Thatch is the spongy, brown layer of organic matter found between the leaf zone and the soil. If it's more than one-half inch thick, it's important to reduce thatch buildup. Leaving grass clippings on the lawn does NOT contribute to thatch buildup. In fact, it's an environmentally responsible way to recycle nutrients and dispose of lawn clippings. The primary factor leading to thatch problems is soil compaction.
Thatch problems are very common on newer sod laid over clay-type soils.
Other factors include excessive nitrogen fertilization, infrequent mowing, and vigorously growing turf varieties. Some lawn pesticides also contribute to thatch build-up, so avoid unnecessary use of these chemicals. To reduce thatch and compaction, aerate the lawn. Make enough passes with the aerator to make plug holes at two-inch intervals. Lawn aerators are readily available from local rental firms and are about as easy to use as a lawn mower.