You can add topsoil to an existing lawn -- and in some cases, you should. Adding a layer of topsoil to your lawn is called "topdressing," and it's a technique you can use to improve the look of your grass. It's important you prepare correctly and choose the right type of soil for a great-looking lawn.
Identify Weak Spots
You might choose to add topsoil to your existing lawn for several reasons. You might have low areas where the earth has sunken down due to tree roots or other changing elements in your yard, causing tripping hazards or pooling of water. You might also have unsightly patches of dead or dying grass that need some improvement. You might have something of the opposite problem too -- thatched parts of your lawn that have grown too thick. Whatever the reason for doing it, the best time to top dress is when the lawn is actively growing.
Test and Prepare the Soil
Your lawn's soil might have high levels of sand or clay, depending on where you live. To be the most successful with topdressing, your topsoil should be similar to what's already in your soil, advises University of Tennessee Extension. Test both your soil and the topsoil you plan to use. If the makeup of your soil and the topsoil is drastically different, get more peat, clay or sand to add to your topsoil so it more closely matches your lawn's soil. Mix it in a wheelbarrow or bucket. Then, use an aerator machine to aerate the parts of the lawn that need topdressing. If you have thatch -- that mossy, thick growth on your lawn -- rake it briskly with a wire rake before aeration.
If you're adding topsoil to fix dead patches, you'll only need to add a 1/8- to 1/4-inch layer of topsoil over dead or dying areas. That translates to between 0.4 and 0.8 cubic yards of topsoil for every 1,000 square feet of lawn. Fill up a bucket and then gently spread the topsoil over the area. For depressions in the soil, add enough topsoil to fix the depression, leaving the soil slightly mounded to allow for settling.
Top With Grass Seed
Before you add grass seed to your newly laid topsoil, use a metal rake to gently work the topsoil into the existing soil -- taking care to add topsoil to the aeration holes you made. Finally, sprinkle grass seed over the area. Some grass seed mixes come with materials added to help distribute the seed more evenly, so read and follow the instructions on the package to use the correct amount of seed product. With the grass seed in place, water the area regularly. Grass seed needs to be continually moist for successful germination and growth.