Fall is an ideal time to improve your lawn’s soil health, and compost can help. A layer of compost – especially after core-aerating your yard – helps improve soil structure, maintain soil organic matter and release beneficial microorganisms into the soil. Although it is not technically a fertilizer, compost can also improve the availability of soil nutrients, like nitrogen, in your turf.
Compost is best applied to lawns in the fall after core aeration, when a mechanical core aerator pulls up plugs of soil. Aerating loosens compacted soil, creates more room for oxygen to penetrate the soil, helps thatch decompose and stimulates turfgrass root growth. Aerator recommendations usually call for running the mechanical core aerator across the lawn several times, in a perpendicular pattern, for ideal lawn aeration. For smaller lawns or small “high-traffic” areas of the yard needing aeration, you can use a garden spading fork, or specialized hand aerators, to open up airways in the soil.
After aerating, you’re ready to spread a layer of compost. Most recommendations call for a layer of compost spread ¼- to ½-inch thick. The compost will lend a darker color to your lawn, but don’t worry; going back and forth over the lawn with a rake will help work the compost down into the holes created by the aerator. That can allow the beneficial, oxygen-loving microbes in the compost to settle more quickly into the soil. Raking also helps break up the plugs the aerator pulled from your yard.
When “topdressing” your yard with compost, remember that not all compost is created equally. “Brown” compost tends to have more original materials higher in carbon, like shredded leaves and wood shavings. Brown compost is a good choice for topdressing lawns with better natural soil fertility. “Green” compost tends to include more composted animal manure and other materials that were higher in nitrogen. All composts will help soil nutrient availability, but regular applications of green compost to a lawn could potentially provide some small nutrient boost to the turfgrass. That makes green compost the topdress of choice for lawns grown on soils that are less than ideal, as found in many lawns of newer homes.
Another way compost can help your yard is compost tea, water that becomes “infused” with nutrients and microorganisms from compost. The microorganisms delivered in compost tea can outcompete with plant disease organisms, resulting in a healthier landscape or garden. Similar impacts from microorganisms in compost can occur in plant roots when compost tea infiltrates the lawn soil. Watering your yard with appropriate amounts of compost tea could be especially beneficial for young grass seedlings growing from a fall seeding.
Although most lawn care experts agree that the rewards from spreading compost on the yard far outweigh any possible risks, there are some things to watch when topdressing the yard with compost. Some compost could take your soil pH level in the wrong direction from the preferred pH level for turfgrass. Commercial compost from reputable sources should have a nutrient analysis available. If you’re interested in the nutrient composition of your own compost, you can check this through the same places that conduct soil test analysis.
Another thing to watch for: weed seeds, which can spread in compost. Spreading compost you didn’t make yourself could introduce new weed species to your yard. But most weed seeds are killed when compost is properly heated, as should have occurred in most commercial composts, and a layer of compost on the yard can help smother out small weeds that may be competing for nutrients with the existing turfgrass.