Well, in my opinion, dirt or soil usually creates a big mess when it gets wet. For example..if the water bowl gets spilled. Also, it is hard to see the area that is defecated in as well, which makes it tougher to keep clean and sanitary. If you want a more "natural" substrate, I would personally use some mulch that has also been oven-baked.
If you still want to use soil bought from a store in a bag, or use it from a natural source of any kind, I would DEFINITELY bake it in the oven for a while to make sure any pests where totally eradicated from it. Soil can contain all sorts of things you don't want in the snake's enclosure.
~Doug
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"a snake in the grass is a GOOD thing" 
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