If a plant is growing with the roots in the water but the rest of the plant above water, does that plant have to absorb all of the nutrients in the ratio that they exist in the water or can it absorb what it needs and reject/return what it doesn't? My (quite basic) understanding is that plants use transpiration to create the pressure difference needed to suck up the water and all the nutrients it contains. This process doesn't allow an obvious way for the plant to either select for certain nutrients, return extra water to its roots, or reject nutrients it already has too much of. Is this because the plants actually can't, or because my understanding is flawed and too simple?
For background, I am asking because I keep aquariums. Like many people in this hobby, I grow pothos (Epipremnum aureum) in several of my tanks with the goal of removing nitrates, which can be deadly to fish in high concentrations, from the water. However if the plant is just taking in water and whatever nutrients are already dissolved into it, without any way to concentrate those nutrients or return excess water, then it stands to reason that having a pothos in the tank will not directly affect the concentration of nitrates. After all, taking a glass of 50/50 water and vinegar, and splitting it into two glasses doesn't give you one half full glass of water and one of vinegar; it just gives you two half glasses of the same mixture. There is still going to be some net effect from me refilling the tank more often with nitrate-free water to account for the water lost via transpiration, but is that it?
I have read through several questions here, but none seems to directly answer my question, or at least not in a way that I understood. While this could potentially be asked on the pets stack, I am hoping it is still on topic here. If I need to refine my question to make it fit better, please let me know.
