Understanding Iron in Aquaponics

Iron is abundant but tricky to manage.

To supplement the iron that your plants need, you have to understand how the element functions within that ecology.

Why Study Iron?

This post gets technical.

You might want to access your inner science geek later on in this slideshow!

Warning:

One of the most plentiful elements on the planet, and one of the oldest used by mankind.

Also crucial to chlorophyll development and photosynthesis in your plants.

Read our full blog post on iron here.

Iron

Fe

Constituent of heme complexes (like hemoglobin).

Essential component in the production of chlorophyll.

IN ANIMALS

IN PLANTS

• No chlorophyll• Retarded plant growth• Chlorosis• No cytochrome• No metabolism

No iron means...

Fe

Unfortunately, because it is highly reactive, iron is typically unavailable.

It flits between soluble and insoluble forms compounds with other minerals and (in aerobic environments generally) plays hard to get.

No iron means...

Fe

Iron exists in a variety of ionic states, but prefers these two in particular:

Abundant but very reactive

Fe3+ Fe2+Transitions between the two readily

Ferric iron Ferrous iron

Fe3+ Fe2+

More oxidizedUnavailable to plants

Less oxidizedAvailable to plants

The more oxidized form of iron is unavailable to plants, but aquaponic systems require mostly aerobic environments (Aerobic = Oxygen)

The problem

As soon as ferrous iron becomes soluble in aerobic environments it is often oxidized, (becoming ferric iron) or reacts with other compounds to become biologically *unavailable.

*especially at high pH values when different hydroxides are formed

The problem

Many practitioners falsely assume that this will supplement system iron.

It does add to the system iron, but not in a constructive or meaningful way. All this does is introduce more ferric iron to the system - a form that was most likely already there in plentiful supply.

Solution 1: add rusty stuff to your system

Other growers use dedicated anaerobic zones where ferric iron will be reduced by the oxygen free, anaerobic environment to produce ferric iron.

Solution 2: intentional anaerobic zones

This can work in low pH systems, but still does not entirely address the problem of getting the reduced iron ion (Fe++) through the oxygenated aerobic zone surrounding the plant roots (especially in high pH systems).

Solution 2: intentional anaerobic zones

Plants have their own solution to this problem.

The good news

You see, plants have some amazing chemical means of hijacking ferric iron ions, tying them up, bundling them into the soluble, biochemical equivalent of the panel van, and delivering them, bound and gagged, to the root surface for plant use.

The good news

Plants also use a few other techniques to make iron available, including:

- Acidifying the root surface by excreting hydronium ions

- Secreting iron reducing compounds

For aquaponic growers, chelation is the key process.

Chelation is an aquaponic iron-fixing technique.

Chelation is the process of tying insoluble ferric iron ions and compounds to organic molecules to make them soluble. It uses special organic molecules called chelatins. These are organic molecules that are specifically designed to capture, or “dissolve” metals like iron.

Chelation

So where do chelatins come from?

1) Produced by plant roots and leaked into the soil (e.g. phytosiderophores, which bind ferric iron, pulling it from various insoluble precipitates and substances. The grasses (Poaceae), and especially barley are particularly effective at producing phytosiderophores for capturing iron.

You need chelatins...

So where do chelatins come from?

2) Others: Amino acids, organic acids (especially humic acids like those from peat), and polyphenols. Can be introduced, but they aren’t always enough to keep iron available to the plants - especially in systems with a pH of 7 or above.

3) Chelated iron is often necessary even if you already have the first two.

You need chelatins...

Admissible under USDA Organic standards, chelated iron is an artificially chelated iron ion - essentially, iron attached to an organic molecule to make it soluble.

By adding chelated iron, iron deficiencies in your plants can be avoided.

Chelated iron

Forms of chelated iron

FeEDTA FeDTPA FeDDHA

This is a slightly toxic form that aquaponic practitioners should not use.

It should not be used because of it’s toxicity, but also because it typically only effectively chelates iron up to the pH range of 6.3 or 6.4. Above this range it is not a stable chelate.

FeEDTA

Recommended for most sytems at pH values between 6 and 7.5. It is commonly available at lawn and garden stores.

FeDTPA

Shop FeDTPA

Recommended for systems above pH values up to 9.0 and the best-all-around form of iron chelate - especially for starting systems.

Maintains iron solubility in almost all of the water conditions encountered by startup aquaponic systems.

*Miller brand can turn water red

FeEDDHA

1) Reactionary - any time you see a deficiency

Two ways to supplement iron

2) Regularly supplement - 2 mg/L every three weeks

Two ways to supplement iron

*Foliar applications can be used for quick response but aren’t highly recommended

At the 2mg/L dosing rate, a 10 pound, $15 bag of chelated FeDTPA will lat well over a year, or less than $1 per month.

Cost of chelated iron

- “Miller DP” - DTPA (On the shelf or ordered through Ace hardware)

- “Sequestrene” - DTPA (5 lb bag on Amazon for $57)

- “Miller FerriPlus” - EDDHA (SunshineGardensFl.com; 1 lb for $20, or 20 lbs for $300)

- “Sequestar Iron 6% Chelate” - EDDHA (RoseCare.com; 5 lbs for $73)

Chelated iron products

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We’ve got plenty of other videos and articles to help you out if you get stuck.

Check out the full iron post here.

If you’re really stuck, feel free to reach out to us.

Happy growing!