A simple model of soil organic matter complexation to predict the solubility of aluminium in acid forest soils

A simple equilibrium model for competitive binding of Al and protons to soil organic matter shows that Al activities in soil solutions of acid mineral soils are controlled by complexation reactions with soil organic matter. The model successfully explains the relation between pH and the activity of dissolved Al in several forest soils. Furthermore, we found evidence that pools of organically bound Al may be depleted fairly quickly. Kinetically controlled dissolution of inorganic soil Al compounds, which may be considerable, is the main cause for the re-supply of the organically bound Al in the soil. The previously reported decrease in Al solubility that accompanies measured decline in organically bound Al in three Dutch soils was found quantitatively consistent with the model for organically bound Al proposed here.