I have a Jordanian Greek, Tallula, and she loves the organic soil I use as substrate. Its good to keep humidity and easy to clean, as you won't need to clean it for a couple months at least !!!
After trying a lot of different combinations of substrates (Eco-Earth, Repti-Bark, Aspen shavings, Playsand, etc.) I have settled on the blend of 70%Eco-Earth and 30% Playsand (as Recommended by Danny.) It seems to be working well. The sand and ecoearth hold moisture well in the humid side, and the cool side of the enclosure is easy for her to burrow into. ALso, 3 blocks of ecoearth cost me $9 dollars, and a bag of playsand was about 3 bucks, so it is economical for me. As opposed to $10 per bag for repti bark and $10 per bag for aspen.