Hydrogen Peroxide or H2O2 has been a common additive used in the hydroponics industry for several decades and is simply water with an extra oxygen atom attached. While this may sound innocuous H2O2 is very powerful and unfortunately with that power it comes with risks as well.
On its own water (H2O) is a very stable molecule, but sticking the extra oxygen onto it makes for instability. This means that it is very reactive and will readily give it up the oxygen when other chemicals are encountered or it can spontaneously decompose into water and oxygen.
H2O2 as a Disinfectant
In higher forms of life such as plants and animals Hydrogen Peroxide is produced under normal metabolic functions. Since it is potentially highly toxic being so reactive, plants and animals are well protected against it having enzymes know as peroxidases which can break it down quickly. On the other hand many types of microbes do not have this ability and this gives us an advantage to exploit. Low concentrations of H2O2 can be harmful to microbes, but since plants can break it down they are immune. At a 29% concentration plants can be fed at 1-2 ml per liter of nutrient solution.
Algae also thrives in hydroponics systems. On its own it is pretty much harmless, and at the worst it will steal some of the nutrients. It is unsightly however and could potentially act as a vector for more serious pathogens. Hydrogen Peroxide works extremely well in controlling it.
Warning: Never us Hydrogen Peroxide in conjunction with beneficial microbes. H2O2 does not discriminate and will kill beneficials along with pathogens.
Along with feeding directly to plants with nutrients, H2O2 can also be used as a general disinfectant around the grow room for things like pots, floors and other equipment. Since it breaks down into water and oxygen it will not leave any residues. To use it in this way, dilute with 20 parts water to 1 part H2O2 along with a drop of detergent to help it penetrate. Also keep in mind that disinfectants work best once a surface is already clean, this way it is not losing potency by reacting with dirt and grime. Always use protective gloves when handling concentrated H2O2, but especially when using it in this way.
If you have ever pulled up a potted plant out of its container along with the soil you will notice that most of the roots wind up clustering around the drain holes. While plants do release more oxygen than they absorb it is only the case for the leaves where the photosynthesis is happening. Roots have to take their oxygen from their environment and that means that in a dense compact soil there's little oxygen being circulated resulting in poorly developed root systems. The roots will seek out any potential source of oxygen they can find, which is why they cluster around the drain holes.
Since there is a lot of material for H2O2 to react with it will quickly break down liberating the oxygen. This is an excellent way of quickly and evenly distributing oxygen throughout all of the soil.
In higher concentrations H2O2 can be quite hazardous and can burn skin, being especially dangerous to the eyes. Since it degrades forming oxygen it means that inside an sealed container it will build up pressure which can cause the container to explode. The hotter the temperature the more rapidly oxygen is liberated and the more the cause for concern. Light can also cause issues, and so it is best to store it in a cool dark place. As with any chemical product, it is recommended to consult the safety data sheet (SDS) for detailed information on safe handling.
There are many claims of Hydrogen Peroxide being a miracle cure, but the scientific evidence to back this up is questionable. Under no circumstances should Hydrogen Peroxide be ingested orally. It will breakdown inside of the stomach forming oxygen which can increase pressures to such a point that ruptures may occur.
As previously mentioned H2O2 is a very unstable molecule and will degrade over time. This depends on many factors; concentration, amount of light, pH, and most importantly the amount of impurities there are to react with. Soil especially has a lot that it can react with and so it won't stay active for very long. Its concentration in a nutrient solution will decrease by 50% effectiveness over a set amount of time and from there that will decrease by 50% over the same amount of time, and will eventually be essentially zero.
The half life of everyone's solution is going to be different, those that are cool and clean will have the longest while a feeding with high levels of organic material (like fish emulsion and guano, mineral based organic products will have less effect) or one heavily infected with root rot will have a very short one. Unfortunately there's no way for the average user to be able to figure this out, but is important that the grower be aware that the potency can go down very quickly and that by the time the next feeding comes around there might not be any H2O2 remaining.
While unrelated to growing, this may be of interest to some readers. It has become trendy to play and collect video games and personal computers from the 80s and 90s. A lot of the time the plastic has discolored into an ugly yellow-brown colour. An application of Hydrogen Peroxide under ultra violet light can eliminate this discoloration, restoring the plastic to its original color. There are several do it yourself recipes and methods available online.