Need to make some mud!

[Mr. 16 gauge]

Hello;
I am an amateur taxidermist (I do it as a hobby....I have no interest in doing it for a living), and I have an idea for a diorama with a couple of pheasants standing in a cut corn fields. I would like to know how I can make the base appear to be moist earth. I've tried in the past using plaster of paris and Ritt dye, but the results were somewhat disappointing, esp. over time.
What do some of you guys use to give the impression of moist earth? I have checked the few taxidermy catalogs that I have, but none have anything specifically for this.
Thanks in advance.