How to prepare a low cost chelated micronutrient solution

Micronutrients constitute only a small portion of a plant’s nutritional requirements but are still vital to growth and development. They are mainly comprised of heavy metals (Fe, Zn, Mn, Cu, Mo) as well as a single non-metal, boron (B). Since they are used in such small concentrations – normally in the 5 to 0.01 ppm range – they are normally put into concentrated nutrient solutions in small proportions and included with other components such as Ca and Mg, which are present in concentrations much more in line with macro nutrients like N, P and K.

Simple model of the metal chelating process

The advantage of micro nutrients is that they are available cheaply and in high purities as heavy metal sulfate salts. These however have the problem of leading to relatively unstable cations in solution, making the preparation of concentrated micro nutrient solutions with pure sulfates impractical (unless you want to see how a gallon of rust looks like). However we can chelate the cations as they come out of these sulfates, using a chelating agent, in order to prevent any precipitation issues. In this article I am going to walk you through the preparation of a DIY chelated micronutrient concentrated solution. This is much cheaper than buying the heavy metal chelates, which can be 3+ times more expensive. To prepare this solution you’ll need to buy the chemicals shown in the table below. The table includes links to buy all the different substances mentioned plus their cost (without shipping).

LinkPrice USD/lbWeight g/gal
Disodium EDTA22.9617.0600
Ferrous sulfate heptahydrate15.999.4211
Zinc sulfate monohydrate9.490.1039
Manganese sulfate monohydrate14.991.1646
Copper sulfate pentahydrate20.990.0595
Sodium Molybdate19.990.0191
Boric acid10.953.3384
Total Cost115.39
List of salts to prepare a DIY chelated micronutrient concentrated solution. This concentrated solution is to be used at 5mL per liter of final feeding solution.

In order to prepare the solution you also need a scale that can weight with a precision of +/- 0.001g (this is my low cost recommendation) and a container where you can store 1 gallon of solution. Please note that these solutions have to be prepared with distilled water, with RO water you might still run into some issues in the process. To prepare the solution carry out the following steps (the weights to be used are specified in the table above):

  1. Wash your container thoroughly with a small amount of distilled water
  2. Fill your container with half its volume of warm distilled water (30C, 86F)
  3. Weight and add the disodium EDTA, stir until it is completely dissolved (this can take a while).
  4. Weight and add all the remaining micro nutrients one by one in the order given above, stirring till each one is fully dissolved before adding the next.
  5. Fill the container to its final volume using warm distilled water.
  6. Let the solution cool before closing the container.
  7. For longer half-life transfer to a container that is opaque to UV light.

This solution is prepared to give you the heavy metal concentrations of the Hoagland nutrient solution (a very common set of ratios used in scientific research for growing plants) when used at a ratio of 5mL per every liter of final feeding solution (18.92mL per gallon). The links given above are for 1lb of each product, with this you should be able to prepare at least 53 gallons of the concentrate, which will allow you to prepare 10,600 gallons of final feeding solution. The first salt you will run out of is Fe, but some are used so sparingly that you should be able to use them for the rest of your life without needing to buy any more (like copper sulfate and sodium molybdate). For less than 120 USD you will be able to have enough solution for probably the rest of your life – if you’re a hydroponics aficionado – or even an entire crop cycle if you’re a commercial grower.

This preparation is not without problems though, since the chelates are all prepared in situ they will take a substantial amount of time to reach their thermodynamic equilibrium, meaning that it cannot be used to soon or some of the metals might not be fully chelated. To obtain the full metal chelating effect an excess of around 25% of disodium EDTA is also used, which means that this micro nutrient solution contains more free EDTA than a solution prepared with the chelates. Another issue is that all heavy metals are chelated with EDTA, which might not be optimal depending on your growing conditions. The EDTA chelates are also less stable against UV light and are also more easily attacked by oxidants. Another final issue is that the solution above contains no preservatives and fungi generally like to feast on this sort of micronutrient containing solutions. It is therefore reasonable to avoid preparing any large amounts of the above, as a solution prepared as instructed is normally expected to spoil in 3-4 weeks.

With this in mind, the above is not a perfect but a low cost and practical solution for those who want to start preparing their own nutrient solutions and avoid paying the high prices of some commercial nutrients just because of their micro nutrient contents. The above gives you a versatile micro nutrient concentrate that is bound to be adequate for growing almost all plants.

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12 Comments

  • Hydro
    May 19, 2020 @ 7:15 pm

    Hi Daniel,
    It is nice to see you back again giving out helpful tips.
    Just some clarification on your article.
    1. The solution in this article is x200?
    2. When weighing for example sodium molydate as it is the smallest in gram quantity in the table. Since the final target concentrated solution is to produce 53 gallons of concentrate to prepare for 10,600 gallons of final feeding solution. So I will multiply 53 x 0.0191(g/gal) = 1.0123g. So I will be weighing 1.0123g of Sodium Molybdate in the precision weighing scale? This 1.0123g of Sodium Molybdate is to be disolved in the 53 gallons?
    Thanks,
    Hydro

    • admin
      May 20, 2020 @ 6:43 am

      Hi Hydro,

      Thanks for writing. To answer your questions:

      1. Yes, it’s 200x.
      2. To prepare 53 gallons you would need to dissolve 1.0123g of Sodium Molybdate. As you see, buying a pound means you probably won’t ever need to buy it again in a lifetime.

      I hope this helps!

      Best Regards,

      Daniel

  • Hydro
    May 20, 2020 @ 9:21 am

    Hello Daniel,
    I missed one question.
    How did you arrived at 8.5387g/gal for the disodium EDTA?
    Thanks
    Hydro

    • admin
      May 20, 2020 @ 10:16 am

      Thanks again for commenting. It’s basically 125% of the molar equivalent needed to chelate all the heavy metals, it’s what has worked for me best experimentally when doing this sort of in situ solution in the past. I hope this answers your question!

      • Hydro
        May 21, 2020 @ 2:00 am

        Hi Daniel,
        Appreciate your prompt reply.
        I maybe asking to much but can you give just basic clue on how the 125% of molar equivalent arrived into 8.5387g/gal of EDTA?
        Thanks,
        Hydro

        • admin
          May 21, 2020 @ 7:35 am

          Thanks for your reply! If you want a more complete explanation I would be happy to provide one through an hour of consulting time.

  • Mike (Skybound)
    May 31, 2020 @ 8:09 am

    I’ve been making my own concentrates from purchased chelated elements for about a year and a half now and am having great results. I can’t comment about the price difference other than to say I think the difference would be negligible, but your tip from an older blog entry of using Sodium Benzoate really panned out for me. I make my concentrates to a 1:1000 ratio and the jugs I use for storage are the common semi-opaque half gallon milk jugs. It takes me roughly 4-6 months to empty a jug and I do use every last bit and as far as I can tell, there is very little precipitation.

    I use Iron DTPA, Zinc, Copper and Manganese EDTA, Sodium Molybdate and Boric acid, load those into Hydro Buddy, set the Volume to 500 gallons (to make 1/2 gal), then target to my desired PPM which is nearly identical to that of Megacrop’s blend and weigh out to the hundredth (0.00) as that’s the scale I have ATM and dump each into the water slowly and shaking it all into the solution. I then add the 0.3 grams of the sodium benzoate and 1 drop of polysorbate as a surfactant, though IDK if that’s helpful or not.

    • admin
      May 31, 2020 @ 10:19 am

      Thanks for your comment and for reading the blog! I’m very glad to hear that you’ve found some useful information. About the cost, you’re right that the difference might be very small depending on where you live and how much fertilizer you use. About the surfactant, it is certainly not necessary but depending on your circumstances it could be helpful – reduce tension with the media and improve wetting – or it could be problematic, causing foaming issues in some cases.

  • Roy
    June 17, 2020 @ 8:42 am

    Dear admin, what is the different of using Na4edta Vs Na2Edta

    • admin
      June 17, 2020 @ 9:06 am

      Thanks for commenting. The EDTA molecule has 4 carboxylic acid groups, so you can actually think about it as EDTAH4, where you have 4 acidic hydrogen atoms that can be replaced by bases in water. Na2EDTA would actually be Na2EDTAH2, where two of the hydrogens have been replaced by sodiums while Na4EDTA replaces all 4 hydrogens.

  • Hub
    September 8, 2020 @ 7:40 am

    Dear Daniel,
    I wanted to modify this recipe to my personal needs and while calculating i probably found a math error. You’re using 8,5387 g disodium edta (dihydrate) per gallon of concentrate, that is ~6,06 mmoles per liter. The sum of cations to be chelated (Fe, Mn, Cu, Zn) is ~10,99 mmoles, even Fe alone is ~8,95 mmoles per liter, so for equimolar (+25%) chelation there is only half the amount of disodium edta that is needed to fully chelate the ions?
    Or am i wrong and 1 mole disodium edta chelates 2 moles Me2+ ions, which i don’t think it does?

    • admin
      September 8, 2020 @ 8:36 am

      Thanks for posting! You’re right, the weight of disodium EDTA has now been fixed.

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