Weathering and Soils

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Weathering and Soils
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Regolith-

• The layer of weathered rock fragments beneath the
  soil and above the bedrock (solid rock).

? Soil ? Regolith ? Bedrock
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Soil Components

• Regolith is the basis for soil
• Soil minerals, water, gases, organic mtl.

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• humus- dark, organic material made of decayed
  animal and plant remains
• Found just beneath the vegetation
• Important part of soil- provides nutrients!

? Humus
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3 types of rock materials make up soil-
classified by the size of the particles.

• Sand (0.05 2 mm in dia.)
• Silt (0.002 to 0.05 mm)
• Clay (lt0.002 mm in dia.)
• The amounts of each of these components depends
  on the parent rock!

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Soil Triangle and Example
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Clay soils- high percentage of clay

• Come from rocks with a lot of feldspar
• Hold a lot of water
• Doesnt drain well

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Sandy soils- high percentage of sand

• Come from weathered granite and rocks with a lot
  of quartz
• Doesnt hold water, drains quickly

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Silty soils

• Very fine particles, have a gritty feel
• Erodes easily
• Often found on riverbanks

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Transported vs. Residual soil

• Transported soil moved from original location,
  will have a different composition than the
  bedrock below
• Residual soil soil that rests on top of its
  parent rock

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Soil profile

• Cross-section of layers of soil and the bedrock
• beneath

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Horizons (Residual Soil)

• A horizon topsoil
• B horizon subsoil (minerals, clay, humus)
• C horizon partially weathered bedrock
  (regolith)
• O- Organic
• E- no organics
• R- bedrock

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• Transported soils DO NOT have horizons, they are
  found in layers or unsorted depending on how they
  were transported!

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Soil and Climate

• Humid tropical climates a lot of rain high
  temps chemical weathering
• Desert climates minimal rain, high temps
  minimal weathering
• Temperate climates temperature range between
  cool and warm, rainfall varies both types of
  weathering

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Tropical Soils

• Laterites Thick but infertile soils produced in
  tropical climates
• Contains irons and other minerals that do not
  dissolve
• Topsoil often thin
• Organic matter
• constantly added

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Desert Soils

• Thin
• Made up of mostly rock fragments (evidence of
  mechanical weathering)
• Arctic soil similar

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Temperate Soils

• All three horizons can reach a thickness of
  several meters.
• 2 main types
• 1. pedalfer- contains clay, quartz, and iron
  compounds, forms in places receiving 65cm of
  rain per year (Ex. states east of Mississippi and
  Gulf Coast states)

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• 2. pedocal- contains large amounts of calcium
  carbonate, formed in places receiving less than
  65 cm of rain per year.

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Soil types around the world correspond to
climates
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Topography and Soil

• Too flat poor drainage
• On slopes- soil is thick and fertile at the
  bottom and thin and nutrient-poor on the slope.
• Ideal soil conditions fairly flat but with good
  drainage

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