Another method: real dirt. Run a few different colors of dried dirt through seive, retain smallest particles. Put in bowl. Small twigs in an old coffee bean grinder, 3 or 4 quick pumps (for exposed roots), add to bowl. Smallest ballast, few different colors, add to bowl. Woodland Scenics Scenic Cement to bowl, mix and bring to a real soft cookie dough consistency. Add cheapest earth tone acrylic to taste. Serve. Before dry, lightly dust with earth tone sands and dusts/pigments to depict paths, roads, etc. Add larger ballasts to edges of roadways, paths and smaller to center. Pat down slightly. Running tires, scale feet, tracks on paths/roads into lightly damp ground will sometimes moisten those tracks and give appropriate shadowing. If not, wait until fully dry and wash tracks ever so slightly with oil wash. Seal with couple of coats of matte varnish.
For grass, weeds, I make mixture of static grasses, cut field grasses, small bits clump foliage and fine turfs. With old paint brush, coat couple square inches of ground with Scenic Cement, add grasses with tweezers. Spritz with more scenic cement. Dry brush grasses to kill sheen. Seal with matte varnish.
Vary the colors of grass and ground mixtures in patches. Play with it. Experiment on scrap wood. Have a blast. This my favorite part of modeling.
SteveM
P.S.- I do love Celluclay as well. I try to change it around, to avoid uniformity in my dios.