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Episode 17: Iron in Aquaponics

Show Notes

Today we’re talking about iron in aquaponic systems. If you have questions, check out the YouTube video that I did, which goes over ferrous and ferric iron in more detail.

What does iron do? (1:17)

Iron is an important and commonly deficient nutrient in aquaponics. The reason for this is that there isn’t usually enough plant available iron in the system, even if there is a lot overall. Iron can only be taken up as ferrous iron, which exists typically in low pH and anaerobic conditions. Both are hard to find in aquaponic system. Most of the iron you have will be ferric, more oxidized, which means that he plants have a harder time taking it up.

Iron is also one of the key components in metabolism (namely, photosynthesis). The problem is that you want your system to be aerobic and that are typically not too low of a pH and the system usually has oxidizes iron

When iron oxidizes it precipitates out, so when it’s un-oxidized (reduced) it is soluble, and the plants can take it up. Otherwise, it’s not available.

Now plants have their own ways of dealing with this, since it happens in soil too. They’ve developed compounds called siderophores (usually carbohydrates of some kind) that leak out into the soil, and when they run into iron that is freshly reduced (sometimes even by the siderophore), they make it soluble. This means that the iron is available even with high pH and in aerobic environments.

Types of iron supplements (4:33)

The nice thing for us is that we can do the same thing as siderophores artificially with chelated iron, which is an iron fertilizer. This chelated iron is what you can use in your aquaponic system. When you look at the package, you’ll see “Fe” (the chemical symbol for iron) and you’ll see a bunch of letters behind that- either “EDDHA” or “DTPA”  or “EDTA”, all describing different forms of iron, and all of which are unique.

FeEDTA is a form of chelated iron that works really well but is typically available at low pH value (if you’re running your pH above 6.5 you’ll want to use another form) and that is phytotoxic (meaning that you probably don’t want to use in an aquaponic system). It is cheap, though, so it’s used in hydroponics and other agricultural applications.  Iron content in FeEDTA is usually around 13%.

FeEDDHA is some of the best chelated iron out there, functioning at a broad pH range. The drawback is that it does stain your water red. If you don’t want water that looks like koolaid, you’re stuck with DeDTPA. The iron content of FeEDDHA is typically in between 7 and 13%.

FeDTPA  is very nontoxic, but slightly lower iron concentration so you have to add more to get the enough.  Iron content tends to be around 7%.

How to dose (7:11)

You’ll want to be dosing your iron at about 2 mG per liter (2 ppm) every two to three weeks. The math can be tricky, but check out Youtube video to see how to do it – I’ll walk you through it. Keep in mind that iron is always entering and leaving your system, so you always have to add iron in excess of what your plants are using. The good news is that once you’ve done it correctly, you can dose at that amount on a schedule.

For use this means we add several ounces of iron on a three week interval, and we almost never see a deficiency.

Most iron deficiencies show up as intervienal chlorosis. If the veins themselves yellow, that’s probably nitrogen. Most of you will see a deficiency at some point in your career.

Other resources

KelpforLess  You can get chelated iron here and sometimes even at a local hardware store.

Iron 1 video  This video descirbes the dynamics of iron.

How to dose your system video This video shows you how to figure out how much iron your system needs.

Blog post on iron


Thanks for joining us today! Stay tuned for more on nutrients in aquaponic systems.

View all of our episodes here.

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