In my opinion, the soil in the garden, meaning your tortoise outside on the native soil in the garden, is the best substrate. When I have to have a tortoise in an indoor habitat I prefer orchid bark or cypress mulch. This is usually a hospital type situation. All my tortoises live outside with heated sheds, 24/7/365.
My guys are loving the new coconut coir in their indoor enclosures. They completely bury themselves in it, and now also emerge to walk around and eat, too. The coco coir seems to be doing a great job of creating a humid microclimate, which is just what Russian tortoises need: dry air with humid microclimate.
I know a lot of people have success with cypress mulch, too, since that also retains moisture well. I used to use it with my redfoot, but I'm not using it now with my Russians.
I'm sure soil outdoors is great, but I have given up on using it indoors; too many insects and fungi growing in it. Yuck.