1. 8th Grade: The Dynamic Earth (Module E)
Unit 1 Lesson Soil Formation

2. The Dirt on Soil – Soil Formation
Unit 1 Lesson 5 Soil Formation
Soil - loose mixture of rock fragments, organic matter, water, and air
supports plant growth
Soil formation
parent rock weathered into smaller pieces to form sediment
Plant roots, rain, wind, etc.
Decomposers (like bacteria and fungi) break down dead plants and animals into humus
Sediment and organic matter mix to form soil
Q: What mixes the soil?
A: animals (earthworms, moles), rainwater
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3. Why do soils differ from place to place?
Unit 1 Lesson 5 Soil Formation
Why do soils differ from place to place?
The type of sediment in soil depends on what rocks are in the area.
Climate
Plants and animals
Time
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4. What factors determine how long it takes for soils to form?
Unit 1 Lesson 5 Soil Formation
What factors determine how long it takes for soils to form?
1. Rock type: rate of weathering depends on the structure of the rock and minerals that make up the rock
2. Climate: Soil usually develops more quickly in warm, wet areas than in cold, dry areas.
3. Topography: Soils usually develop faster in flatter areas where sediments are not easily eroded.
4. Plants and animals: Without a lot of plants and animals, soil tends to develop slowly.
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5. Soil horizons
Unit 1 Lesson 5 Soil Formation
soil profile - vertical section of soil that shows all the different layers
Each layer is called a soil horizon
Each horizon has different properties
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6. A horizon - top layer (a.k.a. topsoil)
Contains the most humus
B horizon - less humus
In Leaching, water carried nutrients from A horizon to B horizon
C horizon - directly above parent rock
Contains the largest rock fragments
Usually no organic matter.
Unit 1 Lesson 5 Soil Formation
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7. Soil Properties Used to classify soil
Unit 1 Lesson 5 Soil Formation
Soil Properties
Used to classify soil
1. Soil texture – amount of differently sized soil particles
Sand – largest particles ( mm)
Silt ( mm)
Clay – smallest particles (less than mm)
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8. Soil Properties, con’t
Unit 1 Lesson 5 Soil Formation
2. Soil color - depends on minerals and organic matter
Iron = reddish/orange
Humus = black / brown
Grey = area may be wet with poor drainage
3. Soil pH: how acidic (pH 0-6) or basic (pH 8-14) soil is
Depends on minerals, sediment & organic matter
Determines what can grow in the soil
Can be changed by adding stuff to soil
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9. Soil Properties, con’t 4. Pore space - the spaces
Unit 1 Lesson 5 Soil Formation
4. Pore space - the spaces
between soil particles.
Water and air are found in
the pore spaces of soils.
5. Soil fertility - how well a soil can support plant growth
Depends on climate and topography
Depends on amount of humus, minerals and nutrients in soil
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10. What are some properties of soil?
Unit 1 Lesson 5 Soil Formation
What are some properties of soil?
How does the pore space of the soil on the left compare to that of the soil on the right?
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11. Soil and Climate Desert soil – dry Tropical soils-warm & wet
Shallow, not much humus
Little rainfall, so chemical weathering & soil development are SLOW
Tropical soils-warm & wet
Rains wash away and leach soils, not much humus
Soil development is FAST
Not good for growing

12. Soil and Climate, con’t. – moderate rainfall & temp
Dark colored, rich in organic matter
Good for crops
Arctic soil – cold & dry
No well-developed horizons
Many rock fragments