SOM questions

Lecture #19 further discusses SOM, in particular pH dependent charge and the dissociation of functional groups. Note that the dissociation of functional groups occurs over a range of pH. As you indicate, carboxyl groups (R-COOH) are largely dissociated over the range of pH common in soils.
You are correct that the dissociation of OH groups contributes to the acidic nature of organic soils (e.g. wetlands), however the relationship with pH is complex. The important thing to remember is that as the pH increases (i.e., more OH^- in soil solution), the greater the dissociation of R-COOH groups, increasing the net negative charge and thus increasing the CEC. Consider:
R-COOH + OH^- goes to R-COO^- + H₂O