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Tom Barr wrote, quoting Jared Weinberg:
> >Tom, I assume you meant 0.05-0.7, since the level suggested on the Flourish
> >Iron level is 0.1 ppm and the range I usually see on this List is 0.05-0.1
> >ppm. A search of the Archives turned up someone's: "Ooops, typing error.
> >The iron level was between 0 and 0.05 mg/l, not 0.5 mg/l. Sorry."
> >Note also the label for Karl Schoeler's Natural Gold fertilizer: "Add
> >enough NATURAL GOLD to keep iron levels at .5 to .75 mg/l."
> Many Companies recommend lesser amounts due to softer waters/new folks
> adding too much etc and other issues that error on the cautious. I would
> also, but might mention that these levels can be raised in a well run higher
> light tank etc. Claus mentioned that mosty of the tanks he saw in SF were
> nutrient deficient even though many had the .5ppm range. We started adding
> more and guess what? Better richer greener growth.
> Typo it is not. I feel it may be low but as you get the PH/KH/CO2 and the
> light balance going well, then a nutrient balance is the next issue to tackle.
I've pointed out before that iron concentrations as high as 0.5 ppm (much
less 1 or 2 ppm) are pretty much meaningless to plants. In fact, it is
orders of magnitude higher than levels that are meaningful. I'll include
a P.S. at the end of this note to show in more detail what I mean. If you
see a response to increasing iron content from e.g. 0.5 to 1 ppm there are
a few things I can think of that could explain the response:
1. Only a very small part (perhaps 1/1000th) of the iron you are
measuring is available to plants.
2. The observed change is not in response to iron, but in response to
some trace nutrient that accompanies the iron, perhaps even as a
3. The test kit it wrong.
The third option here isn't very interesting, so let's just assume that
your test kit is right.
If either of the first two cases is correct then we have a problem. In
the first case, any small variation in the biological availability of the
iron will create a huge difference in your plants' response to iron
fertilizer. I believe that the availability is going to vary a lot with
factors like the kind of chelate, the temperature, pH, type of substrate
and age of substrate. In the second case, the response you observe may not
be seen at all by someone who tries to reproduce your results with a
different iron fertilizer.
Please keep in mind that your results and the results of others are going
to depend *a lot* on the fertilizer you use and the conditions in your
tank. Fluorish Iron, for instance, provides iron as ferrous gluconate;
ferrous iron is what plants actually use. Many other iron-containing
fertilizers provide ferric chelates, like iron-EDTA (e.g. Tetra
FloraPride) and iron-DTPA (Tropica Master Grow, I think). In those cases,
the plants have to reduce the iron to ferrous iron and break the bond
to the chelate before they can use the iron.
At a minimum, a recommended iron level should be accompanied with
information about the fertilizer that's used as a source and preferably
there should also be some information about the substrate used in the tank.
P.S. Just how much iron do plants need?
Short answer: Iron is a trace nutrient; plants need damned little of it.
The table below shows typical concentrations of iron in plants, compares
the need for iron to the need for other nutrients, and lists the amount of
each nutrient needed to provide for normal growth in a hypothetical tank.
Nutrient Concentration Ratio Weekly requirement
element in plant dry ppm element/ parts per *billion*
weight, ppm ppm Fe
H 60,000 600
C 450,000 4,500
O 450,000 4,500
N 15,000 150 440
K 10,000 100 290
Ca 5,000 50 150
Mg 2,000 20 59
P 2,000 20 59
S 1,000 10 29
Cl 100 1 2.9
B 20 0.2 0.59
Fe 100 1 2.9
Mn 50 0.5 1.5
Zn 20 0.2 0.59
Cu 6 0.06 0.18
Mo 0.1 0.001 0.029
The second column shows how plants' needs for iron compare to their
needs for other nutrients. For example, lets say that with fertilizer and
fish food combined you are adding 5 mg/l of nitrate to your tank each
week; that nitrate contains about 1.1 mg/l of nitrogen. The
nitrogen:iron ratio in the plants is 150:1, so to balance the 1.1 mg/l of
nitrogen you would add 0.007 mg/l of iron each week. That's a small
The concentrations in the right column represent the concentrations that
would need to be added to a tank to replace the amount of nutrients that
are removed by normal weeding and pruning. According to values in that
column, the plants only need an added 2.9 ppb (0.0029 ppm) of iron each
The numbers in the column were calculated for a hypothetical tank that
contains about 1 kg of plants/170 liters of water and sustains a growth
rate of 10% per week. That is, where the growth rate is such that you
would have to remove 1/10th of the plant population in the tank each week
in order to keep the plant population constant from week to week. I also
assume that the plants are about 95% water. There's a lot of give in
these numbers, but a wide range of assumptions give similar results.
Here's another small number... Agricultural studies report that crops
(including high-iron crops like spinach) grow normally in soils where the
dissolved iron concentration in the soil water is only 1x10-9 molar.
That's 0.056 parts per billion, or 0.000056 parts per million. Translating
that to an aquarium, the plants need at least 0.000056 parts per million
of biologically available ferrous iron in the pore water of the substrate
in order to sustain healthy growth.
The catch is that the iron needs to be present in the reduced (ferrous)
state, and conditions often don't favor the existence of ferrous iron. Two
conditions favor the presence of ferrous iron over oxidized (ferric) iron:
very low pH and low redox potential. Some plants gather ferrous iron from
their substrate by forcing the substrate pH down. Some plants are also
triggered by iron shortage to use a method that lets them complex ferric
iron in the substrate, import it into the plant and then reduce the ferric
iron to ferrous iron.
These normal mechanisms that plants use to get iron all work in the
substrate, not in the open water. Plants can't generally acidify the
water column to get iron, and exuding ferric-iron complexing agents into
the open water in an attempt to gather up some ferric iron would be a
tremendous waste of plant resources. Finally, some of the physiological
responses that allow plants to gather iron (development of specialized
epidermal cells, for instance) only happen in the roots.