I use what is called a b.a.s! Bio Active Substrate. It is a mixture of eco earth(coconut fiber), soil, and crushed lava rock. Mix it all together. The eco earth and soil are mixed half and half with about a third of crushed lava rock. The lave rock is important for it spaces out the particles so your soil can breath and have room for essential soil bacterias to thrive. The hardest part was crushing the lava rock. You can find the detailed instructions in a book called the art of keeping snakes. The b.a.s that I am talking about was designed for tropical and sub-tropical vivariums. There are others for different needs.

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