---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Message-Id: <9702201541.AA28462@emr1.emr.ca>
Subject: Re: KHCO3 in PMDD
To: Aquatic-Plants at ActWin_com
Date: Thu, 20 Feb 1997 10:41:14 -0500 (EST)

> From: "A. Inniss" <andrewi at u_washington.edu>
> Subject: KHCO3 in PMDD? & substrate Qs.
> 	I've  been considering using KHCO3 in a PMDD.  I like the idea of
> raising my KH (to say, 3 or 4) while also getting Potassium into my tank.
> At present, my KH is'0', and i don't believe the water here in Seattle
> ever gets over 1 KH.

	If you add enough KHCO3 to the tank to get to KH 4, you will
have lots of potassium there:

	at KH 4, the concentration of HCO3- is 1.42 mM (millimolar),
and if it all comes from KHCO3, the concentration of K would be the 
same (before the plants started consuming it).  This means you would
have about 56 ppm of potassium in the water.  To do this, you would
add 142 mg of KHCO3 for each litre of water.

> Would I be better
> off using Potassium Carbonate (K2CO3)?  Which is more soluble? Which will
> give me better long-term KH/pH stability? 

	As far as the long-term effects are concerned, the carbonate and
bicarbonate are identical, because the carbonate will be converted to
the bicarbonate by reaction with CO2.  In the short term, adding a large
quantity of the carbonate to the tank would cause a very big jump in pH,
enough to be lethal to its inhabitants.  Adding small quantities (under a
ppm) and waiting for the CO2 situation to stabilize again before adding more
would be O.K..
	You will have problems dissolving these salts as part of a PMDD mix.
The carbonate will cause precipitation of MgCO3 and possibly other things,
and even the bicarbonate will almost certainly give difficulties.

	In any case, I think you are better off treating the KH issue and 
the fertilization issue separately.  I think that it is better to adjust
the KH of water added at water changes to what you want, rather than
putting in a bit of bicarbonate each day (slowly rising KH, until the
next water change?).

	Does anyone know, when a K+ ion is absorbed by a plant, what ion
is realeased to the solution to maintain charge balance?  I'm wondering
what the effect the plants have on KH.  If they absorb more +ve metal ions
than (negatively charged) strong acid anions, making up the difference in
charge with interactions involving H+, OH-, HCO3-, the KH would fall.
Does this happen?

> 	I was also thinking of getting ahold of some Calcium Chloride
> (CaCl?) to help with the Calcium levels for my Swords.

	Calcium chloride (CaCl2) should be O.K..

Paul Sears        Ottawa, Canada

Finger ap626 at freenet_carleton.ca for PGP public key.