Moss growing in a lawn can be a symptom of any or all of the following conditions: poor soil fertility, compacted soil, overwatering, poor airflow and the wrong grass growing in a shady area. Chemical moss killers are available for purchase by the general public, but University of Illinois Extension horticulture educator Bruce Spangenberg advises that addressing the causes of the problem will choke out moss growth naturally, allowing your lawn to flourish where the moss once grew.
Test your soil. Home test kits are available, but submitting soil samples to your county extension office yields more reliable results, and usually costs about the same.
Amend your soil according to the test's findings. Raise or lower the pH, if necessary,with agricultural lime or sulfur; grass likes a neutral, non-acidic pH level. Consult your county extension office for specific application advice, as it differs by area and circumstance.
Aerate your lawn once every few years, usually in the fall, to allow better nutrient and water absorption into the soil. Rent an aerator from an agricultural rental service in your area, or contact a lawncare professional in your area if you do not want to do it yourself.
Dethatch your lawn in spring and fall whenever thatch levels reach above the 3/4-inch level. Rent a dethatcher, or buy a dethatching rake to do it by hand. Use the dethatcher to remove most of the existing moss in problem areas.
Apply grass seed with a spreader, following package instructions. Choose a grass seed suitable to the light level and growing conditions in the area of the lawn you are working.
Apply a non-burning, slow-release fertilizer at the time of seeding, using a spreader. Continue fertilizing on a regular basis as the package directs. Like gardens, lawns need constant replenishment of nutrients.
Rake a thin layer of topsoil or compost (no more than 1/2 inch or so) on top of the seeds and fertilizer. Water with a garden hose with an adjustable sprayer attachment set to "shower" or another gentle setting, or a sprinkler. Seeds should germinate in 10 days.