What Is Soil?

Soils-1-1Image: T. Loynachan

Soil is a layer of unconsolidated material on the Earth’s surface in which plants grow.

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Image: Louis Maher

Image: Professional Soil Classifiers Assoc. of Alabama

Soil is a mixture of mineral grains from the Earth, decomposing organic matter,

and voids filled with air or water.

Soils-1-3Image: Professional Soil Classifiers Assoc. of Alabama

Soil is the Earth material that links living things (biosphere), air (atmosphere), water

(hydrosphere), and the solid Earth (geosphere).

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Image: Bruce Molnia

A soil profile is an exposure from the surface downward through a soil to its

parent material.

Soils-3-1Image: NRCS

Image: Martin Miller

Soil profiles generally expose three layers or soil horizons.

Soils-3-2Image: National Cooperative Soil Survey

The upper “A” horizon contains organic matter mixed with mineral particles.

Percolating water dissolves and removes some mineral constituents from the “A”

horizon.

Horizon “A”

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Some constituents dissolved from the “A” horizon are deposited below in the

“B” horizon.

Horizon “A”

Horizon “B”

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The “C” horizon is the lower soil layer developed on the underlying parent

material.

Horizon “A”

Horizon “B”

Horizon “C”

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A mature soil profile with well developed A, B, and C horizons takes

hundreds to thousands of years to develop.

Horizon “A”

Horizon “B”

Horizon “C”

Soils-3-6Image: NRCS

Physical weathering breaks Physical weathering breaks rocks into small mineral rocks into small mineral

particles that accumulate on particles that accumulate on the Earth’s surface.the Earth’s surface.

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Images: Martin Miller, NRCS

How Soils How Soils FormForm

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Image: T. Loynachan

Chemical weathering Chemical weathering dissolves and changes dissolves and changes

minerals that have minerals that have accumulated on the Earth’s accumulated on the Earth’s

surface.surface.

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Images: National Cooperative Soil Survey, University of Nebraska

Decomposing organic material Decomposing organic material from plants and animals mixes from plants and animals mixes

with accumulated soil with accumulated soil minerals.minerals.

Soils-2-4Images: NRCS, Soil Classifiers of Michigan

Accumulated soil Accumulated soil materials contain voids materials contain voids filled with air or water.filled with air or water.

Soils-2-5Image: T. Loynachan

Continued physical and chemical Continued physical and chemical changes over hundreds to changes over hundreds to

thousands of years produce thousands of years produce layers called soil horizons.layers called soil horizons.

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Image: National Cooperative Soil Survey