The umbric horizon (''Latin: umbra, shade'') is a thick, dark coloured, surface
soil horizon
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
rich in organic matter. It is identified by its dark colour and structure.
Normally it has a pH of less than 5.5 representing a
base saturation
Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with o ...
of less than 50 percent. An indication for
soil acidity
Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
is a rooting pattern whereby the roots tend to be horizontal.
Similar thick, dark coloured, organic-rich, base-desaturated surface horizons occur in
anthropedogenic horizons An anthrosol (or anthropogenic soil) in the World Reference Base for Soil Resources (WRB) is a type of soil that has been formed or heavily modified due to long-term human activity, such as from irrigation, addition of organic waste or wet-field cul ...
from human activities such as deep cultivation, kitchen middens and the addition of organic manures etc.
These horizons can usually be differentiated by the presence of artifacts, spade marks or by checking the agricultural history of the area.
In the
World Reference Base for Soil Resources The World Reference Base for Soil Resources (WRB) is an international soil classification system for naming soils and creating legends for soil maps. The currently valid version is the fourth edition 2022. It is edited by a working group of the Inte ...
(WRB), the umbric horizon is a diagnostic horizon. The diagnostic criteria are:
*Soil structure sufficiently strong that the horizon is not both
massive and hard or very hard when dry.
*
Munsell colours
In colorimetry, the Munsell color system is a color space that specifies colors based on three properties of color: hue (basic color), chroma (color intensity), and value (lightness). It was created by Professor Albert H. Munsell in the first ...
with a
chroma and value ≤ 3 when moist and a value ≤ 5 when dry, both on slightly crushed samples.
*It is described as
base-desaturated in that it has a base saturation of < 50% on a weighted average throughout the depth of the horizon; and
*Soil organic carbon content of ≥ 0.6% throughout the thickness of mixed horizon; and
*A thickness of ≥ 100 mm if resting directly on hard rock; ≥ 200 mm else.
References
Pedology
{{soil-sci-stub, acidification, soil