Many of my gardening friends have complained in the past few years about the quality of the standard potting mixes that are available in most garden-stores. One friend in particular, who starts a thousand or more plants each spring for school gardens, community gardens, and a Plant-a-Row-for-the-Hungry garden, has lost many hundreds of plants as a result of "burning" from a bad potting mix.
Last year, my friend used a bag of a professional potting mix, Faford brand, that she bought from a greenhouse supply store up in Ballground. The mix did not include fertilizer, so she was able to regulate the amount of nutrients that were available to the plants and to avoid burning her seedlings, which are so very sensitive to the salts in conventional fertilizers.
In my own shopping for potting mixes, I've also noticed the variable quality issue, even in seed starting mixes that are no-fertilizer-added products. Some are just very finely textured peat, and some are the finely textured peat plus perlite and possibly something else. For seed starting, the finely-textured aspect plays an important role in keeping very small seeds from washing too deeply into the soil to push their seed-leaves up into the light.
This year, I've started some seeds in Jiffy Pellets, but yesterday I also mixed up a batch of my own mix, filled a flat, and planted some seeds. The recipe I used is based on one from ATTRA, from its publication "Potting mixes for certified organic production." If you go the webpage to view (or download!) the publication, the recipes are way down toward the end, in appendix 3.
My mix is equal parts compost and peat moss and a quarter-part of perlite. This mix filled a 5-gallon bucket about 2/3 full. To these main components, I added a couple of Tablespoons of an organic fertilizer mix. If my compost had been "heavier," I would have used more perlite. Also, if I'd had vermiculite in my supply area rather than perlite, I might have used that, instead. Basically, I used what I had on hand.
For those who might not want to sort through the numerous mixes in the ATTRA publication, Organic Gardening offers a pared-down list that was gleaned from the ATTRA set.