I prefer using a soilless mix when starting seeds. Here's the recipe:
• 1/2 cubic yard peat moss or coconut coir
• 1/2 cubic yard perlite
• 10 lbs. bone meal
• 5 lbs. ground limestone
• 5 lbs. blood meal
ATTRA or the National Sustainable Agriculture Information Service has a webpage called Potting Mixes for Certified Organic Production. It's loaded with all kinds of information to help organic producers find commercial sources of organic potting soil…or make their own!
1) Clear a work area outside where you will not be concerned about making a mess. Use a drop cloth or newspapers if working inside.
2) Gather your gardening supplies such as gloves and pots. You will also need a large container to mix and possibly store the organic potting soil.
3) Collect your organic potting soil ingredients. This should include coco fiber, expanded shale, organic compost or earthworm castings, and wheat bran.
4) Place equal parts of the organic potting soil ingredients into the container.
5) Mix these supplies together by hand until they are distributed evenly.
6) Use this mixture for potted plants, gardens and growing herbs. It can also be mixed in with the native soil in your organic flower beds.
As with any garden, soil preparation is what really counts and good seed starting mix is no exception. For best results, make sure it is lightweight and drains well, yet contains enough organic matter to hold moisture — you don’t want it drying out too quickly. I’ve included a link to an article of ours that includes several recipes for making your own potting soil.