A horizon

noun Geology.

the topsoil in a soil profile.

Compare B horizon, C horizon.

Origin of A horizon

First recorded in 1935–40

Also called zone of leaching.

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

British Dictionary definitions for a horizon

A horizon

noun

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

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for a horizon

A horizon

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