Substrate is a personal choice. You first have to find out what the tortoise needs to be healthy, then provide that any way you can. Russian tortoises, in fact most tortoises, need a substrate you can moisten a bit, yet it dries out on the top layer. So that when the tortoise digs down he can have a bit of moisture in the substrate. In order to provide that, I like to use orchid bark or cypress mulch. Some people use coco coir, some use clean dirt from the garden.