Soil Development in Arid Environments

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Soil Development in Arid Environments
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Soil Formation

• Parent material geologic material soil forms
  on.
• Soils grow from the top down.

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Soil Forming Processes

• Parent material geologic deposit that soils
  forms on.
• Examples glacial till, flood sediments, volcanic
  ash, lava
• Soils form on stable surfaces.
• soil thickness increases with time
• erosion decreases soil thickness

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Soil Forming Processes
surface
parent material
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Soil Horizons in Arid Environments

• Master horizons A, B, and C (or R)
• A horizon thin, some organic matter, mineral
  matter dominant
• B horizon accumulated clay/Fe oxide, highly
  weathered

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Soil Horizons in Arid Environments

• Subdivisions of B horizon
• Bw reddish color from 2ndary iron oxides
• Bt accumulation of translocated or in situ
  precipitated clays
• Bk accumulation of secondary carbonates

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Soil Horizons in Arid Environments

• C horizon slightly altered to unaltered parent
  material
• R horizon bedrock on which soil formed, may be
  altered

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Soil Age

• What does soil age mean?
• How old are these soils?

What factors control the soil formation rate?
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Distribution of Calcic Soils

• Soils with significant accumulation of secondary
  (pedogenic) carbonate.

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Soil Age and Carbonate Accumulation

• Carbonate accumulates over time.
• Accumulation rate related to climate and calcium
  input.
• Stage 1 thin discontinuous coatings
• Stage 2 continuous coatings
• Stage 3 continuous carbonate layer
• Stage 4 thick layer, platy or lamellar structure

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Soil Age and Carbonate Accumulation

• A thick, continuous carbonate layer takes gt105 of
  years to form.

Stage 4 carbonate
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Soil Age and Clay Content

• Clay content and thickness of Bt (argillic)
  horizon increases with age.

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Other Ways to Identify Soils

• Color use Munsell color charts of hue, value
  and chroma (e.g. 10YR 4/6)

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Other Ways to Identify Soils

• Plant assemblages
• ocotillo and prickly pear thrive on eroded,
  petrocalcic soil