Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil it is composed of a variable mixture of small particles such as sand, silt and/or clay, but it lacks the organic matter and humus content of topsoil. Below the subsoil is the substratum, which can be residual bedrock, sediments, or aeolian deposits. As it is lacking in dark humus, subsoil is usually paler in colour than the overlying topsoil. It may contain the deeper roots of some plants, such as trees, but a majority of plant roots lie within the surface topsoil.