I know others may think differently on this, but I'm doing it and its working great. It's called a 'bioactive substrate system' something along the lines of this:
Bottom layer being lava rock, ontop of that a 2-3" layer of unfertilized pitting soil (regular boring dirt) then I've put a bit of reptibark mixed on with the dirt and as the top layer. I then have some vine plants potted into the substrate. In a corner I've put a tube that goes right to the bottom. That is how I water the plants, WITHOUT getting the top section wet or anything (the lava rock seperates the dirt on top from where the water would be, keeping the dirt dry). The plants are thriving and the garters absolutely love it. There's different ways of doing this and a little research helps. I'll post pixs later on when I git some time on the computer instead of my phone.