Factors affecting soil phosphate distribution and reactions in cape forest soils

The amounts of various forms of phosphate in some Cape soils and the adsorption of phosphate by these soils were investigated. The levels were low and only 2% was in available form. The major fraction of phosphate in the soil was organic phosphate. Aluminium phosphate and iron phosphate were the next largest fractions. Aluminium phosphate concentrations were related to soil organic carbon, and iron phosphate concentrations to clay and silt content. Adsorption of phosphate was low to moderate and was correlated to organic carbon, pH in sodium fluoride and fine silt content. In these soils, exchange sites associated with the fine silt fraction seem to be more important to phosphate adsorption than those associated with the clay fraction. The amount of phosphate fertiliser calculated from buffering capacity was lower than the current recommended rate for most of the soils tested. From these studies fixation of phosphate does not appear to be a major problem in plantation soils in the Cape regions; loss of phosphate owing to leaching from coarse textured soils is likely to be a greater problem. © 1989 Taylor & Francis Group, LLC.