Aggregation and Dispersion Behavior in the 0‐ to 2‐ µm Fraction of Luvisols

The migration of particles smaller than 2 µm in soil pores is responsible for the preferential transfer of various contaminants and for soil textural differentiation 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‐crystallized clay minerals rather than pedogenetic particles and have often been performed under physicochemical conditions, which are poorly representative 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 concentrations and have identified mechanisms associated with the formation of these different phases. We find 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 determine the expected role of these mechanisms on the transport of particles in gravitational soil water.