Fulvic Acid: CEC and acidity

Fulvic acids have higher density of functional groups per molecular weight relative to humin.
Functional groups on organic compounds are the places where negative charge is formed. Hence, higher number of functional groups on fluvic acid will lead to higher number of negative charges (once those functional groups are deprotonated), which in turn will lead to higher CEC relative to humin.

I also had the same question. I understand now that fulvic acid have higher CEC because it has high number of carboxyl groups which dissociates and carry negative charges. However, I don't really understand why increasing acidity corresponds to increasing CEC (which is stated in lecture 17, slide 6). Unless increasing acidity does not refer to lower pH. Thank you!

Aini, fulvic acids have a high number of functional groups per unit mass (e.g. R-COOH) which when dissociated and release H+. In other words, functional groups on organic compounds are places where charge will form. Thus SOM high in fulvic acids have a large exchangeable acidity.