Soil layers, also known as soil horizons, are the different sections of soil as it extends from the surface (the topsoil) down into the Earth. Each layer has a designated letter and name to identify it.  These are the soil layers in order, from top to bottom: -- O horizon: Humus, a thin layer of decomposing organic materials (such as fallen leaves) that may or may not be present on top of the soil.  -- A horizon: Topsoil, the layer of rock particles and organic matter that you work with when gardening.  -- E horizon: Eluviated layer, a layer that contains no clay or organic matter but only sand or silt particles, which is only found in some soils.  -- B horizon: Subsoil, a layer rich in minerals, where plant roots will extend if they're allowed to grow and spread for a long time without being disturbed. -- C horizon: Parent material, the original material which the soil developed from, such as granite, limestone, or quartzite.   -- R horizon: Bedrock, the mass of rock below the soil.