I use a "tushhog". has rock teeth and will grind up to about 6" below soil. Will grind through rock or concrete in a stump too! Grinds 4' wide at a time and can do a huge stump quickly and efficiently even if it is hard as rock. Only drawback is it cuts 4' wide and only half a foot below grade unless you want to spend extra time letting it mill the earth on it's way below grade.