I'm one of those that doesn't pasteurise my mixes. I kind of like the lottery of what else comes up in my seedling boxes. Does pasteurising reduce damping off, Lorn?
Finding a good potting media is hard, and I always have my eyes open for promising looking sands/gravels. For some reason there doesn't seem to be anything commercially available in Australia that isn't basically composted pine bark or wood chippings. Sure there's stuff added, but the bulk of the mix is still composed of fine (and not so fine!) chippings.
So... resigning myself to what I can buy, I modify it with extra sharp sand (gravel) and/or peat and compost. A few years back I made my own seedling mix with coir peat and gravel. It worked fine. Had to be aware that nutrients needed supplying regularly (I like seaweed and fish emulsion), but as time went on the coir would wash out, leaving 2-3 year old plants basically growing in sharp sand - no rot! - but very hard to keep cool and watered.
(My seedlings usually start life in styrofoam boxes and remain there for 2-3 years as I have no where to plant them out. Those seedlings I then keep get potted into commercial potting mix in large plastic pots.)
The greatest resource I ever had access to for making seed mixes was tree fern and humus from the bush on the farm I grew up on... but such fantastic materials aren't always easy to acquire. Saying that, I've noticed recently that a company in Tasmania is making shredded tree fern available, so I'll buy that for my special babies and mix it with gravels and potting mix. If you can find leaf mulch it makes a great base layer in seedling boxes. Something my Aunt taught me was to crumble some charcoal over this bottom layer too - it is a great growth stimulant. I presume because it captures and holds nutrients from being washed away.
