I just use topsoil, but robyn has a really good mix he came up with. I would read the posts with robyn about how to get a dirt mix, but he also said that you have to play around and try to find a good mix, but 100% topsoil works good for me because the surface is dry and about 1-2 inches below the surface it is moist for digging burrows. You just have to play around and eventually you will get it right.
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Mike
1.1.3 maliensis (Spike, Marshmellow, Brown, Green, Orange)
0.2 Female U. Acanthinurus-Werneri x "coming soon"
0.1 Pit Bull/German Shephard/ Lab/ ect. mix (Kodak) - looks like an over grown toco bell dog.