SOILS

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SOILS

• Chapter 7.3

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Soil

• Is a loose covering of weathered rock particles
  and humus (organic matter) overlying the bedrock
  of the Earths surface.
• Its function is to act as a medium for growth of
  plants
• It is the product of thousands of years of
  weathering biological activity.

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Soil Development

• Begins with
• the breaking of rock into smaller and smaller
  pieces
• Organisms breaking down organic matter, add
  nutrients, add passageways for water and oxygen
• As nutrients are added, the texture changes its
  ability to hold water increases

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Mixture Components of Soil

• Air
• Water
• Mineral matter
• Organic matter

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Soil Layers

• During formation, soils develop layers
• Most volume is from weathered products from its
  source called the parent material
• Two types occur
• Residual parent material is the local bedrock
  (no major lateral movement of materials)
• Transported develops from parent material
  thats been moved from its original source

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Soil Horizons

• TOPSOIL (O) dark color, nicely textured, high
  organic matter content, very important to plant
  growth. 
• SUBSOIL (A) below topsoil horizon, has
  accumulated elements from topsoil, but only minor
  amounts of organic matter. 
• PARENT MATERIAL (B) unconsolidated rock material
  below the subsoil, the material directly broken
  down into soil. 
• BEDROCK (C) solid rock below parent material.

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Particle Sizes

• Soils have a combination of soil particle sizes
• Clay less than 0.002 mm
• When dry Hard feel / When wet Sticky, plastic
  feel
• Can be seen with an electron microscope
• Sticks to fingers
• Silt 0.002 to 0.06 mm
• When dry Powdery smooth feel, flour-like / When
  wet Creamy slick, slippery feel
• Can be seen with a hand lens or microscope
• Coats hand, able to brush off
• Sand 0.06 to 2.0 mm
• Gritty feel, can be see with naked eye no
  residue left on hand
• Gravel greater than 2.0 mm.

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Soil Types
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Factors of Soil Development

• Climate
• Topography
• Parent material
• Biologic activity
• Time

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Factors of Soil Development

• Climate
• Single most important factor of soil development
• Climate consists of average temperature,
  rainfall, and wind
• Three classifications
• Humid climates (i.e. USs East Coast) have both
  chemical mechanical weathering resulting in
  soils that are high in iron oxides and aluminum.
  (Pedalfers)
• Tropical climates have deep soils void of most
  soluble minerals. (Laterite)
• Arid climates contain less organic matter and
  much of water content evaporates up through the
  soil. (Pedocals)

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Factors of Soil Development

• Topography
• Relief difference between high and low points
• Slope angle direction
• Steep slopes little or no soil due to fast
  erosion
• North-facing slopes vs. south-facing slopes in US
• South gets more sunlight
  resulting in thicker soils
• Valleys of steep slopes
• Thick soils

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Factors of Soil Development

• Parent material
• Residual vs. transported materials
• Type of bedrock dictates how thick the soils
  above it will be.
• Ex. A soil that contains sediment from limestone
  will be high in calcium, and will also have a
  basic pH. This will be much different than one
  that derives from granite, which will have a
  higher sodium or aluminum content and a pH that
  tends more toward neutral or acidic.
• Biologic activity
• Microorganisms affect the soil order
• Time
• Entisols new soils have not had time to develop
  thick layers
• Ultisols old soils have weathered and many of
  the original content has been washed away