Thanks for the comment above Somenewguy.
All the stuff Hans mentioned will work for the rough work(bigger areas) ,but you know what "The Man said"...."never touch s**t, even with gloves on ! (Ok I specify that as comedy) Although I don't like getting dirty when I work--gloves are a great tip!
For detail sculpting of terrain, these are my fav.
From bottom to top, a probe(or awl--or just a toothpick), pallete knife(oil painters spatula), putty applicator( about 2/$1 @ Harbor Freight),and last, but in no way least, a tongue depressor(popsickle sticks ok, but no where as good) swipe a few of these next time at the Dr. !
For making impressions of tires and tracks, any spares are good, or even saran wrap between model & ground medium can work. For the Ardenne Dio I made a simple stamp with some x-tra Sherman lengths and some scrap styrene and this worked perfect(with some mould release(or light oil even)on it to avoid build-up)of course I didn't clean it too well afterwards , as you see....