If I buy over-the-counter iron supplements at a drug store, mix with water and spray on dandelions, will it kill them?
If so, how much iron do I need and is it more cost effective to make this yourself than buying iron-based weed killers.
Probably not. The active ingredient in the new iron-based weedkillers is a specific iron chelate - specifically, iron HEDTA (Hydroxyethyl Ethylenediamine Triacetic Acid). AFAICT, this isn't used in diet supplements.
I wouldn't try using the weedkiller as a diet supplement, either. ;-)