What Is Soil?

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What Is Soil?
Image T. Loynachan
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Soil is a layer of unconsolidated material on the
Earths surface in which plants grow.
Image Louis Maher
Image Professional Soil Classifiers Assoc. of
Alabama
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Soil is a mixture of mineral grains from the
Earth, decomposing organic matter, and voids
filled with air or water.

Image Professional Soil Classifiers Assoc. of
Alabama
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Soil is the Earth material that links living
things (biosphere), air (atmosphere), water
(hydrosphere), and the solid Earth (geosphere).

Image Bruce Molnia
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A soil profile is an exposure from the surface
downward through a soil to its parent material.
Image Martin Miller
Image NRCS
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Soil profiles generally expose three layers or
soil horizons.
Image National Cooperative Soil Survey
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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
Image NRCS
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Physical weathering breaks rocks into small
mineral particles that accumulate on the Earths
surface.
Images Martin Miller, NRCS
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How Soils Form
Image T. Loynachan
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Chemical weathering dissolves and changes
minerals that have accumulated on the Earths
surface.
Images National Cooperative Soil Survey,
University of Nebraska
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Decomposing organic material from plants and
animals mixes with accumulated soil minerals.
Images NRCS, Soil Classifiers of Michigan
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Accumulated soil materials contain voids filled
with air or water.
Image T. Loynachan
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Continued physical and chemical changes over
hundreds to thousands of years produce layers
called soil horizons.
Image National Cooperative Soil Survey
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