The toxicity of dissolved aluminium to many plant and animal species is one of the major deleterious effects of the acidification of the environment. Al mobilization from insoluble forms in minerals and rocks is governed mainly by the pH of the solution surrounding weathering minerals. To a first approximation, the solubility product of gibbsite gives an indication of Al concentrations expected in solution. Secondary silicate minerals such as kaolinite, imogolite and allophane, and sulphate minerals such as alunite, maintain lower Al concentrations in solution than does gibbsite, particularly at pH 5. The cation exchange system of acid soils provides a large reserve of ionic Al, which can be brought into solution when soluble salts percolate through soils. Ligands such as fluoride and organic anions, which form soluble Al complexes, combine with much of the Al in solution when they are present in suitable concentrations, and therefore maintain higher concentrations of Al than might be expected from mineral solubility equilibria, particularly at pH 5–7.
Slow reactions involving solid phases, and uncertain data for possible Al species present, limit the rigorous application of equilibrium thermodynamics to these systems, but the factors mentioned provide a reasonable understanding of the major features of aluminium mobilization.