Before reusing a potting mix, consider its condition and what happened to the plant that grew in it. A container from a sound seasonal display presents a different question from one with unexplained root decay or persistent pests.
Remove debris and inspect the structure. A mix that has broken down or become compacted may no longer perform well, even if a treatment could reduce some unwanted organisms.
Treatment does not renew everything
Heating or other treatment cannot automatically restore the physical properties or nutrient balance of an old medium. Avoid treating 'disinfected' as a synonym for 'suitable for any new plant'.
For an unidentified disease problem, seek appropriate advice rather than choosing a household heating or chemical method by guesswork. The cause matters when deciding whether material should be reused at all.
Keep reused batches separate and labelled. If you use some material in a new mix, record the proportion and the plants receiving it. For delicate seedlings or valuable plants, consider whether fresh suitable medium is the more practical choice. Reuse should be a considered decision, not a way to conceal an unknown problem beneath a new plant.
Oklahoma State University Extension limits its reuse recommendation to mixes in which plants were healthy and showed no disease. It also discusses nutrient depletion and compaction. A successful treatment for one problem does not by itself restore an old mix.
Related reading: Potting Soil Treatment: Steam, Heat and Other Methods.