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Re: Questions about PMDD

On Tue, 2 Feb 1999, Grover wrote:


> I'm
> using terralit as a substrate additive and have never had insufficient
> iron level in my tank <.1 ppm (also no signs of iron deficiency in my
> plants).  In addition recent posts have been pointing out that the test
> kits used are not measuring chelated iron.  The forms of iron
> registering on the tests are not the ones being used by plants.

You may want to check your test kit.  It could be giving you a false

>     This leads me to think that I should be using another method to
> determine dose size.  Plant condition might be a option but I have never
> observed an overt element deficiency in my plants.  Just slow growth in
> some species, rising nitrate levels, and outbreaks of algae.

If you have rising nitrates, then PMDD probably isn't the indicated
solution.  PMDD is formulated for the case where nitrogen is limiting
(absent in the water and in the substrate) and the water contains elevated
phosphorus levels.  In your case with rising nitrate levels it appears
that something other than nitrogen is limiting.  That could be the light.
It might be something else.

> Is there
> another trace element I can measure effectively to determine PMDD dose
> size.  Ideally it would be something used by the plants readily and
> quickly maybe sulphur (.05% to 1.5% concentration in dry weight if
> plant) or manganese (.0005% to .15% concentration in dry weight of
> plants).  How quickly is sulphur and manganese used in plants, and can
> they be measured effectively.
> Any input anyone can give would be greatly appreciated.

Sulfur isn't a trace element.  It's a minor part of plant tissue, but it's
a major part of the salts content in *most* water so it's usually present
in surplus even without fertilization.  It wouldn't be a good indicator.

But then, it doesn't sound like you need an indicator for your PMDD.  If
the algae really are problematic (slow growth isn't necessarily a problem
and rising nitrates can be fixed with more plants, increased water changes
and/or different feeding schedules) then you might want to look for some
other solution.  Good algae eaters - if you don't already have them - are
one possibility.  You can also look for and fix other limiting factors;
without knowing more about your tank I can't give you much input about
which factors to look at.

Roger Miller