Fig. 4. Solubility diagrams for (a) first segment, first experiment, (b) second segment, first experiment, (c) first segment, second experiment, (d) second segment, second experiment. Parentheses indicate activities. Al³⁺ activities calculated from total Al concentrations using ECOSAT, accounting for AlSO⁺₄ and CaSO⁰₄ complexes. Error bars are for log(SO^2-₄) - 1 and log(SO^2-₄) + 1 respectively. C = Control, L = CaCO₃, LG = CaCO₃ + gypsum, n = no compost, c = 68.8 g kg^-1 compost.