iron supplements?

I have yellowing grass and I was told that my lawn probably needed some iron supplementation. But I'm confused--here's what I saw at Home Depot but I don't know which supplement to purchase.
-Miracle Grow lawn food. -Scotts liquid turf builder. - Natural Lime - get-a-lawn. -ironite plus.
Most of these contain 1 to 2% of iron. But I don't know how much iron my lawn might need.
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