First I painted the lower hull on earth paint. Once dry I coated the lower hull in mod podge and the sprinkled the hull in sived garden dirt and secured that with scenic glue made from mod podge and water in a 1:3 mix.
Once that was dry I added the wheels back on and then made a mix out of earth tone pastels and enamel thinners and painted this on liberally.
Once that was dry it possible to knock it back a bit with a stiff brush if desired.
You can get mod podge and pastels at the warehouse. Quite inexpensively...