I do the same thing as edweather. I get a large sharp carving knife from the kitchen, yank the whole thing out, shake as much soil off the root ball as I can, cut out the large pieces of roots, cut up and put the smaller ones back in the bottom of the box, cover with soil and maybe put in just a few handfuls of new soil to fill it up. If you chop up some of the roots of helps create more surface area for microorganisms to work on breaking them down.
