Play Sand/loam (coir) mix: This is by far the best substrate. The amount of moisture can be easily regulated. Also it easy to create a higher moisture/humidity (substrate moisture is more important than humidity) area as well as a dry area in the same pen. This way the tortoise has a choice of micro-climates.his is by far the best substrate. The amount of moisture can be easily regulated. Also it easy to create a higher moisture/humidity (substrate moisture is more important than humidity) area as well as a dry area in the same pen.