Excellent question!

Always wondered about that.

Can't wait to see the feedback. =)

Perhaps it's because when H2O2 comes in contact with certain
organisms and blood it releases oxygen and water. Maybe some
believe it helps reduce infective agents and gets more oxygen
to the roots. Just sayin'.
Karen

One reason I'm seeing for this suggestion is to combat fungus gnats. I don't have a problem with them, so not a bit interested in that but some might be. Talk amongst yourselves...! ;)

The other reason is to 'oxygenate the soil.' It is this that interests me. Seems like a bunch of hooey, but my gut reaction isn't much to go on... a little science learnin' would be better...

I'm very suspicious because TONS of links to sites that seem to be selling something were the majority of results in the searches I could think to try. Nothing at an .edu site or similar reliable source of info.

I think this is just something you use when you need it. Using it all the time could have detrimental effects on soil composition and the balance of soil organisms.

Just as an example of what i mean, hydrogen peroxide can be used as a mouth rinse to whiten your teeth. BUT (and this is a big but) a week or so after I started using it for that purpose, I developed oral thrush. The yeast that is naturally present in your mouth is normally kept in check by good bacteria. The hydrogen peroxide killed off all the bacteria, good and bad. So the yeast multiplied unchecked to the point of being harmful. For the same reason, I cured it in a couple days just by eating yogurt (probiotics). Probiotics, to those who dont know, are beneficial bacteria. Since soil has both harmful and beneficial organisms in it, I can see a similar thing happening here.

Its used in hydroponics to disinfect the systems and to increase O2 levels in the nutrient solution.

For potted plants, with long term regular use, the peroxide will break down organic components in the soil and completely undo any benefits you think you'd be getting.

i do not do chemical analysis of my soil, so i have no idea
what lives and for how long in it. i just look at my plants to see if they do ok.
i've started using a very weak solution of 3% peroxide (2-3tsp per gal) about 5 years ago.
i read that is one of the tricks that is used for growing larger plants in containers, when most of the soil media is consumed by root mass and the plant can only be rejuved by removing a mere 1-2" around the root ball in-situ. so the rest of the rootball is mostly untouched and very compacted (some top dressing should be done too). peroxide gives roots extra oxygen in a compacted root ball.
but solution has to be fresh, not allowed to stand and be used immediately.
since i have a number of very large and heavy plants that i cannot repot - i started experimenting. after some time i started watering most if not all my plants with above solution regularly (most of the time, except when i flush).
then i started putting my moisture lovers on water-wicks for sub-irrigation (i need to be away for longer then sev weeks at a time and i keep close to 100 plants indoors - so no irrigation system is possible beyond passive wicks and ceramic spikes in city apt).
naturally i add peroxide into the water reservoir too - to suppress algae growth (that i learned from hydro forums) and to keep water solution from going briny. of course, i do liquid feed too thru water.
i don't see any visible detrimental results on my plants. they are all doing well. it's been 4-5 years.i also started to add water for cacti/jades/sansi - so just everything really at this point.
my medium contains 40-50% perlite and the balance is divided between peat/coir peat/fine bark/scoria and some LF sphag or sphag peat depending on the type of plant.
i read that peroxide solution also will prevent mildew/mold/fungal infections in the soil - fine with me.
since i grow only indoors (+some balcony in summer with no insects to speak of on a high fl except scale) i use exclusively soiless medium and do not use organic ferts (only kelp from time to time, but only for top watering, or very occasional spray with fish emulsion, but spray only, not in soil).
so, so far i've had only positive results.
plants that i grow: sev ficus lyratas, f benji, f. alii, sev large polyscias; aroids like spath, ags, alocasia,anthuriums, syngonium, caladiums; amaryllis, begonias, stroms, african violets, holiday cacti, tropical ferns; mature jades, var cacti, sansi, broms and ivies.
i think that about covers it :).
i have subsequently read that watering/misting young seedlings/sprouts/seeds helps to prevent mold/damping off. since i was sprouting some tropicals that required sev months on the heat mat and in moist condition - preventing seeds from mildewing was necessary (peroxide to the resque again: done that, like it, recommend it).
by the way for these hard to germinate seeds it is advised to soak the seeds either in peroxide or alcohol solution (much higher concentration) to kill off surface 'flora' - to sterilize the seed surface to keep contaminating the medium.

purple,
i don't know why you can't find anything on peroxide watering/hydro. there's plenty. since i don't need extra studies to cont doing what i am doing i'll limit myself to just a quick superficial search here: a more thorough search requires more time and you can do it yourself and report.
you'll need to look thru pretty hairy pdf docs with all the specialized horti stuff (it's too much for me, but i can get the gist of it by looking at summary and discussion - but it does take an effort to plough thru) - they won't necessarily be on the 1st page...
i entered: watering plants hydrogen peroxide
and pulled a bunch of articles, then i added another word to the string: study . you can try 'research'.
this pops up on the very 1st page:
The increased biomass
of the aerial portions of plants in treated soil indicates that injecting H2O2 into heavy loam clay soils may be a useful
management tool in poorly aerated soil.
http://www.scielo.cl/pdf/chiljar/v69n1/at12.pdf
http://www.actahort.org/books/889/889_71.htm - same presentation at int symposium

there are various studies on how absorption of h2o2 effects other nutrient uptake, reduces heavy metal stress in plants, etc.
then there's hydro applications, aquarium/aquaculture applications (it reduces algae, but does not harm the fish)...i don't want to spend time to look thru ev.
here's what i call 'a fun article': very entertaining

http://foodgradeh2o2.com/garden-uses/growing-tomatoes/
if i were to grow tomatoes myself, i'd deff would try the method :).

then there's some reg articles for greenhouse growers, which often have interesting info in them (for me at least):
http://www.quickgrow.com/gardening_articles/hydrogen_peroxide_horticulture.html
'can use it all the time' ��" gnats problem here:
http://www.gardenandgreenhouse.net/index.php/past-issues-mainmenu-18/104-2011-garden-greenhouse/september-2011/1192-efficient-gardening

mind you, i am not trying to convert or convince you. i am just responding to your 'can't find any reliable studies' post and calling it 'a fad'. i think, it's deff NOT a fad.