One way to stretch seed-starting mix is to add pine bark fibers or small nuggets, for example screened from a bag of clean mulch. Avoid damp, smelly or dirty mulch.
It's especially useful if you want your commercial mix to hold less water and more air. Instead of adding coarse Perlite, just use bark fibers or small nuggets the same size.
At $3.50 - $4.25 for 2 cubic feet, bark is CHEAP. Say 50-55 cents per gallon. I've used up to 75% bark. Pine, fir or hemlock are great, probably any evergreen bark.
For fine seeds that need light and surface sowing, the surface has to be fine and not have holes and crevices. I lost a whole tray of petunias, but the Lobelia came up OK. A top layer of pure mix or vermiculite should fix that.
P.S. My mojo for preventin g damping off includes fast-draining mix and coarser bark chips on top as soon as the tips emerge. The coarser bark dries right away even if you top-water, and it does not wick as much as peat.
