The goal of this investigation was to establish a theoretically applicable model of, and develop an approach for, the direct measurement of cadmium loading capacities under laboratory conditions. Data from the literature were fitted to a model indicating a direct, linear relationship between the logarithm of the activity of Cd2+ in the soil solution and the logarithm of plant uptake of cadmium. These results support the model for passive absorption of cadmium by plants. It was postulated that the diffusion of Cd2+ from the soil-adsorbed phase to plant roots results from the electrochemical potential gmCd, gradient established between soil-adsorbed Cd2+ and that in solution, which is depleted as a result of passive absorption by plants.
Laboratory methods for relating the negative logarithm of the activity of Cd2+ in solution, pCd, to soil-adsorbed cadmium and plant-absorbed cadmium were developed, and test results for each are presented. It was concluded that this approach is theoretically valid and will provide routine laboratory methods for measuring soil loading capacities to protect the human food chain from undesirable concentrations of cadmium.