خاک زیرسطحی

thumb|Subsoil layer 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 clay, but with a much lower percentage of organic matter and humus. The subsoil is labeled the B Horizon in most soil mapping systems. Because it has less organic matter than topsoil, subsoil soil colour is mainly derived from iron oxides. Iron oxides and clay minerals form due to weathering. Rainfall moves these weathering products downward as solutes and colloids by rainfall. The subsoil is the depth wher