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.• 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, freeze-thaw

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

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Classifying a soil: Texture

• Soil texture • Twelve soil texture classes• Based on percent sand, silt, clay in soil

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Soil Texture

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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

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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• 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

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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

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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

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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

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