I didn't know that. Where can I read more on this topic? I knew plants can utilize nitrites but prefer nitrates. I've never known about ammonia and ammonuim. Although, now that I am thinking about it, ammonium nitrate, NH4NO3 is a common nitrogen fertilizer.Basicly in plant filtration plants prefer to eat ammonium so that has to become depleated befor the plant will chose to eat nitrats