Under-soil, also referred to as subsoil, is an important layer of the earth's surface that lies beneath the topsoil. This layer is usually less fertile than the topsoil and comprises a mixture of rock fragments, organic matter, and mineral particles. Synonyms for under-soil include subsoil, substratum, substrata, under-earth, and bottom soil. Subsoil is often used interchangeably with under-soil as it describes the layer between the topsoil and the substratum. The substratum may refer to any rock that underlies soil, while the term under-earth refers to anything that lies below the earth's surface. Bottom soil is used to describe the lowest layer of soil, usually lying beneath all other soils.