Question on ground cover...

I use actual mud. I have some finely sifted dirt from my backyard. I just mix it with 50/50 mix of water and white glue, then apply that. For grass, I've been using tinted sawdust, but I think you should go with the woodland scenics stuff. Been meaning to try it myself.

I use Cellu clay for my ground cover (dirt and mud). Works great, it's cheap and can be colored when mixed. On top of that I use Woodland scenics static grass. To make the celluclay mud all you have to do is darken the color and add a coat of future when you're done.

Heres a Pic of  one of my older dios where you can see the ground work ok.

and another oldie

Anthony,
For the ground itself, I used regular spackle compound for drywall patching in this 1/87 diorama but there are a bunch of things you can use like plaster of paris for instance. Since it takes a while for the spackle to set up, I have time to run a few old tires through it to create ruts.
http://pg.photos.yahoo.com/ph/stryper50/detail?.dir=730e&.dnm=e936.jpg&.src=ph
Then it's just a matter of painting it. The low spots have some Future floor wax in them to replicate puddles.
http://pg.photos.yahoo.com/ph/stryper50/detail?.dir=730e&.dnm=9f86.jpg&.src=ph
For grass I just use Woodland Scenics selection of grasses or fine sawdust tinted green.