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The Nature of Soil
Chapter 7, Section 2
The Formation of Soil

What is Soil?

Mixture of weathered rock,
decayed organic matter, mineral
fragments, water, and air.
Formation of Soil

How does
soil form?


Weathering breaks down rocks
and minerals into fragments.
Plants and animals add organic
material to the soil.


May take thousands of years to
form.
May range in thickness from a few
centimeters to 60 meters thick.
Composition of Soil

Rock and
Mineral
Fragments


Mostly small particles of sediment
such as clay, silt, and sand.
Some larger rocks can be present.
Composition of Soil

Organic
Material

Dead and Decaying Plants



Make up the majority of organic
material.
Leaves, stems, roots, etc.
Dead and decaying animals and
microorganisms
Composition of Soil

Organic
Material

Humus

Dark colored material formed from
decayed plants and animals.



Decay process is due to bacteria and
fungi in the soil.
Serves as a nutrient source for
plants.
Mixed with other components of
soil by burrowing rodents, worms,
and insects.

Good-quality surface soil has about
50% humus.
Composition of Soil

Water and
Air

Fill the spaces between the
particles of soil.
Soil Profile

Horizons

Different layers of soil.


Typical soils have an A, B, and C
horizon.
All the horizons form a soil profile.
Soil Profile

A Horizon





Top layer of soil.
Also known as topsoil.
Contains a higher percentage of
humus.
Generally dark and fertile.
May be covered with litter.

Litter consists of leaves, twigs, and other
organic material.

Helps prevent erosion and holds water.
Soil Profile

B Horizon

Contains material moved down
from the A horizon through
leaching.

Leaching is the removal of minerals
that have been dissolved in water.
Soil Profile

C Horizon




Consists of partially weathered
rock.
Coarser than other horizons.
Contains less humus.
Similar to the parent rock found
beneath the horizon.
Soil Types

What affects
formation of
soil and soil
type?






Glaciation
Climate
Type of parent rock
Type of vegetation
Slope
Time
Soil Types

Glacial
Deposits


Unsorted mass of clay, silt, sand,
and boulders.
Creates very fertile soil.
Soil Types

Climate

Desert soils



Prairie soils


Contain little organic material.
Thinner than soils in wet climates
Thick, dark A horizons due to the
large number of grasses.
Temperate Forest soils

Thinner A and B horizons enriched
in minerals due to leaching.
Soil Types

Parent rock


Affects the type of soil that
develops.
Can also affect the vegetation that
grows in the area.
Soil Types

Slope

Steep slopes


Soil horizons are often poorly
developed.
Bottomland

Soils are often thick, dark, and full
of organic material.
Soil Types

Time


At first, soil characteristics are
determined by the characteristics
of the parent rock.
Over time, soil resembles the
parent rock less and less.