I mentioned this proceedure a year or twoago to another fellow modeler who asked this question. Here where I am the humidiy is very high most of the time so we have to bake any foreign dirt we use because thing's do grow. Elmers glue doesn't stop anything. Ever see a fresh paved asphalt driveway which contains oil, tar and other chemical's that can burn the skin but a year later grass has broken through it without a crack even being there? Well, same concept except it's fungis and mold that devolop and that's not good. It can actually cause damage to a layout later on down the road so regaurdless of where you live, it's best to take the precaution and add the extra step of process and save yourself the trouble later if you happen to have any. 350-400 degree's for about 30 minutes give or take, remembering to stir and turn it over from time to time to make sure it all get's the treatment. If ever in doubt. Just go get something off the shelf from the local HS of home improvment store. Just not potting soil.