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Re: Potassium (but not CF Lighting)

Steve Dixon wrote:

> I would say I have confidence in the kit picking out increments of
> say, 5 ppm K.  For $36 its seems to me a fairly inexpensive way to get
> some baseline info on this important macronutrient.

Thanks for the review, Steve.

> So my tentative conclusion is that potassium, as we have found with
> nitrate, is not so problematic at higher levels.

I think this is because the range of potassium concentrations you're
looking at are all high enough that potassium isn't limiting algae growth.

> When we were first discussing K a couple of years ago, dr. dave
> (Heubert), our resident Ph.D. aquatic botanist, (are you still out there
> dr. dave?) suggested that we should target K levels in the 5 - 10 ppm
> range.  A personal friend of mine, a Ph.D. viticulturalist with an
> undergraduate degree in aquatic biology, offered just a guess that 100
> ppm might be a desirable range.  I have always aimed for the lower
> level. Having said that, I have also always been of the view that some
> extra potassium has given my plants a very nice boost and vibrant
> growth, definitely one of the keys for improving my skill in the hobby.

> What is our current thinking on K?  Does the information I'm reporting
> square or conflict with your views and observations?  Thanks.

Well, I can't address *our* current thinking, but *my* current thinking
runs along these lines...

Potassium concentrations in natural fresh waters rarely exceed 10 mg/l.  
In the NAWQA data that I pulled up and talked about a while back there
were no analyses from undeveloped watersheds with potassium over 10 mg/l;
the median was actually under 1 mg/l.

I find that my plants grow better with 10 mg/l or more in the water.  In
my case the number could be elevated because of the high sodium content of
my tap water, but I'm pretty sure others with water more normal than mine
could verify your observation that plants grow better with unnaturally
high concentrations of potassium.

So why would plants growing in an aquarium need ten times the amount of
potassium that they could have gotten from the water in the setting they
evolved to grow in?  I think the answer is in the substrate.

The cation exchange capacity in natural substrates concentrates potassium
and provides the plants with much more potassium than they can get
directly from the water.  Our artificial substrates usually have much
smaller exchange capacities than can be found in natural substrates.  As a
result, the plants don't get a concentrated supply of potassium from the
substrate.  We need to provide artificially high values in the water to
offset the low values we provide in the substrate.

The difference between Dr. Dave's 10 mg/l advice and your friend's 100
mg/l advice probably reflects the role of cation exchange in the
substrate.  If I recall correctly, your friend's advice was based on
information about potassium in water from agricultural soil.  It might not
take very much cation exchange capacity to concentrate 10 mg/l potassium
in the water column to 100 mg/l potassium (or more) in the water in the

Roger Miller