Re: Ca/Mg/K

Experimental evidence clearly indicates that K, Mg and Ca are required in 
the water column for the 'normal' growth of at least a few submerged 
aquatic plants. The reasons for this are less clear but can be discussed 
on the basis of their chemical properties and physiological/biochemical 
roles. As mentioned, potassium, while a mobile element, is required for 
ionic balance and so must be present both internally and external to the 
cell membrane ... remember that aquatic plants, unlike terrestrial 
plants, have their apoplasm (that part of the plant external to the 
membrane) in direct contact with a solution into which ions can diffuse. 
Calcium is also necessary external to the cell membrane for normal cell 
wall development ... internal Ca requirements are actually quite low and 
in fact ionic Ca is extremely toxic to plant cell function. Both Ca and 
Mg in land plants do not 'move' as readily in the xylem as other ions. 
The hypothesis is that because the movement of water through aquatic 
plants is less than in land plants, Mg requirements cant be satisfied by 
upward movement. At no time have I said that Ca and Mg cant move within 
the xylem of aquatic plants ... in fact experimental evidence clearly 
shows that at least Ca can move up or down in aquatic plants tissues 
(just not at a rapid enough rate to satisfy external requirements).