Clay is a hard, slow-draining soil that is difficult to dig in and leaves standing water from rain and irrigation. If you have weeds in your lawn, then your problem is compounded by this second issue. Clay soil is full of nutrients, which are used up by the weeds, leaving little for your grass. If your lawn is grown in this type of dirt, it is possible to improve the soil and give your yard a better chance at survival.
Spray a selective herbicide to remove weeds without harming the grass. Wait a few days to make sure that the weeds has died. Look at the label to see if there is a waiting period before planting new grass seed.
Aerate the lawn to break it up and put air back in the soil. You can rent an aerator from an equipment rental store or buy one at a lawn center. There are several different types from which to choose. Some are manually pushed, like a mower, and a round metal drum with spikes rolls across the lawn, creating holes. Others are pulled behind a riding mower or are gas-powered machines that spike the ground and pull out plugs of dirt. Weight is needed for the manual aerator and the one that is pulled behind the riding mower. This helps push the aerator spikes down into the ground, and can be something as simple as cinder blocks.
Add fertilizer to the lawn base on the results of a pH test. Grass grows best between levels six and seven, which is mildly acidic.
Spread seed to fill in the gaps left by the dead weeds. You don't need to spread as much as you would if you were sewing an empty area. The seed bag will tell you the amounts for overseeding. Spread it in one direction and then in the opposite direction to get full coverage.
Cover the seeds with a small amount of hay to keep birds away.
Kill the grass and weeds with an herbicide. Remove the vegetation with a shovel. Check the label to see if there is a period of time that you must wait before proceeding.
Test the soil to find its pH level. Add fertilizer to adjust if needed.
Till up the top 6 inches of soil. Pour on a 3-inch layer of sand and some organic material, and till it into the soil. This will help improve the clay's drainage.
Spread the seed over the area. Use the recommended amounts listed on the bag.
Cover the newly sewn grass seed liberally with hay to keep the seed from washing and the birds from eating it.