Which mineral ion does a plant need in order to make chlorophyll?

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The green pigment cannot be assembled without magnesium, so a plant short of that one mineral makes too little pigment however much light falls on its leaves. Nitrate is the tempting alternative because it is the better-known mineral of the two, but nitrate feeds protein production, which involves a completely different set of molecules, and no other mineral in a fertiliser bag stands in for the one the pigment needs.