Relationship between pH and CEC

Soil pH only has an effect on CEC of the soil colloids (organic matter, sesqui oxides) that have pH-dependent charge.

Here is an example how a sesquioxide may develop a pH-dependent charge: Al(OH)3 + H+ Al(OH)2+ + H2O

A clay-size particle of hydrous aluminum oxide has a positive charge under acid conditions (as indicated in the reaction above). Such a particle being a part of the soil matrix, rather than a cation in solution, contributes to the soil’s anion exchange capacity (AEC). At sufficiently high pH, sesquioxides may even contribute to cation exchange capacity, by acquiring extra hydroxyl groups: Al(OH)3 + OH- Al(OH)4-

However, most soils in which sesquioxides are abundant are naturally acidic, so they are contributors to AEC.