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Nutrients (aka, Roger & Me)
Roger Miller's response to Michael Rubin's "what is missing" comment has
stirred me up quite a bit this morning. So I thought I would share some of
my thoughts. No personal attack on Roger is intended in any sense; in fact,
I'm delighted that Roger has "forced my hand" so-to-speak, and made me
rethink some of the rationales for my own views! :-) Roger's comments are
in quotes and my responses 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
I suspect that Roger may be right on this point about half the time. ;-)
Roger's note seems to be saying if you have an algae problem, ONE OR MORE
NUTRIENTS MUST BE IN EXCESS. (This statement is nothing more than a
necessary corollary of Roger's "nothing is missing" statement. Let's call
this Roger's Corollary! :-)) While I certainly think this is true in some
cases, it is by no means true in all cases, and certainly not patently
obvious in any but a very small percentage of cases. Let's start with an
example where Roger's Corollary IS TRUE. I can take a healthy, growing,
reasonably well-balanced, stable planted tank with minimal algae and wreak
havoc on that tank by adding large doses of excess iron to the water column.
Unfortunately, I have created this exact situation in my living room tank on
several occasions! ;-) The solution has been to reduce iron levels to lower
levels, say, under 0.05 ppm in the water column.
Now let's take a much more complicated situation where Roger's Corollary is
NOT TRUE. I have been quite surprised recently to learn that the presence
of significant levels phosphorus in the water column is not necessarily in
and of itself problematic. We now have numerous reports (including Karen
Randall's) which I have confirmed with my own anecdotal experiments that a
reasonably well-run planted aquarium can operate quite successfully with PO4
levels at least as high as 1 ppm (water column) SO LONG AS NITROGEN IS NOT
ALLOWED TO BECOME THE LIMITING NUTIENT. Let's suppose you have a chunk of
phosphorus in your tap water. Imagine trying eliminate that problem by
reducing phosphorus (as so many people try to do) while ignoring the hard
won knowledge (thank you Paul and Kevin) that one limits nitrogen in a
planted tank at one's own peril!
Back to Roger's original "nothing is missing" statement. What is Roger's
scientific justification for that statement? Answer: Leibniz Law, which
states that the limiting growth factor will limit plant growth. According
to Roger, nothing is missing because the algae is growing!!! If any
essential nutrients were missing, the algae wouldn't be growing, would it!
Say what? Nice hat trick Roger, but NO CIGAR! If growing buckets of algae
was our purpose I would say fine, let's declare victory and retire to the
nearest pub to lift a few. But we're trying to grow higher order plants,
more or less to the exclusion of most algae. Why aren't the higher order
plants growing? That should be the focus of our inquiry.
"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 relative 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."
This is where the rubber meets the road folks! Roger supposes a tank that
is nitrogen-limited, but has enough other nutrients. He says that nitrogen
limits both the growth of algae and higher order plants. He says that
increasing nitrogen will "certainly [result in] more growth from the algae."
WRONG! We have observed for years and years now that increasing nitrogen in
nitrogen limited situations will REDUCE ALGAE GROWTH, and increase higher
order plant growth. And those of us who have run amok playing around with
nitrogen also know that even monster amounts of nitrogen (say, 50 - 100 ppm
NO3) are not in and of themselves particularly problematic in an otherwise
well-run planted tank. (I have always assumed that this knowledge explained
why portions of Europe with very high nitrate levels in the tap water could
do so well with planted tanks. Recall that the large Dupla tank was
reported in "The Optimum Aquarium" to retain ~25 ppm NO3 with tap water
inputs of ~55 ppm, if memory serves.)
Before we leave this, what the hell happened to Leibniz Law in this example?
There wasn't much algae in the tank; we added some nitrogen; and we got even
less algae. Huh? While we didn't limit the other nutrients, could the
faster growing higher order plants have limited other nutrients? But if the
higher order plants result in some other nutrient(s) being limited, why are
they still growing? After all, as Roger (and Leibniz) says, if the
necessary nutrients were limited, the plants would not be growing, would
they? And if the higher order plants are growing, why aren't the algae
growing? Could the higher order plants be able to grab all of some
particular nutrient(s), store them in their tissue for later use, and as a
result starve out the algae? Could Leibniz Law be wrong? I DON'T KNOW the
answer to these questions, but I do think many of these observations have
been repeated for years by aquarists from around the world.
As an aside, when Claus Christensen was here on the West Coast, while he
paid homage to Leibniz, he went on to say that in practical terms Leibniz
law was somehow wrong. I have been trying for days now to recall and
analyze exactly what Claus said on this point, but can't get it clear in my
mind. If he made the same point in his East Coast speeches I would love to
hear your best understanding of the point Claus was making. While I can
dream up limiting factor theories that might explain why higher order plants
seem to be able to "outcompete" algae, in all honesty I'm completely baffled
by the notion.
"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."
In point of fact, we observe the exact opposite in case after case after
case. The higher order plants do grow better than algae as we ease the
nitrogen limited situation.
"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."
Test kits can be an invaluable aid to tank observation. The most recent
example for me concerns phosphorus. Had I not taken my Hach PO4 test kit to
Marin County to measure Tom Barr's tap water in connection with the feature
article on Tom for the first issue of Planted Aquaria Magazine (see, I got
it in Dave! :-)) I'm not sure it would have dawned on me what a valuable
contribution a little bit of PO4 in the water column can be for a variety of
plants. This, I believe, turns out to be the secret to what those of us in
Northern California have always referred to as "Marin's magic water"! That
is to say, in an otherwise well-run high-growth planted aquarium, one can be
phosphorus-limited to the detriment of the plants.
And while there are half a dozen important "rules of thumb" for the proper
use and analysis of test kits and their results, like everything else in
life, after you figure it out, it's pretty easy. There is no truth
whatsoever to the MAD-SCIENTIST-RUN-AMOK image that has been applied to the
intelligent use of test kits.
"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."
YEP, you can't tell much about substrate nutrients by testing the water
column! Except Leibniz' point, if they're growing, they must have what they
"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."
Uh-oh, can't be anything missing if the algae is growing though, right
Roger? :-) My apologies, couldn't restrain myself on that one! While it is
true enough that if the growth is robust and healthy the nutrient supply is
fine, how do we know by visual observation alone what the problem is if the
tank is a wreck? You suppose, wrongly in my view, that some or other
nutrient is always in excess (if the algae is growing). But which nutrient
is it and how do you tell the difference between one excess nutrient and
another. Or heaven forbid, combinations and permutations of multiple excess
nutrients? The mind reels. I wish I could do that but I can't, and some
times my test kits help me eliminate certain issues and improve the
likelihood that a particular course of action will help the aquarist with a
"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
Not so fast. Claus Christensen and I talked non-stop for 10 hours last
Thursday as we drove to and from and toured the Monterey Bay Aquarium. We
talked about potassium at some length and I was surprised to hear Claus say
that many if not most of our aquatic plants do well with 10 - 15 ppm
potassium, but that several species do much better with 25 - 35 ppm
potassium in the water column. So to answer your specific question: They
just might like the extra potassium :-) which is not to say it might be just
as well to add some potassium in the substrate.
While I hate to be the bearer of bad news, I can report more or less for
sure, that 20 - 30 ppm of potassium in the water column is not necessarily
EXCESS and in an otherwise well-run aquarium, presents no algae issues that
I'm aware of.
It occurs to me that perhaps Roger has his own version of "magic Marin
water" and so he naturally thinks about excess nutrients, while those of
working with very soft pure waters constantly think about what nutrients to
Time for some lunch. It's been a fun morning, and I needed the break from
way too much work. Thanks Roger.
Regards, Steve Dixon in San Francisco where the sun is shining brightly