In a lot of the recent discussion about substrate heating, everyboody seems
to agree that there is an an advantage to keeping plant roots warm. The
other attributed benefit is the increased circulation in the substrate
which is necessary to get nutrients to the roots.
Stephen Pushak wrote (Aquatic Plants Digest V1 #293):
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.
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.
Assuming Stephen's flow estimates and tests are anywhere near realistic, it
does not appear to be a very good way to get nutrients to the roots.
Additionally, any nutrients delivered to the roots have to first be
dissolved in the water, where they are available first for algal growth and
may stress the fish. It appears to me that a better solution would be use a
pad under the tank, and if you want to deliver nutrients to the roots put
an "I.V." tube in the substrate. This "I.V." could be a under gravel filter
or a grid of fine tubing with holes punched in it. It would allow the
injection of concentrated nutrients directly to the substrate, without
exposing fish or algae to the nutrients.
In southern California where its dry, warm, sunny, and you can see 50 miles!