I've read in a few places that it's a good idea to sterilize your soil-less mix before starting seeds, and it seemed like a good idea.
The directions I saw said 6 minutes in the microwave would do it, but of course I had to improvise. I thought, well, I sterilize canning jars and so forth for 10 minutes, so wouldn't that be better? Well, the soil is in the microwave, and I'm on the phone with the mechanic, when I smell smoke... I dash around the kitchen, sniffing, phone to my ear, and I track the source to a little tendril of smoke emerging from the microwave. Egads! I turn off the microwave and peer inside, and I see tiny coals among the soil particles. Egads!! Fortunately, the tupperware container isn't melting or anything, and I slap the lid on to deprive my little fire of oxygen. I quickly finish my business with the mechanic, who is wondering why I seem suddenly distracted, and I take the container of soil outside to cool down. Egads!!! Who knew??!?!! Well, no harm done, and I'm pretty sure my mix is sterile now!
I though I would pass this story along as a word of warning not to exceed that recommended 6 minutes in the microwave!!
BTW, the Pro-mix plus perlite mixture in question has been dampened for use, but still has a smokey smell (imagine that). Do you suppose there's any reason not to use it for winter-sowing?
UPDATE I've learned a few things since this incident, although it still cracks me up, especially the mechanic's reaction when I said, "Aaagh! 'Scuse me, smoke... fire... " and then set the phone down ("Hey, lady! Lady??!? Are you OK? Lady!!").
You MUST add enough water to dampen the potting mix; I'd say 1 cup water per quart of mix. I use a big plastic container that I get from the commercial kitchen shelf at Costco (I think it's for dough raising; it has measurement marks and a yellow lid). I add water, put the lid on loosely, and microwave for 12 minutes or until I see good steam. Then I carefully tamp the lid down into place for another 15 minutes so the steam can do its work.
In addition to killing pests/pathogens, this gives me nice fluffy moistened potting mix to work with. I do add extra perlite and/or polymer moisture crystals to my mix before nuking (just for convenience), but I don't add time-release fertilizer (like Osmocote), because overheating can cause it to dump its fertilizer all at once (not good for seedlings).