Iron dose

Description

Iron is the micronutrient that runs out first, and it runs out visibly. FAO's aquaponics manual puts it in the chloroplasts and the electron transport chain, so a shortage stops photosynthesis rather than slowing it. The progression it describes is yellowing between the veins, then whole leaves pale and chlorotic, then white with necrotic patches and distorted margins. Where the symptom appears is the diagnosis. Iron is a non movable element, so it cannot be pulled out of old leaves to supply new ones, and a genuine iron deficiency therefore shows in new growth. A plant yellowing from the bottom up is telling you about something else; the same manual points at nitrogen for that pattern. Chemistry decides what is worth buying. FAO's instruction is that iron has to be added in chelated form, sequestered iron or Fe-EDTA, because iron is apt to precipitate at pH greater than 7. An unchelated iron salt in a tank sitting at 7.5 is on its way to becoming a solid. Elsewhere the same manual notes that a pH of 7.5 can lead to deficiencies of iron, phosphorus and manganese, so hard alkaline water can produce an iron deficiency with iron in the bottle and iron in the water. In that case more iron is not the fix. On quantity FAO is loose and says so: about 5 millilitres per square metre of grow bed whenever a deficiency is suspected, with the note that a larger quantity does not harm the system but can discolour tanks and pipes. That is a grow bed rate, not an aquarium one, and no aquarium figure exists to offer instead. One curiosity from the same paragraph, flagged there as unsettled: submerged magnetic drive pumps have been suggested to sequester iron, and the manual calls it the subject of current research. Deleted with the old description: the claim that red stemmed plants show the most dramatic colour response. Nothing in this literature ties iron dosing to colour in any named aquarium plant.

Source

• openknowledge.fao.org — https://openknowledge.fao.org/items/e4d11efd-d97a-4959-8ae2-8b7d3fd32c5e

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