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Iron is immobile in plants. This means that plants cannot divert iron from older leaves to new leaves. Therefore, deficiency symptoms first appear on new or young leaves. Since plants use iron to produce chlorophyll, a lack of iron leads to chlorosis or yellowing of younger leaves. Stems may also appear short and thin. If the deficiency is severe and prolonged, each new leaf emerges paler than the previous one.
When choosing an iron supplement, it is important to know the distinction between the two forms of iron. Iron will be in one of two oxidation states: ferrous having a charge of +2, or ferric having a charge of +3. Ferrous iron, the preferred form of iron, is soluble in water at any pH. Ferric iron, however, is only soluble below a pH of about 5.5; but if the pH is above 5.5, which is more than likely in a planted aquarium, ferric iron will become insoluble and precipitate, settling in the root zone. Once this happens, foliar absorption becomes impossible.
To overcome this precipitation, competing products use a ferric iron chelate: iron-EDTA. While this keeps it soluble, it has a few drawbacks regarding foliar absorption of iron. (1) The iron-EDTA bond is very strong, so very little iron will be available to plants over a given period, and (2) physiological energy must be expended by the plant to extract ferric iron from iron-EDTA and convert (reduce) it to ferrous iron. Our approach is different in that we use a complex (non-chelated) ferrous iron in Flourish Iron™.
Flourish Iron is a highly concentrated (10,000 mg/L) ferrous iron gluconate supplement. Plants can benefit much more easily from Flourish Iron because ferrous iron gluconate is already in ferrous form and therefore does not expend energy to reduce it. Despite what other manufacturers may intimate, gluconate is not harmful to plants or fish. In fact, ferrous gluconate is better suited for foliar feeding than iron-EDTA due to the relatively weaker iron-gluconate bond than iron-EDTA. In addition, ferrous gluconate has the added bonus of being a carbon source.
