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I was always a bit leary of .5ppm of iron every other day (3x a week). That's 1.5 ppm a week. I've noticed problems with higher iron concentrations. I know some people like Mr. Barr will tout the benign nature of almost any macro or micro nutrient in typical EI dosing regimen, but the fact remains that EI dosing does NOT reflect typical (or even close to) concentrations in nature.

Iron problems have been shown in plants at 1mg/L :

Duckweed is a suitable plant model because of its small
size, rapid growth, and ease of culture [46, 57, 58]. Lemna
minor L., Lemna paucicostata Hegelm., Lemna gibba L.
and Spirodela polyrrhiza (L.) Schleid are all widespreadly
used in toxicity evaluation experiments. In our previous
study, S. polyrrhiza was used to evaluate its physiological
responses to excess iron (1, 10, and 100 mg L-1 Fe3+, added
in the form of FeCl3·6H2O) [46]. After a 24-h short-term
exposure, 10 and 100 mg L-1 Fe3+ caused plants necrosis or
death and colonies disintegration as well as roots abscission.
Moreover, significant differences in chlorophyll fluorescence
(Fv/Fm) were observed at 1-100 mg L-1 iron. Furthermore,
the synthesis of chlorophyll and protein as well
as carbohydrate, and the uptake of phosphate and nitrogen,
were inhibited seriously by excess iron. In addition, with
the increase of iron concentration, malondialdehyde (MDA)
content increased, but proline content decreased.

Though submerged macrophytes and emergent plants
are extensively studied, effects of iron on physiology of
them are relatively rare. At higher iron solution concentrations,
plants exhibit visual symptoms of possible iron
toxicity, including root flaccidity, reduced root branching,
increased shoot die-back and mottling of leaves [44, 59,
60]. Moreover, Basiouny et al. [43] pointed out that contents
of iron and chlorophyll in Hydrilla verticillata (L.f.)
Royle increased with the increase of iron concentration
(0-8.0 ppm). Batty and Younger [44] found a threshold of
iron concentration (1 mg L-1) above which seedling growth
of Phragmites australis was severely inhibited. In addition,
P. australis is proposed as a more appropriate biological indicator
of iron and manganese pollutions [61]. Like phytoplankton,
the activities of antioxidative enzymes in aquatic
plants, such as Elodea nuttallii (Planch.) H. St. John,
are inhibited seriously by high iron concentration (beyond
10 mg L-1 [Fe3+]) [47].

Where they using chelated iron? No. Another factor not explained by many is how EDTA is typically broken easily by light from visible and UV spectrums during the day, leaving Ionic forms of iron floating around in a whitish haze. This also happens based on pH.

Instead of dosing tons of iron I have now moved to doing what plants expect: using a rich substrate with laterite for iron uptake via the roots. I still add some micro's, but 0.05ppm 3x a week is PLENTY.

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