The uppermost layer of topsoil in a well-developed or mature soil profile. It is characterized by well-weathered parent material and a relatively large amount of humus. The A-horizon is generally considered to be fertile and well suited to agriculture. It may be subject to the processes of leaching or eluviation in regions with relatively high rainfall. This causes the removal of some of the nutrients down through the soil profile to leave behind a nutrient-poor eluviated horizon within the main A-horizon. Immediately above the A-horizon is the O-horizon, which comprises the rotting humus that will slowly be incorporated into the A-horizon by the action of worms and other organisms.