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Re:Do substrate heating coils fertilize the substrate?

>Date: Thu, 22 Oct 1998 01:51:42 -0700
>From: Steve Pushak <teban at powersonic_bc.ca>
>George goes on at length deflecting from the issue that he has made an
>unsupported assertion that heating coils are in any way effective at
>bringing nutrients into a laterite substrate.

This thread is annoying me to no end, so I think I'll respond to it.

In preface, I have never used any form of substrate heating.

Steve has pointed out studies proving that root-feeding plants produce a
small transpiration flow from their roots, to their leaves, and back into
the water column.

George has pointed out for years that his plants grow better in tanks with
heating cables than those without, and that the heating cable tanks last
longer between teardowns than those with plain substrates.  The assumption
based on Dupla theory is that the heating cables produce convection
currents in the substrate that transport nutrients from the water column
into the substrate, where they are locked in by the substrate's CEC until
the plants can use them.

While there is no documentation proving that heating cables introduce any
form of convection into the substrate, they do seem to do something.

Let me propose an additional hypothesis that works within what we know to
be proven:

Transpiration occurs, carrying water and nutrients from the roots to the
Heating cables warm the substrate and thus, the roots.
Warmth increases metabolism.
Increased root metabolism means faster root growth.
Increased root metabolism means faster transpiration.
Faster transpiration means more water and nutrients move from the roots to
the leaves.

Whether or not heating cables promote substrate flow via convection, I
think it's fairly obvious that heating cables will promote increased
transpiration by both increasing the amount of root volume per plant and by
increasing the transpiration rate of the roots.  This increased
transpiration rate can be observed by noting the difference in growth rates
between plants in otherwise identical tanks, one with heating cables, one
without.  I know of no such experiment, and I don't really expect anyone to
go fiddling with their tanks to see if they can mess one up by turning off
their heating cables, but there is a sufficient amount of independently
documented anecdote regarding tanks with and without heating cables for me
to believe that plants with heating cables are getting more nutrients from
somewhere, given similar lighting conditions.  Also, knowing that the
substrate on these tanks does not deteriorate in the way that it might in a
tank without heating cables provides evidence that some form of water
movement is occurring within these substrates that is not occurring at
least as quickly in tanks without these substrates.

It is possible that convection is a contributing factor.  It is also
possible that convection is not occurring.  In this case, I would argue
that the increased transpiration rate of the plants due to the increased
temperature in the substrate is responsible for both substrate longevity
and plant growth, given similar lighting and fertilization considerations.
Actually, upon examining the two possibilities, it is preferable that
convection does not occur at all, but rather that increased transpiration
cycles water from the water column, into the substrate, to the roots, to
the leaves, and back into the water column, filtered of nutrients.

George has documented the lifetimes of many, many tanks on his web pages
and on the Krib.  Based on this documentation and based on the known points
of fact stated above, I would state emphatically that substrate heating
coils will in fact contribute to a slow movement of water in the substrate
of an aquarium.  This movement, if we assume that the water contains
nutrients, will "fertilize the substrate".  It will also remove nutrients
from the water column.

My answer to the question: "Do substrate heating coils fertilize the
substrate?" is: "Yes, although indirectly.  They contribute to the plants'
ability to fertilize the substrate."

David W. Webb
Live-Foods list administrator