Limited experience and totally anecdotal - I found that adding fulvic acid boosted growth on bucephalandra that had stagnated. So I would have to assume something in the uptake allowed for better nutrient uptake that then boosted growth?
Lol - I see where you are going with your train of thought. Dead and decaying wood should among other things dump some form of humus into your tank, but does this make plants grow ... now this ain't peat or coal you are talking about but just a bit of wood. Over time in a balanced tank sure why will it not play a part in helping the ecosystem support plant growth. As to the OP's question about humic acid in the context of the aquarium - I would rather look at the more soluble fulvic acid over a wider pH range to potentially play a part in nutrient uptake. If you are going to add "snake oil" - give it a fighting chance and let it play nice in water at our typical pH levels.
I was thinking in a natural setting were aquatic plants grow. There are diffrent sources where plants get there nutrients from. One them could be from the decay of diffrent leaf or wood material. One of the substances in their being humic acid could contribute to plant growth because it is part of the system where all other nutrient sources are part of in a natural growing aquatic environment. I know that in land plants that humic and fluvic' acid helps in increase plant root growth and metabolism. So was wanting to know if it was observed in aquatic plants.
Here are quotes on this subject from the article "Dissolved, Particulate and Microbial Biomass Organic Carbon (DOC, MBC, and POC)" of BarrReport:
"Humic substances are polymeric mixtures derived largely from plant material and are resistant to decay. They are divided in to three main groups based on chemical characteristics: Fulvic acids, Humic acid and Humin. They hold high concentrations of metals in souliton as well as reduce toxicity of certain metals by complexing. This can increase the bioavailabilty of metals."
"Humic acids can bind enzymes causing inhibition by binding one or more sites on the surface to form a monolayer on the protein’s surface producing hydrophilic (less water loving) than the protein itself. This impacts photosynthesis. Humic acids can lower growth due to this effect and remove divalent cations (Ca etc)."
Does humic acid absorb into the substrates cec too?
Ive been adding a lot of humic acids into my tank every few to couple of days and ive seen erios, syn, and toninas do very well but that could be anecdotal. Its good for the ph lowering so was wondering on the soil, i use africana/malaya mix of soil. So if absorbed by the soil i can keep the soil buffering longer
Water column vs clay based soil sources are different. Generally water column= not really good. Sediment, generally good.
So add peat to the sediment or buy clays with it already mixed, eg ADA aqua soil etc.