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>On the opposite side of the thought, what is it about salt that is
>bad for plants? I would guess it's not the sodium. If so, the
>addition of sodium bicarbonate for buffering would have fallen from
>favor a long time ago. So how do the Cl ions hurt plants?
It is the total solute concentration, not necessary NaCl, that can be
detrimental to plants. If the solute concentration surrounding a
plant is higher than the concentration within its' cells, then water
will move out of the plant via osmosis. Since most plants do not
require sodium, and NaCl is a common solute, it is usually the
culprit. You could dehydrate a plant by immersing it in a solution of
sugar, for example, if the concentration was high enough.
Marine plants can live in high salinity water by excreting excess
sodium or by maintaining a high cellular concentration of other
solutes, e.g., K, or mannitol, for osmotic balance.
Many aquarists have reported difficulty growing plants in water with a
high (500 ppm +) mineral content. As I mentioned above, it is the
osmotic concentration of solutes, not necessarily the total weight of
soultes, that determines suitability, and for that physiologists
measure concentrations in osmoles, i.e., the total number of moles of
solute per liter of solvent. Given the recent discussion on units of
hardness, I won't even touch this.