The mechanism of reaction of most anions, and of the heavy metal cations, with soil has not been fully understood. However, the observations can be closely described if it is assumed that ions react with variable charge surfaces, that the surfaces are not uniform, and that an initial adsorption is followed by a diffusive penetration. With these assumptions, it is possible to describe the relation between the amount sorbed and the solution concentration; the effects of pH, of electrolyte concentration, of time and of temperature on sorption; the slowness of desorption; and the effects of previous additions on the reaction with further additions.