Iron is an important plant and animal nutrient and is thus very crucial to your aquaponics system. In plants, iron serves many functions but is an essential component in the production of chlorophyll, the site of photosynthesis. Without enough iron, plants cannot produce enough chlorophyll, leading to retarded plant growth characterized by interveinal chlorosis.
The Issues with Iron in Aquaponics
Because systems are generally aerobic (and certainly aerobic in the root zone), iron deficiencies can often arise- even when there is technically plenty of ferric iron within the system. In the aquaponic solution, iron is commonly available in one of two forms- reduced, soluble Ferrous Iron (2+) and insoluble, oxidized Ferric Iron (3+).
Ferrous iron is available to plants as it is soluble whereas Ferric iron is not due to it being insoluble. This is important to understand that, because ferric iron is the more oxidized form, whereas ferrous iron is not. In short, 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).
Many practitioners throw rusty iron items into their systems falsely assuming that this will supplement system iron. In a sense it does add to the reservoir of system iron, but not in a constructive or meaningful way. All this does is introduce more ferric iron to the system- a form of iron that was most likely already in plentiful supply.
Iron is one of the plant nutrients that must be supplemented in almost all aquaponic systems.
To supplement iron, chelated iron must be added to systems.
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.
Iron is generally the only essential nutrient that is not available to the plants in the system as it is not included in most commercially available feeds. It is advisable to run your aquaponic system at a pH of close to 6.8 which is the perfect balance for all living components in the system, however, running your system at this pH forces available iron to precipitate out, making it unavailable to your plants. If you operate your system at a lower pH such as 6.0-6.5, you may not have to supplement iron as often because it is less likely to precipitate out of solution.