Preparing the soil before sowing grass seed is the first crucial step to ensuring a healthy lawn. Improperly prepared ground can have poor drainage, an uneven surface or deficiencies of vital nutrients that make the grass strong and lush. Before breaking ground for a lawn, consult your local Extension service for city, county and state laws and restrictions about lawns. Many regions have watering, pest control and fertilization restrictions to protect the environment, and there may also be requirements about where lawn drainage can go. Ground preparation should begin at least a month before sowing.
Test the soil for pH and the presence of elements such as nitrogen, potassium, iron and phosphorus. You can have this test done by a professional landscaper or the local Extension service, or you can do a home soil test, available at garden stores.
Clear the area of stones, trash and any debris left by builders. Any vegetation has to be completely removed, or the lawn can fill with weeds before the seeds are established.
Dig up weeds by hand in smaller spaces, making sure to pull them up by the taproots. For larger spaces or areas choked with weeds, use an herbicide to get rid of them.
Burn the weeds or dispose of them in a plastic bag away from the garden to prevent the seeds from spreading. Do not compost weeds, as many seeds can survive the composting process.
Slice off any existing turf with a flat shovel or mechanical turf stripper. Set the turf aside to lay down under the topsoil later. It will add beneficial organic material to the soil.
Remove the topsoil with a shovel. If there is less than 4 inches of topsoil, you will have to add more to give the grass roots a minimum depth in which to thrive. Break up the topsoil with a shovel or tiller and set it aside to be replaced later.
Check the surface of the subsoil for buried debris, humps and holes. Also consider the slope of the area. A lawn should be sloped toward the street or drainage pipes, so move the subsoil around with a shovel or soil rake to create an even slope.
Fill any holes with dirt from the humps and rake over the surface to make it even. Let the area settle for up to a month to see if any new holes appear.
Choose soil amendments based on the information from the soil test about your soil's quality and pH. Heavy clay and sandy soils both benefit from an addition of aged manure, compost or leaf mulch to improve drainage. PH can be adjusted with lime if the soil is too acidic, or sulfur if it is too alkaline.
Spread half of the soil amendments over the surface of the subsoil before replacing the topsoil. If you saved old turf, lay it over the subsoil grass-side down.
Spread the topsoil with a rake, breaking up any clumps and removing new weeds as you go. Grass seed requires a fine tilth, so any pieces larger than a pea should be smashed.
Spread the remaining amendments over the topsoil and dig them in with a shovel or tiller.
Tamp down the soil before you sow either by walking over the entire surface or pressing it down with a garden roller, and water the area thoroughly.