Most grass seed will grow in various soil types. However, clay soil is perhaps the most difficult to work with. That's because it is extremely heavy and thick. Clay soil is made up of tons of small particles that stick together, clinging to moisture. There are very few air pockets under the soil surface and grass needs air pockets for root spread and development. The key to planting grass in clay soil is to first amend the soil to make it looser, drier and more fertile.
Dig up rocks with a shovel to remove them from the planting area.
Rent a gas-powered sod cutter and use it to remove old grass, weeds and their roots.
Loosen the top 6 inches of clay soil with a rototiller. Run it over the area until the soil is no longer packed down.
Fill in dips with topsoil to create a level surface. Rake until smooth.
Determine the soil pH with a test kit obtained from a nursery or planting center. If it is between 6.0 and 7.5, no amendments need to be made. If the pH needs to be adjusted, you will do that in step 7.
Amend the soil with compost and other organic materials. Leaf mold, well-rotted manure and compost can all be used, according to Gardening Know How. Put 3 to 4 inches of the material on top of the soil and use the rototiller to mix it to a depth of 4 to 6 inches.
Adjust the pH based on the results of the pH test (step 4). The Garden Helper says to boost the pH of acidic soil with hydrated lime. Apply 12 ounces of lime per square yard using a spreader to increase the pH by 1.0 point. Lower the pH of alkaline soil with ground rock sulfur. Apply 3.6 ounces of sulfur per square yard of clay soil.
Apply a starter fertilizer with a broadcast spreader. Refer to the information on the seed bag to determine the setting for the machine. The fertilizer will add the right combination of nutrients to the clay to make the grass seed germinate faster and more successfully.
Rake the fertilizer and pH soil amendment, if any, into the top inch of soil.
Spread the grass seed evenly across the planting area. Use a push spreader for large areas or a hand spreader for a small lawn. The grass seed packaging will specify the size of the area that its contents are intended to cover.
Rake the grass seed gently into the top 1/8 to 1/4 inch of soil using the backs of the tines on a leaf rake. Covering the seed will help it germinate faster.
Water the seed twice daily, for 5 to 10 minutes. After two weeks cut down watering to once a day. Keep the clay moist because if it dries out, some seed may die.