[Tilted Irish Kilt]

greeneyes:"If mixing your own (could be cheaper) you could try a soiless mix of equal parts sphagnum peat moss (or coir), perlite, and Vermiculite. "

Just as a reminder, " perlite, and Vermiculite" are not ferilizers , but will increase the frability of your soil (light , less dense).
"Spahagnum peat moss or (choir)" will retain soil moisture, and allow for soil moisture during dry summer heat, while being an almost sterile soil medium.
Also , it incidently adds to soil friability, and keeps the soil a lighter texture and adds organic matter to soil, especially important to people with heavy clay soil.

[greeneyes]

Yes, those are good points. The final transplant to the garden needs to be into rich soil. The link does point out that nutrients will need to be added once the seedlings get going.