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Re: The purpose of Laterite and CEC (even longer)



I followed the recent thread on iron with some interest, but I don't
recall posting on it at all.  The chemistry of iron under natural
conditions is complex compared to the chemistry of many common solutes and
it has biochemical roles that leave me with more questions than answers. 

> 
> The ability of colloids in a soil to adsorb cations is called the
> cation-exchange capacity (CEC). In theory, any positively charge ion could
> be attached to a negatively charged particle. 
> In their order of attraction, the cations usually included are:
> 
>         Al > Ca > Mg > NH4 = K > Na
> 
> Among these, four are considered to be nutrients. However,
> Ca, K and Mg are needed in the water column by the plants, so there may be
> limited added value to have it attached to substrate particles. The last is
> ammonium which is going to be sucked up quickly by the plants or converted
> NO3. I don't know if detritrification creates NH4 (I saw this on APD, but
> always thought that the endproduct was N2). So that leaves Al which is not
> a nutrient and in fact can be toxic and Na which may be a substitute for
> plants when K is unavailable.  
> 

Many sources show the list as you show it here.  I've also seen hydrogen 
ion included in the list, but hydrogen ion is typically present at very 
low concentrations (0.1 part per billion at pH 7), so its position on the 
list may not be very important.

> Cu, Zn and other metals can occur as cations. I don't know if these are the
> nutrients which need to be attracted to substrate particles, because they
> tend to form insoluble precipitates which will keep them out of the water
> anyway. When added as an aquarium fertilizer, they are chelated which
> allows them to be used in the water column.

These metals also may be strongly complexed by "humic" substances and 
by bicarbonates.  In either case their bioavailability is reduced 
compared to the uncomplexed ion.

> 
> If you ever inspect Dupla laterite, you will notice that it does not
> consist of ultra fine clay particles... it is more granular. This is good
> and may be the reason that it settles out quickly and does not remain as a
> colloid like other clay. Regardless of particle size, I have tested Dupla
> and other laterite and they been shown to have very low CEC. If CEC was
> going to be important for an aquatic substrate, then it would be more
> relevant for clay- and organic-soil.
> 

I think that the pure iron hyroxides in laterite probably would only 
have a CEC as the result of chemical accidents in their rather loosely 
knit crystal structure.  But there are mechanisms by which iron hydroxides 
with no native CEC can acquire a CEC.

The iron compounds that comprise much of laterite (sometimes abbreviated
"fo", for ferric oxyhydroxides) have a a chemically active surface; many
molecules bind to different kinds of sites available on the surface of the
fo.  Phosphate is an example (probably the best known example) of a solute
that forms a strong "surface complex" by binding with the fo.  There are 
others.

When a negatively charged ion like phosphate (or hydrogen phosphate, or
dihydrogen phosphate) binds to the fo the attached ion still retains its
negative charge.  Now the surface complex formed by phosphate and fo has
all the characteristics necessary for an ion exchanger:  it's a solid with
a net negative charge that can be satisfied by attracting positively
charged ions in the water.  In the case of a phosphate-fo surface 
complex, the CEC would also be pH-sensitive, increasing as the pH goes up 
and the hydrogen phosphates disassociate from H2PO4- to HPO4-2 to PO4-3.

> So, IMHO, particle attraction is not the purpose of laterite (or soil).
> Moreover, I think the entire subject of CEC for aquarium substrates may be
> overblown. Call it APD heresy and let me be the first to be burned at the
> stake!!!
>
> I believe that the major advantages for laterite (and other iron bearing
> aquarium substrate materials like 'clayey' soil) is to be an INITIAL source
> of iron (and possibly Mg) and to be a long term sink for phosphates.
> Laterite, soil and aquatic sediments will chemically bind with phosphate.
> This has been demonstrated. Chemical bonds are different than electrostatic
> attraction. So, P may be THE nutrient which needs to be brought down into
> the substrate. Even without substrate circulation, phosphates can
> precipitate and end up in substrates with Fe (this happens in aquatic
> sediment). IMHO, everything else about the function of laterite is less
> certain. 

Probably also quite variable with the laterite source, details of its 
composition, its time in the tank and a few other things as well.

I too have to wonder about how sweeping the importance of CEC really is to
aquatic plants.  I can imagine that their CEC dependence should vary
greatly.  A plant that can obtain most of its nutrients directly from the
water (in the fashion of algae) could be largely independent of the
substrate characteristics, even if the plant normally grows rooted.  On 
the other hand, a plant that might normally grow rooted underwater but 
with most of its top out of the water (like many Echinodorus) might not 
be well adapted to getting nutrients out of the water column and so be very 
dependent on being able to get nutrients exchanged on substrate materials.

Looking at the plants I grow, I expect that most of my crypts and
echinodorus and probably H. corymbosa would depend on their roots
supplying the plant from the pool of nutrients concentrated in the
substrate by CEC.  H. polysperma, Val, egeria, and cabomba I expect would
be less dependent on it.  Of course Java fern, Bolbitis, hornwort and
duckweed aren't rooted in the substrate and might actually prefer a tank 
where nutrients aren't being concentrated in the substrate. 

> Even with water slowly moving thru the substrate, I would imagine that the
> cations have the difficult choice of staying near the negatively charged
> substrate particle when they can disperse/diffuse into the water column. It
> seems to me that cations have different choices in the aquatic environment
> (but this admittedly is speculation on my part). I note that most
> everything written about CEC applies to terrestrial or agricultural soils.
> Here, the nutrients (including Ca, Mg, K, NH4 and trace elements) need to
> stay put and not get "washed away" from rain water, etc. I imagine that
> this situation may be entirely different in a permanently submerged
> substrate which is under a large body of water.  

Cations remain associated with the ion exchanger even when fully saturated
with gradually flowing water.  The exchanger and their associated cations
form what is sometimes called the "electrical double layer".  The
electrical double layer is responsible for much of the electrical
conduction of sediments containing fresh water.  Ion exchange in flowing
groundwater also has important effects on the chemical composition of
water moving underground for long distances.  Water softening by natural
ion exchange is the reason why my tap water contains over 100 mg/l of
sodium, but only about 7 mg/l of calcium. 

The importance of CEC is not just that it binds nutrients to the soils,
but that it creates concentrations of nutrients near the soil particles
that are much higher than the nutrient concentrations in the rest of the
space between soil particles.   

It's too late in the day for me to get to the data now, but I may be
able to show at least for a laterite-free substrate how different
nutrients will tend to be found in the water, in substrate pore space and
in association with exchange media in a substrate.  It will probably be a 
few months before I can repeat the same exercise for a substrate 
containing laterite.
 

Roger Miller