However, for the onion seeds, I didn't bother with the soil blocker and put them into a tray. The soil will be dumped later on and they will be individually planted so no need to mess with the blocker for them. The onions were a good test to see if the mix worked.
I stayed with a sterile mixture, no compost or soil.
3 gallons peat moss (bought in a big square at lowes)
about a half gallon of vermiculite
2 Tablespoons limestone
So far so good!
I think I may need to add some kind of fertilizer once they get bigger.