I disagree with @peer's answer. I've been making my own seed mix for the past three years, I have never sterilized it, and I only once had a problem with damping off when I covered a flat with plastic wrap. (So that's 1 flat out of maybe 80 or more that had a problem, and I'm pretty sure that was caused by too much moisture and not enough air flow.) Elliot Coleman agrees with this; I think I read his position on not sterilizing potting soil in "The New Organic Grower".
Most "sterile seed starting mix" that I've seen in the garden center is a mixture of peat and vermiculite, or something similar: there's nothing in there to feed the plants. I think the key to getting good germination is that the mix stays moist, thus the peat component. If the mixture has a fertilizer component then I think (at least in the US) it is required to be labeled as such, and you'll see the N-P-K values printed on the package.
I've posted my recipe for seed starting mix here before:
- 5 gal sifted garden soil
- 5 gal sifted mature horse manure compost (We use a wood-based bedding product in the horse stalls, and the compost has some of the desirable properties of peat -- mainly that it holds moisture well.)
- 5 gal coarse sand
- "sprinkling" of equal parts lime, greensand, and rock phosphate -- strength depending on what I'm potting
You'll want the strength of the fertilizer to be lower for seed starting, especially if your compost and garden soil are fertile. If your compost is acidic (or if you use some compost and some peat), then add a little extra sprinkling of lime to raise the pH. When you pot up your seedlings to larger pots, you can use a slightly stronger potting soil, and you may want to include just a little dried blood for a small nitrogen boost. (Overfeeding seedlings seems to make them leggy.)
This is not an exact science. I've mixed seed starting soil using variations of the above because I didn't have enough of one material or the other on hand at the time, and the seeds came up and the plants grew just fine. Experiment with whatever you can get locally in bulk for cheap. (Note that my recipe uses "on farm" ingredients except the fertilizers.)