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N-limited/P Limited

Karen Randall, Jeff Kropp and I have had an off-line conversation that
might be of interest to the group.

Steve wrote:
Jeff Kropp and I have been chatting (almost daily recently) thinking
about trying to reconcile your reports on your tanks with the whole
N-limited PMDD story.  Paul Sears' note yesterday got us going again.
What Paul described was a P-limited situation where he added P (to the
water column, I presume) and watched the nitrate levels drop noticeably.
 The thought was that the extra P allowed additional uptake of nitrates
that otherwise would have been left in the water column.  This confirmed
the P-limited situation in the tank Paul described, as I understand it.

Karen wrote:
That certainly sounds accurate, and is what I would expect to see.  You
just have to hope that the higher plants capture those nutrients before
the algae kicks in.

Steve wrote:
You report fairly regularly that your tanks almost always have zero or
near zero nitrate levels and 'just detectable' P levels in the water
column -- without significant algae problems of any kind.  Your
situation is complicated in my view by two additional facts:  You do add
nitrogen to your tank, but only in the substrate (Jobe's spikes, if I
recall, punched deep into the substrate).  You also keep certain plants
in pots which contain soil (with at least some additional nutrients,
perhaps including N).

Jeff and I keep wondering whether your tanks are N-limited?  They may
not be N-limited if you are successful at introducing N in the substrate
and keeping it out of the water column.  We do know that your tanks are
not P-limited because you report at least some P in the water column.

Karen wrote:
I also think that plants are generally excellent scavengers.  I think it
is entirely possible for them to use smaller amounts of nitrogen from
the water than we can test for.  Let's face it.  You won't find _any_
nitrate in the water unless some ammonium is getting past the plants and
being processed by nitrifying bacteria.  (BTW, I think this is also true
for iron.  I have never tested to make sure I have specific levels of
iron in the water,  I think that specific iron levels are irrelevant.
Plants can and do store more iron than they need for growth.  Therefore,
if there is _any_ measurable iron in the water, you are meeting the iron
needs of your plants.

Steve wrote:
We were thinking of a simple experiment (essentially the opposite of
what Paul was describing yesterday) which might resolve the question.
If you added nitrates to your water column and observed a noticeable
drop in the P levels, we think this would suggest that your tanks really
are N-limited.  (The addition of the limited nutrient N resulted in
additional uptake of P.)  If there were no drop in P levels, this might
suggest that your tanks aren't N-limited after all; that you are putting
enough N in the substrate to avoid this condition.  

I have the feeling this could be significant information.  If the P
levels dropped upon the addition of NO3 to the water column, I would
want to theorize that one can have a very successful planted tank which
is N-limited (though not N-starved) so long as the P level remains in
the 'just detectable' range.  

If it turned out that your tanks were not N-limited, then this puts
another notch in gun stock of the N-limited folks, it seems.
[Inartfully put to say the least.  I  didn't really mean to suggest a
competition of that sort. <g>]  We might conclude that so long as we
avoid the N-limited situation (however we accomplish that) we can
tolerate a bit of P in our tanks; that we don't need to reach a
P-limited situation so long as we avoid an N-limited situation.

What do you think?  Any interest?

Karen wrote:
I am quite sure that my tank WATER _is_ N.  limited, at least in my more
strongly lit tanks.  If I don't add the Jobes sticks, my plants show
clear signs of N deficiency.  In fact, I'd love it if we could talk some
fertilizer manufacturer into formulating a solid N-K fertilizer for use
in planted tanks.

Jeff wrote in a later note:
Stump remover [KNO3] is a solid, but needs dilution some how... powdered
clay, Knox blocks?

Karen wrote:
I don't want to make my own... I want someone else to do it for me!<g>
could always roll it in clay, but you'd have to experiment on the
quantities needed.

Jeff wrote:
Tetra Hilena Crypto Total N 1% from Ethylendiaminetetraacetic acid
(proprietary name?) 0.5% water soluble N from Ethylendiaminetetraacetic
acid iron salt soluble K20 25% from Potassium Sulfate

Karen wrote:
I haven't seen that Tetra products do much of anything for my plants.  I
suspect 1.5% N is just too low to have much effect.  My Jobes sticks
work much better.

Karen continued responding to my note:
In a few low light tanks, where I have not had much in the way of fast
growing plants, but also haven't had algae problems because of the lower
light levels, I have sometimes in the past measured N levels as high as
20mg/L.  In these tanks the plants show no signs of nitrogen deficiency,
and I would not, of course, supplement either N or P.  Another telling
point is that the Java Ferns, which, of course, must meet their needs
directly through the water column grow consistently larger in these
tanks than they do in the more strongly lit  nitrate limited tanks.

I just feel that it is "safer" for the display type tank to supplement
macronutrients via the substrate, if at all possible, rather than the
water column, where it can be accessed by algae.  The plants tell me
fast enough if they need food, and it's cheaper than all those test

I think I'll pass on the experiment of adding N to my water, because my
tanks are happy, healthy and algae free as it stands now, and I feel
that I can adequately meet their needs via substrate supplementation.  I
really don't want to turn my tanks into pea soup just before the
holidays.<g>  As I think I've mentioned before, all of my tanks are in
the living area of our house, not tucked away in a "fishroom" where I
could experiment without the tanks being seen.

I _do_ know that this is not an isolated occurrence effected in some way
by local tap water concentrations, because I have been instrumental in
the setup of many other tanks in areas with very different tap water,
and they all behave similarly.  I can't give you absolute readings from
these other tanks because, again, unless there's a problem, I don't
spend much time using test kits.  I only know where mine stand because
I've tested out of curiosity from time to time.

I think the bottom line is that you can probably limit algae by limiting
one of several nutrients in the water column.  Diana Walstad does it by
limiting iron in the water column.  She's got way high N and P levels by
our standards, but I've seen her tanks in person, (in fact, I used a
photo of one of them in my Dec. AFM column) and the plants are healthy,
and there is no visible algae, even with few or no algae eaters in the
tanks.  In fact, the tank that I used the picture of, has one 1" long M.
boesemani fry in it.  Diana "feeds" the tank fish food, even though
there are essentially no fish in the tank.  She says she has done the
same in the past with tanks that were totally fish-less.  

This certainly isn't an approach I'd use, or advocate, particularly in a
strongly lit tank.  But it clearly works for her.  (BTW, her tanks are
soil based, which gives the plants their prime nutritional source)

Another random thought that needs to be somehow factored in to this
discussion is the "Dupla" style tanks in Europe.  These tanks routinely
run well, and have no real algae problems even when each water change
floods the tank with a new supply on nitrate from the tap water.  I have
been told that these tanks often run relatively algae free with nitrate
in the range of 20 mg/L.  I can only assume that either the tanks are
phosphate limited, or that even though they are open topped and lit with
MH or MV lighting, that the lights are high enough that the effective
light level within the tank is actually quite low.

I am convinced that Kevin and Paul's PMDD system works, not only for
them, but that it is also reproducible by others.  (If you've been
reading my Aquarium Frontiers column, you'll have noticed that I've
directed people to it at least twice)  I also know that my method works
and is reproducible under a variety of conditions.  For the people I
work with, my method is probably preferable, because they are mostly
novices who would be intimidated by the idea of lots of testing, let
alone the need to brew your own supplements.  I encourage them to use
their eyes and common sense instead.<g>  

I also would not be happy with the progression of various algae types
that I seem to hear is quite common during the calibration stage of
working with PMDD's, whether in a new or established tank.  I don't
_want_ to have to wait out various algae problems... I want an
attractive tank that is free of visible algae from day one if I can at
all manage it.  I think this is very important for people who are just
getting started with planted tanks as well.  I think that many would get
discouraged and quit if they had to wait through multiple blooms of
various algae types. Not to sound too smug, but my method produces algae
free tanks with healthy plants even in the hands of most novices.