Dirt as substrate.

[Web Bacon]

This was copied from the Garter Snake Care Sheet...........Dirt - plain dirt dug from outside. Contains mold spores, organisms, and microorganisms that while outside can be fine. But indoors can mold, bacteria populations can explode. Bottom line is, dirt is dirty. Best to avoid it. So now I am a bit confused.

[Albert Clark]

Bio active enclosures seems to be what you are describing and there are several types that are commercially available for your consideration. I will try and get the link posted for you.