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conceptual traps


This note isn't aimed at Michael -- he has just provided me with a soap
box to climb on.  There have been any number of posts to which I could
write this same response.

On Tue, 28 Mar 2000, Michael Rubin wrote:

> I've got hair algae out the kazoo in my 50g and I was casting about
> looking for a magic bullit.  There's really no such thing - magic
> bullits always come at a cost.  I think the best idea for me is to
> hunker down and figure out what's missing in my nutrient balance.

CAUTION, strong opinions follow.

A lot of people on this group when faced with an algae problem (their own
or that of others who they are trying to help) have fallen into a similar
conceptual trap; they ask "what's missing?"

The answer to the question is patently obvious.  THERE IS NOTHING MISSING.
If some essential component were missing then the algae wouldn't be
a problem.

No matter what you do with your tank there will always be something that
limits plant and/or algae growth.  It may be light intensity, it could be
circulation, it could be any one of the essential elements.

Let's take an example that is limited by the availability of nitrogen.
The plants and algae grow at a rate that is determined by the rate that
plant-available nitrogen is added to the tank.  All other essential
nutrients are (by definition) added at a higher rate relatve to the
demand and tend to accumulate in the tank.  When you increase the
nitrogen supply you get more growth -- certainly more growth from the
algae and hopefully more growth from the plants.  In fact you should get
a burst of growth that lasts until the excess accumulation of some other
essential element is depleted.  In the meantime your tank may be a mess.

At some point the accumulated nutrients will be used up and the growth
rate will slow down.  Whatever the growth rate settles down to will be a
*higher* rate than it was before you started adding extra nitrogen.
That's because all of the other potentially growth-controlling nutrients
are added at a higher rate than nitrogen was back when the nitrogen supply
was growth-controlling.  This doesn't solve your problem.

As long as you're adding those nutrients to the water column you're
feeding both the plants and the algae and there's no reason to believe
that plants will be favored over algae.  That initial burst of growth and
the subsequently higher growth rates may be seen in algae growth as easily
(or more easily) than in plant growth because the dissolved nutrients are
available to plants and to the algae.

There is another conceptual trap that victimizes people in this group, and
it seems to be closely related to the first problem.  People try to manage
their tank with test kits.  This approach seems to work for some but it's
a pretty long and winding route for others.

When you test nutrients in the water column you are -- under the best
scenario -- testing the nutrient supply that is available to algae and to
floating plants.  The nutrient supply to rooted plants is something
different.  The test kits don't necessarily work right and even the best
test kits may not give an accurate measure of nutrients that are available
to the algae and floating plants.  The actual nutrient availability may be
higher (in the case of phosphorus) or lower (in the case of iron) then the
value indicated by a test kit.  As a result, the common scenario probably
is substantially different from the best scenario.

The nutrient values you test in the water column certainly aren't the
nutrient levels available to rooted plants.  Unless, or course, your
tank's substrate is inert and nutrient-free.  Most aren't.  Normally the
substrate can provide a substantial amount of the nutrient needs of
rooted plants.  Iron and phosphorus tend to settle into the substrate.
Exchangeable nutrients like potassium, calcium and magnesium are
concentrated in the substrate by cation exchange.

You can only judge the nutrient supply to your plants by observing their
growth.  If their growth is robust and healthy then their nutrient supply
is fine; it doesn't make any difference what the test kits say.  If their
growth is weak and unhealthy then something - maybe a nutrient - is
missing and what your test kits tell you may or may not be useful.

What's more, some test kit results get passed around on this list with
little apparent thought to what they might mean.  Dissolved iron
concentrations that are a substantial part of a ppm are probably more than
an order of magnitude above levels that are actually meaningful to your
plants.  If you have those high levels and your plants are showing
symptoms similar to iron deficiency, what might that mean?  I doubt
there's a plant on earth that evolved to required 20 or 30 mg/l of
potassium in the water it grows in.  If your plants seem to need that
much, what might the problem be?

Roger Miller

P.S.  Some possible answers to the iron question:  1) it's not iron
deficiency 2) the iron shown in the test isn't available to the plants 3)
the test result is bogus.  A possible answer to the potassium question...
most natural surface water will contain only 2-3 ppm of potassium; higher
concentrations are provided in aquatic soils, and that's probably where
rooted plants satisfy most of their demand.  You might be able to dose
your water up to a potassium concentration that would be normal in soil
pore water, but why would you?