Answer: Most gardeners prefer a relatively sterile mix for seed starting, so you might try one part milled sphagnum peat moss with one part vermiculite and possibly some perlite. (This is often what is basically sold -- sometimes with a wetting agent added -- as a soilless mix.) This provides good aeration and drainage as well as a good moisture-holding capacity.
Here is a potting mix recipe from the Natural Organic Farmers Association: 16 quarts sphagnum peat moss, 16 quarts vermiculite, 7 ounces of bonemeal, 3 1/2 ounces of dried blood meal, 3 1/2 ounces of ground limestone. This makes about a bushel and would be fine for transplants.
Home made mixes are as varied as the gardeners who use them!
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