Aerate your lawn too. If you haven't aerated your lawn in a couple of years, now is the time. Lawns get compacted by foot traffic, pets and children playing, making the soil hard. Grass roots have a difficult time growing in compacted soil, and aeration is a way to loosen it up. Rent an aerator from your local home improvement store and always follow the manufacturer's guidelines when operating the machinery. Top-dress the lawn with 1/2 inch of compost afterward and water in well for a healthy lawn.