Environmental Engineering Reference
In-Depth Information
Fig. 4.2 A diagram of soil
horizons (Published with
kind permission of US
Department of Agriculture,
Natural Resources
Conservation Service
(
2010b
). Figure is public
domain in the USA. All
Rights Reserved)
typically increases with depth and diurnal fluctuations in soil temperature are
minimized with depth. Because of this, soil forming processes are not uniform
with depth. As a result, soil morphology typically changes with depth and displays
distinct horizontal layers of soil called
horizons
(Fig.
4.2
). Horizons can be com-
posed predominately of organic matter (O horizons) or composed predominately of
unconsolidated mineral materials (designated as A, E, B, and C horizons).
Consolidated bedrock is designated as R.
O horizons form at the soil surface because they are composed primarily of plant
roots and leaves in various stages of decomposition. O horizons are dark brown or
black. A horizons, commonly called topsoil, are predominately mineral, but distin-
guished from other mineral horizons by organic matter enrichment. As a result, they
tend to be dark brown or black in color. A horizons form at the soil surface or below
an O horizon. E horizons represent zones of elluviation, the loss of soil components
such as clay, Fe, or organic matter. B horizons represent zones of illuviation, the
gain of soil components such as clay, Fe, or organic matter. C horizons display little of
the soil forming processes and are similar in composition to parent material. Gener-
ally, the horizon you are in does not matter when identifying a hydric soil. However,
it is important to understand when you are in an A or E horizon as the requirements for
those horizons for some indicators are different than for other horizons.
A soil may lack one or more of these horizons or may have similar horizons at
multiple depths. O or A horizons, which form near the soil surface, may be found
deeper in the soil due to subsequent formation of horizons above them following
