Fig. 3. Predicted activity of Al³⁺ and Al complexes (a_i) as a function of depth in soil extracts. Activities were modeled using data from water extracts input into Visual MINTEQ. The insert graphs show the percentage of total Al (Al_T) present as Al³⁺ or organically complexed Al as a function of pH for each treatment; • = Al³⁺, = fulvic acid (FA) as FA₂Al⁺, = FA₂AlOH, = AlSO₄⁺, = AlHPO₄⁺. Note: The 20- to 30-cm depth interval of the broadcast S treatment values are based on one sample. Concentrations of Al_T were below the detection limit of the inductively coupled argon plasma–atomic emission spectroscopy for the other replicates.