One of the most misunderstood topics in tortoise husbandry is how to pick the right substrate for your species. There is a lot of bad advice going around on the Internet, plus a lot of good, but very incomplete advice. People tend to be great advocates of what worked for them, and think everybody should do the same, but different species of tortoise require very different substrates.
Your choice of substrate is going to have a huge impact on your tortoise’s life. Whatever material you choose, your tortoise will have its belly pressed up against that pretty much 24 hours a day. If it is too moist, upper respiratory tract infections are almost guaranteed. If it is too dry, dehydration and associated shell problems are probable.
As that last paragraph implies, the most important factor in substrates is the amount of moisture it will hold. Some substrates – like compost – are pretty soggy, and this will lead to a humid microclimate just above the substrate (i.e. where your tortoise lives). Others – like sand – hold much less moisture, and create much less humid conditions.
So what are some good substrates? Sphagnum moss is good for many tropical species, either on its own or mixed in with compost and sand. You can get it either in horticultural stores, where it is sold for growing orchids in, and some, but not all, reptile shops stock it.
Perhaps the best substrate is a mixture of play sand and loamy compost. When I was first given that advice, I had no idea what either ‘play sand’ or ‘loamy compost’ were, so let me help you out: play sand is the sort used in children’s sandboxes and is sold in hardware shops. Loamy compost means compost with intermediate water retaining capacity – not too soggy and not too sandy – and is sold in gardening shops.
The neat thing about using these as a substrate is that you can alter the ratio to any desired level. More sand makes for a drier substrate; more compost for a wetter one. Tropical species, that are adapted to the rainforest floor, might need a mixture off 60% compost to 40% sand. For a Horsfield’s tortoise, it might be as high as 80% sand to 20% compost. You can also add in a little sphagnum moss too, so your finished substrate might contain 75% sand, 15% compost and 10% moss.
If you keep your senses open, your tortoise will tell you whether you have the right humidity balance in the substrate. Does your tortoise spend all its time up on a basking rock or log, off the substrate? That likely means he doesn’t like it down there.
Besides humidity, there are a few other considerations with substrates. One obvious rule: don’t use things that are poisoned. A lot of woodchips that are sold have been treated with fungicides and things like that; steer clear of all but organic woodchips. Resinous woods like cedar should also be avoided. Another rule: don’t use thing that will catch fire. This is pretty much the opposite of rocket science, but there are keepers out there who have used shredded newspaper as a substrate. Put that under a basking lamp, and I think the warranty on your tortoise becomes void.