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Re: Laterite Definition for Newbies
On Mon, 25 Jan 1999 Coline Anderson wrote:
> All I gather from this is that we can't depend on the kaolinite portion
> 'often' found in laterites to deliver any iron exchange capability.
Or ion exchange capacity. Iron isn't usually involved in ion exchange
> Question: What ions can Kaolinite colloids exchange? Can iron be
> exchanged by Smectites or Illites? Another question on clays and
> colloidal properties--Is cation exchange capacity really and effective
> and/or accepted term for emersed clays? It seems to me that CEC is a
> term derived for the description of non-saturated soils.
Exchange reactions are usually considered for H+, Li+, Na+, K+, Rb+, Cs+,
NH4+, Mg++, Ca++, Sr++, Ba++ and Al+++. Under aquarium conditions we
could probably limit the list to H+, Na+, K+, NH4+, Mg++, Ca++. I think
that potassium (K+) has very little tendency to get involved in exchange
reactions, but I included it because of its importance as a nutrient. H+
also plays only a small role in exchange reactions unless the water is
fairly pure (like rain water).
The list is the same regardless of whether you're talking about kaolinite,
smectite (=montmorillonite=bentonite) or illite. Kaolinite has a low
exchange capacity, but that doesn't change the list of ions that it might
exchange, only the amount of the ions it can hold.
The exchange capacity and the somewhat more obscure values called
"selectivity coefficients" are variable and usually if they're important,
then they have to be measured for whatever clay or soil is being used.
Generalized properties are of mostly conceptual value.
CEC is a valid measure regardless of whether the soil is saturated or