This thesis is an investigation of the behaviour of phosphates (P) in southern Ontario agricultural soils with the development of a soil assemblage model. The model was used to predict P availability in soils with varying pH and soil texture. The model considers the aqueous speciation of P and the complexation of P onto two reactive surfaces: clay minerals and iron oxides. Clays were separated from four representative soils from southern Ontario and were used to develop the soil assemblage model. Goethite was chosen as the representative iron oxide and was synthesized in the laboratory. Clay and goethite proton and P binding constants were determined with potentiometric titrations and batch adsorption experiments. All data was analyzed with the program FITEQL. The model was tested with soils sampled from an agricultural field in Parkhill, Ontario, which were characterized for soil texture, pH, and calcium, magnesium, dissolved organic carbon, extractable iron and P content. Model predictions were most accurate for the sites with the lowest soil clay content and in general were accurate around the actual soil pH of the site soils.