I've always used GW gravel & sand (which I believe is very finely-shredded cork), glued onto the base with PVA and left overnight to dry.
Firstly I then apply a dark brown ink wash and leave to dry. Once dry, I then drybrush with a lighter brown shade, depending on what sort of earth effect i'm after (with additional drybrushes to achieve a lighter / parched effect. For a bare earth effect then this is the final stage, but if you wanted to go for a grass / forest battlefield effect, glue on some static grass in irregular patterns and/or some small pieces of flock or modelling twigs.
For larger models or mordor-style rough earth, you could use GW gravel or some tiny stones (washed first) from the garden. With this option, I would recommend undercoating the stones black once cleaned before gluing them in place. Then paint them a similar shade to the ink you use (i normally go for doombull brown) before applying the ink all over as above.
I find this method keeps the detail on the bases, without needing excessive painting and overall seems pleasing to the eye. It also comes in handy for basing lots of models quickly if you have some free time to paint, but not enough time to be painting lots of layers / shades on clothing / faces, etc.
Hope this is useful for you
