Transcript How Soil Formsppt

How Soil Forms
What is Soil?
• Soil is the loose, weathered material on Earth’s
surface in which plants grow.
One of the main ingredients of soil is:
• Bedrock which is the solid layer of rock beneath the soil.
Soil Composition
• Soil is comprised of a mixture of rock,
particles, minerals, decayed organic
material, water and air.
• The decayed organic material in soil is
called humus. Humus provides plants with
the nutrients they need to grow.
• Soil that is rich in humus has a high fertility
rate supporting plant growth.
Soil Texture
• Soil texture depends on the size of the
individual particles.
• The largest particle size is gravel.
• The smallest particle size is clay.
• Loam is the best type of soil for growing
plants since it is made up of about equal
parts of clay, sand and silt.
Soil Particle Size
The Process of Soil Formation
• Soil forms as rock is
broken down by
weathering and mixes
with other materials.
• 3 horizons (layers)
– A horizon: topsoil
– B horizon: subsoil
– C horizon: partly
weathered rock
Horizon A
• Horizon A is made up of topsoil.
• Topsoil is a crumbly, dark brown soil that
is a mixture of humus, clay and other
materials.
• This soil horizon is most
important for growing food.
Horizon B
• Horizon B is consists of clay and other
particles washed down from Horizon A;
very little humus.
• Horizon B is often called subsoil.
• Horizon B forms last as particles
wash down from the A horizon.
Horizon C
• Horizon C is weathered rock.
• The C horizon is most similar to bedrock.
• It consists mostly of pieces of bedrock.
Rate of Soil Formation
• The rate of soil formation depends on the
climate and type of rock.
• Weather occurs most rapidly in areas with
a warm, rainy climate so soil will form
faster in these areas.
• Soil forms more quickly from limestone
than granite since limestone weathers
faster.
Soil Types
• Classified based on climate, plants and soil
composition
Types of Soil
• Fertile soil forms in regions with hot, wet
climates.
• Thin soil forms in mountains and polar
regions with cold and dry climates.
• The thickest and most fertile soil forms in
moderate temperatures with moderate
rainfall.
Major soil types in North America
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Forest
Prairie
Desert
Mountain
Tundra
Tropical
Living Organisms in Soil
• Two ways organisms contribute to soil:
A. Forming Humus
B. Mixing the Soil
Forming Humus
• Plants contribute most of the organic remains
that form humus.
• As plants shed leaves, they form a loose layer
called litter.
• Humus forms in a process known as
decomposition where organisms that live in the
soil turn dead organic material into humus.
• Decomposers are the organisms that break
down the remains. These include fungi,
bacteria, worms, and other organisms.
Mixing the Soil
• Earthworms mix humus with other materials in
soil.
• They also help move air and water into the soil
through a process called aeration.