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Abstract
Soil acidity related parameters like pH, extractable and soil organic matter bound Al, amorphous Al phases and Al substitutions in secondary iron minerals were determined on 57 samples from 15 representative soil profiles from the Lusikisiki area. The pH (H2O) in the uppermost soil horizon ranged from 5.20 to 6.92 and was poorly (R2 = 0.31) related to the annual precipitation. Acidity was derived almost exclusively (R2= 0.99) from extractable Al, which increased exponentially with decreasing pH. Similar pH values, however, could be associated with widely contrasting Al contents. Elevated extractable Al proportions were closely related (R2 = 0.83) with the presence of primary 2:1 phyllosilicates. Soil organic matter may contain significant amounts of Al, which is more closely related to the organic carbon content (R2 = 0.58) than to the pH (R2 = 0.16). Aluminium associated with poorly ordered mineral phases varied between 3.44 and 23.42 (cmol kg−1) and generally increased towards the top of each profile. Percentage ratios of SiO2/Al2O3 in the A horizons ranged from 1.2 to 0.19, while in the lower parts of the profiles more Al-rich compounds dominated. The Fe/Al ratios of secondary Fe minerals in the A horizons varied between 1.82 and 8.74 and indicates that goethite and hematite may be a major sink for Al.