(ox)). A significant relation between crystalline iron (Fe(d)-Fe(ox)) and total P sorption was not found. Reversibly adsorbed phosphate (P(i)), measured after 40 d reaction time, was a function of clay content and content of amorphous iron and aluminium (hydr)oxides.