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Re: adding iron
Folks,
Fertilizing with high levels of iron is a great example of a nearly
complete disconnect between science and aquarium lore. I have long
wondered why.
The best summary article I've ever read on iron use in plants is an
article originally written in Spanish that I translated (poorly) to
English and which now resides at Steve Pushak's web site. The article's
emphasis is on food crops, but much of the material is sufficiently
general that it certainly applies to aquatic plants, as well.
According to that article, terrestrial plants (even those with high iron
contents) don't show iron deficiency unless iron in the soil water drops
to relatively low concentrations -- 1x10^-9 molar, or about 56 parts per
*trillion*. Some of the higher recommended iron levels we've heard
recently on this list are more than 20000 times higher then that
necessary level.
Plants use a number of different methods to obtain additional iron when
the iron concentration drops too low -- that is, below about 56 parts
per trillion -- including using organic chelated iron. I read in a
couple sources that plants generally don't use those mechanisms except
as a response to iron stress.
Incidentally, when it's desirable to add iron with a foliar application
the spray is mixed to a chelated iron concentration of 0.6 to 1.5 mg/l
and applied once or twice in a crop cycle. Those of you who dose your
tanks up to 1 mg/l or so can rest assured that if your roses ever start
looking a little pale then you can just dip some water out of your
aquarium and spray it directly on the plants. No mixing necessary.
Once a year should do it.
I'm not disagreeing with the observation from some list members that
high iron levels are desirable, or that they may be necessary for good
growth -- at least under some conditions. Karen Randall (who's
experience and observations I trust implicitly) told us some time ago
that she had to maintain iron levels at 0.5 mg/l with Tropica Mastergrow
or her plants showed symptoms of iron deficiency.
So why this big difference between what plants are supposed to need and
what they appear to need in our tanks?
One simple conclusion seems unavoidable; the plants are *not* using the
chelated iron that we diligently add and carefully measure. Rather,
they are using the much lower levels of ionic iron that are released as
the chelate breaks down. Plants probably won't use the chelated iron
directly unless they are already iron-stressed, which appears as a
visible iron deficiency. Some of us avoid the visible symptoms of iron
deficiency by adding very high concentrations of chelated iron to the
water column. Ionic iron is only briefly available as the chelate
breaks down, and keeping high levels of chelated iron makes sure that
the ionic iron is always sufficient to meet the plants' needs.
High iron levels could cause a number of different problems. The main
problem is just that you apply way more fertilizer than your plants
actually use. Also, many of us add iron in a trace mix and use iron
measurements as an indicator for the adequacy of trace element
concentrations. It is very possible that as iron levels are pushed
higher and higher that some other trace element in the mix will reach
toxic levels. Conceivably any of copper, zinc or boron (just to name
three) could reach concentrations where plant growth is inhibited and/or
invertebrates or fry are damaged.
Complicating matters even further, the level of chelated iron that's
necessary to provide the desired concentration of ionic iron is going to
depend on a number of poorly constrained variables. Commercial iron
chelate mixes use at least three different chelating agents (EDTA, DTPA
and gluconate), all with different decomposition rates. The
decomposition rate of the chelate (and hence the supply of ionic iron)
also depends on light intensity, light quality, temperature, and
probably on bacterial activity. Further, some aquarists keep substrate
conditions (generally, acidic) that might make it unnecessary to *ever*
add iron.
If you need to add iron at levels that are pushing 20000 times the
essential concentration then maybe it's time to look for be a better way
to fertilize your tank.
Roger Miller