All grass forms a layer of dead plant material, known as thatch, 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. This is known as scarifying. Sprinkling a thin layer of topsoil or compost over the lawn (top-dressing) will also help. In a healthy lawn, micro-organisms and earthworms help keep the thatch layer in balance by decomposing it and releasing the nutrients into the soil.