Understanding Vivarium Substrates. By A. C. Highfield. Libyan tortoise-this tortoise requires a sandy, dry substrate. If maintained on grass or upon a moist substrate it will suffer shell and respiratory infections.Along with heating and lighting systems, the choice of substrate in tortoise vivaria and terraria often causes a great deal of confusion to keepers.Some substrates are excellent, others positively lethal.nderstanding Vivarium Substrates. By A. C. Highfield. Libyan tortoise-this tortoise requires a sandy, dry substrate. If maintained on grass or upon a moist substrate it will suffer shell and respiratory infections.