All the books I've read have strongly suggested against using dirt (in any amount) as a substrate. Its not sterile and promotes the breeding of bacteria. Of coarse its alright in nature, but once a snake is confined to a cage, there's no escaping the bacteria and such. I would stick to the more commonly used substrates. Ease of cleaning is also something you might want to take into consideration unless this is your only cage and you don't mind spending the amount of time required to clean the cage properly. I've learned a lot through trial and error, I'm sure that will be your best teacher. I like to use ground walnut shell and sometimes mix it with small bark chips to give it more of a natural look. Its always best to feed your snakes outside their cages in a box or paper bag so they don't swallow the substrate in their cage with their meals. Sometimes the substrate accumulates in the belly of the snake and they are unable to digest their food anymore. It happens a lot with gravel. I would also suggest you use plastic plants. Snakes crawl all over everything and a live plant just wont hold up. Good luck.