Steve,
I'm a fan of good 'ol Celluclay, another "instant" papier-mache mix for groundwork. It's essentially shredded cellulose ("newsprint") that you mix with water and a dollop of white glue. It has a really nice earthy texture as is, or you can mix static grass, sand or other crud into it, and can be pre-coloured with acrylic paint. It comes in a 6" x 6" "brick" like the Aves product Bob recommended (I've never tried the Aves stuff, but if a miniaturist of Bob's calibre recommends it, who's to argue ?) and you can find it in crafts stores like Michaels for around $7.00 Cdn.
Anyway, it's another option.
Cheers,
Brian