"Humic acids" is an indeterminate chemical name. It refers to what is left when organic compounds in nature are broken down by natural processes. It will contain various organic molecules, both small and large. Its effect on plants is not proven. Plants generally absorb ions from inorganic molecules for nutrients, not big organic molecules. Plants use CO2 as a source for carbon, because it is the most efficient way they have to get carbon. Glutaraldehyde is a compound that plants can get some carbon from, but nowhere near as easily as from CO2. I know of no other molecule that plants can use for carbon, and certainly none that they can efficiently use. As I recall, Tom Barr was referring to the solid powder form of Diamond Black when he suggested it could be used as a thin layer under a soil substrate. Liquid humic acids could be a big problem in the water if used in the substrate - any liquid in the substrate will soon be in the water, too. But, I haven't tried it, so maybe someone who likes to experiment will try it.