DEVELOPING A KINETIC ALTERNATIVE IN MODELING SOIL ALUMINUM

Soil chemistry models often use gibbsite solubility and similar equili brium models to predict Al concentrations in soil solution. A kinetic alternative was developed with the goal of finding universal rate cons tants instead of the site- and depth-specific solubility constants usu ally associated with the equilibrium approach, The behavior of the two approaches was studied within the framework of the steady-state soil chemistry model PROFILE using data from Selling, Germany and Gardsjon, Sweden, two sites with different mineralogy and land use history. The kinetic alternative uses a mass balance to predict Al concentrations. The sources of Al in soil water are deposition, weathering and minera lization. The sinks are leaching and the formation of an aluminosilica te precursor. The precursor slowly transforms into an ordinary clay mi neral. Both formation and transformation of the precursor are treated as irreversible processes. The kinetic model introduces a new relation ship between pH and Al and produces a systematic pattern of different apparent gibbsite equilibrium constants at different depths. Results s how that the kinetic model systematically underestimates Al concentrat ion in the upper horizons, which indicates that there may be additiona l sources of Al in the upper horizons not accounted for in the model. Predicted values of pH and Al concentrations are comparable with field observations.