Despite common misperception, there is nothing special about topsoil. Topsoil is the first few inches of any soil. When you purchase bags of topsoil, you risk the chance of purchasing soil from an area that may have more noxious weeds, insects or chemicals in it. Always know where your topsoil comes from; ask the garden center professional you're working with or the supplier. If possible, ask for a decent sized sample of the topsoil so you can have it tested for nutrients and other properties.
Test your existing soil for pH and nutrients before planting a lawn or garden. A healthy soil pH for sod is between 6.0 and 7.0; however, this can vary for different gardens or landscape plants. Your local university cooperative extension service should perform soil tests for you. Along with the results of the test, you should get recommendations on fertilizers, how much organic matter to add and pH level. A soil test is often the most important, yet most overlooked, deciding factor for preparing your soil.
Amending your soil means adding nutrient-rich organic materials into your soil to improve drainage and your plants' ability to extract the nutrients from the soil. Compost is an ideal organic soil amendment because it uses the natural biological breakdown of organic matter and puts it right back in your soil. You can purchase compost or make it yourself. Other organic materials include peat or manure. Peat is a material mined from natural bogs, and because of this, you should use it sparingly. Only use composted manure when amending or fertilizing with manure. Manure releases nitrogen and some other nutrients into the soil for your plants. Cow and chicken manure are two common examples of commercially available manures.
If you still insist on topsoil, know that you'll need a lot of it to complete a lawn. Even a small vegetable garden or flower bed may require a truckload of soil. You'll need at least 4 to 6 inches of topsoil for a lawn and about that for a garden. Tilling the topsoil into your existing soil can help spread the benefits of your existing soil. For example, although clay is hard to grow in, it holds several nutrients. Sandy soils allow for excellent water drainage.