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re: Iron (was What is the best liquid fertilizer)

> Date: Fri, 10 Sep 1999 14:08:30 -0600 
> From: Chuck Gadd <cgadd at cfxc_com> 
> Subject: re: Iron  (was What is the best liquid fertilizer) 
> > I can accept that your iron is getting used up, but not that it won't stay in 
> > the water column.  Iron is one of the few things that comes in my tap water, 
> > the others being Ca, Mg, Na, CO3, silicate, and chloride.  It's pretty pure 
> > stuff.  The Fe comes in at 0.6ppm.  If I leave the water in a bucket for a 
> > week, or sitting in an empty tank for a week, the Fe is still at 0.6ppm. 
> But is the iron still available to the plants after a day or two?
> Doesn't it oxidize?

Short answer: If it oxidizes, it becomes completely unavailable: it
precipitates out, and isn't there any more -- at least in the water.

But the short answer is of no use, because there are lots of

First off, if the iron is in the ferrous state (Fe++), and it oxidizes to
ferric (Fe+++), and it's not complexed by something, and the pH is
reasonable, then essentially _all_ of it drops out as plain old rust.
Fe(OH)3 -- or Fe2O3, effectively the same thing --  is soluble at pH 2
(not recommended for plants or fish), very slightly soluble at pH4 (still
not recommended) and very insoluble at pH 7.  Its solubility depends on
the _cube_ of the hydrogen ion concentration (acidity).

But, as was recently pointed out, there may be compunds in the water,
miscellaneous organic acids lumped together as humic acid, that bind
tightly to the iron and keep it in solution.  They have to bind it with
such enthusiasm that they don't leave as much as 0.1% of the iron free,
else the iron would precipitate out at any pH above 5.

But if this is tap water, we wouldn't expect it to be full of humic
acids, right?  (At least, outside of the Scotch highlands, where the tap
water is visibly brown from soaking thrugh peat.  Makes you reluctant to
drink the rusty-looking stuff, till you recall that this is the magical
Highlands water that's essential for making Scotch whiskey.  But I
digress.)  I wonder what form the iron is in in this tap water that holds
the iron for a week.

As to becoming unavailable to plants when it oxidizes, even that is up
for grabs.  As was pointed out here a while ago, the roots of
many terrestrial plants can reduce Fe+++ to Fe++, after
dissolving it by secreting acid; and it's not unreasonable to expect
plants to reduce Fe+++ if it's already dissolved in chelated form.  In
fact, we know they must be able to, or most of the successful aquarium
fertilizers wouldn't work.  Only one of the widely used products uses

Dan "gnashing his teeth at Lamotte, which supplied weird iron reagents on
the one occasion he has an experiment going and _needs_ accurate tests"

Dan Drake
dd at dandrake_com