a horizon

noun Geology.

1. the topsoil in a soil profile.

noun

1. the top layer of a soil profile, usually dark-coloured and containing humus and from which soluble salts may have been leachedSee B horizon, C horizon

1. In ABC soil, the uppermost, darkest zone that is rich in organic matter. The upper section of the A horizon usually contains humus along with plant and animal matter in varying stages of decay. The middle section usually contains a high concentration of quartz or other minerals that remain following the leaching away of clay, iron, and aluminum. The lower section is typically transitional in nature between the A horizon and the B horizon. Also called zone of leaching