When I'm not recycling whatever dirt from whichever old pots (or, for that matter, native soil dug out of the ground), I do a 50-50 mix of fine redwood mulch (pieces are dime-to-nickle size) with a planting mix from a local chain (Summerwinds), which has soil but also some amount of amendments such as chicken manure. I add fertilizer and alfalfa meal to this. (The redwood mulch is for drainage, and it breaks down to add organic material. I gather people on the east coast and in the south use pine fines for roughly the same purpose.)
