Download Soil Formation and Composition

Soil Formation and
Composition
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I. Soil Formation
– A. When bedrock is exposed, it weathers.
– B. Particles of rock mix with other material.
– C. Soil forms - the loose, weathered material
on Earth’s surface in which plants can grow.
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II. Soil Composition
– A. Soil is a mixture of:
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1.
2.
3.
4.
5.
Rock particles
Minerals
Decayed organic material (humus)
Air
Water
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III. Soil Texture
– A. Rock particles come in 4 sizes:
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1.
2.
3.
4.
Gravel (2 mm and up)
Sand (less than 2 mm)
Silt (less than 1/16 mm)
Clay (less than 1/256 mm)
microscopic view
– B. Permeability
• 1. Clay soils are dense, heavy and hold a lot of
water, so plants may drown.
• 2. Sandy/gravel soils have larger air spaces, so
water drains quickly.
• 3. Loam is made up of equal parts clay, sand, and
silt and is good for growing most plants because it
holds both air and water.

IV. Soil Horizons
– A. A layer of soil
– The O horizon is leaf litter- a thin layer of organic
matter.
• 1. Topsoil (The A horizon)
– a. Dark brown soil, mixture of humus, clay and other
minerals. Develops from C; plant roots weather rocks
mechanically and chemically.
• 2. Subsoil (The B horizon)
– a. Materials washed down from the A horizon, but with
little humus. Only roots of very large plants
– reach B.
• 3. C horizon first step – weathering of
bedrock; partly weathered rock
• 4. Bedrock –solid layer of rock

V. Life in Soil
– A. Soil organisms:
• 1. mix the soil
– a. burrowing mammals and worms mix soil.
• 2. create spaces for air and water
– a. burrowing mammals and worms do this too.
• 3. make humus
– a. most comes from plant leaves on the surface,
called litter.
– b. decomposers break the dead matter down.
• i. fungi, protists, bacteria, worms