Potassium is needed in the water column because it is used to maintain
the ionic balance within the plant. There is a dynamic equilibrium
between the K inside and outside the plant. Calcium and Magnesium are
required in the water because they do not transport readily within the
xylem of aquatic plants. Calcium in particlular is necessary in the water
because there is an extracellular requirement for Ca ... if the Ca level
drops too low then the cell wall does not form properly.