First, outside soil probably isn't a good thing to put in the tank. There's no telling what could be in it. I use aspen, but I have no idea what you could use to grow plants in.
For the humidity, anywhere from 50-70% should be fine. I keep mine 60-70% personally.
Ball pythons don't really need baths unless they decide to paint the tank after going potty or to help remove mites. I'm not sure where you heard that.