We are building a new tortoise table based on storage bins (design and photos to come), and needed to figure out what we would put into those bins when the time comes. Below are some opinions from the web...

A second group was reared using an identical diet, with identical lighting and hearting provision, but this time in an open-topped ‘table-top’ terrarium that featured a 2.5” (7 cm) deep substrate comprised of 50% sterile loam and 50% play pit sand. Additionally, some rocks, herbaceous plants and hollowed out logs were also included. It was immediately evident that the tortoises utilized this substrate extensively, often burying themselves completely overnight, or during particularly hot weather. This behavior closely mirrors that observed in the same species in the wild, where juveniles stay buried for by far the greatest proportion of their day, emerging only briefly to bask and graze.

http://www.tortoisetrust.org/articles/microclimates.htm

Calci-Sand

Very prone to clumping. Can cause eye irritation. This product has been reported to be implicated in numerous cases of gut impaction. We encountered one such case only recently. The tortoise died. This product is also very dehydrating. We do not recommend use of this material under any circumstances. The tortoises (G. sulcata) shown below were acutely dehydrated after being kept on this material in a reptile store's display vivarium.

A recommended approach to substrate

The substrate that we have found to be the most effective and safest by far consist of a mixed loam compost base mixed with varying amounts of soft (play) sand. This needs to be of sufficient depth, about 50mm or 2 inches is recommended as a minimum. Increase the proportion of sand for very arid habitat species (our Testudo kleinmanni have a 30/70 loam/sand mix, for example).

http://www.tortoisetrust.org/articles/substrates.html

when they must come indoors, I would be highly wary of the following substrates:

1. Coconut husk (Bed-A-Beast). This is the material that expands to 7-8 times the packaged size when unpacked. Unfortunately, this same material if eaten by your animal will also expand in their stomachs thus causing an impaction and possible death. I know of several deaths in tortoises from this material and do not recommend it.

2. Small stones/gravel. Tortoises like bright, shiny objects that fit into their mouths. Most species will eat almost anything placed in front of them. Because of this, small stones and gravel should not be used as substrates. While in some cases these foreign bodies are able to pass through the animals, there is always the possibility of the intestines twisting on themselves when they try to pass the stones. This twisting can lead to sudden death due to shock and a rapidly developing toxemia

3. Sand (including CalciSand). While some species actually live on sand in the wild, their foraging behavior is such that the food they eat grows above the sand rather than lying in it. Unfortunately, when the animals are fed on sand in captivity, the sand sticks to their food and a large amount of the substrate ends up being ingested. This in turn can build up to dangerous levels in their GI tracts leading to sand impactions and a twisting of the guts called colic (a common problem in horses). Feeding species that normally exist on sand in a completely separate enclosure without substrate is one way around this.

4. Aromatic/soft wood shavings such as cedar and pine. These types of shavings give off aromatic hydrocarbons which have been found to elevate liver enzyme levels in rodents which are housed on them. While tortoises are not rodents, I'm not one to take chances and would recommend not using these substances as substrates.

http://chelonia.org/Articles/march_2001.htm

The water dish in the habitat should be large enough to allow the tortoise to soak in it if it wishes - it must also be shallow enough to protect from drowning. As a substrate in the dry portion of the environment a mixture of topsoil and children’s play sand or cypress bark works well.

http://chelonia.org/Articles/hermannstortoisecare.htm

DRAINAGE MATERIAL. Use 1/4"-1/2" gravel, small stones, or lava rock (which is great because of the light weight). It usually looks better if you stick to natural colors like beight, brown, gray, etc., because the gravel will show through the glass. Bold colors tend to detract from your overall design. When you lay down the drainage material, the depth proportion is 1/3 drainage material to 2/3 soil mix. Usually the minimum is 1/2" and the maximum you would use is 1-1/2".

CHARCOAL. Use horticultural grade charcoal if you can find it; it's cheap and already cleaned and bagged. Otherwise, you can use crushed barbecue charcoal; just make sure it's quite fine, no more than 1/8"-1/4" pieces. Sprinkle roughly a double layer directly on top of the drainage material. Some will stay on top, and some will work down between the stones. This will help keep the water "sweet" instead of letting it turn foul. There's no smell in the substrate mix when you add charcoal.

SOIL SEPARATOR. This is a step most people have never heard of, but it makes a tremendous difference in the success of maintaining a healthy substrate for your plants. Basically, it prevents the soil from sifting down onto the drainage layer. A soil separator needs to be something that won't rot, is porous enough to let water through but not so porous as to allow soil to filter through. The best thing I've found is rolled fine mesh fiberglass from the home supply. It's available by the yard, resembles open weave fabric, and is more flexible than window screening material, but either works. I fit this in a single piece flush with the sides of the terrarium because there's less likelihood that the turtle or tortoise will claw it aside and allow a lot of soil to trickle to the lower layers. In a pinch, you can use overlapping layers of women's pantyhose, but if your turtle or tortoise does any digging at all, it's going to get messy pretty quickly.

SOIL MIX. This is the final (top) layer of your substrate. Now's the time to add the tropical soil mix recipe below. The amount is going to depend upon the height of your tank, meaning how much of your animals' space you can afford to give up to the plant's roots. If you're not including plants, don't worry about this. Generally, though, you will wind up with about 2-1/2" to 3" of soil. Here's the soil mix: 3 parts of sterilized potting soil; 2 parts sand (NOT seashore sand, it's loaded with salt!); and 1 part peat moss (NOT sphagnum moss).