More specifically the mycorrhizal and trichoderma fungi. The mycorrhizal fungi form a symbiotic relationship with the roots. There is endomycorrhizal fungus which forms an intracellular relationship (on the inside) and ectomycorrhizal which forms an extracellular relationship, a 'sheath' on the outside. The fungus serves various purposes for the plant roots including the uptake of water and nutrients and even protects the roots from unfriendly fungus and bacteria.
Then there are several species of bacterial microorganisms that also assist in breaking down organic matter, preventing salt accumulation and fixing nitrogen (from the atmosphere). These are all part of the microherd, which is of particular and extreme importance in organic agriculture (including indoor organic ganja gardening).
Along with the products toquer mentioned, these microbes are also found in some Fox Farm products: soils like Ocean Forest and Peace-Of-Mind dry fertilizers. Typically products will say on the front if that they contain mycorrhizae and/or trichoderma, then they'll list the species and the populations.
The point is: if you are using these products and your plant has formed a symbiotic relationship with the microherd, then you don't want to wipe the microherd out by using hydrogen peroxide. This will put negative stress on the plant, which is now suddenly forced to process and uptake nutrients itself. Growth and potentially yield and potency could be negatively affected.
Hydrogen peroxide isn't the only thing that is horrible for micro-organisms, and especially in organic soil grows; you should avoid it, along with chlorine/chloramine (tap water), synthetic nutrients and chelating agents such as ethylenediaminetetraacetic acid (EDTA).