Thatch is an organic layer that sits just below the grass and is composed of both living and dead plant material.  A little is okay, however it may reach a stage where by physical removal is appropriate. A thick layer can prevent air, water and nutrients from accessing the root zone.  A spongy lawn underfoot and scalping grass when mowing may be signs that your lawn has too much thatch.  Scarification is a mechanical process carried out to remove the thatch from your lawn.  It should also be noted that a balanced nutrient programme and routine aeration will reduce the rate of thatch production.