Like @Maryland Guppy
, I’m struggling to understand the implication. I don’t think we can use surface area in the way that they do in agriculture. I think that lbs/acre is used for application convenience. Soil test reports usually reference ppm and then convert to the quantity needed to adjust. In a tank, particularly with stems, we have a 3D environment, as though in agriculture you could grow everything underground and look for saturation levels in the soil strata.
If you are saying that a shorter tank may have more relative plant mass, because of the third dimension (height), then isn’t ppm still valid? Seems to me that a better approach would be to determine plant mass as a function of available space, but I don’t know how you would do that.