The best substrate for an American Toad?

We had been using dirt from outside, from our (uncertified ) organic yard, but can't due to the weather. Also, it contains *some* sand, which I have just read isn't great for toads, and especially realized that this is a concern now that we have begun using earthworms as food since its too cold to get crickets routinely here right now. Ted's dirt is sticking to the earthworms and we don't want to impact him. But we need a new substrate anyway,because i feel like its time to clean his tank.

What's a good interval to change the substrate anyway? 10 gallon, one American Toad.

TIA!