substrate iron for plants

[Steve Pushak <teban at powersonic_bc.ca>]

From: Neil Frank <nfrank at mindspring_com>
Subject: Re: Fe2+ / Fe3+ [long but hopefully interesting]

> >> I told Neil Frank I thought heating coils might be helping provide iron
                                    did say --->       
^^^^^^^^^^^^^^^^^^^^
> >> in George's laterite substrates and he countered: "I don't buy this....
> >> maybe he can test it out by not using his daily dupla drops which is
> >> essentially only iron." This is a reasonable experiment and maybe George
> >> could try it for a few weeks.
> [snip]
> 
> I actually was smiling to myself when I told Steve (in an off-line
> conversation) that you might do such an experiment. BTW, this out of
> context statement was responding to Steve's assertion that heat coils were
> the ONLY source of iron to all plants.
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ <- did not say

I did not say "that" at all!!!!  :-)  (good natured smiley, not joke
smiley) 

I said in an offline message to Neil that materials which I have
gathered from George Booth's web site and from George Booth's articles
in TAG _state_ that heating coils increase bacterial activity in the
substrate which stimulates the reduction of iron which increases the
availability of iron to rooted plants. That is not the same as saying
that "the ONLY source of iron is from the substrate". I suspect that
Neil might have misunderstood what I wrote the first time. I apologize
for quoting Neil out of context but I hoped I had made the context
clear. Sorry Neil! Sorry George!

In the context of my message to the APD, I was wondering how much
PROPORTION of iron is ACTUALLY being provided by a MATURE laterite
substrate to ROOTED aquatic plants. I asked the question: is it POSSIBLE
for mature rooted plants to get ENOUGH iron to continue to grow RAPIDLY
from a mature laterite substrate? Since I do not have any laterite
substrates, it will be up to somebody else to try this out. I suspect
the answer could be no, not enough for RAPID growth but enough to
sustain growth.

George recently said simply stopping chelated iron additions to the
water will not eliminate the chelated iron which may have arrived in the
substrate from the water. It is true that we do not know if any chelated
iron from the water goes into the substrate and is stored there.
Personally I do not think it is significant. 

If somebody with a laterite substrate wants to continue not adding
chelated iron to the water for a long enough period we could see if the
plants slow down their growth rates. I think they WILL slow down their
growth rates because I think the laterite MAY not be providing ENOUGH
iron to ensure rapid growth. I think that the plants will keep on
growing a little slower however and will not show any symptoms of iron
deprivation such as chlorosis. I stress that this is an EDUCATED guess. 

There are many factors which will determine how much iron could be
gotten from the substrate including whether or not the plants are
actually pushing so much oxygen INTO the substrate that it is no longer
anoxic enough to actually reduce iron (as Neil states). In my current
substrate I have used peat which uses up the oxygen in the substrate and
ensures that the redox potential stays low enough to reduce iron. I
would like to fine the minimum amount of peat necessary to perform this
function over a long term (say a year or two) but that's for another
day.

Another factor is that the texture of laterite may not be FINE enough to
permit a high degree of chemical availability of iron. A lot of it could
be held unavailable inside the granules of laterite whereas iron rich
clay or micronized iron especially will have a MUCH larger surface area
of available iron.

Steve

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