Abstract The retention of phosphate in soils is to a large extent determined by insoluble metal-oxides. In this paper, model studies are described on the uptake and release of phosphate by gibbsite (γ-Al(OH) 3). At low phosphate concentration c p , the uptake is a pure monolayer exchange adsorption on the edge surfaces of the gibbsite platelets. At high c p and long reaction times, a precipitate is formed on top of this adsorbate. Kinetic equations are given to discriminate between these processes. Our analysis leads to arguments in favour of applying slow-release phosphate fertilizers. In addition we analyse the effect on the precipitation of some ionic admixtures such as K +, Na + and NH + 4 that may be inadvertedly or intentionally present in the soil.