Soil Formation and Composition
EQ: How do you scientifically
describe soil? How is soil formed?
G2-2
Soil Formation
• Soil- the loose, weathered material on Earth’s
surface in which plants can grow
• 3 steps to soil formation
1)Begins as bedrock (solid layer of rock
beneath the soil)
2)Breaks down (weathering)
3)Mixes with other materials
EQ: How do you scientifically describe
soil? How is soil formed?
G2-2
Soil Composition
• Soil – a mixture of rock particles, minerals,
decayed organic material, air and water
• Rock particles- sand, silt, clay
• 2 factors affect the rock particles and minerals
found in a soil
- bedrock
EQ: How do you
scientifically
- type of weathering
describe soil?
How is soil
formed?
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Soil Composition
• Humus
– dark colored substance that forms as plant
and animal remains decay
– Creates space in soil for air and water
– Rich in nutrients for plants:
• nitrogen, sulfur, phosphorus, and potassium
EQ: How do you
scientifically describe
soil? How is soil formed?
G2-2
Soil Color
• Color of soil can vary from gray, yellow, deep
red brown to black.
EQ: How do you
scientifically describe
soil? How is soil
formed?
G2-2
Soil Texture
• Size of particles.
• Rub soil between your fingers to test.
• Different names are given to soils depending
on the proportion of sand, silt, and clay.
• Every soil on earth is defined as sand, silt, clay.
EQ: How do you scientifically
describe soil? How is soil
formed?
G2-2
Soil Texture
Pebbles/gravel are NOT
considered soil!
G2-2
EQ: How do you scientifically
describe soil? How is soil
formed?
Soil Texture is:
• grainy -large-sized pieces and feels gritty, like
sand.
• silky -medium-sized pieces and feels powdery,
like silt.
• sticky -small pieces and feels gummy, like wet
clay.
EQ: How do you scientifically
describe soil? How is soil
formed?
G2-2
Soil Consistency
EQ: How do you
scientifically
describe soil?
How is soil
formed?
• how easily the soil clumps can be
broken apart.
The soil consistency is
• loose - soil breaks apart when held.
• friable - soil breaks apart with a small amount
of pressure from one finger.
• firm - soil breaks apart with a lot of pressure
between two fingers.
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EQ: How do you
scientifically describe
soil? How is soil
formed?
Soil Horizons
• A layer of soil that differs in color and texture
from the layers above or below it
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Soil Horizons
• Scientists classify soil into 3 horizons
– A Horizon - topsoil-crumbly, dark brown soil
– B Horizon – subsoil- contains clay and other
particles washed down from A. Contains little
humus
– C Horizon – contains partly weathered rock
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EQ: How do you
scientifically describe
soil? How is soil
formed?
Rate of Soil Formation
• Depends on the climate and type of rock
– Warm and rainy climate – soil develops quickly
– Cold and dry climate – soil develops slowly
EQ: How do you scientifically
describe soil? How is soil
formed?
G2-2
Life in soil- organisms mix the soil,
create spaces, make humus .
• Plant “litter”, a
loose layer over
the soil where
plants shed leaves
or die
• Decomposers,
organisms that
break down the
remains of dead
plants and animals
live in the soil
• Bacteria, fungi,
earthworms, snails
and beetles
EQ: How do you
scientifically describe
soil? How is soil
formed?
G2-2
Soil Types in the United States
• Scientists classify soil based on:
– Climate
– Common plants found in the region
– Soil composition
EQ: How do you
scientifically
describe soil?
How is soil
formed?
G2-2
Class Work and Homework
• Classwork: Copy questions 1-5 on notebook
paper.
• Homework: Questions 1-5 using notes. See
page G55.
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