Introduction to Soil Description
Transcript of Introduction to Soil Description
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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
Soil Horizon - A layer of soil, approximately parallel to the surface, having distinct characteristics produced by soil forming processes.
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
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
Soil horizons designated by a combination of: CAPITAL LETTERS-lower case letters-ARABIC NUMERALSMaster 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, CgVertical 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
O horizons or layers: Layers dominated by organic material.Field Criteria
>20% organic matterDark color (never used by itself)Low strength, light dry weight, high fiber contentTypically a surface horizon-if buried may indicate disturbance
Subordinate Distinctions of O HorizonOa-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
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
Mineral soil materialMix of well decomposed organic matter and mineral materialSurface mineral horizonTypically dark in color-darker than underlying horizons
Typical Subordinate Distinctions of A HorizonAp-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
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 thickField Criteria
Zone of eluviation - removal of clays, Fe, Al, and humusLighter in color than over or underlying horizonNear 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
A horizons have a high OM contentE horizons have low OM contentE horizons often feel sandier (coarser) than the A horizonE horizons have a higher value
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E vs. B horizon
E horizons are sandier (coarser) than B horizonsE horizons are higher in value B horizons have more clayB horizons have more Fe
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B Horizons
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
Subsurface horizon formed below an O, A, E horizon and above the C horizonFormed as a result of soil forming processesWeakest expression is color developmentIlluvial concentration-zone of accumulation
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B Horizons - Subordinate DistinctionsBg-Strong gleying, indicates prolonged periods of saturationBs-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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Bs Horizon
Bh Horizon
Bhs over a Bs horizon in an Aquod. Often a Bhs will overlie a Bs
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C HorizonReferred to as substratum. These horizons and layers are little affected by soil forming processes (unweathered geologic material).Field Criteria
Little affected by soil-forming processesGeologic layeringLack of color development, color of unweathered geologic material
Subordinate Distinctions of C HorizonsCg-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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Soil ProfileHorizons
determine if restrictive by rule
Ap
Bw1
2BCd
2Cd
Bw2
Bw3
Restrictive
Trench bottom depth
Soil wetness
Parent material
Most limiting
Tota
l dep
th
Tota
l bel
ow tr
ench
ad
just
ed fo
r wet
ness
Tota
l bel
ow
tren
chUsa
ble
dept
hDepth
to parent materialto soil wetnessto most limiting layerfor trench bottom
Usable depthsUsable depthBelow trench bottomAdjustment for slope and/or wetness condition
Bt under a lighter E horizon.