[genewitch]

take the part of the plants that isn't used for anything else and ferment it with yeast. Free CO2. I'm not sure how old this trick is, as i used it a decade and a half ago with 2-3 liter bottles with holes drilled in the caps and a bubbler to remove solids to provide CO2 for a heavily planted aquarium bootstrap.

My theory here is that running all the stems, stalks, and roots that you don't use from stuff like annual food crops through a wood chipper or something should macerate the cell walls enough to allow the yeast to feast, at least.