Introduction to Soil
Descriptions
Part 3 of 3
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Acknowledgement
This work was supported [in part] by the National Decentralized
Water Resources Capacity Development Project with funding
provided by the U.S. Environmental Protection Agency through a
Cooperative Agreement (EPA No. CR827881-01-0) with
Washington University in St. Louis. The results have not been
reviewed by EPA or Washington University in St. Louis. The
views expressed in this presentation are solely those of NCSU, and
University of Arkansas and EPA and Washington University in St.
Louis do not endorse any products or commercial services
mentioned in the presentation.
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Soil Profile
Description and
Horizons
Rules .1939, .1941 (a4),
.1943, .1944
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Soil Profile Descriptions
¾ Soil
Horizon - A layer of soil,
approximately parallel to the surface,
having distinct characteristics produced
by soil forming processes.
¾ Soil
Profile - A vertical section of the soil
extending through all its horizons and
into the parent material.
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Soil Profile Descriptions
¾
¾
¾
Soil Profile - A vertical section of the soil
extending through all its horizons and into the
parent material.
Soil Horizon - A layer of soil, approximately
parallel to the surface, having distinct
characteristics produced by soil forming
processes.
Soil Layer - A layer in the soil deposited by a
geologic force (wind, water, glaciers, oceans,
etc.) and not relating to soil forming process.
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Soil Horizon Nomenclature
¾
¾
¾
¾
Soil horizons designated by a combination of: CAPITAL
LETTERS-lower case letters-ARABIC NUMERALS
Master Horizons and Layer: Major breaks in the soil: O,
A, E, B, C, and R.
Subordinate Distinctions: Lower case letters used as
suffixes to designate specific master horizons. i.e. Ap,
Bw, Cg
Vertical Subdivisions: A horizon designated by a single
combination of letters which needs to be subdivided. i.e.
Bw1, Bw2, Cg1, Cg2.
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Organic (O) Horizons
¾
¾
O horizons or layers: Layers dominated by organic
material.
Field Criteria
z
z
z
z
¾
>20% organic matter
Dark color (never used by itself)
Low strength, light dry weight, high fiber content
Typically a surface horizon-if buried may indicate
disturbance
Subordinate Distinctions of O Horizon
z
z
z
Oa-Highly decomposed organic material (Muck)
Oe-Moderately decomposed organic material (Mucky Peat)
Oi-Slightly decomposed organic material (Peat)
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Pungo series, organic from top to bottom.
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A Horizons
¾
¾
Referred to as topsoil and typically ranging from 6-30
centimeters thick. Mineral horizon formed at the
surface or below an O horizon. Characterized by an
accumulation of humified organic matter intimately
mixed with the mineral fraction.
Field Criteria
z
z
z
z
¾
Mineral soil material
Mix of well decomposed organic matter and mineral material
Surface mineral horizon
Typically dark in color-darker than underlying horizons
Typical Subordinate Distinctions of A Horizon
z
Ap-Plowing or other disturbance
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A Horizon
Cecil series with poorly defined A horizon typical of the piedmont in that it is
only slightly darker than the horizons below.
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Ap Horizon
Ap horizon or plow layer. Note how abrupt the transition is from the Ap to
the lower horizon.
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E Horizons
¾
¾
Mineral horizon in the upper part of the soil typically
underlying an O or A horizon. Light colored, leached
horizons ranging from not being present to several
centimeters thick
Field Criteria
z
z
z
¾
Zone of eluviation - removal of clays, Fe, Al, and humus
Lighter in color than over or underlying horizon
Near surface, below O or A horizons and above a B horizon
Formed by weak organic acids that strip coatings from
sand grains. Light color due to the natural color of the
mineral grains.
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E Horizon
Thick E horizon in a Conetoe Series. The fine reddish brown streaks are
lamella composed of Fe and clay. The B horizon shows a slight increase in
clay.
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E Horizon
E horizon in a Aquod (wet Spodosol). Fe and OM have been removed and
translocated lower in the profile.
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A vs. E horizon
¾A
horizons have a high OM content
¾ E horizons have low OM content
¾ E horizons often feel sandier (coarser)
than the A horizon
¾ E horizons have a higher value
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E vs. B horizon
¾E
horizons are sandier (coarser) than B
horizons
¾ E horizons are higher in value
¾ B horizons have more clay
¾ B horizons have more Fe
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B Horizons
¾ Referred
to as subsoil. the zone of
accumulation with in the soil. In well
drained soils it has the brightest colors.
May extend over a meter below the
surface.
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B Horizons - Field Criteria
¾
¾
¾
¾
Subsurface horizon formed below an O, A, E
horizon and above the C horizon
Formed as a result of soil forming processes
Weakest expression is color development
Illuvial concentration-zone of accumulation
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B Horizons - Subordinate Distinctions
¾
¾
¾
¾
Bg-Strong gleying, indicates prolonged periods
of saturation
Bs-Illuvial accumulation of sesquioxides (spodic)
Bh-Illuvial accumulation of humus (spodic)
Bt-Accumulation of clay
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Bt Horizon
Bt under a lighter E horizon.
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Btg Horizon
Btg horizon at 40 cm and continuing to 100 cm. It is a g as it formed under
saturated conditions and ha a matrix chroma of 2 or less. It is a t because it
has a higher clay content than the Eg horizon above.
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Bh Horizon
Bs Horizon
Bhs over a Bs horizon in an Aquod. Often a Bhs will overlie a Bs
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C Horizon
¾
¾
Referred to as substratum. These horizons and
layers are little affected by soil forming processes
(unweathered geologic material).
Field Criteria
z
z
z
¾
Little affected by soil-forming processes
Geologic layering
Lack of color development, color of unweathered geologic
material
Subordinate Distinctions of C Horizons
z
Cg-Strong gleying, indicates prolonged periods of saturation
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C horizon that looks more like rock but it offers no resistance to excavations.
Note the foliation pattern is still obvious.
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Total below trench
adjusted for wetness
Usable depth
Total below
trench
Total depth
Ap
Trench bottom depth
Bw1
Soil Profile
¾
z
¾
z
z
Bw3
z
¾
determine if restrictive by
rule
Depth
z
Soil wetness Bw2
2BCd
Parent material
Restrictive
Most limiting2Cd
Horizons
to parent material
to soil wetness
to most limiting layer
for trench bottom
Usable depths
z
z
z
Usable depth
Below trench bottom
Adjustment for slope
and/or wetness condition
Bt under a lighter E horizon.
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