In the bottom of the lawn, a layer of mixed living and dead material is gathered, most of the lawn is partly or fully mowed. The layer is called felt. The felt is subsequently broken down by soil animals (insects and deworms) and microorganisms (fungi and bacteria) to moles and nutrients that the grass can reuse. The condition that this will happen continuously is that the oxygen supply and moisture in the soil layer are good. The process also requires an excess of nitrogen at its initial stage.
Under certain conditions such as drought and lack of nutrition, decomposition is slower than the supply of new plant material. The felt grows in thickness and forms almost a lid over the ground so that neither air nor water reaches the grass roots.
It is the care of the lawn that determines the thickness of the felt layer.
The grass then has a poorer growth and the entire carpet may have a bad appearance . Under such conditions it will be necessary to remove the blanket and "air" the lawn. With a vertical cutter, this work is done quickly and efficiently. The modern mulch-type lawnmowers make up the lawn and it is likely that the decomposition will accelerate.