Ask most aquaponic gardeners or growers when their system needs iron(Fe) and the answer is usually always the same, when the plants indicate that they are iron deficient. Typically low iron looks very similar to a Nitrogen deficiency and is often confused for one. However, a lack of iron has a somewhat unique indicator. Usually an iron deficiency in new growth is evident when the entire leaf shows a yellowing or Chlorosis, however in mature leaves, there is a very noticeable and characteristic yellowing between the veins and the veins maintain their dark green color, as indicted in the image below.
This low iron condition has several adverse effects on the plant and is much more serious a condition then just looking pale and yellow. Low iron levels compromises the plants ability to make chlorophyll. It also is essential for the plant to synthesize proteins and absorb necessary nutrients such as nitrogen and phosphorous. For the backyard gardener, waiting for this indication is certainly tolerable however for the aquaponic farmer whose livelihood depends on healthy, desirable crops, how can we insure that a Basil crop is at the utmost quality and color every week or every day when it goes out to multiple customers? Waiting for that deficiency to show its color, or lack of, for a crop such as Basil that has to go into a Chef’s vibrant green Pesto is too late. So what can we do?
One option is to routinely add iron to the system. This can surely insure that there is an adequate amount of iron, but if that iron is not being absorbed as frequently as it is added, how would we then know if it is too much? There are indicators for excess iron as well and it can actually cause greater problems then having an inadequate amount. Whereas plants need iron to create Chlorophyll, too much can cause the chlorophyll to change and then inhibits the plant’s ability to absorb sunlight and the energy it derives from it. Too much iron can also impede the plants ability to take up nutrients and absorb sugars and once these vital processes are impaired the plant begins to fail from within and eventually will die. Really then we can actually be creating a greater problem by oversaturating our systems with iron. The other effect is that Chelated iron at a 13% strength, which is what we use, is not cheap. Depending upon where it is purchased it can be as much as $16 per pound so having a way to manage how much iron to add can be crucial.
If waiting for the deficiency to appear or risking adding too much iron are neither a desirable alternative, then we need a way to identify an adequate presence of iron in our systems and the solution to that is an iron test kit. This can be found usually at a jacuzzi supply store. Here this simple test can indicate iron levels in your water in parts per million or ppm. After much testing and monitoring we have finally identified where the low range is for our just over 20,000 gallon system and how much iron to add to get to what is our desired range. A reading of 1.5ppm is just a touch too low and when getting a reading of 1.5ppm, the plants are just beginning to show a deficiency. Therefore with our weekly monitoring we are looking for a reading of 2.0ppm and then we add either 2 or 4 cups of a powdered, 13% chelated iron. Each 2 cup addition is first mixed in a bucket of water to dissolve the powder and is then added at various places throughout the system. Each 2 cup addition will move the iron measurement 0.5ppm. In addition to raising the iron level, adding the chelate also causes a noticeable yellow or amber hue to the water and there is a quick “greening up” in the plants sometimes as rapidly as within 12 hours or a day. Of course, your water source will play a role in this and if there is a high iron concentration in your water, it will translate into having to add less in the form of chelates.
Since implementing this process at our farm, we no longer see iron deficiencies in our plants and have minimized the risks of adding an unmeasured amount of iron that might create an excess and also avoid incurring an additional expense from adding too much. Some aquaponic gardeners advocate using things such as a rusted piece of metal as a source and this may be suitable for backyard or hobby systems but we recommend using a chelated product and implementing a similar procedure so iron additions are a metered and measured addition to your system much like additives to alter pH, especially if you are growing for profit. As farmers, controlling as many variables as possible can help insure a good quality crop, satisfied customers and a difference for your bottom line.