I mix up some white glue, fine sand/dust, static grass, some baking powder/baby powder, and an earthy brown acrylic paint. To put it on thick I use an old paint brush, or thin it down with water and using an old toothbrush, splatter it on by pulling on the bristles.
As its only one colour, I dab different earthy tones on with paint or pastels or both!
If you are depicting fresh mud blow some fresh static grass onto it when it is in place!
If you want it to look dry, use a lighter shade. If you want it to look wet use a darker shade and give a light wash with gloss varnish! Both look good together!