Recently I bought some potting soil from a hardware store. When I opened it, I discovered that there were small insects (perhaps fruit gnats?) in the soil. Since I was already having problems with spider mites (grr) on my indoor roses,
It turns out to be very easy, although it requires a bit of patience. Essentially, you put the soil on a cookie tray (probably on aluminum foil). You moisten the soil (enough to be damp to the touch but not runny or squishy), then cover the tray with aluminum foil. As the soil heats, the steam from the water sterilizes the soil. When you open the aluminum foil (carefully away from you!), steam should come out. If it doesn’t, then the soil hasn’t been sterilized.
The internet say maybe 1.5 or 2 hours was enough, but I found that my oven always took something closer to 4 or 5 hours to steam the soil… :/ 180F I think is the max the internet recommends (something about releasing toxins at higher temperatures).
It’s also possible to microwave the soil in a ziplock bag with the top open for 2 or 3 minutes, or alternatively in a tupperware with a few holes in the lid for 1 or 2 minutes. I think I will definitely try this next time — the oven took HOURS.