Re: Heating Pad v.s. Substrate Heating Cables

Enrique wrote:
> >From Paul Nicholson:
> >Can anyone justify why internal substrate heating cables should be 
> >used over an external heating pad?
> two paragraphs have been excerpted from two messages sent by George
> Booth to the Aquatic Plants Mailing List:
> >Keep in mind the main purpose of the coils: provide slow circulation
> >in the substrate.  The substrate heating thing is secondary and the
> >tank water heating thing is just a useful side effect.  The "ideal"
> >(as far as we understand it) is continuous slow circulation in the
> >I have also found that a substrate with heating coils works somewhat
> >"better" for at least 4 years (so far).  I therefore conclude that
> >coils DO NOT help plant growth to any great extent but promote long
> >term SUSTAINED growth.  I conjecture that this is due primarily to
> >fresh nutrients being brought into the substrate by slow convective
> >currents.  I conjecture that coils are the best way to do this.

The theory behind heating coils is to create slow convection currents.
The reason heating coils and other heating devices show demonstrable
positive results MAY be different.

Permit me to make a number of observations for the basis of further
conjecturing. :-)

1) The coefficient of thermal expansion for water is on the order of
.001 (density change per degree C). Convection currents are induced
by horizontal temperature gradients in the fluid, that is, they are
powered by the differences in hydrostatic pressure between different
regions. With cables separated by only a few inches and in a substrate
only a few inches deep, these pressure differences will be extremely
low, on the order of .01 inches of water (hydrostatic pressure of
a column of water .01 inches tall). 

In a compact substrate containing soil, fine sand, clay or silt this
amount of pressure differential will be insufficient to produce flow
rates of around 1cm/day which would be about what we wanted (please
verify). I did some measurements using a slow RUGF system powered
by about 1" of hydrostatic pressure through a substrate containing
soil and I think the results were about ten times slower than that
with 100 times the pressure. George was going to do an experiment
with gravel and Dupla Laterite granules and an UGF plate to see what
the time constants were for that but I don't think he ever got
motivated enough to do it because we dropped the discussion.

It could be that heating cables would work dandy at inducing slow
circulation in a coarse substrate but likely pretty useless for
soil substrates except as a source of heat.

2) Plants have a pretty good vascular system and do a good job of
inducing water into the substrate and also of providing oxygen in
air channels right out into the roots. In nature plants find most
of their nutrients in the substrate and this includes CO2, ammonium
and soluble Fe ions as well as other macro and micro nutrients.
In fact, ANAEROBIC bacterial action in the substrate performs two 
very beneficial actions that we know about: reducing iron to a 
soluble state and reducing nitrates to useable ammonium. Another
bacterial activity is the production of CO2 by the process of rotting
organic material. These activities are enhanced by a moderate increase
in temperature.

3) CO2 in nature is in very short supply in streams and lakes. 
Submerged plant leaves have a thin layer of water molecules which
forms a boundary at their surface and CO2 diffusion rates through
the water are very slow. Floating plants can take advantage of 
atmospheric CO2. Rooted aquatic plants get most of their CO2 from 
the substrate. One theory is that we help our submerged plants
respire by providing water circulation in the aquarium.

4) The substrate in an aquarium is not deep enough to mimic the
processes in nature completely. A densely planted aquarium probably
has too much oxygen in the substrate to efficiently resolublize
Fe. Most of us supplement weekly or daily with chelated Fe to
get around this. We also fertilize with CO2 for the same reason.

5) From what I've read, we don't know completely about all the
beneficial chemical/biological processes that occur in the substrate. 
Suffice to say that these processes are usually enhanced by a moderate
increase in temperature.

6) Laterite contains lots of iron. Some of that iron becomes available
to the plants (just as in soil) but laterite alone does not seem to
provide enough Fe long term. One property of the iron compounds is
that they can "fix" phosphorous atoms within a crystalline structure.
One theory about laterite is that it helps combat algae by reducing
the amount of phosphates in solution.

My conjecture is:
1) heating coils will be no better than heating pads in a soil, fine
clay or silt substrate but still probably beneficial.
2) in a granular substrate, heating coils might help circulation but
the plants are probably doing most of the work on their own.

Vancouver Steve