Re: Aquatic Plants Digest V2 #108

Michael Eckardt <mike at argosys_odg.com> wrote Thursday, May 30:

>Subject: UGH
>I want to pick up on a thread that was discussed here
>a little while ago - under-gravel heaters.
>Kasper Horst of Dupla fame, wrote in his book
>"Pflanzen im Aquarium" that _on average_ aquatic
>plants can absorb nutrients from any surface [i.e.
>roots, stem, leaves], that they can transport
>nutrients, and metabolic end [and by!-] products in
>ANY direction within their structure, and they
>excrete compounds into their environment. Diana
>Walstad, in her excellent paper on allelopathy
>published in TAG, elaborates on this and gives us
>actual examples and data.
>Let me apply this to the substrate of a new tank. It
>will go through a number of stages:
>1) New plants start growing their roots in the
>substrate. Because there are no allelochemicals
>present, the initial spreading of the root is not
>limited by them, and bacteria start to populate the
>2) After some time, the allelochemicals excreted by
>the roots become concentrated enough to kill
>bacteria, and rootlets of some plants. Some of the
>released compounds are not easily broken down and
>3) After a long period of time (years(?)), the
>substrate becomes "old", saturated with growth
>inhibitors that slow root growth, and consequently
>reduce O2 release from the roots. The accumulated
>organic matter is broken down, using up yet more O2
>The result is poor plant growth, a phenomena hinted
>at by Horst.
>4) In the final stage, the substrate become anaerobic
>and needs to be replaced.

I read Diana Walstad's articles on allelochemicals, and I don't recall
seeing any support for the notion that allelochemicals might build up in
the soil over long periods of time.  I think she did cite some evidence
that allelochemicals were broken down by bacteria over time peroids of one
or two months.  The scenerio presented here basically has plants doing
themselves in by accumulation of their own allelochemicals.  As I recall,
Diana cited evidence of one species or group doing in another, but not

I have a 55 gallon at home that has gravel with soil underneath that I have
not disturbed or changed for 8 years.  The plants (Echinodorus hormanii,
Echinodorus sp?, [same size as hormanii, but narrower leaves], Aponogeton
rigidifolia, and Hygrophila polysperma) are still doing fine.  I havn't
even changed any of the water for the last 8 years, but I think I will do a
partial change because the swordtails have stopped having babies.  Over
this time period the hormanii sent up a second plant which grew in a
different direction, and now the two plants are about 15 inches apart.
Just recently, I saw a third and a fourth hormanii coming up from the
rhizomes of the older plants.

Plants in old, long-established tanks with a soil plus gravel substratum
can run low on iron.  I know this is true because I have seen increased
growth when I added chelated iron to some of these old set-ups.  I think
they become iron deficient because they get the substratum filled up with
roots to the point where the entire substratum becomes aerobic and all the
iron compounds become oxidized and relatively insoluble, compared to
reduced iron compounds.  There are other micronutrients that can become
deficient in old set-ups.  Many times I have seen a deficiency that looks
like boron deficiency that I have cured with a micronutrient mixture which
includes boron.  I have never tried only boron, so I havn't proved that the
deficiency is boron, it just looks like boron deficiency (new growth is
distorted and small; roots are short with death or slowed growth of
meristematic regions) The belief that old soil becomes toxic may be
actually due to experiences where the old soil actually became deficient.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In cool, humid, Mississippi where we finally got some rain!