Fig. 4. Soluble P_i in reaction mixtures in the absence and presence of Fe or Al ions (3.22 mM each). Each reaction mixture contains 0.322 mM P. Pi, KH₂PO₄; IP6, phytate; EHa, Elliott soil standard humic acid; EFa, Elliott soil standard fulvic acid; WHa, Waskish peat reference humic acid; WFa, Waskish peat reference fulvic acid. Values are averages ± standard deviations (n = 6).