Characteristics and properties of sorne Dyslrandepts in the Lake Region, southern zone of Chile. II. Charge and specific surface

Gloria Sepúlveda W.1 y Eduardo Besoain M.1

Three Dystrandepts of South Central Chile were studied. The total specific surface (TSS) seems to be strongly affected by the lyophillization procedure, when using Na as saturating cation, as well as by the remaining primary minerals in the clay fraction and the organic residues, after peroxidation treatment. These factors, together with the removal of extractable-Fe-oxides, influence the value of the zero point charge (ZPC). Cation exchange capacity (CEC) is strongly dependent on the equilibrium pH: for each unit of pH increase the CEC of the allophane increases by 10.6 meq/100 g. The shapes of the electric charge curves of allophane seems to confirm that, together with the pH dependent charge, there exists a small permanent negative charge, which could explain the adsorption of cations at low pH values. Since t depends on the CEC, the surface density of charge is directly proportional to pH and inversely to TSS. The SDC decreases while the content of Al203 of the minerals increases. Not being stable but circumstantjal, it is evident that the determination of CEC at pH 7.0, performed by the usual methods, is not adequate to characterize Andisois. For the same reason, the use of SDC is not adequate neither. Although with sorne lirnitations, the application of ZPC may be more useful.

1 Estación Experimental La Platina (INIA), CasilIa 5427, Santiago, Chile.