Relationship between pH and CEC

Hello Maja, I was wondering if you could explain the relationship between the pH and the CEC. Thank you.

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.