Aquaponics systems are deficient in one essential thing, and that is iron. As anyone in AP will tell you, it is a metal you must add to your system periodically. Yellow leaves on your plants are usually a good indication that some sort of nutrient deficiency is occurring. Since iron is the one substance AP systems do not naturally contain, and because it has such a tight PH absorption window, it is usually the culprit.
For most people, adding iron is quite simple. Chelated iron is available in most countries at nurseries or from hydroponics suppliers. For most home systems, the amount to add ranges in the teaspoon-measurements range every few months, so it goes a long way.
Chelated iron is simply iron that is loosely bound to a chelating agent, which makes it more soluble and absorbent by cell walls. Commercially available chelated iron for agricultural applications, often uses ethylenediaminetetraacetate (EDTA) as the chelating agent. Chelated iron for human use, marketed as a dietary supplement, often uses an amino acid as the chelating agent.
The reason chelated iron is important is because free-floating iron is difficult for plant cells to absorb, though not impossible. If it is attached to an amino acid or other chelating agent, plant cell walls absorb it readily. Once inside, the chelating agent and iron atom disassociate and the iron can be used for cellular processes. It is especially important for the formation of chlorophyl (itself a chelate), which is responsible for photosynthesis and gives plants their green color. As for how iron is absorbed in nature, or chelated in nature, I do not know - any plant biologists, please chime in!
Some people, myself included, have rusty pieces of metal somewhere in their systems, constantly introducing some iron into the system all the time. I have a rusty chain hung over my tank wall dipped in the water, and Ive buried several rusty nails in my grow beds.
If my PH were the perfect match for the iron absorption profiles and needs of all my plants, this should theoretically work. But as Yogi Berra once said, "In theory there is no difference between theory and practice. In practice there is."
The fact of the matter is that my spinach has suddenly turned a really bad yellow. So has one of my oregano plants, but to a much lesser extent. The strange thing is, everything else looks great. All my tomatoes are thriving, blooms coming out everywhere. The cuke plant must have tripled in size since last week. Everything else - all the herbs, lettuce, even the onion and radish which I didnt expect to do well, are doing great or at least look healthy.
So why just the spinach, and to a lesser degree, the oregano? I remember being told spinach is good for you because "it contains lots of iron". If that is true, I suppose it must need lots of iron as well. Im just speculating, but that would make sense, wouldnt it?
My problem is I cannot find chelated iron anywhere around here. And despite the iron added by my rusty friends, there is apparently no natural chelator in the system to help get it into plant cell walls.
What I did find out, however, is that molasses, the sugar-cane extract, is supposedly a good iron chelator. That is according to this and this.
It is also supposed to itself contain a lot of iron, without any need to add it. I have no idea if this is true, but I am willing to try.
So, last night I sprayed some molasses on all my plants. I diluted it with water a bit. The reason I had some around is because I had once heard it works as a bug repellant. The sweet smell apparently is not liked by many bugs. I actually sprayed it back when I first started 4 months ago, so who knows, maybe I inadvertently added a chelating agent and possibly chelated iron to my system back then.
This morning, noticing the spinach plants were still quite yellow, I mixed some molasses with water (1:9 ratio) and poured it into a small bucket that had rusty nails and even a nice little pile of "rust dust" at the bottom. I am going to let that soak the rest of the day and apply some tonight and the rest tomorrow morning.
Will report back here with whatever the results are.
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