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Cation Ratio's - How Important is It?

James Purchas wrote

Or does it really not matter if the ratio of the cations in our
tanks and in a tropical stream is different, given sufficient Ca,
Mg and K for plantgrowth?

my 2 cents:

The important cations of Ca, Mg, K should exist in proportion to
TDS on the assumption that these are largely taken up by osmosis
and an inbalance of one will effect the uptake of the other.

Additionally, it has been written somewhere that a TDS in access of
500mg/L is bad for plants. So with that as a cap, I would try to keep
the ions of Na, Ca, Mg, and K in a proportion of 4:2:1:1.  In your
case with R/O water, you have some control over your Na
(I understand that dosing Kent R/O Right at 1tsp/10Gal) yields about
210TDS of which about 70-80% is sodium as NaCl).  Others won't
and I think that the harder the water, the more of each supplemented
ion must be added to maintain the overall proportion to total ions.

I'm sure this could spark some debate as most prescriptions for
ion supplementation are based on absolute numbers of 5-20mg/L
or whatever.

Christopher Coleman
christopher.coleman at worldnet_att.net