I just posted some remarks on this subject. I cant remember which forum so here it is again
Everyone has a different opinion on this subject. I remove the old roots from last years plantings and dig out about 3 or b4 inches of old soil. I mix this with new potting mix and compost and put that back in the planter. It has worked well for me.
Oh. I also heap the new added soil and let everything settle a week before planting.
