confusion regarding carboxyl group dissociation on SOM and CEC

Hey, Not sure about the first question... Hope Sandra will clarify our doubts.

but for the SOM CEC thing, my understanding is： when the soil pH is high the functional groups dissociates (R-COOH + OH- --> R-COO- +H2O), giving SOM a negative charge. So this makes the soil CEC high (because of the negative charge the soil colloid can hold more cations).

When the pH is low, R-COOH doesnt dissociate (and dissociated R-COO- reacts with H+ --> R-COOH). This makes the CEC low because now the soil isnt charged and it cant hold as much cations.

MelodyFu (talk)‎

Excellent Melody re: pH dependent charge

SandraBrown (talk)‎

thanks! but where does the OH- in the first equation come from?

Katrine‎

The OH- from the first equation comes from:

In soil, when compounds containing base forming cations (ie. fine minerals) meets water, they will hydrolyze:

Na2CO3 + H2O --> 2Na+ + HCO3- + OH-

The Na+ then binds to the soil cation exchange sites. OH- is what makes the soil alkaline. This OH- then reacts with the soil colloid functional group (R-COOH) to make R-COO- + H20 (first equation)

MelodyFu (talk)‎