The upper layer of soil within a soil profile. This is the fertile topsoil of a mineral soil, that contains much organic material (from the decomposition of litter) and minerals, and is the location of much biological activity (by earthworms and other soil organisms). The organic‐rich layer in the A‐horizon is termed the O‐horizon. Fine materials (such as iron, aluminium oxides, and silicate clays) are washed out (leached) from the A‐horizon and deposited in the lower horizons, by eluviation, so this horizon is also sometimes known as the zone of leaching. See also B‐horizon, C‐horizon, soil horizon.
Subjects: Environmental Science.