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Why are recommending many Internet sources to add "iron" to treat chlorosis, when chlorophyll has no iron, but magnesium in it?

(note I am not asking about putting rusty nails to soil, because I do know that form is unconsumable for plants, just us putting rusty nails to apple, for human anemia)

Instead I am asking about adding gardening iron supplements, to treat chlorosis, which would make sense if chlorophyll would contain iron, but it does not.

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Chlorosis, as you say, just means a shortage of chlorophyll - it can be caused by nutrient shortages other than iron, and may have other causes, such as poor drainage or compacted roots. The more common cause is actually shortage of iron, sometimes because the soil ph is very alkaline, but if its iron shortage, the younger or terminal leaves will show signs of it first; other deficiencies present in a different order, see here https://web.extension.illinois.edu/focus/index.cfm?problem=chlorosis

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    The structure of chlorophyll is very similar to heme, the precursor chemical for hemoglobin in blood. The main difference is whether the trapped atom in the organic compound is iron (heme) or magnesium (chlorophyll). Iron is beneficial for the synthesis of chlorophyll in plants, even though there is no iron in the final product. (Interesting footnote: Vitamin B12 also has the same molecular structure, but with cobalt instead of iron or magnesium.) – alephzero Jul 22 at 12:32

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