Iron sulfate, also known as iron vitriol, helps strengthen the lawn, resulting in a denser, greener lawn. The iron content in iron vitriol plays an important role in the biological processes that nourish plants, thereby improving lawn growth. Iron vitriol is derived from natural minerals and does not burn the grass. To strengthen the lawn, it should be limed in the spring or fall, and to promote growth, it should be fertilized with lawn fertilizer in the spring, summer, and fall.