A Horizon Definition
In ABC soil, the uppermost, darkest zone that is rich in organic matter.
The uppermost soil zone, containing humus.
Webster's New World
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.
American Heritage Science