I have been experimenting here with iron ferts. In the following I make reference to claims made by SeaChem. To verify my citations go to the Discussions section of their website and surf around in the Sunken Gardens forum. Not all the info available from SeaChem is printed on the bottle.
As previously noted, I turned off the UV Wednesday AM, and as of Thursday AM I had brought up the iron to 0.56 ppm. No iron ferts were added until this AM. Everything else was done regularly and the plants spent those two days under the normal light cycle. I tested several times throughout the day Thursday and Friday. Friday evening the iron tested at 0.52 ppm. No change. The UV was turned back on at that time and ran all night until its regularly scheduled 6:00 AM shutoff. Then I allowed the normal dose of MICRO/TRACE which contains Ferric EDTA, but withheld the Ferrous Gluconate. I tested 4 hours later, after the lighting was well under way, and the iron was back down to 0.07 ppm.
SeaChem asserts, and I accept, that Flourish Iron is the superior iron fertilizer for foliar uptake because it is a carbohydrate that plants can metabolize readily, thereby being a good carbon source as well as having the advantage of releasing iron in the ferrous state. Something I have noticed since I continuously monitor ORP (redox), is that the ORP drops a full 100 mV every morning at dosing time (6:00 AM). The moment the gluconate goes into the water, the redox drops like a rock, then gradually rises to its normal "real" value by midday. Both days the gluconate was withheld, the redox did not fall off and has stabilized at about 380 mV on a flat curve. The ORP meter is keeping tabs on the relative abundances of reducing agents and oxidizing agents. In a system that is highly oxidative as a rule (in which plants produce lots of O2 and keep the dissolved oxygen at a high level day after day) a reduction in the redox can be interpreted as an increase in DOC, which is comprised largely of reducing agents. Gluconate is a dissolved carbohydrate and is a reducing agent. No other additive used, including Excel (gluteraldehyde), affects the redox in this way.
SeaChem asserts that the plants will consume a prescribed dose of Fe Gluconate so rapidly that it will virtually disappear from the system in half an hour. Of course, I think this depends on some variables such as rates and patterns of water circulation, and, as I assume the plants must be photosynthesizing and metabolizing normally for this to occur, the total plant mass, character of the light, and other things. In any case, I accept that the plants will uptake Fe Gluconate rapidly and I fully expect it to be consumed within a couple of hours in my system. SeaChem states that if you test 0.1 ppm iron within one half hour of dosing Flourish Iron, that is a sufficient dose of iron for the aquarium. This may be a better way of stating the case. But I should think that a heavily planted tank with highly productive specimens could do with a somewhat larger dose than that.
For cations to be bioavailable, they can be complexed with relatively stable organic molecules to preserve them, at least for a while, from being immediately bound to anions in the solution and present them to the plant as food it recognizes and can use. Both EDTA and gluconate are subject to photodissociation. Probably a UV sterilizer is a more expeditious way to get that done than is overhead visible light, but both will do. The gluconate ligand is probably fairly weak, which is a desirable characteristic in a plant fertilizer, as the idea is to require as little energy input from the plant as possible in order to get the iron ion out. Also the gluconate is a carbon source, and is likely a source, rather than a sink, of energy. EDTA is a hexadentate chelate and is a strong ligand. The plant has to pry the iron loose and then reduce it to ferrous state before it can use it. This requires energy from the plant. Many of the desirable metals in micronutrient fertilizer are made bioavailable by EDTA, or similar, chelation. Probably all such complexes can be decomposed by exposure to light. Some ligands are stronger than others, and rates of decomposition may be intensity and wavelength dependent. When these substances are exposed to light, the molecule is denatured and oxidizing agents and orthophosphate anions grab the metal, forming insoluble inorganics that precipitate out of solution and are no longer bioavailable, at least in terms of foliar uptake.
SeaChem says that you should not keep iron and phosphate ferts in the same bottle. Since Flourish iron is consumed so rapidly, there is little opportunity for it to decompose in the water column, so there isn't much concern about the timing of phosphate dosing. I think that does follow from the theory of what gluconate is and how it works, but I still dose these two nutrients 12 hours apart. It's just good karma. I also suspend UV exposure from 06:00 to 12:00 every day to maximize the gluconate's presence; that is, I give it 6 hours - 4 hours with lights on - before I subject it to UV.
Pretty clearly, the UV denatures EDTA fairly rapidly, as this experiment shows. And since EDTA complexed nutrients are not rapidly consumed, there may be a lot lost to UV exposure. This, I think, must be accepted and dealt with in much the same way as the inevitable loss due to water changes, especially by those who practice EI for example. It seems to me that many (most?) micro/trace nutrients are more ideally dosed via substrate, for root uptake. Getting this stuff into the roots from the water column is the hard way for the plant, but the easy way for the aquarist. I should probably make better use of root tabs, but in a large densely planted tank there are practical difficulties, and I wouldn't rely on that method completely in any case.
I will not be going to an "active" (soil-based) substrate. I like my inert gravel and unchanging Flourite mix. It is eternally stable.
All this explains also why I do not routinely use activated carbon. It is good at removing DOCs from water, and it won't remove desirable dissolved inorganics such as those that comprise macronutrients. But it will remove organic molecules, and that includes all ligands and the metals bound to them. Activated carbon is a good way to defeat dosing of anything that is complexed, and it will also take out gluteraldehyde (Excel), phytohormones, humic (blackwater) additives, and any allelopathic toxins the plants may be generating to suppress algae (and yes, I do believe in that). And the same goes for other chemical filtration media designed to latch onto organic molecules. I don't use them, and haven't missed them since I stopped.
So, there is no doubt in my mind that ferrous gluconate is the preferred way to provide iron to the aquatic macrophyte, provided certain things are borne in mind about when and how much to dose it. It does, however, play merry hell with my data curves.
