I use Summerwinds (a nursery chain here) organic planting mix for potting. Depending on what I'm planting, I may mix in fine redwood mulch and vermiculite, and maybe some alfalfa meal and/or compost. Often, though, I will just recycle dirt from old containers before adding in new, unused planting mix. (If the old containers last held nursery plants, the soil in there is often very light.) And then, of course, add in whatever time release and/or organic fertilizers.
Curiously enough, I seem to be having a bit better success seed starting with Miracle Gro organic planting mix, as opposed to any kind of seed starting mix. This is with daylily seeds, though... YMMV for irises and other plants. (And I still have to play games to keep the gnat population down.
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