Hi Gary, you might want to check the diorama section of this site, would mean a bit of hard searching but am sure you will find all the answers to your queries.
If you want a bit of lay advice, try mixing an earth colored acrylic paint with your plaster, am not sure what the ratio of paint to plaster should be so you might need to experiment, this helps rid you of any unrealistic plaster colour showing through in you finished piece. If you can afford it go for the ready mix filler instead, i believe it has a more in scale texture then plaster of paris. Then again i could be way off the mark there.
For earth covering try the gutter in front of your house sift through the dirt and sand you find there its used the world over by diorama makers and its free, you should find enough for your needs, if not try your neighbors thats free too
don't forget to include some of the small stones you find as including them in your dio will aid in realism and break up an otherwise uniform appearance. Diluted white clue or pva (mixed with h20)is the norm for attaching your earth textured ground work static grass etc. one more thing, if you are planing on using anything that will affect the polystyrene under your ground work it might pay to employ a sealant of some kind best inquire at your LHS. hope i haven't misled you in any way and hope this helps. cheers.
P.S Try and find Shep Paines Making or building dioramas its bible as far as dioramas go.
