Soil Science Society of America Journal Soil Sci. Soc. Am. J. 79:43–54 doi:10.2136/sssaj2013.12.0538 Received 20 Dec. 2013. *Corresponding author (labille@cerege.fr). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Aggregation and Dispersion Behavior in the 0- to 2-mm Fraction of Luvisols Soil Chemistry The migration of particles smaller than 2 mm in soil pores is responsible for the preferential transfer of various contaminants and for soil textural differ- entiation in Luvisols. Aggregation vs. dispersion mechanisms of clay particles are suspected to play a major role in this migration process. However, these mechanisms have mostly been studied with respect to pure and well-crys- tallized clay minerals rather than pedogenetic particles and have often been performed under physicochemical conditions, which are poorly rep- resentative of soil conditions. We studied the respective impacts of pH and Ca concentration on aggregation and dispersion behavior of clay particles in a Luvisol under conditions encountered in the soil solution. Both static and dynamic approaches were followed in studying particle interactions and dynamics under transient phenomena. Based on these experiments, we have drawn a phase diagram for soil clays as a function of pH and Ca con- centrations and have identiied mechanisms associated with the formation of these different phases. We ind that soil particle behavior in suspensions is similar to that recorded for model clays in that they are driven by both pH and Ca concentrations. These two parameters are interrelated and tend to favor aggregation at higher Ca concentrations and/or lower pH. These effects are reversible over the gravitational water time scale, with the exception of dilution-induced dispersion. In situating these physicochemical mechanisms within a literature review of rainwater and soil solution chemistries, we deter- mine the expected role of these mechanisms on the transport of particles in gravitational soil water. Abbreviations: CCC, critical coagulation concentration; I, scattered intensity; q, wave vector; SEM, scanning electron microscopy. P article migration in soil is responsible for: (i) the preferential transfer of vari- ous contaminants bound to surfaces (Amrhein et al., 1993; de Jonge et al., 1998, 2004; Jacobsen et al., 1997; Laegdsmand et al., 1999; Ryan et al., 1998), notably pesticides, microbes, pathogen viruses, and heavy metals and (ii) lessivage— or argilluviation—which is deined as signiicant particle migration (Mercier et al., 2000) from a departure upper soil E-horizon (eluviated) to an accumulation Bt-horizon (illuviated) at the subsurface at the pedogenesis time scale. he latter phenomenon is the major pedogenetic process for Acrisols, Alisols, Albeluvisols, Lixisols, Luvisols, and Solonetz formations of the World Reference Base (WRB) soil classiication (Bockheim and Gennadiyev, 2000; FAO, 2006). It also serves the pedo- genetic process involved in the formation of Argid of Aridosol subgroups, Molisols, Oxisol kandic groups, and Spodosol alic subgroups of the soil taxonomy classiica- tion (Bockheim and Gennadiyev, 2000; Soil Survey Staf, 1998). Romain Van Den Bogaert INRA UR 1119 Géochimie des Sols et des Eaux F-13100 Aix en Provence France Jérôme Labille* Aix-Marseille Univ. CNRS IRD CEREGE UMR 7330 F-13545 Aix en Provence France Sophie Cornu INRA UR 1119 Géochimie des Sols et des Eaux F-13100 Aix en Provence France Published January 13, 2015