if you want to use soil i would sudjest linning a baking sheet with tin foil, then spreading an even amount over it again covering it with tin foil. Pre-heat your oven to around 400 degree f and bake the soil for around 20 to 25 minutes. Freezing will not really do much, as some insects and other criters you dont want in with your snake will just go into suspended animation when frozen. Which is for winter and is just a big sleep. When you defrost it they will all wake up a probably be hungry
.
You can use the same method with bark. Also if you cut branches off of trees it is a good idea to wrap them in tin foil and again bake them.