SOM questions
On the SOM summary, it says that non-humic substance is a primary component and humic substances are secondary components. What does it mean to be a secondary component? Are humic substances created from the further breakdown of non-humic substances?
It also says that "at pH 4-6, 85-90% of carboxyl groups are dissociated". Does the dissociation further lowers the pH? Kinda like a positive feedback loop?
Thank you so much!
Non-humic substances are primary components such as carbohydrates, proteins, lipids, lignin. Primary in this context means that these components are derived from the organic residues/litter.
Humic substances are decomposition products, i.e., secondary components which have been microbially transformed. As indicated in the slide "SOM components - a summary" posted with lecture #8, micro-organisms through decomposition processes synthesize new organic compounds (i.e., secondary components)
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