Fig. 4. A comparison of equilibrium values of Al³⁺ activity [(Al³⁺)] from 1:1 water extracts (data points) with theoretical values predicted from Al(OH)₃–controlled (Al³⁺) (lines). Equilibrium expressions were derived from Visual MINTEQ Version 2.32 thermodynamic database. Points falling above the equilibrium line are supersaturated with respect to the mineral while points falling below the equilibrium line are considered undersaturated with respect to the mineral. A log (Al³⁺) of approximately –6 corresponds to the level where Al phytotoxicity has been expressed by many crops (dashed line).