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Re: The purpose of Laterite and CEC (long)

I got an email message from Christopher Coleman in response to something I
briefly mentioned on the APD. He suggested that I respond on the APD, so I
repeat his mail for context and then provide my reponse below. I certainly
invite other reponses.

At 04:05 AM 4/29/98 -0400, Christopher Coleman wrote:
>In the recent APD thread entitled "Fe2+ / Fe3+" concerning 
>absorbion of iron by the water column and/or substrate, you 
>mentioned that (Apr 23) that you felt the long term purpose 
>of Laterite was to serve as a bonding site for nutrients
>entering the substrate but that to date the only nutrient
>you were aware of that Laterite served in this capacity was
>P. I may have misunderstood your context so I'll include
>the post here:
>  George Booth Wrote:
>  >> I think the long term purpose of laterite in 
>  >> the substrate is to act as a bonding site for 
>  >> nutrients brought into the substrate by whatever
>  >> circulation is provided. 
>  (Your response)
>  > Yes... mostly P. I have not seen anything written 
>  > to mention any other nutrient.
>I'd like to preface by indicating I found that thread of great
>interest. You will recall it began with a question by Steve
>Pushak that possibly Laterite was not a significant source of 
>iron to plants and that only chelated iron forms from 
>Dupladrops, etc. were significant. (And further we could test
>this by having George Booth skip a couple of weeks of 
>Dupladrop supplementation!)
>>From what I have learned of Laterite soils their primary
>long term purpose is to serve as bonding site for nutrients
>through the CEC mechanism. My confusion from your post is that
>the only beneficiary nutrient would be P. I thought from my 
>understanding that  _any_  positively charged nutrient could be 
>bonded, including fe++.
>Perhaps you could explain. Please also feel free to post this
>to APD. 

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.  

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.

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.

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

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

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.  

I would be interested in hearing from the limnologists and aquatic
botanists and others who are on this list. 

Neil Frank,
Aquatic Gardeners Association