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trg-s338 wrote:

I'm building a 4 x 8 foot greenhouse planter and would like to embed
an electrical heating system, some kind of grid/mesh in the soil.
Just need to keep the soil around 70 degrees. Accomplishing this with
the least use of electricity is a plus if possible. Is this something
I can build or salvage off of an existing system? I would appreciate
any suggestions or direction where I can research further. Thanks.

"accomplishing with the least use of electricity" is the tricky part.
Resistance soil heating cables are a common greenhouse item, plenty
cheap enough to buy new. It's running them that costs money.

More complexity, less electricity if you used the same electricity to
power a heat pump - but prohibitively expensive for heating one planting
bed.

Insulate the whole planter well - that will make a big difference. The
whole thing - including underneath. Use good quality double or triple
glazing for the greenhouse part. Use at least 2" extruded styrofoam.

Depending how much sun you get and how handy you are (not cost effective
if you are not handy and have to buy the collectors) a solar thermal
collector (perhaps with PV powering pump or fan) coupled to the soil
(via pipes or ducts) could do well, and relegate your electric to
night-time & cloudy-day back-up.

If you have hot water heat in the house, often the most cost-effective
(to run) route is to couple a heat exchanger to a antifreeze filled loop
running out to the planter, if the planter is near the house. Insulate
the pipes well, of course.

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