Relationship between CEC and pH

Soil pH has an effect on cation exchange capacity (CEC) of soil colloids that have pH-dependent type of charge (ie soil organic colloids adn sesquioxides). During yesterday's review session, I went over the graph from your textbook (I believe from the page 257) that relates soil pH and CEC. I cannot copy the graph here, but it is included into the final exam from 2005 posted on "Old exam" page.

The explanation of that graph is as follows:

Below pH 6 the charge for the clay mineral is negative and relatively constant. This charge is considered permanent, since it is formed as a result of the isomorphic substitution during mineral weathering. Permanent type of charge on montomorillonite (or any other phylosilicate) does not change under the influence of soil pH. Above pH 6 the charge of mineral colloid increases slightly because of ionization (deprotonation) of hydrogen from exposed hydroxyl groups at crystal edges. Note – these exposed hydroxyl functional groups are responsible for the formation of small portion of the pH-dependant charge on smectite (montmorillonite) clay minerals. In contrast to clay (i.e. montmorillonite mineral), essentially all charges on organic colloid are pH-dependant and as such are under the influence of pH of the surrounding soil solution. Consequently, as soil pH increases, ionization (deprotonation) of hydrogen from functional groups (e.g. carboxyl, phenolic hydroxyl, alcoholic hydroxyl) on organic colloid increases (i.e. number of negative charges on the surface of organic colloids increases) resulting in the increase of the cation exchange capacity.

If you need more explanation on the relationship between soil pH and CEC, pls refer to your textbook.