Re: "K2O", cation exchange

> From: "David W. Webb" <dwebb at ti_com>
> Subject: Cyanobacteria problem - Journal
> I haven't opened the K2O yet, and don't know of how to test for potassium
> levels in the water.

	It won't be K2O.  The fertilizer manufacturers state potassium
content as K2O equivalent, and phosphorous as P2O5 equivalent.
The potassium will be there as a salt.  Unfortunately, there are no 
easy ways that I know to measure K in water.  If anyone finds one,
I would like to hear about it, as it would be very useful!
	Actually, it is easy using an atomic absorbtion spectrometer,
but I don't have one!

> What effect should I expect from adding K2O to water?

	Since K2O is the anhydride of potassium hydroxide, if you _did_
add some to water, I would expect considerable evolution of heat, and
a _very_ caustic solution.  What does it say on the bag, exactly?

> > From: spush at saudan_hac.com
> > Subject: Re: Calcium and aquatic plants
> > 

> > I'm also unsure that I could establish a reasonable Ca dosage for my tank 
> > since the CEC capacity of my substrate will probably act as a _damping_ 
> > _factor_ for Ca. Ca ions readily displace other cations from cation 
> > exchange sites. Incidentally, this in itself can produce improvements in
> > growth by liberating NH4+, K+ as well as trace elements like Fe, Cu,
> > Mn & Bo.

	I don't think either NH4+ or K+ will be held much on your cation
exchange sites.  Both these tend to be pretty mobile.  Boron (B) won't be held
at all, since it is there as anionic species.  The same is true for 

> > 
> > Again, let me refer this question to the various experts in each field.

	I'm not sure I qualify, but...

> > How do we best establish a reasonable Ca dosage?

	I don't think dosage should be much of a problem for anything
one can _measure_ in the water; if there isn't enough, add more, if too
much, add less.  The question then is; what is enough?  We seem to have
some empirical ideas about this for Fe and NO3-, and they work reasonably
well.  It is easy to measure Ca++ + Mg++, as general hardness, and the two
measurements can be separated without too much problem by a bit more
analytical chemistry, so if you are really worried about Ca++ availability,
why not do it that way, instead of worrying about _dosing_.  Base things
on the concentrations in the water.
	This is why I would like a K test!

Paul Sears        Ottawa, Canada

Finger ap626 at freenet_carleton.ca for PGP public key.