I have a young Sulcatta tortoise and was wondering if it is safe to use sphagnum peat moss as a base layer for substrate with grass hay mixed in and layered atop it?
There are some very good threads for new sulcata keepers at the top of the Sulcata section. You will learn much about substrates there and your question will be answered.
I use sphagnum peat moss as a substrate. I don't mix anything into it, though. Mixing in grass hay may encourage ingestion of the substrate because it is actually a main food for a sulcata, plus it will promote mold. I would not recommend using it as a substrate, but rather as food. I do have grass hay sitting on top for my sulcata to graze on or play in. I love the moss as a substrate and use it for many other reptiles. It is a great way to keep up the humidity in any enclosure. I like to keep it moist, but not wet. It shouldn't be crumbly or too dry or dusty. It should be where you can grab a handful off it and squeeze it without dripping water, but when you let go it holds the form of your hand. It's great for burrowing, I keep it churned up in my humid hide and a little more 'wet' in there. My sulcata is very healthy and growing very well. I recommend using this substrate to anyone!