Smithsonian National Museum of Natural History

A Top to Bottom Guide

O horizon

The organic layer of newly deposited debris and partially decomposed matter on the surface

A horizon

A mineral layer at or near the surface, with more soil organic matter than lower layers, also called topsoil

E horizon

A layer of loss, as water carries clay, iron, aluminum, and organic matter into lower levels

B horizon

A layer of gain, as minerals and organic matter wash in from higher layers or form in place through weathering

C horizon

A mineral layer that represents the condition of the soil when it first started to form; also called parent material, it is relatively unaffected by biological activity or the processes of loss and gain in higher layers