Well Russian tortoises are found in grassland areas with a mixed topography of dense to loose soils under brush. Any earth will work great for them. They are limited on their activity as the areas they are found have very cold and harsh winters. Peat moss is a great substrate, it's very similar to coco coir. It's very loose so they can burrow(which they tend to do) or even just top soil mixed with coco coir or peat moss. Your going to get more bag for your buck with peat moss or coco coir.