Iron is one of the most abundant elements on earth and is vital for the health and well being of all living organisms.  Fish require iron to bind oxygen molecules to haemoglobin and facilitate the distribution of oxygen throughout the body, whereas plants require iron to synthesise chlorophyll and for leaf development.  In aquaponics the fish require very low levels of iron whereas plants thrive best when the iron concentration is around 0.5mg/?.  Higher iron levels do not quickly pose a threat to the plants but levels around 1mg/? and above can be dangerous to our fish, with sensitive fish species dying at an iron concentration of 1.5mg/?.
 
Despite its natural abundance, iron may not be available to our plants as the solubility of iron varies according to pH and dissolved oxygen in the water.  At the desired pH for aquaponics of 6.5, iron is fairly soluble if supplied in the chelated form.  Non-chelated iron is relatively unavailable to plants and as such it is ineffective to add nails and other iron-rich material to the system in the hope that they will rust and release iron for the plants to utilise.
 
If you are the sort of aquaponist who watches the crops for deficiency symptoms, then you will notice yellowing of the younger leaves if iron is deficient.  This could also mean that any one of a dozen other things are wrong and as such is not the method I recommend (more on this matter next month).  I recommend monitoring your iron levels by measuring the iron concentration in the water using an iron test kit.  These kits are available on our online store or in larger aquarium retailers.  Measuring gives you a clear indication of what the concentration is, enabling you to manage the iron concentration to be optimal for the plants without harming the fish.
 
After measuring the iron level in our aquaponics systems to be below 0.5mg/? we dissolve the required mass of chelated iron in some water in a bucket and add it directly into the fish tanks.  In our system we use EDTA chelated iron with great success and find that we require roughly about 1g/kg feed added to the system.
 
Failure to manage the iron concentration will result in weak, slow growing plants, so be sure to manage the iron concentration and supplement as required.