A substrate is important to maintain the health of your pet's environment. It also helps to make the enclosure feel like a real home. However, not only is the substrate important, but also that you choose the right one!
Gravel is a common substrate for aquatic turtles. Such rocks are probably the only substrate you could use anyway, because anything else would clog up the filter. Gravel is not required for turtles--mine seem to do just fine in a tank with a bare bottom. But the gravel does help to improve the efficiency of filters, gives the turtles another thing to play with and something to chew on to wear their beaks down without destroying aquarium accessories, and generally makes the place feel more homey and pretty.
Be careful in choosing the gravel, though. Turtles tend to "explore" their environment, which means tasting everything, and I have seem some play with gravel. If the gravel is ingested, it could cause an intestinal blockage and can, at the least, cause an intestinal prolapse. This condition is usually not serious and usually fixes itself, but it can be serious and is generally not a good experience for the turtle! One of my friends had a hatchling red-eared slider die possibly from this very problem.
To prevent this, choose river rocks, not aquarium gravel. Aquarium gravel are usually large rocks that have been shattered, so they have very sharp corners and can cause a lot of internal damage. River rocks have been smoothed and worn down by water and generally go through easily if they aren't eaten in large quantities (which rarely happens anyway). Another thing I like to do sometimes is to use river rocks, then cover the surface with large flat river rocks or large marble which are too big for the turtles to swallow. These are available at some department stores such as Wal-Mart in the craft section.
Another good choice is crushed coral. This helps to keep the PH level of the water higher where the turtles like it. It's also harmless if they chew on it and will actually provide them with supplementary calcium. This is the recommended substrate for brackish terrapins.
A variety of substrates can be used for semiterrestrial turtles and tortoises. To make for more ease of cleaning, I like to use old carpeting, old towels, or artificial turf in the bottom of the terrarium. I have heard of people using rubber shower mats, which I imagine would be very effective since it allows waste from the animals to drain off so they are not walking through it, and then it can be easily hosed and cleaned.
Since these animals need a burrowing medium, I like to use cardboard boxes with a hole cut in the side. The hole should be cut so it is barely wider than the turtle and is about 1-1/2 inches from the bottom and has a little room on top to allow the turtle to stand when it goes inside. Then I fill the bottom up with the burrowing medium. Many of my turtles treat this box as a bedroom of sorts, coming to the "door" whenever I approach the tank. For turtles or forest tortoises that have a high humidity requirement, such as hatchling redfoots, I put the substrate in half the enclosure including under the basking lamps and use the cardboard box as a dry retreat so they do not develop pneumonia from being kept too wet.
The burrowing medium for semiterrestrial turtles should be ones that hold moisture so as to provide humidity. (These animals get their humidity from the substrate, NOT from the air!) Good ones would be plain unfertilized untreated potting soil that is watered down to the moisture you would pot a plant in, plain unscented kitty litter that is also watered down, or ZooMed's Eco-Earth/Bed-A-Beast coconut fiber substrate that is mixed with sand and moistened. Long-fiber sphagnum moss is also a good substrate that holds alot of moisture and it may be required for hatchlings as young turtles dry out very quickly. The substrate should never be so wet that you can squeeze water from it. Never use pine or cedar wood chips, cage accessories, or similar things as they have toxic oils that will damage your turtle's lungs and/or eyes. Also never use any of the above substrates dry as they get dusty and will irritate your turtle or forest tortoise's lungs. The only good dry substrate is a pile of hay in one corner, but I find that simply providing a few dry hides is sufficient.
Arid-land tortoises need a drier substrate, but you would not want something that is bone-dry. Good substrates for tortoises would be a mixture of 40% play sand and 60% plain, untreated, unfertilized potting soil or cypress bedding moistened or ZooMed's Eco-Earth coconut fiber substrates or Bed-A-Beast mixed 50/50 with play sand and watered down. Never put them in plain sand, as this will cause intestinal blockages if it is ingested, and never use pine or cedar beddings as it has fumes that are poisonous to tortoises. They also need a dry area and I usually achieve this by giving them a pile of Bermuda or Timothy hay in one corner. Do not lay the hay on top of the wet substrate--give it a corner to itself--or it will get moldy. Corncob beddings and rabbit/alfalfa pellets are not good either as they tend to get moldy fast and pellets contain too much protein for tortoises. Kitty litter is not a good dry bedding either because it is very dusty when dry and can irritate the tortoise's lungs.
My pets also enjoy piles of leaves or straw *for semiterrestrial turtles) or hay (for arid-land tortoises, as they will eat it!). In fact, spreading 3 inches of straw/hay or leaves in an outdoors enclosure will encourage a good population of earthworms, crickets, and other food (so do not do this for grazing grassland tortoises as they are herbivorous).
Newspaper is a good choice if your shelled friend is sick. It's easy to change... But the newspaper does not provide any traction, so it makes it difficult for the shelled ones to climb onto basking platforms or to run for hiding places, so I don't use it every day. I like to use leaves or hay/straw (hay kept dry for arid-land tortoises) as a substrate when transporting my turtles as it accomplishes everything at once: it provides traction, can be moistened for aquatic or semiterrestrial turtles, and allows them to burrow and hide therefore providing a sense of security at a time when the animal is wondering what's going on... If there is no substrate or just newspapers in the crate, oftentimes your pet will slide about as your car turns or hits a bump, so I think leaves are more comfortable for them.
Choosing the right substrate will help you to avoid skin fungus problems, intestinal prolapses, and other possibly major problems. It can also make the tank easier to clean (or keep clean!) and will keep your babies happier. Feel free to get creative as long as you stick to what's appropriate.
