Heating a natural vivarium

[RAD House]

I am wondering how most of you heat your planted vivarium. I currently have an under tank heater with a thermostat ready to use when I finally set it up. Two things concern me. First that the uth will not be effective enough to heat 4 inches of soil substrate. Second that the heater will dry the dirt out killing the crucial bacteria. It is possible for one third of the aquarium to be in direct sunlight. I would like to avoid using a lamp for several reasons. Mainly these are fire hazard, energy consumption, and that they make opening the tank awkward. Radiant heat panels are out of my budget at the time.