It would seem to me when screening pine bark 1/8 - 1/4 for the 5--1-1 mix, the material passing thru the 1/8" screen could be combined with some spagnum and pearlite and would make a good seed starting medium. Has anyone tried this? Any problems?
The 5:1:1 mix usually doesn't require screening for fines - only for very large particles in the 3/8+ size range. If you're screening for the gritty mix, which IS best with 1/8-1/4" bark, you can use both the fines & large particles up to 3/8" for the 5:1:1 mix.
For seed starting, I like a very coarse medium. Most often what I do is sow seeds on top of the 5:1:1 or gritty mix, then cover them with peat or Turface fines & keep the surface misted until I see the first sign of a new plant emerging, which means the seed radicle (taproot) is already burrowing it's way downward. I realize that's not the correct physiological term, but it LOOKS like that's what is happening.