Nitrogen is a “macro” feed for the leaves of your grass plant. Iron is an added organic feed that helps give the grass a VERY Dark green color.
Assuming your grass is fairly green and healthy already, you can add a light dose of iron as a way of achieving a richer, deeper blue-green color. This is will surely set your lawn apart in the neighborhood. In fact, it is iron that makes Kentucky Blue Grass, “blue.” Iron can also be used to correct soil deficiencies (nitrogen gives the lawn a greening but also pushes growth) If your turf grass looks dull yellow, chances are a good dose of iron will help restore it.
Iron feed should be applied in April to green up the grass during the growing season, and in August when the grass may find itself losing the "green luster" it had throughout the season.