Fe(o,o-EDDHA) isomers: Stability and uptake by dicots (cannabis)

Ralf · August 27, 2022, 9:33pm

Hi @danielfp. I just read your good blog post, Are Iron chelates of humic/fulvic acids better or worse than synthetics? Excellent material as always!

I have a few questions, and I thought it would be helpful to post them here for a wider audience of dicot growers. And because this is a long message, the formatting is easier to read here.

You wrote:

An additional concern is the oxidation state of the Fe. While Fe chelates are usually prepared using ferrous iron (Fe2+), these iron chelates are quickly oxidized in solution to their ferric iron (Fe3+) counterparts, especially when the solution is aerated to maintain high levels of oxygen. Since Fe3+ is both more tightly bound to chelates and more reactive when free – so more toxic when taken up without reduction – plants can have an even harder time mining Fe3+ out of chelates

Please correct me if I’m wrong, but from what I have read, Fe(o,o-EDDHA) is prepared with Fe(III), not Fe(II), and the same for Fe(DTPA) and Fe(EDTA); at least for Dissolvine (E, D, and Q) and Sprint. Are you aware of manufacturers that prepare Fe(o,o-EDDHA) or Fe(DTPA) using Fe(II) for horticulture?

I reviewed your references, and I didn’t see anything showing an aerated nutrient solution affecting Fe(EDDHA) chelates or the form of iron in the chelate. I may have missed something, though. Can you refer me to research showing aeration of a nutrient solution affects the form of iron (II vs. III) within Fe(EDDHA) chelate? I am especially interested to learn of research on DO2 supersaturation using nanobubbles.

As one of your references showed, it appears dicots preferentially uptake the meso isomer compared to the d,l-racemic mixture. In contrast, monocots use forms equally well. And that the chelation strength of the two forms may be the issue, as the d,l-racemic mixture is 500-times more stable than the meso isomer. For example, pepper showed zero to minimal uptake of the d,l-racemic mixture, and tomato showed a significant preference for the meso isomer. According to Alcañiz et al. (2005), “In Strategy I, Fe(o,o-EDDHA) must enter the root apoplast to get in contact with the reductase on the cell membrane. Part of the chelating agent is then temporally retained in the free space of the apoplast and again turned back to the nutrient solution. It would explain the oscillation observed in Fig. 5.”

Due to preferential uptake of the meso isomer by dicots and slow transformation of the racemic form into the meso form, it appears the expected equilibria of d,l-racemic and meso forms is not maintained in hydroponic solutions for dicots. So, a greater concentration of the racemic form is expected in recirculating solutions after some time. Which can lead to iron deficiency in recirculating hydroponics from the limited uptake of the racemic form and slow transformation of the racemic form into the meso form.

According to Cerdán et al., (2006), “The results suggest that the use of Fe(o,o-EDDHA) products with a higher percentage of meso isomer could be more efficient, at least in hydroponics for strategy I plants, than the use of the present products containing 50% of each isomer.”

Do you think Fe(EDDHA) with higher meso isomer ratio would be benefical for cannabis? And are you aware of a commercial or research source of Fe(o,o-EDDHA) with a higher percentage of meso isomer? It’s possible to separate the racemic and meso from a solution of Fe(o,o-EDDHA) where they are in equilibrium, but for growers that doesn’t seem feasible.

The use of humic/fulvic acids complimented with either unchelated Fe or with some lower proportion of stronger iron chelates, seems to be a better overall choice in terms of both plant uptake and economic expense. As shown by several studies mentioned in this post, the effect of humic/fulvic acids and synthetic chelates might actually be synergistic, with both providing different advantages that can be complimentary in hydroponic solutions. These humic/fulvic acid solutions might also be much more favorable for monocot species, where the use of highly stable Fe(EDDHA) chelating agents does not cure deficiency symptoms.

What concentration ranges of fulvic acid in solution or relative to Fe(EDDHA) would you expect to provide the described synergistic effect?

danielfp · August 29, 2022, 9:28am

@Ralf.

EDTA is most commonly made with Fe(II), the DTPA and EDDHA are both indeed made with Fe(III), so they won’t change much in a solution with high oxygen. Some manufacturers might choose to oxidize the Fe in EDTA as well.

So there are a couple of problems with EDDHA.

When I talk about the isomers of EDDHA I am not talking about the isomers of the chelate (the meso and racemic you mention) but I am talking, more importantly, about the isomers of EDDHA itself. Because EDDHA not only contains o-o EDDHA but also all combinations of the ortho, meta, para isomers of the EDDHA molecule (o)(m)(p)-(o)(m)(p) EDDHA.

These EDDHA isomers are the ones that are, in practice, very problematic in terms of the quality of the chelate. Many commercial sources of EDDHA will contain large amounts of the meta or para isomers of EDDHA, which do not generate stable chelates and are quite likely to precipitate Fe (which is a common occurrence in concentrated solutions that use EDDHA chelates).

Before we could even think about the structure of the chelate we would need to solve the problem of just making pure o-o EDDHA in sufficient quantities.

Regarding cannabis, I don’t think we have an issue with Fe uptake of the cannabis plant, so I don’t think we would need to go to such extents to improve Fe uptake.

About the beneficial synergistic effects, generally I would recommend feeding fulvic/humic acids at no more than 50 ppm in solution. I would conjecture that those concentrations plus 1-2ppm of a normal synthetic Fe chelate should provide the best case for Fe uptake. At least if the papers also apply to cannabis. In the case of humic acids, only feeding once every weeks could be better as humic acids can accumulate and cause problems in roots if you use them non-stop.

Ralf · August 29, 2022, 3:16pm

I’m aware in part you were discussing the positional isomers of EDDHA (o,o, o,p. p,p), and I use EDDHA with the highest o,o I’m aware of (Sprint 138 at 5.2% o,o). So I didn’t discuss that issue because most people who use EDDHA, I assume, are aware they want the highest o,o % they can find.

This is an interesting paper on positional isomers for EDDHA:
Synthesis of o,*p-*EDDHA and Its Detection as the Main Impurity in *o,o-*EDDHA Commercial Iron Chelates

I focused on the racemic and meso forms of o,o-EDDHA because that seems to be a topic not often discussed. And one where the chelate can be improved for dicots in recirculating hydroponics where the solution isn’t replaced weekly, bi-weekly, etc. With drain to waste irrigation, it’s not an issue because the isomers should stay in equilibrium within the media because fresh Fe-EDDHA is continually added. However, with recirculating hydroponics using, for example, using the mass balance refill method described by Bruce Bugbee in his older paper and the recent paper co-authored by you and Bruce, the topic of racemic:meso ratio and uneven uptake is interesting and an area where mfgs could provide a new product to address the issue.

Is it necessary to produce a new type of Fe-EDDHA with greater meso than racemic? No. But I think it could improve plant growth while increasing sustainability by reducing the required Fe-EDDHA concentration for dicots. Which Especially because cannabis seems to respond better to 3-4 ppm Fe than 1-2 ppm (from my personal experience, the experience of others, and academic research); plus, I use UV-C, so I run Fe a bit higher than I otherwise could, and I use EDDHA rather than DTPA due to UV-C.

This is intersting reserach from NC State:

Are you lumping fulvic and humic acid together when you recommond agsint running “humic acids” non-stop? Or, in yhour opinoin is it less of a concern if using fulvic acid?

danielfp · August 29, 2022, 3:54pm

The most important issue for Fe uptake is the root zone pH. If your media pH is lower than 6, then you can likely grow without needing much more iron than 1-2ppm of Fe. The plants don’t seem to be bothered by higher Fe though, so it is common to go to higher values, especially if you’re in media like coco, which can fix some of that iron into the media.

About humic/fulvic, I generally use only fulvic sources, as they are usually less problematic than humic acids. When I used humic acids, I tend to only do drenches once in a while, instead of apply them with nutrient solution everytime. For fulvic acids, I have never seen daily applications be a problem.

Ralf · August 29, 2022, 4:53pm

I agree about fulvic. That’s also what I use. And I use it with each irrigation without noticing any issues. I’m glad to learn you also find no problem wiht its daily use.

If you haven’t read that research summary in the link above for cannabis Fe, it’s worth checking out. Experimentally, I see increased biomass yield and chlorophyll concentration from Fe 3 ppm compared to 1.5 ppm when using Fe-DTPA in Rockwool and coco.

Thanks

danielfp · August 29, 2022, 5:18pm

@Ralf Thanks for sharing your experience!

Ralf · August 29, 2022, 7:07pm

You too!

Ralf · October 5, 2022, 9:04pm

@danielfp do you have Brendan Fatzinger’s contact info? Or could you get him a message from me?

I read his research “Residue from Sequestrene 138 Fe EDDHA Clogs Filters: Alternatives for Chelated Iron in Liquid Fertilizer Systems,” and I have a couple of questions/critiques.

It’s unclear to me why he and Dr. Bugbee drew those conclusions:

Without mentioning the o-o content of the various Fe-EDDHA sources he recommended over Sequestrene (Sprint) 138. I looked for the o-o content of Miller Ferriplus, but it’s not listed, so I left a voicemail for a sales rep.
Without reporting solution pH
Why he didn’t study the addition of Sequestrene 138 to a typical stock A solution (nitrate and cations) without the unchelated Cu and Zn with their relatively high EDDHA stability constants. (The EDDHA stability constants of Ca and Mg are much lower than Fe(III), Ca, and Zn.)
And why he recommended other EDDHA sources without testing them in the same Peter’s/MKP/CuCl stock solution (it appears he tested them at 0.4g/L in DI water in the second study).

EDIT:

Also, whehter he was able to identify the sediment in the Fe-DTPA solution and some of the sediment in the two Fe-EDDHA solutions. From the study description, it seems he did identify the sediments. Still, there was sediment from the Fe-EDDHA solutions he couldn’t identify. It would be interesting to know what comprises those sediments.