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Re: Plenums, slurries, laterite and such.

Tom Aquatect2 at aol_com writes:

> Steve wrote several good comments regarding the use of plenums in freshwater
> aquariums, but I'd still like to pursue it, even if it is only "imaginative".

We should never discourage imagination! Pursue away.

>  1) I don't think it will be practical since clay slurry is not going to
>  be easy to filter out of the circulation water.
> This was just a way to get it into a tank already set up, there is laterite in
> the gravel and it doesn't wash out, so I doubt it would get out of the plenum.
> I'd expect the laterite slurry to settle into a thin layer at the very bottom
> of the tank.

Ok, it wasn't clear that you don't intend to have circulation in the

>  2) why bother with plenums when they do nothing to improve the
>  availability of phosphates in the substrate? In fact, they will tend to
>  circulate interstitial phosphates and other nutrients back into the
>  aquarium water, exactly where you DON'T want them.
> This gets close to answering my original question, which was, does laterite
> and/or vermiculite have properties that cause them to attach to and hold on to
> such nutrients? 

Yes, they can act to sequester nutrients.

> If so, and such materials are present in the plenum, would
> they effectively sequester such nutrients in the plenum?

The key word is effectively. I don't think it will be effective because
I think that the nutrients would be more available to the plants in a
continuous substrate.

If your question is do they sequester nutrients and thus effectively
reduce the nutrient levels in the aquarium in order to discourage algae
growth, then no, I'm sure they will only be able to capture a small
amount of nutrients.

> 2) continued: Iron, phosphorus and  other low solubility minerals are the ones
> which are advantageous when  supplied in the substrate. Soluble nutrients such
> as nitrates, Ca, K, Mg  and S do not need to be absorbed into the substrate by
> a plenum  circulation system in order for aquatic plants to use them.
> I thought that the substrate heating cables were intended to do exactly this?

what are you suggesting that heating cables do? circulate water in the
substrate at low speed? There is no great benefit from circulating the
relatively low levels of nutrients in the aquarium water into the
substrate. Plants are able to absorb these nutrients through their leaf
tissues. Dr Dave might correct me on a few points here. I seem to recall
him saying something about sulpher as a nutrient.

> I'm wanting the plenum to serve as a passive replacement for the heating
> cables since PMDD is a totally waterborne fertilizer method.

now I'm confused. Is the plenum being pumped or creating circulation?

I think PMDD aims to supply most nutrients in solution but not
necessarily via the substrate. Iron in particular may be available from
certain kinds of substrates and this could be a method to decrease the
availability of iron to algae. With PMDD I think we are more reliant
upon the accuracy of the iron test kit since we need to achieve a low,
safe level of iron. Perhaps it would be better to modify the PMDD
approach to calculate a target concentration for iron and dose it based
upon water changes? My current experiment is to determine if the
substrate can provide enough iron on its own over a long term (i.e. a
year or more) without supplementation. At some point, I may add a
measured dose of iron to see if there is an increase in growth rates.
I'm also concerned about boron since I wonder if the substrate is going
to provide a long term source of boron with a tank of growing plants. I
have in fact added a small dose of boron with my last mineral addition
on the weekend.

>  3) plenums and other open spaces beneath the substrate only collect iron
>  oxides and other insoluble precipitates where they are not available for
>  plant roots. It is better to have a continuous substrate so that
>  precipitates settle into the anaerobic zones where they can be reduced
>  and dissolved.
> It's my understanding that plenums -are- anaerobic.

That entirely depends upon how much oxygenated water is being circulated
into the plenum. Even if you have a zone of dead water under the
substrate in a plenum, I'm not sure that this is going to be an ideal
environment for minerlizing nutrients. Roger's explanation about micro
environments describes the situation eloquently!

>   Steve - I've read the archives and your website, I know you advocate the
> rich substrate method.

There is a whole range of meanings to "rich substrate"!! I don't
advocate rich substrates because they cause problems with algae. I do
advocate a substrate which provides micro nutrients especially iron. And
if you desire to increase the rate of growth of plants, I suggest a
method to increase your substrate fertility incrementally with a minimum
of risk. (i.e. clay balls or fertilizer sticks)

I think we should clarify that a soil substrate is not necessarily a
rich substrate. Soil is a very general term.

> I'm experimenting here and don't understand the
> chemistry as well as you, but I like the controlled dosage method that PMDD
> offers.  If I mess it up, a couple of water changes gets me back to square
> one.

If you get an outbreak of algae, a couple of water changes won't get you
back to square one! But your point is well taken. If you build a
substrate using bagged (compost) soil, you are likely going to have a
tricky situation! The more you can learn about it, the better your
chances of success with selecting a natural soil component for your

> After ordering and reading the ECO catalogue on hydroponics, it struck me that
> they are doing the same thing that PMDD does, we just have the additional
> problem that a fully submerged condition presents.  I'm looking at the gravel
> substrate as serving the same purpose the rockwool that hydroponic systems
> use, and the plenum as the place where nutrients are collected and fed to the
> roots.  The denitrification benefits that plenums serve in a reef are probably
> counterproductive here, unless pure nitrogen is easier for plants to
> assimilate.  Is it?

No, elemental nitrogen is not directly available to plants.

Hydroponics is a very difference method of growing plants which is based
upon flooding the roots of plants with relatively high nutrient
concentrations and then draining them. I'm not sure that there's much of
the technology that is transportable to a submerged system...

>  5) Certain botanists have demonstrated that a water transpiration system
>  exists even in FULLY SUBMERGED aquatic plants. This water circulation
>  stream draws water down into the substrate, into plant roots and up
>  through the plant stems to the actively growing regions of the plant
>  where they can be absorbed. This phenomenon was documented in a recent
>  article in TAG 10:6 by Ole Pedersen of the University of Copenhagen. It
>  was originally published in Aquarium Heute No 3, 1996.
> Okay.....Not sure what this means. 

I'm not sure what it means practically either. I don't know how strong
this transpiration stream is. It might be significant. It's something to
consider. I mentioned it because it seems to bear upon the circulation
theme of substrate heating coils and slow under gravel circulation
ideas. I guess it's not really a counter-point to your idea but I stuck
it in there. Nobody seemed to complain that I had two 5) points.

> Anyway, the basic question remains, would
> laterite and/or vermiculite in a plenum act to hold nutrients?

Yes, but not necessarily for the benefit of the plants.

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!