The reason potting mixes should be sterile is because the pathogens have no competition inside the way they do outside. Seedlings in particular are highly susceptible to diseases caused by soil born pathogens. If you haven't heard of it google on 'damping off'.

If you don't wish to buy peat or 'expensive' mixes (not sure what qualifies as expensive), but do have compost then I think we both know what you will be using ;-)

Seed starting is usually done in very small containers or seed trays and doesn't require a lot of pre-made mix. Consider going with something that hasn't been in contact with the ground for a long time to give the plants a healthy start.

Damping off is caused by a fungi that functions best in cool, moist soils so the medium used for seed starting should not be watered from above, only from below, and once the seedlings emerge the top of the soil should not be more than just barely moist. Camomile tea drenches have been found to be an effective preventer of damping off, if there is a problem with that. Once I learned to not water from above I never saw a problem with damping off.

Thanks for the replies. I have some finished compost from last fall. Do you think I can use that or will it likely have too many pathogens? Would it help to pop a small batch into the oven? What do you/other folks use? And thanks for the tip on watering, kimmsr!

Compost is supposed to be "alive". I wouldn't sterilize it.

Lorna

I use a mix of extra vermiculite and peat moss added to a potting mix. Do NOT put compost in the oven. A friend cooked some outside and it stunk to high heaven.

tj

Ten percent to forty percent vermicompost by volume in a potting mix is supposed to be a great way to fight off damping off and promote seedling growth. It's the beneficial microherd which gives vermicompost the value though, so you don't want to ever sterilize it.

All the best,
-Patrick

By vermicompost, you do mean worm castings - right? I thought that worm castings used in more than 20% of a mix would give a rather heavy compacting effect to your mix? I guess it would depend on the porousity of your remaining mix, but then wouldn't it settle down to the bottom of the containers and compact there? I am a huge fan of worm castings- don't get me wrong. I use them in all my container mixes, all my teas, and top dress several times a year with them. I just thought the % of addition to a container mix would be best in smaller amounts. Not trying to rile ya up, I agree that a healthy microherd is the ONLY way to go. I mix all my container mixes in early March, they sit until first week of May before I add the transplanted peppers, tomatoes etc. Results speak loudly.

Aye Rootdoctor, I meant worm poop. :)

The 10-40% figure I gave was a quote from memory of a post by KellyS, my favourite Vermigoddess. However, Kelly repeatedly said in her many posts here on GW and elsewhere, that what worked best for people was the best plan to follow.

This year I'm hopeful of starting a few seedlings indoors and will keep your 20% suggestion in mind when mixing the soil. Thank you!

All the best,
-Patrick