Kim, microwaves vary as to heating so to give how many minutes would vary.
Wet soil heats more quickly, but even when using dry potting soil, I heat it till
it steams without melting the plastic pot. In trying this, I did slightly melt a
couple of pots. It does get really hot, and have to run cold water through
just to handle it.
Last year I had seedlings in basement and upstairs. The ones upstairs
had the potting soil nuked, but basement not. Used same potting soil
for all. The ones downstairs had gnats so bad we had to resort to
chemicals. No gnats upstairs. Used distilled water and peroxide as
described above on all. No problem this winter, as yet anyway.
I have in past years used the vacuum cleaner to rid each crop of tiny
gnats. That saved the seedlings without having to use chemicals inside
the house, but it did take constant surveillance. This is why I tried
the microwave.
