Two methods

Easiest

Mix light brown acrylic paint (i.e. emulsion or big bottles of Rowney/other acrylic from a hobby store) with Pollyfilla Woodflex. It Must be this product - it is water based, not bleached - hence takes colour & chips and is flexible. Slap that on with a palette knife. Let it dry (only 20 mins if you don't add too much paint). Then dry brush with a very light mushroom/cornmeal colour (I buy the emulsion testers from Hardware stores for this) any near white creamy colour will do. Enhance with flock & mini pebbles to suit. Some grassy bits break up the dull surface. I paint on rivulets of thinned PVA glue and sprinkle on dyed green leafing material (crumbled cork/sieved sawdust) from railway hobby shops (they use it for trees). You may want to dry brush this with very light green/yellow.

Advanced

Disguise edge of bases with coloured (see above) Pollyfilla Woodflex. Cover whole base with slightly thinned PVA glue. sprinkle a rivulet of mini pebbles then cover the rest with course dry sand. Let it dry less than an hour if you have not diluted the glue too much. Then thin down "Burnt Umber" acrylic paint (see above). This really brings out detail on mini pebbles and sandy texture. Dry Brush with creamy wholemeal colour (see above). Add Grassy material (as above).

For both methods dry brushing is the key. I favor a largish (#3) old brush loaded with paint. Then take much of it off onto the palette for a semi dry brush that scrapes over to pick out the highlights.