Soil is not really a good choice.

I'd recommend either bed-a-beast or eco-earth (both are cocoa beddings for reptiles). You can be sure that they are fertilizer-free and safe. You could put a drainage layer of stone under a couple inches of the substrate. It holds moisture well and with the drainage layer it should not mold.

You can also grow plants in it. Pothos is a good one, it's cheap, grows to the perfect sized leaves for them and doesn't need UV, it's also extremely easy to grow (like a weed). Just make sure you remove the soil you buy it with, wash the leaves and roots and replant either directly into the tank or into a pot with bedabeast.