Lawn grasses are perennial grasses that come in two types, cool season and warm season. While sod is the easiest way to get a ready-made lawn, it does not always last as well as it should. Due to environmental or other factors, sometimes the sod starts to thin. Instead of laying it over with more sod it is easier just to seed over the existing sod with grass seed. This helps fill in a thin lawn or rejuvenates a worn-out lawn.
Remove any thatch, weeds or rocks from the areas of the sod you want to seed over. Use the rake to remove large rocks and thatch. Pull the weeds up by hand, being sure to remove the roots.
Add 4 to 6 inches of compost per square feet to the sod you want to seed over and work it into the soil to a depth of 6 to 8 inches with the cultivator, according to the University of Illinois. This helps improve your soil's condition for the new seeds.
Water the area in which you want to plant the seed deeply, adding about 1 inch of water. Allow the soil to dry on the surface before you plant the seeds.
Mix ¼ lb. of lawn seed with 1 lb. top soil and sprinkle it by hand over the prepared areas. Hold the seed and soil mixture in your hands and broadcast it through your fingers onto the prepared sod. Be sure about 30 to 50 seeds are planted evenly for every square foot.
Water the seeded lawn deeply, applying about 1 inch of water. Be sure to keep the soil moist until the seeds sprout.