i use peat moss for my black soil dragon. he loves to dig in it and he is sleeping un a burrow under a log in it now. he is almost 6 months now and has been growing really well. my mate had two form the same batch of hatchlings and he is amazed when he see's mine.
the peat was from the tank setup for a bird eating spider, a littl clean up and it was ready for him. the substrate is available in bricks and when water is added trun into a rich looking soil that holds mousture and therefor heat. he seems to love it and at no time have i had any problems with impaction even when he was only a few weeks old he was chasing crickets around on the peat.
i woudl highly reccomend this substrate, with a little bit of maintenance cleaning the fecal waste. (i have some tweasers and pick up the "big ones" :-P)
the rest seems to break down nicely in the ecosystem that is created in the tank, it's a live tank full of surprises for him and he seems to love running around and the occasional "big dig" haha i will come home some days and he will ahve escavated the tank in three corners!! ha ha the funny thign is if you leave it he will eventually end up with an even tank over a few days, he seems to level it back out by himself haha