EFFECTS OF ORGANIC MATTER AND IRON OXIDES ON CATION EXCHANGE EQUILIBRIA AND POTASSIUM SELECTIVITY IN A VOLCANIC ASH SOIL OF CHILE Itilier Salazar, 1, * Mauricio Escudey, 2 Julio Guajardo, 1 and Keith W. T. Goulding 3 1 Departamento de Ciencias Quı ´micas, Facultad de Ingenierı ´a, Ciencias y Administracio ´n, Universidad de La Frontera, Casilla 54-D, Temuco, Chile 2 Departamento de Quı ´mica de los Materiales, Facultad de Quı ´mica y Biologı ´a, Universidad de Santiago de Chile, Av. B. O’Higgins 3363, Casilla 40, Correo 33, Santiago, Chile 3 Soil Science Department, IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2IQ, UK ABSTRACT The volcanic ash soils of Chile are naturally acidic, of variable charge, relatively rich in organic matter (OM) and contain little reserves of potassium (K). Ion exchange controls the availability of nutrient cations such as K and is strongly influenced by the OM 3663 DOI: 10.1081/CSS-120015914 0010-3624 (Print); 1532-2416 (Online) Copyright q 2002 by Marcel Dekker, Inc. www.dekker.com * Corresponding author. E-mail: itilier@ufro.cl COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS Vol. 33, Nos. 19 & 20, pp. 3663–3677, 2002 ©2002 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc. MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016