So I contacted the local wildlife sanctuary about the super friendly, outgoing box turtle I had found.
She said that he was probably a pet, and since he can't be released, I could keep him as long as they get to inspect his living space.

Well, I have build a nice little pen, made a nice little pond and hide, but I'm missing one major thing.
The substrate.
I've looked and looked for soil that isn't tampered with using pesticides or fertilizers, but I just can't find any.
I'm going to need a lot, so the stuff at pet stores is a bit out of the question.

Suggestions?