The Definition of a Soil Profile

Surface Horizons or Topsoil

• When decomposing organic material is present, the soil profile begins at the surface with an organic O horizon. However, sometimes an older O horizon is covered, for example, by sediments, and in that case, the O horizon is said to be buried. Lowercase a, c or i are sometimes used with the O horizon to describe organics that are highly, intermediately or slightly decomposed respectively. Below that, the surface A horizon is sand, silt and/or clay minerals, mixed with decomposing roots. Lowercase p is used to indicate that plowing has disturbed the native A horizon and d is used when compaction prevents roots from entering the A horizon.

Subsoil

• Water leaches topsoil minerals into subsoil or carries soluble minerals out of subsoil. The mineral subsoil B horizon is the parent substrate altered by leaching, and it is therefore either accumulating or losing clay, iron, aluminum, carbonates or gypsum. B horizon descriptions include notations such as lowercase h leached humus, lowercase g gleying of reduced iron in stagnant soil water, or lowercase jj for churning by freezing and thawing. Lower in the profile, soils may have a weathered C horizon of minerals that are not chemically different from their parent materials, or an E horizon where loss of clay, iron or aluminum has caused sand or silt-sized quartz to accumulate.

Parent Substrate

• R is for rock, which may occur at the bottom of the soil profile. Soil scientists are practical people, and they consider the R horizon to be present when they can no longer dig the soil by hand with a spade, even if some taproots are able to penetrate it. The R horizon may be any sort of bedrock, and when cracks in the bedrock are coated with accumulations of clay leached from higher in the profile, the clay is noted with a lowercase t.

Water

• The presence of ice or water in a soil is noted as W for water, whether the liquid is wetland bog or tundra permafrost. Lowercase f notes permanently frozen soils found in the Arctic and in some Alpine locations that contain ice lenses, although the surface of the profile may thaw seasonally. Lowercase ff indicates dry permafrost in the cold deserts of the Antarctic, where the soil profile is frozen but liquid is absent.