SUMMARYThe pH of samples of a soil was altered by mixing them either with acid or lime, and incubating the moistened samples at 60°C for a day. The sorption of selenate and of selenite was then measured using as background electrolytes, 0.01 M, 0.1 M and 1.0 M sodium chloride and also 0.01 M calcium chloride. The results were compared with previous studies with phosphate and fluoride.Selenite was sorbed more strongly than selenate, but not as strongly as phosphate or fluoride. Sorption of both selenite and selenate decreased with increasing pH. This decrease was more marked for selenate than for selenite; more marked in a sodium system than in a calcium one; and more marked with a dilute background electrolyte than a concentrated one. Under certain conditions, the steeper curves for the dilute electrolyte crossed the curves for the concentrated electrolyte giving points of zero salt effect. For selenite, these points of zero salt effect occurred near pH 6 and the greater the sorption the lower the pH for zero salt effect. For phosphate, the analogous value was near pH 5. For selenate, if a point of zero salt effect occurred, it was at such a high pH and such a low amount of sorption that it could not be measured. Thus, the larger the amount of sorption the lower the pH for the point of zero salt effect. This generalization applied both within and between different kinds of sorbates.The results were closely described by a model that had previously been applied to phosphate and fluoride. The model postulates that ions react with charged surfaces. The electric potentials of the reacting surfaces are affected by the identity and concentration of the background electrolyte and this produces the interactions between pH and electrolyte concentration. The model also postulates that there is a distribution of electric potentials. Anions react with surfaces which occur in the more positive tail of this distribution. The smaller the amount of the reaction the more positive the potential of the reacted surface and, therefore, the higher the pH required to decrease this potential to zero