Chemical species of Al reacting with soil humic acids

The formation and presence of aluminum tridecamer (Al₁₃, [AlO₄Al₁₂(OH)₂₄(H₂O) ₁₂]⁷⁺) in solution was strongly inhibited by soil humic acids (HA), as shown by liquid-state ²⁷Al-nuclear magnetic resonance (NMR) analyses. It was also observed that Al₁₃ coprecipitated with HA in the form of Al₁₃-HA complexes, as shown by solid-state magic angle spinning (MAS) ²⁷Al-NMR analyses. Tetrahedral Al in the Al ₁₃-HA complexes was gradually converted to octahedral Al, indicating that Al₁₃ in Al₁₃-HA complexes was decomposed into Al-HA complexes with reaction time. When HA was present in the acidic Al solution before the partial neutralization, the precipitates formed after the partial neutralization did not contain any tetrahedral Al, indicating the absence of Al₁₃ and that HA is a strong inhibitor of Al₁₃ formation. These results indicate that Al₁₃ is not formed from Al complexed with soil HA after partial neutralization. If Al₁₃ was added to soils, it would preferentially form precipitates of non-phytotoxic Al ₁₃-HA complex in the presence of excess HA (i.e. a COOH/Al ₁₃ molar ratio of more than 9) and gradually be converted to an octahedral Al-HA complex. Therefore, Al₁₃ is not a major plant-growth inhibitor in humus-rich acidic soils.