The goals of the study were to investigate the effects of the soil-water phase ionic strength, mainly monitored by the calcium ion (Ca2+) concentration, on the stability behavior of easily dispersed topsoil colloidal clay-sized particles (<2 mu m). The aggregation kinetics as a function of the Ca2+ concentration was monitored by measuring the increase of the particle size over time with photon correlation spectroscopy. The critical coagulation concentrations (CCC) of Ca2+ were measured. The Hamaker constants (A) characterizing the attractive chemical properties of the topsoil colloid surface were thus scaled according to the Derjaguin, Landau, Verwey, Overbeek (DLVO) theory by taking into account the electrokinetic behavior of the particles, measured by the zeta-potential. Effective values for the Hamaker constants Of topsoil clay-sized colloids, clay minerals, and metal oxides were calculated by referring to reported values for crystalline silica or sand (quartz) particles. Potential-energy diagrams of interacting topsoil clay-sized colloids were calculated. The primary energy maximum and secondary energy minimum were used for modeling the aggregation kinetics along the Ca2+ concentration by employing Marmur's model. Coagulation in the secondary energy minimum can only explain the aggregation efficiency of topsoil colloids at low Ca2+ concentrations (<2 mM Ca2+) under unfavorable electrostatic conditions. The effect of surface-associated organic matter on the colloidal electrosteric stability was also investigated by comparing the topsoil colloid stability after the removal of organic matter. (C) 2009 Elsevier Inc. All rights reserved.