Wetland Soils
What is soil?
• Soil: A natural body of mineral (non-living) and organic (living)
material that forms on the surface of the earth, and is the medium for
plant growth.
• Forms over time from the decay of plants and organisms on the
surface, and the break-down (weathering) of rocks, minerals, and
sediments at, and beneath, the surface.
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Soil Horizons
• Soil can be divided into horizons: layer that have similar
characteristics. 6 horizons.
Nutrient-rich
topsoil
• O horizon: decaying organic material at the surface (leaves,
grass, etc).
• A horizon. Mostly mineral, but it still has a lot of organic
material. Often dark brown or black.
• E horizon: materials are leached out of this layer. Light-colored,
nutrient-poor
• B horizon: Materials that have been removed from layers above
accumulate in this layer. Zone of accumulation.
• C horizon: Weathered parent material. The original rock or
sediment on which the soil formed
• The bedrock or un-weathered parent material is not actually
soil, but is sometimes called the “R horizon”
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Soil formation
• Soils will be different from location-to-location.
Several factors influence the characteristics of
soils:
• Climate: temperature, moisture, humidity, freezethaw cycles
• Organisms/Organic material: The organisms that
live in the soil, plants that grow in the soil
• Relief/Topography: Whether the soil forms on a
steep hill, at the bottom of a hill, etc.
• Parent Material: What kinds of minerals are in the
parent material? A soil that formed on sand will be
different from a soil that formed on clay
• Time: How much time has the soil had to form and
develop?
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Classifying a soil
• Soils can be classified by examining three basic properties:
• Structure
• Color
• Texture
Classifying a soil: Structure
• Structure
• How the soil clumps together
• Determines how well water drains
through soil
• Can be granular, blocky, prismatic,
columnar, platy, single grained
http://www.ctahr.hawaii.edu
Classifying a soil: Texture
• Soil texture
• Twelve soil texture classes
• Based on percent sand, silt, clay in soil
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Soil Texture
http://www.soilsensor.com
http://aces.nmsu.edu/pubs
Classifying a soil: Color
• Color can be influenced by the amount of organic material in soil, the
types of minerals , or the amount of oxygen available in the soil.
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Color
• Humus: Decomposed organic material
• Causes soil to be dark brown or black
• Nutrient-rich
• Soil nearest the surface is highest in
organic material
• O and A horizons
http://three-z.com
Color
• Water moving through soil can carry minerals throughout the soil column
• Iron is one of the most common minerals that can affect soil color. When
iron encounters oxygen, it oxidizes—or rusts—turning a reddish orange
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If the soil is too wet, not enough oxygen is present to oxidize the iron
Usually, oxidation happens when the soil is able to dry out
Red soils show presence of iron and oxygen
Soils with sufficient oxygen are called “aerobic soils”
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Color
• If a soil that has iron becomes too saturated with water it becomes
depleted of oxygen
• This is called an “anaerobic soil”
• Iron minerals will become reduced, which turns the soil to a greenish-gray
color
• These soils are called “gleyed soils”
• Gleyed soils are usually found where soil has been saturated for long
period of time (such as a wetland!)
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Wetland Soils
• What is a wetland?
• An area that is inundated or saturated with surface water or
groundwater often enough, or for a long enough duration, to support
vegetation that is adapted to grow in saturated soil conditions
• Swamps, marshes, bogs are types of wetlands
http://www.dnrec.delaware.gov
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Wetland Soils
• Soils that develop in wetlands are called “hydric soils”
• Permanently or seasonally saturated with water
• Have an organic layer and a mineral layer
• Soils are generally classified as “mineral soils” if they have less than 20%
organic material
• Organic layer: partially decomposed plant material
http://science.nature.nps.gov
Soils in wetlands
• Mineral layer
• Can have different characteristics based on amount of water
• Affects amount of oxygen in the soil
• Upper layers may not be saturated, so they can resemble “dry” soils
• Grayish brown or red in color
http://conservationmaven.com
Soils in wetlands
• Soils that are permanently saturated are often gleyed
• Soils that are seasonally saturated may be mottled
• Mottling: gleyed soil has red “splotches”
• Called “redoximorphic (or redox) features”
• Iron that was reduced by being in an anaerobic environment becomes
oxidized when soil dries out and becomes aerobic
http://courses.soil.ncsu.edu
http://nesoil.com/images/redox.jpg