Paula Messina

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Soils and Geomorphology

• Paula Messina

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Pedology The Study of Soils

• Biology
• microbiology
• botany
• Chemistry
• organic
• physical
• Geology
• mineralogy
• geomorphology

Lateritic Soil
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Soil Its not just dirt

• Soil A product of in situ chemical and physical
  weathering, with additions by biological
  processes
• Soil-forming factors
• Climate
• Organisms
• Relief
• Parent material
• Time
• Stupid human tricks

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Master Soil Horizons
O

• (O organic layer, topsoil, humus)
• A Zone of maximum organic accumulation
• B Zone where weathering products accumulate
• C Weathered bedrock (saprolite)

A
B
C
Non-weathered parent material
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A Horizon

• A.k.a. Zone of leaching
• Contains humus (decayed organic matter), plants,
  roots, burrows
• Usually dark in color

A
B
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B Horizon

• A.k.a Zone of Accumulation or subsoil
• Zone of maximum weathering-product accumulation
• Fe, Al oxides
• clay minerals (sheet silicates)
• salts
• CaCO3
• CaSO4
• NaCl
• Thickness depends on climate

A
B
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Bt Horizon

• Clay, hydrated oxides fill pore spaces between
  particles
• B horizon becomes hard and impermeable
• Bt horizons may contain swelling clays, that form
  desiccation cracks when dry
• Typically red in color (iron oxide)

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Bk Horizon

• Arid/semi-arid climates
• Calcium carbonate precipitates in the B horizon,
  coating particles with caliche crust
• If the CaCO3 coats all grains in the, and if it
  constitutes more than 50 of the horizons
  composition, it is known as a K Horizon
  (calcrete, caliche)

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Bw Horizon

• CaCO3 absent
• No cementation, brittle consistency
• Distinctively red or brown

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C Horizon

• A.k.a. Weathered bedrock, or saprolite (rotten
  rock)

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Major Soil Orders
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Major Soil Orders (1)

• Oxisols (Lateritic)
• Tropical, hot humid areas maximum eluviation
• 0.02 USA, 9.2 World
• Aridisols
• Desert, hot, dry areas minimal alteration of
  parent rock, carbonates often present in B
  horizon (Bk)
• 11.5 USA, 19.2 World

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Major Soil Orders (2)

• Mollisols
• Grassland Soils, sub-humid, semi-arid dark with
  organic material, well-structured horizons base
  saturation high
• 24.6 USA, 9.0 World
• Alfisols
• Moderately weathered humid/temperate forest
  soils B horizon high in clays
• 13.4 USA, 14.7 World

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Major Soil Orders (3)

• Ultisols
• Highly weathered sub-tropical forest soils
  similar clay-rich B horizon, but redder than
  alfisols
• 12.9 USA, 8.5 World
• Spodosols
• Northern coniferous (cool/humid) forest soils
  highly leached, strongly acidic, B Horizon rich
  in Fe/Al clays
• 5.1 USA, 5.5 World

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Major Soil Orders (4)

• Entisols
• Recent soils, profile undeveloped, present in all
  climates horizons lacking, many inherited
  characteristics from parent material
• 7.9 USA, 12.5 World
• Inceptisols
• Weakly-developed, humid-region soils embryonic
  development, few diagnostic features
• 18.2 USA, 15.8 World

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Major Soil Orders (5)

• Vertisols
• Tropical/sub-tropical swelling clay-rich forms
  large cracks when dried
• 1.0 USA, 2.1 World
• Histosols
• Organic, wet places peat, bog, gt20 organic
• 0.5 USA, 0.8 World
• Andisols
• Volcanic generally fertile
• gt1.0 USA, gt1.0 World

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Soils of the Midwest USA
7.0-8.0 pH
5.5-7.0 pH
Aridisols Mollisols Alfisols
Increasing precipitation
Dry Line Average annual precipitation 20
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Major Soil Types
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Pedalfer (old nomenclature)

• B-Horizon is rich in Al and Fe
• Common in temperate humid areas
• Major soil-type of the Eastern United States

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Pedocal

• A Pedocal is rich in calcium carbonate
• They form in arid climates, such as the
  southwestern U.S.
• These soils commonly contain caliche (or
  hardpan), a calcium carbonate deposit which
  accumulates in the soil.

Caliche
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Laterite

• Depleted of nearly all elements except iron and
  aluminum oxides.
• Derived from the weathering of mafic-rich parent
  rock.
• Typical of tropical climates with very high
  rainfall.

• The high rainfall causes leaching of most of the
  elements, humus, and nutrients from the soil.
• When used for agriculture, the small amount of
  nutrients is quickly depleted, and the soil dries
  to become as hard as a brick.

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Bauxite

• Bauxite is also depleted of nearly all elements
  due to intense tropical weathering.
• Rich in Al oxides, which are all that remains
  after the more soluble Ca, Na, K, and Si have
  been leached out.

• Derived from the weathering of granite (sialic
  parent rock).
• This Al-rich soil is the principal ore of
  aluminum.