I agree that there is no need to bake dirt. Just spread it out in the sun for an afternoon to get it nice and dry. It will be pretty sterile after that, not that it matters.
OTOH, I also endorse coffee grounds. I use them all the time. As O. T. said, it has been used forever, and provides a very nice contribution to your color and texture palette. Just make sure they are fully dry before applying (the sun will accomplish this, too). You may get many posts from those who will tell you all the bad things that will happen if you use it. I recommend trying it for yourself.
How do you glue anything down without getting it wet after drying it?
Hmm. Good point. Well, maybe it doesn't matter how dry it is. Not sure.
All I can say is that I have used coffee grounds regularly since I was 8 years old and have never had any problems. I don't see why it would be any worse than lichen, paper, homasote, papier-mâché or any other organic material. They will all absorb moisture until they are in equilibrium with the ambient air. Shouldn't be a problem.
