I use a microwave. A small one. OUTSIDE (it smells)
It’s 800 watts and I cook my home made compost for 18 mins.
I use it in my mix for seedlings and indoor growing
I reintroduce various microbes when appropriate
Soil is not " sterilized " but I am not buying an autoclave.
here are a couple articles, it looks like 180 degrees and 30 minutes in the oven. Beware the whole house will smell like a peat fire and the oven will smell like burning peat the next few times you use it after this.
For smaller amounts of soil I’ve had good luck putting it in a container outside and pouring boiling water through. I never noticed any harm from the dropoff in soil microbes although it must have happened.
There are many ways to sterility, dry or steam/pressure heat. UV, chemicals, gases, radio frequency etc.
For dry method
180°C for at least 2 hours
Or
250°C for 30min to 1 hour.
To steam sterilise in a pressure cooker
120°C for 30 min at 1 bar
The articles temperatures and times are way off, at best they pasteurize, not sterilise.
Hope this helps.
Nutrient content won’t change much. 15cuft is one hell of a batch to sterilize.
Exactly… Either a big oven or a huge pressure cooker!!
Or use a old oil drum and adapt like some people for mushroom cultivation.
I’m a bottles man, no organics, but here you can find useful info … 
RR Roger Rabbit, some.of you might know from other forums dedicated to mushroom farming.
Mushroom mountain oil drum tech
https://www.mushroomvideos.com/pmwiki.php?n=55-Gallon-Drum-Sterilizer
oven to 180 degrees thru out for 60 minutes. (they say 30)
or Ozone. That has been my go to lately for sterilizing dirt. (has to be stirred up)
The info on the site isn’t the most accurate. And it should be 180° Celsius not Fahrenheit. Big difference. But that’s why I was saying that they got the info wrong. They telling one to sterile when it’s pasteurization
Thanks everyone, it’s 1.5 x 10.
Got a lot to think about, probably gonna do a small trial run. In separate room with pest control
I should have more clear also. Get the heat of the dirt to 180 (and i meant F), and hold it there for 60 minutes. I used to use 350 in the oven to get it there. But now, I have an ozone generator I use. Like it far better.
Fahrenheit bo Celsius in this case → 180f is 83°C in this case it’s a pasteurization, ideally it should be 200f approx 85°C. I guess in this case it’s enough to cook the eggs, insects, nemotods and so on. But most bacteria won’t die, only the pathogen ones (for us that is). Sterility includes a much wider scope of thermophile bacteria and also viruses. Plus many bacteria will sporulate (a latent form) and don’t get destroyed hence the higher temps.
But if the ozone works, cool 
It’s not sterilization because you don’t want sterilization - the OP said he wanted to kill fungus gnats. Most insects die at much lower temperatures than true sterilization. For example beg bugs die at 113 degrees F. I think getting the soil to 140 degrees F is more than enough to wipe out gnat larvae.
I microwave my soil in small batches for 2 to 3 min at a time.
Could you give me a little knowledge about ozone generator, I need to clean over 15 cubic ft. Don’t think oven is way too go
The topic does say “Sterilizing soil”.
I was just informing and alerting to the inaccuracie of the article that’s all.
One thing is sterile and another is pasteurized.
Anyway. I hope it helped. Not trying to diss anyone. Only the article.
For all that amount…
I suggest this…
https://www.mushroomvideos.com/pmwiki.php?n=55-Gallon-Drum-Sterilizer
or else spread it out as much as you can over tarpaulin and let the soil bake in the sun.
pasteurize soil your soil, sterilize you containers 
. I’ll leave it to you guys 
Would it suffice to fill a kiddie pool with the soil, cover with black plastic (trash bags?) and let it ‘bake’ in the sun. I’d think that you could get to the right temps assuming it wasn’t too cold outside… says the guy who has never done it before or performed any research