Soil needs air and water in order to support the turf grasses used in lawns. Some topsoil has a good balance of silt, clay and sand with a healthy helping of organic matter but most soil can benefit from periodic aeration to maintain its "loam" quality. Lawn care for clay or silt-heavy soils requires more frequent interventions.
Aeration is a process of piercing the surface of soil or extracting vertical "plugs" of soil with machines that make openings at least 2 inches deep with long wire tines or hollow circular cutters. Periodic aeration helps topsoil remain loose and loamy. Aeration with top dressing can improve heavy soils by depositing clay or silt plugs on the surface where they can be removed. Aeration provides an intermediate soil improvement alternative to tearing up the old lawn and re-seeding.
Plant roots gather more than nutrients from soil. They need oxygen and light in order to grow and provide food for plant growth. Heavy soil locks out air and light because silt and clay particles are very small; packed closely, they form a solid material that works well for pottery but chokes plants. Regular aeration allows air, light, water and fertilizer to get directly to plant roots. In addition to allowing access to roots, aeration aids in reducing soil compaction, a major cause of thatch.
Aeration may be performed in several ways. The most effective aeration is performed with rollers of spikes that remove plugs a centimeter or more in diameter in a pattern across a lawn or field. The heavy machine has a motor because it is quite heavy; models are manufactured to be operated by hand or be pulled by tractors. Aerators that look like lawnmowers with a rotating brush of tines are marketed directly to homeowners. Lawn tractor attachments with these tine rollers are also available.
The most intriguing---but least effective---method of aeration is to wear golf or athletic "spikes" while gardening. Some purveyors offer special "aerating shoes" with short, sharp pikes on plates that can be strapped on shoes or boots.
Aerators are rolled across lawns in rows, much like mowing or fertilizing a lawn. The lawn is then aerated again at a right angle to the first passes. Where thatch---the compaction of dead plants and roots that closes up the topsoil surface---is thicker than a half-inch, plugs should be removed to reduce its growth.
With good quality topsoil, light fertilization with a slow-release nitrogen fertilizer may be performed. If plugs are removed, the holes can be filled with humus or compost scattered around and watered into the surface. No matter which methodology is used, turf should be thoroughly watered to reduce stress, provide moisture and settle disturbed soil.
Aeration is generally performed in the late summer or fall when improved soil conditioning and feeding will stimulating root growth will provide the greatest benefit to plants. Early spring aeration must be carefully timed; the ground and air must be reliably warm to avoid frost damage to exposed roots.
Lawns with thick topsoil consisting of friable (well-drained) loam may not need aeration for decades but clay soils with thatch build-up may need annual or even bi-annual aeration. Mature lawns with average quality topsoil benefit from aeration every other year or more depending on condition.