ISOELECTRIC POINT OF SOILS DETERMINED BY THE DIFFUSION POTENTIAL METHOD

The diffusion potential method was used to determine the Isoelectric P oint (IEP) of the surface horizons of five soils (Typic Hapludoll, Typ ic Argiudoll, Vertic Argiudoll, Rhodic Kandiudalf and Lithic Kandihumu lt) in the following conditions: without treatment (natural), homoioni zed with K+ (K+-soil) or Ca2+ (Ca2+-soil) and after organic matter (OM ) removal. Soil samples were characterized through their mineralogy, t otal organic carbon (TOC), clay percentage, cation exchange capacity ( CEC), basic cations in the exchange complex, pH in water, Fe2O3 conten t and zero point of charge (ZPC). The IEPpH of soils gave the followin g order: Ca2+-soil > natural-soil > K+-soil. These results were explai ned in terms of the higher affinity of bivalent over monovalent cation s for negative surfaces. IEPpH of K-soils was dissimilar to ZPC(pH), i ndicating that K+ did not behave as an indifferent cation. Data of IEP pH after OM removal of Anguil (TH), Cepeda (VA) and Cerro Azul (LK) so ils were lower than those IEPpH of the same natural samples and higher for Castelar (TA) and Virasoro (RK) soils. Results could not be expla ined in a simple way. The IEPpH of natural soils correlated with ZPC(p H), pH in water and Fe2O3 and day contents. The diffusion potential me thod showed to be rapid, simple and adequate to measure IEPpH of natur al or treated soils. (C) 1998 Elsevier Science B.V. All rights reserve d.