Environmental Science / Unit 5: Land and Soil
5.1 Soil Formation and Composition
Notes
Soil forms when parent material (bedrock) is broken down by weathering and mixed with organic matter. Soil formation takes hundreds to thousands of years, so soil is treated as a nonrenewable resource on human timescales.
- Factors in soil formation: parent material, climate, organisms, topography, and time.
- Soil is made of mineral particles, humus (decayed organic matter), water, and air.
- Particle sizes: sand (largest, 0.05 to 2 mm), silt (medium), clay (smallest, under 0.002 mm).
- Loam is a balanced mix of sand, silt, and clay and is good for farming.
- Sandy soil drains quickly but holds few nutrients; clay holds water and nutrients but drains poorly.
- Example: a soil that is 40% sand, 40% silt, and 20% clay is a loam.
Practice
Which soil particle is the smallest?
Clay particles are smaller than 0.002 mm.
Why is soil considered nonrenewable on a human timescale?
Weathering and organic matter build up very slowly.
A farmer wants soil that holds nutrients but also drains well. The best choice is:
Loam balances water retention and drainage.
Ready for more? Take the Unit 5 test.