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A simple equilibrium model for competitive binding of A1 and protons to soil organic matter shows that A1 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 A1 in several forest soils. Furthermore, we found evidence that pools of organically bound A1 may be depleted fairly quickly. Kinetically controlled dissolution of inorganic soil A1 compounds, which may be considerable, is the main cause for the re-supply of the organically bound A1 in the soil. The previously reported decrease in A1 solubility that accompanies measured decline in organically bound A1 in three Dutch soils was found quantitatively consistent with the model for organically bound A1 proposed here.

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The role of soluble Al in catalyzing the abiotic oxidation of phenolic compounds has been investigated by monitoring the slow oxidation of aerated aqueous solutions of catechol containing Al. Spectroscopic analysis of the various oxidation products has demonstrated that aqueous Al3+ increases the rate of oxidation of catechol by O2, favoring the formation of highly colored polymeric products which may possess charge‐transfer properties. Spectral (UV‐visible, nuclear magnetic resonance, infrared, and electron spin resonance) properties of these products suggest that they have some similarities to polymers generated by alkaline oxidation of polyphenols, but they also have properties which suggest that they are related to melanins generated enzymatically byu fungi and plants. A proposed low‐pH mechanism for the role of Al in promoting oxidative polymerization is based on the concept that Al3+ cations tend to stabilize o‐semiquinone radicals at low pH and direct the manner in which these radicals polymerize. Aluminum may also stabilize charge‐transfer complexes by bridging oxidized and reduced molecules. Preliminary evidence is presented to suggest that the surfaces of Al oxides in soils may, by a similar mechanism, catalyze the oxidation of phenolic compounds by O2.

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