Why is iron content important?
Aquatic plants need iron as a nutrient. If the plants do not have enough iron available, they show the deficiency by a so-called chlorosis. The leaves become light green and pale.
At the latest when signs of chlorosis appear, it should be fertilized. Some plants, such as dwarf spear leaves, are particularly sensitive to iron deficiency and react quite quickly with chlorosis. Duckweed is also a good indicator of iron deficiency. If enough iron is available, duckweed multiplies quickly. If there is a lack of iron, they stop multiplying.
Why is my iron test not finding any iron?
Iron fertilizers contain chelators so that the iron does not oxidize from divalent Fe(II) to trivalent Fe(III) and does not collect in this form in the filter sludge. Without these chelators, the iron in the water would quickly oxidize and precipitate. This means that the plants cannot utilize the iron. Not all commercially available iron tests are able to measure chelated iron. These tests then show no iron, even though there is enough iron in the water. By heating the water sample, iron and chelators are separated, so that the iron content of the water can also be determined with these tests.
Iron tests that also measure chelated iron, e.g. B. from Dupla, need some time until iron and chelators are separated. Therefore, the iron value can only be read after some time, about 10 minutes.
How much iron is needed?
On the one hand, plants need iron as a nutrient, on the other hand, heavy metals such as iron are plant toxins that are harmful in excessive doses. Bacteria can develop in the aquarium, which destroy the chelators so that iron is precipitated. In turn, precipitating iron precipitates numerous trace elements.
Approx. 0.1 mg iron per liter of water is a dose that provides the plants with enough iron and is harmless.
What happens to precipitated iron?
Precipitated iron collects in the filter sludge and is removed when the filter is cleaned. Precipitated iron that gets into the substrate can still be used by the plants. They excrete an acid with their roots, which dissolves the iron from the soil and is absorbed and utilized by the plants. Good plant growth without chlorosis can also develop in an aquarium without detectable iron in the water.
Of course, plant species that prefer to take up their nutrients via the leaves can derive less benefit from such iron in the soil.
Iron in the substrate can be detected by diluting some sludge from the soil with a few drops of HCL and then doing an iron test with the liquid that has formed on the sludge.
In an aquarium, natural chelators are created, e.g. B. Humic substances, through which iron from the substrate can get back into the water. In addition, in anaerobic, ie low-oxygen, areas in the soil, the precipitated trivalent iron Fe(III) can be reduced back to bivalent iron Fe(II). Since bivalent iron is easily soluble, it can diffuse upwards into the water. If there are no complex-forming chelators in the water, the Fe(II) in the upper aerobic soil layers oxidizes again to Fe(III). It falls out again and does not get into the free aquarium water. Only if complex-forming chelators are present does the iron remain in solution and finally find its way into the open water.