Availability to plants of phosphate adsorbed on goethite: experiment and simulation

A model has been developed to predict the availability of sorbed phosphate to plants. It is based on uptake by a growing root system. Phosphate sorption is assumed to be reversible and the rate of desorption is assumed not to limit uptake. The model was tested against observed availability of sorbed phosphate estimated from uptake by plants growing in phosphorus-deficient conditions. The plants were grown on quartz sand coated with goethite on which phosphate was adsorbed, and which was mixed with nutrient solution. The influence of the phosphate loading of goethite and the goethite content of the sand on available phosphate was examined. Phosphate uptake was predicted well by the model for treatments with high phosphate loading and different goethite content of the sand. However, for a treatment with low phosphate loading, uptake was underestimated by the model. In this treatment the pH of the 'soil' solution decreased from 5.5 to 4.2 during the experiment. The phosphate concentration in solution increased with a lowering of the pH in this pH range, resulting in increased phosphate uptake.