I just love your thread title, Roger, it did catch my eye!
Seriously, for thick accumulations of mud or dirt, I like to use ground-up pastel chalks, mixed with a drop or two of water, and a drop of dishwashing liquid. I can make a slurry with this, and the dishwashing liquid helps provide some cohesion and adhesion. When it dries, it can have a grainy texture, and I can add things like old tea leaves to it, or bits of twine, to make it look like there's grass or leaves mixed up in it. I hit on this mix building armor and aircraft models.
Prost!
Brad