If your lawn is bumpy or feels spongy underfoot you may have thatch build up. You can check by removing a small wedge of lawn deep enough to get some soil. Thatch is a layer of living and dead plant material that lies under the green blades of grass and above the soil. Some thatch in a lawn is inevitable and creates no problems but if it becomes more than one half inch thick it may reduce the amount of water, air and nutrients that reach the roots, resulting in grass with shallow roots. A lawn of shallow rooted grass dries out quickly and is more susceptible to the effects of heat and cold. In addition, thatch provides a favorable environment for pests and disease.
Thatch is composed of stems and roots. It builds to excess because the roots and stems are produced faster than they are breaking down. There are several factors that contribute to thatch build up some of which you can easily control. Here is list of possible causes of thatch build up.
Overwatering: Frequent waterings of short duration encourage the growth of shallow rooted grass.
Overfertilizing: High nitrogen levels lead to lush growth that will accumulate as it dies.
High or Low soil pH: Reduces microbial activity that break down thatch.
Infrequent Mowing with a high blade. Long blades of grass can not decay quickly and add to thatch.
Soil compaction: Reduces the amount of air, water, and nutrients that reach the roots leading to shallow rooted grass.
Type of Grass: Some kinds of grasses are more likely to form thatch than others. Creeping bent grass and Bermuda grass, for example, have a high tendency to produce thatch, while Bahia grass has a low tendency.
Heavy clay soil: Reduces the amount of air and nutrients that reach soil leading to shallow rooted grass.
Notice that allowing average sized grass clippings to remain on the lawn is NOT mentioned as a cause of thatch buildup. Most experts agree that such lawn clipping disintegrate quickly and will not add unwanted organic matter to the lawn.
The most common cause of thatch build up areoverwatering and overfertilizing. They are both easy control and lead to less work and expense. Soil compaction and pH problems are more difficult and costly to solve but are doable. For more information on treating soil compaction read my post Coring your lawn.