>> I actually do mess around with the first part of the pro mix that I add to the potting soil and then I just add more promix.
Great, that removes any risk that the changeover will be so abrupt as to interfere with capillary "connectedness".
>> Does hydrogen peroxide help anaerobic soil.
I read that in many places, and finally read it somewhere that REALLY seemed to know what they were talking about. And it makes sense. 2 H2O2 --> 2 H2O + O2.
>> the odor told me it was anaerobic.
The nose knows.
>> I decided to hit it with some H2O2. A day later the soil smelled very fresh again. Is this soil now usable??
Maybe or probably. Has the water evaporated enough that the mix is light and filled with air again?
Maybe the H2O2 only killed enough microbes that they aren't fermenting enough to stink enough for you to detect.
Either way, I would first remove enough water to let the air back in! And turn it and spread it around enough so that the fermentation products (root-hair poison!) can evaporate.
I think the careful answer is that you should FLUSH away the fermentation products, then drain and dry it so it's aerobic again.
The more careful answer would be that it only stunk if enough bacteria grew in it to produce enough acid and alcohol for a human nose to detect. That's kind of a lot of microbes. Did you want fairly sterile seed-starting mix, or just potting soil? I would not worry about using it as p;otting soil, with good drainage.
Now my drainage fetish comes out full-force.
ANY mix will go anaerobic or hypoxic if the mix is water-logged. If the voids fill with water, there is NO air. If the voids MOSTLY fill with water, air can't diffuse through channels into the lower inches of the mix. Again, no oxygen. Dead roots. Soil bacteria fermenting instead of oxidizing humus and releasing nutrients. Mycorrhiza suffocating instead of helping roots absorb water and nutrients.
Either never water containers enough to saturate the mix, or provide adequate drainage so that excess water drains away. That's as important to a plant as never holding your head completely underwater for longer than 5 minutes.
I like to add bark nuggets or chips or shreds to open up peat-based mixes. Others like coarse Perlite.
