Chemical and charge characteristics of kaolinitic soils of south-east Queensland

Abstract

Cation exchange capacity at field pH and net negative charge at pH 8.5 of 39 kaolinitic soils were studied in relation to surface area and other soil characteristics. All soils exhibited an anion exchange capacity at field pH, but this was very low compared with cation exchange capacity. Little evidence could be found for the presence of amorphous aluminium or silica, or short-range order alumino-silicates on the basis of acid oxalate-extraction, in contrast to Tiron extraction. All samples had significant permanent negative charge as deduced using the method of Gillman and Uehara, in addition to pHdependent charge. The former was strongly related to the exchangeable divalent cations, and this may explain the great variation in base saturation observed. Most of the charge in these soils is derived from the small poorly crystalline kaolins, although at high pH oxalate-extractable iron contributes significantly to the negative charge. Based on the results for those soils containing less than 5% iron oxide, surface area determined by ethylene glycol is linearly related to the net negative charge at pH 8.5. The average surface charge density on the soils containing <5% hematite and goethite measured at this pH value is 0.9 nm2/charge. Hematite and goethite show much lower negative charge densities and effectively dilute the active surface.