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Cation Exchane Capacity and Planted Aquarium Substrate

Discussion in 'Talk to Tom Barr' started by Gautam, Dec 19, 2008.

  1. Gautam

    Gautam Prolific Poster

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    Hi Tom and Plant Guru Team

    I have been wondering over last few months to come to a firm conclusion on CEC and it's role in choosing a good planted aquarium substrate.

    Here are some of my doubts.

    1> What is the precise role of CEC? Why is it important? All specialist that I have come across including Tom himself has suggested to go for substrate with high CEC. Is high CEC important because that indicates that the particular soil has better nutrient holding capacity?

    2> I read Tom's report on Sediment Analysis. SMS and ADA AS both have high CEC. But then on basis of CEC alone which substrate should be chosen SMS or ADA AS?

    3> Should CEC be the only measure of selecting a substrate? What other parameters should one consider while choosing a subsrate?

    4> I understand that Laterite too has got high CEC. What are the advantages or disadvantages of using laterite only as a subsrate. It seems that it is advisable to mix laterite with other natiural material like clay. Why is that?

    5> Is ADA AS a recommended substrate by almost all experts as it has a high CEC capacity or beacuse it has got other advantages too

    I am not an expert in chemistry and my questions might sound foolish. I request you to excuse my ignorance and help me clear my doubts
     
  2. Tom Barr

    Tom Barr Founder
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    CEC, and in a way, AEC, are simply the ability to hold to the ions, then the plants' roots use them from the particles as nutrients.

    Some important aspects:

    How do plants extract the nutrients that are bound to the particles?
    This has to be done somehow obviously. Many over look this.
    Total "capacity" may not be as as good as Biological extraction capacity.
    Texture plays a role, as does the grain size.

    But CEC is good for holding a lot more nutrients than say...plain silica sand.
    That holds none pretty much, just the accumulated mulm in there, not the sand itself.

    A clay can have many layers of CE and AE sites, and if it's semi soft, roots can still get at it. Something harder like EC or Flourite, while less messy and tougher, does not have the sites available as much for roots even if the CEC was the same. This might be fine for some like bacteriam but less so for plant roots. You can see where the roots bore into the grains with root hairs for most any high CEC sediment. Not with sand.

    After the plant root extracts the nutrient, then the site is available to re attach more nutrients(this requires a source of those nutrients- CEC is not a source, it's just a place where exchange occurs)

    When you uproot, then you pull a lot of that up, and then replant, in a few days, weeks, months etc, the roots re invade that area, and hopefully by then, the grains have had some time to acquire more nutrients at the CEC sites.

    That's the general idea anyway.

    I would not rely too heavily on CEC in and of itself. Aesthetics, nutrients/plant availability of nutrients play big roles. Also, how the nutrients are locked and folded into clays, where roots can get them, but where they are not exposed to the outside water.

    Realtive to the amount you use for ADA AS or SMS etc, the total CEC for laterite is very small. I'd not even bother with that. I never found that much utility with Laterite truth be told. Mulm +peat did better. But I also added plenty of Fe to the water also.

    Regards,
    Tom Barr
     
  3. Gautam

    Gautam Prolific Poster

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    Dear Tom,

    Thanks.

    Can we then say:

    1> ADA AS is good because not only does it have good CEC so that it can replenish the nutrient uptake by plants by binding the same from water column and also it is high in nutrients which are not exposed to water column but exposed to roots only

    2> Laterite has CEC but its main importance is it's source of iron. So if we go for peat + mulm we are getting high CEC as well as nutrients. The only thing missing is Fe which you are supplying thru water column fertlization

    Regards,

    Gautam
     
  4. Tom Barr

    Tom Barr Founder
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    Pretty much.

    Regards,
    Tom Barr
     

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