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| Soil colloids (clay and humus) have 2 different types of charge... pH dependent and permanent.
permanent charge occurs when the cations within a clay mineral do not balance all the negative charges from oxygen atoms... this phenomenon is called isomorphic substitution and causes some clays to be highly negatively charged (e.g., montmorillonite and vermiculite) regardless of pH.
Other clays like kaolinite have very little permanent charge.
Most of the charge on these clays (often called low activity clays) and all of the charge on humus is pH dependent and increases as pH increases (and decreases as pH decreases).
see attached diagram.
So... humus contributes very little to CEC under very acid conditions but is often the dominant source of charge under neutral to alkaline conditions.
hope this makes sense and is of interest to a few.
Joel
WIU Agriculture
 (pHdependentcharge.jpg)
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