AVAILABILITY TO PLANTS OF PHOSPHATE ADSORBED ON GOETHITE - EXPERIMENTAND SIMULATION

A model has been developed to predict the availability of sorbed phosp hate to plants. It is based on uptake by a growing root system. Phosph ate sorption is assumed to be reversible and the rate of desorption is assumed not to limit uptake. The model was tested against observed av ailability of sorbed phosphate estimated from uptake by plants growing in phosphorus-deficient conditions. The plants were grown on quartz s and coated with goethite on which phosphate was adsorbed, and which wa s mixed with nutrient solution. The influence of the phosphate loading of goethite and the goethite content of the sand on available phospha te 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, upta ke was underestimated by the model. In this treatment the pH of the 's oil' solution decreased from 5.5 to 4.2 during the experiment. The pho sphate concentration in solution increased with a lowering of the pH i n this pH range, resulting in increased phosphate uptake.