Establishing a lawn around a new home often involves the addition of topsoil. Developers remove or cover the native topsoil, leaving a surface with little organic matter and poor soil structure for plant growth. There's some disagreement among experts on just how much topsoil is needed to establish a lawn, but some common threads run throughout the recommendations of university researchers from coast to coast.
To calculate the volume necessary to spread a given amount over the entire lawn, begin with the square footage of the lawn and divide by 9 to find the area in square yards. For example, a 5,000-square-foot lawn is 556 square yards. Divide the desired depth in inches by 36 to find the depth as a fraction of a yard. For example, 6 inches divided by 36 is .167. Multiply the fraction of a yard in depth by the square yardage of the lawn to find the volume of topsoil necessary. To continue the example, 556 times .167 equals 93 cubic yards.
Recommendations range from "never add less than 6 inches" from the University of Vermont to as little as "whatever your budget will allow" from Michigan State University. The condition of the existing soil and the current grade of the yard relative to sidewalks, driveways and the home's foundation also contribute to the decision. Final grade before seeding or sodding should be 1 inch below the level of sidewalks and driveways, and the yard should slope away from the house in all directions at a grade that will not cause either erosion or puddles.
There is universal agreement that whatever you add should be at least partially incorporated with the existing soil. Adding topsoil without incorporating it into the existing soil will create a stratified layer, resulting in a shallow root system and poor water infiltration. Soil will likely be compacted from construction traffic. Use a tiller to loosen the existing soil. Spread half of the topsoil. Till the added topsoil into the existing soil below. Spread the remainder of the topsoil and and any soil amendments that are to be added and till once more. Rake out to final grade and your yard is ready for seed or sod.
If the existing soil is better than most construction sites and the grade is too close to final to allow several inches of topsoil to be added, you might consider incorporating organic matter into the existing soil, instead of topsoil. Compost, or other sources of organic matter, will improve water retention in sandy soils and soil structure in clay-heavy soils. Don't add organic matter without incorporating it into the existing soil. It would be vulnerable to both erosion and compaction during the installation process. Even if it doesn't get compacted during installation, it likely will under any foot traffic. Incorporating 1 or 2 inches of compost into the top 6 inches of existing soil could be a more economical alternative than adding topsoil.
Soil Testing and Amendment
Once the seed is spread or the sod is laid, it becomes difficult to make significant changes to soil chemistry. When topsoil and/or compost has been incorporated into the yard and final grade has been established, sample the soil and have it analyzed. Most area extension services offer low-cost soil analysis and will give you recommendations for soil amendments. Armed with a soil test you can adjust the pH of your soil and add nutrients as needed to prepare the soil for your new lawn.