I want to switch my russian's substrate to a mix of 70/30 topsoil/playsand. However, I don't know what to look for to be sure it isn't harmful. I can't get the soil out of my yard since I have two dogs, so I need to buy it. Am I looking for sterilized top soil? And sterilized playsand? Does anyone know where I can find this and how to make sure it does not have any pesticides?