Here is Lesson 5 in our Course "Learn about Lawns"
All grass forms a layer of dead plant material, known as thatch. Thatch builds up between the grass blades and the soil.
When thatch gets too thick, deeper than one-half inch, it prevents water and nutrients from penetrating to the soil and grass roots. Some grasses tend to form a thick layer of thatch. Overuse of fertilizer can also create a heavy layer of thatch.
You can reduce thatch by raking the lawn or using a machine that slices through the thatch layer to break it up. Sprinkling a thin layer of topsoil or compost over the lawn will also help.
Thatch is desirable when is in proper balance. The thatch layer helps to keep moisture in the soil by reducing evaporation. In a healthy lawn, microorganisms and earthworms help keep the thatch layer in balance by decomposing it and releasing the nutrients into the soil.