Iron Sources in Hydroponics : Which One is the Best ?

Definitely one of the most important problems dealing with the stability of hydroponic solutions is the availability of the iron (Fe+2 or Fe+3) ions. Since iron easily forms hydroxides and insoluble salts with other ions present in hydroponic media it becomes essential for us to provide iron in a way which is accessible to the plant and does not "come out" of the hydroponic solution through precipitation. Within the next few paragraphs I will talk to you about different iron sources available to hydroponic growers and which source is actually the best one we can use in hydroponic nutrient solutions. We will go through the different factors that make an iron source better or worse and finally we will be able to choose one as the ideal source for our nutrient needs.
What is the problem with iron ? The main problem we have with iron is that - unlike most other transition metal ions in hydroponic solutions - it is a very strong hard lewis acid which easily forms insoluble salts with many of the hard lewis bases within our hydroponic solutions. When iron is added to a nutrient solution in its "naked" form (for example when adding iron (II) sulfate) the ion easily reacts with carbonate, phosphate, citrate, oxalate, acetate or hydroxide ions to form insoluble compounds that make the iron effectively unavailable to our plants. To put it in simpler terms, iron ions have a chemical nature which is similar but opposite to that of many other constituents of our hydroponics solution meaning that when they meet together they form a "perfect match" that does not easily separate.

There is not only a problem with the higher inherent chemical match-making of iron with the anions present within the solution but we also have the problem that iron is always present at a much higher concentration than the other micronutrients. So even though some transition metals like copper would suffer from similar problems the fact is that they do not simply because of their much lower concentration (Fe is usually around 3-5 ppm while Cu is usually around 0.05-0.01 ppm).
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The solution to this problem is actually easy and comes in the form of chelating agents that "wrap" around the iron ions and make them disappear to anions that may want to form stable salts with them. There are many of these chelating agents with the most commonly used being EDDHA, EDTA and DTPA. They are different due to the fact that their stability is different and their abilities to dissolve iron are also different. While all of them make sure iron stays within solution EDTA only allows this to happen until pH 5-6 while, DTPA takes it until about 8 and EDDHA to more than 9. The most stable iron complex is definitely FeEDDHA but this does not make it necessarily the best candidate for hydroponic growing.

The fact is that although EDDHA binds iron much more strongly it decomposes easier within the hydroponic solution than EDTA or DTPA (this is due to the fact that EDDHA is composed of several different isomers, some of which are not very stable), reason why this complex appears to be but is not the best solution for hydroponic nutrient solutions. The best compromise between stability and durability is earned by DTPA which gives us a very stable complex and a strong resistance to decomposition. So next time you are looking into getting a new complex for your Fe needs, try FeDTPA (this salt can also be used with my hydroponic calculator).

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16 Responses

  1. [...] fernandez gives a case for iron dpta being a good compromise between stability, and durability. http://scienceinhydroponics.com/2010...-the-best.html __________________ none are more hopelessly enslaved than those who falsely believe they are [...]
  2. Axel
    I found Fe EDDHA in a store, it says that is in the form of 0-0 isomer (100%) Can that be true? Or it is a combination of isomers?
    • admin
      hi Axel, It is probably true although it doesn't make a huge difference if you're going to be using EDDHA for chelation in hydro, Daniel
  3. BlueCode
    Hi Mr Daniel, Most agri stores here usually only sell Fe-EDTA, there is supplier of FE-DTPA but required to buy in bulk 25Kg that is so expensive. Finding on local chemical store, they have "Dissolvine D 40 / DTPA / Diethylenetriaminepentaacetic acid, pentasodium salt". How we can make ourself FE-DTPA ? Do we only mixed iron powder / iron sulphate with this DTPA ? How about other micro (Mn, Zn, Cu), is it simply mixed the salt (MnSO4, ZnSO4, CuSO4) with DTPA ? Thanks for your kindness
  4. Mast
    Hi Mr Daniel, I want to to know the procedure of preparation of Fe-EDTA, Fe-EDDHA and Fe-DTPA from Iron Salt(like iron (II) sulfate). Is it possible to use Humic Acid as a chelating agent? If yes, How to use it in hydroponics as well as in general agriculture. Thank You
    • Curtis
      A couple things: A published report on making the DTPA complex of FeSO4 vaguely describes mixing H5dtpa (the normal acid form of the ligand) with FeSO4 in deoxygenated water and the EDTA complex from FeSO4 and Na4edta. (Inorg. Chem. 1990, 29, 1705) The deoxygenated water part of this synthesis is important because the same paper describes the aerobic oxidation of DTPA chelated Fe(II) to Fe(III) with data that imply a half-life of the Fe(II) complex of about 24 hours under normal hydroponic conditions, assuming the solution is saturated in oxygen. Under the same conditions, the half-life of the EDTA complex is about 30 minutes. Does it matter if one adds Fe(II) vs. Fe(III) to their hydroponic solution? Fe(II) is the usable form for plants, and any Fe(III) that is absorbed by them has to be reduced to the former. This takes energy for the plant to do. So it's probably better to get Fe(II) to the plant. Fe(II) is also absorbed preferentially. That being said, most of the iron that we generally provide to plants is in the form of Fe(III) because of this oxidation behavior. If you have a KSCN or NaSCN on hand, you can confirm this by treating an acidic solution of your iron source with either thiocyanate salt. A strong blood red color indicates the presence of Fe(III), while Fe(II) does not change color at all. Even freshly opened bottles of reagent grade FeSO4 heptahydrate test positive for small amounts of Fe(III), and the oxidation of Fe(II) is much more rapid in solution than in the solid state. Fertilizer grade FeSO4 contains large amounts of Fe(III). Anhydrous FeSO4 does not generally oxidize under atmospheric conditions unless it gets wet. However, if you're concerned about dechelation and preceiptation, Fe(II) binds about 10^11 times <i>less</i> strongly that Fe(III) to both EDTA and DTPA. For reference, DTPA's stability constant for both Fe(II) and Fe(III) is about 100 larger than EDTA's (Pure Appl. Chem., 2005, 77, 1445). Though the pH of the solutions in which these values are measured is not explicitly mentioned, in my experience the conditions that are used probably lead to a pH of about 3. The standard dogma for chelation of iron is that EDTA is useful up to pH 6.5 and DTPA is useful to pH 7. Above these values, dechelation will probably become significant, and any dechelated iron will precipitate in the presence of phosphate ions, becoming effectively unavailable to plants. If you're trying to avoid oxidation to Fe(II) because you're bored, unemployed, and fastidious, like I am, you can see problems with performing the published procedures at home. The most glaring issue is that most of us don't have access to deoxygenated water. You could significantly reduce the amount of oxygen in your water by boiling it immediately before use if you want, but I don't. You could also sparge it with a non oxygen containing gas. Preparing Fe(dtpa) from H5dtpa takes a long time to complete the dissolution of the latter, during which time Fe(II) oxidizes to Fe(III). What I do is prepare a stock solution of K2H3dtpa from H5dtpa (Acros, %65/kg) and two equivalents of KOH to make a 0.20 M solution (39.3 g H5dtpa, 39 g 29% KOH diluted to a total volume of 500 mL and heated gently for several hours in glass). Then I dissolve the required amount of FeSO4 (for the target ppm of iron) in an equimolar amount of the DTPA stock solution. Complex formation is immediate. I then add this to my hydroponic solution and avoid storing it. If you don't want to form Fe(III), stock solutions of FeSO4 are best avoided, since they oxidize in the presence of air, forming a yellow precipitate of iron oxide sulfate complexes. If you don't care, you can use either Na4EDTA or whatever DTPA salt you want. Na4EDTA is cheaper, the whole thing is easier because you can make stock solutions, and your plants still won't get iron deficiency chlorosis. The things I do are probably too much work to be worth it, but I have too much time on my hands these days. I would stay away from using humic or fulvic acid as the sole means of keeping iron in solution. Though I don't have any hard data to support this, my feeling is that iron won't be particularly persistent in your hydroponic solutions without a true chelating agent, especially if the pH rises above 6. I have seen FeSO4 visually precipitate from such solutions, even when it is added in small concentrations.
      • Curtis
        A correction: The formation of Fe(dtpa) from H5dtpa is pretty rapid, and it happens faster than oxidation of the complex. The color of the Fe(II) complex at 0.2 M is about the shade of a chlorotic leaf, the color of the Fe(III) complex at the same concentration is about the shade of an autumn leaf. You can visually observe the oxidation by the darkening of the solution. So here's the bottom line for the synthesis question: You can make Fe(dtpa) from FeSO4 and either H5dtpa if you want to use the solid or from K2H3dtpa if you want to use a solution. The pH of either solution is 2.6-2.9. You can make Fe(edta) from Na2H2edta or Na4edta, but the latter is easier to make solutions of because the former dissolves only very slowly. I would personally use Na4edta. An addition: I don't use EDDHA, but here's the synthesis of Fe(III)(eddha), which is by far the most common form used, and one that has been repeatedly demonstrated to be effective as both a root and foliar treatment for iron chlorosis: "FeCl3.6H2O (13.5 g.) was added to a slurry of [EDDHA] in 100 ml. H2O, 8 g. NaOH added, the mixt. cooled, and the product filtered off, and dried in a vacuum desiccator over P2O5 to give 20.7 g. monohydrogen ferric chelate of [EDDHA]." (US Patent 2,921,847) The commercial sources I've found for EDDHA are much more expensive than DTPA and especially EDTA, but I was only looking for reagent grade.
  5. zahid
    Mr Daniel, I want to to know the procedure of preparation of Fe-EDTA, Fe-EDDHA and Fe-DTPA from Iron Salt(like iron (II) sulfate). Thank You, Regards, Zahid
  6. Hector
    Hi, i'm new to hydroponics, just wanted to know if i could use iron sulfate instead of it's chelated form ?
    • admin
      only under very limited circumstances or with some special care. If you use this it will probably precipitate. If you want a more detailed answer please make a 20 USD paypal donation.
  7. Toby Hazlett
    Hi Daniel. I have been having some dramas with finding DPTA Chelated iron in australia. Same old, having to get 20kg bags etc. I have found a supplier who does powdered 11% strength in 1kg bags which is great. However some of the chelated iron i have been finding from suppliers comes in various strengths. Some say 7% w/w others 14% w/w and 11% so on. If a hydro formula asks for say eg: 1 gram of 7% chelated dtpa iron. Can i simply use .5 gram or halve as much of some %14 chelated iron? Cheers am almost there with making my own batch of nutrients from scratch. Just getting the final details of chems i need. Cheers!
    • Oxyandy
      In know is old, but I will answer in case it helps others. The answer is: In HydroBuddy "Substance Selection" you tweak the database - edit the existing entry "Iron DTPA" change the default % to your source percentage (14% w/w and 11% so on) click "Update" and done, you should do this with all your sourced 'Substances' if they don't match the database exactly.. I left the default ones and added custom ones to match my sources. I have only now read this post.. I sourced & used what was listed in the default 'Substances Used for Calculations' column in HydroBuddy after initially installing it - there it stated EDTA, The name of the product: Valagro EDTA FE (I had to buy 5Kg @AU$82 - ouch) useless ? I am getting a cloudy (white) solution not too long after mixing - tried everything - rainwater - tap water - mixing techniques - order in which I dissolve things - direct addition - pre-mixed concentrates etc etc 100% something in part A is the cause, this I am sure ! Only thing I haven't done is drop Ph below 6 - is around 6.3 (I was happy with 6.3) I guess next step is to make a mix without Iron, everything else 'as is' - see what happens.. starting to feel hopeless.
      • Oxyandy
        Preparing Your Own Hydroponic Nutrients : A Complete Guide for Beginners http://scienceinhydroponics.com/2010/07/preparing-your-own-hydroponic-nutrients-a-complete-guide-for-beginners.html#comment-139539 Also under "What you need" lists EDTA not DTPA.
        • Oxyandy
          Well, I am staring at 50Lt of 'crystal clear water' - has been stable long enough, only thing missing is 1.8 grams of "Valagro EDTA FE".. My supplier's catalogue lists this product in the "hydroponic section", it is supposed to be stable till Ph 7.0 NOTE: However, this is a Chilli (maxyield) blend, so higher EC(3'ish) lower Ph.(under 6) 1. take an UNdiluted sample add a sprinkle of FeEDTA - check, wait. 2. take a sample diluted to EC 2'ish which will up Ph - check, wait. if stable add a sprinkle of FeEDTA - check, wait. 3. foolishly, make my 'original blend': confirmed turned white (cloudy)and make that without FeEDTA, check EC & Ph.. wait, if stable add a sprinkle of FeEDTA - check, wait.
  8. Oxyandy
    Well, I've solved it... Thought I better share. Making a solution with either trace or Iron left out, results in a stable solution (Clear, no precipitation) left for 24hrs Using either DTPA or EDDHA as a substitute for EDTA makes stable mixes. Conclusion: I can not speak for other brands of EDTA, but don't use “Valagro EDTA FE” in hydro mixes. Using EDDHA will result in a tank of 'apple & blackcurrant cordial' you will not see the bottom.
  9. Say if making a stock solution (100x) concentrate that results in a pH of ~3, would that make FeDTPA "dechelate?" Is there a "too low" on the pH scale for FeDTPA?
    • admin
      Certainly chelate formation depends on the concentration of deprotonated species of DTPA, EDTA, etc, which depends on pH. This pH can be calculated with the equilibrium constant of the different chelates formed.

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