Measuring Iron In Your Live Aquaponic System Water
There’s a lot of nonsense in the aquaponics forums and among the “experts” about how it’s neccesary to measure iron with this test kit or that; and then to “keep it within certain limits”. In eight years of commercial aquaponics operation, we have found that it’s totally unnecessary. First, there’s absolutely no need to spend any money on a test kit for iron. This is because you already have a built-in iron tester (it’s called your eyes!).
(Below) A healthy leaf on the left, an iron-deficient leaf on the right. Nitrogen deficiencies look exactly the same (read below).
We don’t use an “iron test” to measure iron, we simply use our eyes and observe: you have an iron deficiency when you see yellowish new growth on plants between the veins on the leaves. Add chelated iron to fix this. We add a pound or so of 13% organically certified iron chelate (Biomin brand is approved by the certifying agencies) each time we see yellow growth. Iron deficiencies don’t occur on a schedule, so just keep an eye on your plants; one variety or species will usually start to show an iron deficiency before any of the others. Use that one as your guide, and add iron. We usually see a need to add iron once every month to three months in our large commercial systems.
Now, there is one condition your plants may have that you could confuse with an iron deficiency, but you won’t, because you’re going to read and remember the following: the leaves of a plant with an iron deficiency look just like the leaves of a plant with a nitrogen deficiency, they both have yellow between the veins of the leaves. Except, the plant with the iron deficiency has yellow on all the leaves, and the plant with the nitrogen deficiency only shows it on the largest, oldest leaves. That’s how you tell the difference. Also, you will hardly ever see a nitrogen deficiency in a system once it’s past its first two or three months of operation. You frequently experience nitrogen deficiencies during startup (read our posts on Starting Up Your Aquaponics System to find out more about this phenomenon).
Adding iron when your plants have an iron deficiency is very satisfying: the plants green up sometimes within six hours or so of the iron going into the system. It takes us a year and a half to go through a $150, 25-pound bag in our commercial operation with three systems totalling 6,000 square feet.