Brief Overview of Iron in Aquaponics
Iron (Fe) is an essential element for plant development which is typically limiting in aquaponic systems.
Iron requirements and toxicities of fish vary by species though in general, these levels are much higher than what we require in aquaponics. Iron may be added to feed in various forms to meet fish requirements; however, relatively low amounts are included in fish feed and do not provide enough for proper plant growth. What little Iron that is excreted by the fish is bound up in the organic waste and unavailable to the plants without further biological processing
Iron is generally present in two forms: the soluble form Ferrous (Fe +2) and the insoluble form ferric (Fe+3). In the presence of oxygen or in a high pH environment, the usable ferrous iron quickly becomes ferric iron and is unusable by plants (*). To overcome solubility issues, ferric Iron is bound to an organic chelating agent. Chelated iron must be used to ensure its availability to plants. There are several forms of chelated iron each of which has a specific range of pH in which they can operate.
The three most commonly used chelated forms are Fe EDTA, Fe DTPA, and Fe EDDHA (See chart below)
FeHEDTA— is treated as a biochemical herbicide commonly used to kill and deter broadleaved plants, in lawns, golf courses, and other grassy areas. In the U.S is not approved for use in food crops. FeHEDTA is known to have low acute toxicity when exposed to orally, dermally, or inhaled, if exposed to it before it dries. There are only a few decoupled aquaponic applications that this would be suitable for and should not be used it traditional aquaponics.
FeEDTA- has been widely used in hydroponics as a foliar and water addition, however, in pH’s above 6.3, and in well-aerated systems, it quickly oxidizes and becomes ferric iron making it insoluble and unavailable to the plants; this makes it the least ideal to use in traditional aquaponic systems.
FeDTPA- is widely available, cost-effective, and is the preferred form aquaponic systems that operate in a pH up to 7.5.
FeEDDHA- is a more expensive form of chelated Iron though it is available to plants at wide pH range and is stable up to a pH of 10. If your system is operating in a pH of greater than 7.5 then it is suggested that you use this form. *Some manufacturers use red dyes in this compound which tint the color of your water red.
Ideal pH range for most aquaponic systems is between 6.8-7 making the preferred form of chelated Iron to be FeDTPA. Individual needs and operations exist outside of this range so chelated Iron should be chosen based on your pH.
Irons that use synthetic chelates are not-approved for use in organically certified operations. For those seeking organic certifications, you should look at using Irons that have organic chelating agents such as amino acids or citric acid. Another option is the use of use Iron Sulfate, which is an inexpensive iron additive. However, these form of Iron quickly oxidize into the Ferric (insoluble) form of Iron in pH’s of 7 and greater; for this reason, it is better recommended for use as a foliar spray or in decoupled aquaponic systems that can maintain a lower pH, without compromising fish or bacterial health.
Recent research has shown the viability of using specific bacteria, fungi, and companion plants that produce natural chelating agents known as siderophores and phytosiderophores to help nutrients such as iron, zinc, phosphorus, etc. become bioavailable to the plants.
For hydroponic and aquaponic applications the optimal iron concentrations of the water are 2.0 mg/l or 7.57 mg/gal. To measure your iron concentration try our Hanna Iron Checker.
* Most plants have developed some means to assist with absorption of the insoluble ferric Iron and Some can even directly uptake it