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.
All Science Journal Classification (ASJC) codes
- Inorganic Chemistry