Introductory Soil Science

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Introductory Soil Science

• Lecture Week 5
• Surface Processes and the Formation of Soils

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Soil Forming FactorsClimate

• Climate
• Temperature and water have a profound effect on
  weathering

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Soil Forming FactorsClimate
Temperature
- - - - -
50 40 30 20
Clay content
l l l l
10 15 20 25
Ave. annual temperature
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Soil Forming FactorsClimate
Water
- - - - -
50 40 30 20
Clay content
l l l l
10 20 30 40
Ave. annual rainfall (inches)
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Soil Forming FactorsClimate

• Climate
• Hot wet climates produce fine textured, deep
  soils
• Climate also affects soil colour

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Soil colour

• Soil colour results from staining of mineral
  grains
• Humic acid
• Black
• brown

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Soil colour

• Soil colour results from staining of mineral
  grains
• Iron oxides
• Red,
• yellow or
• green

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Soil colour

• Soil colour results from staining of mineral
  grains
• Salts
• White

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Soil Types

• Climate and colour
• hot-wet climate removes organic material leaving
  iron oxides
• cool dry climate allows humus to build up
• arid climate are not humus producing and increase
  the potential for salt build-up
• water-logged soils cause iron oxide build-up

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Soil Forming Factors

• Factors determining soil type
• parent material
• climate
• topography
• time

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Soil Forming FactorsTopography

• Topography-slope affects soil depth
• steep slopes have thin soils
• colour texture change down slope
• results in a series of related but different
  soils from top to bottom
• called a catena

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Soil Forming FactorsTopography
Catena
Thin soils on steep slopes
Thicker soils near bottom of slopes
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Soil Forming Factors

• Factors determining soil type
• parent material
• climate
• topography
• time

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Soil Forming FactorsTime

• Time
• the development of a soil over time is called a
  seral succession
• mature soils take 100s to 1000s of years to form
• changes in conditions during the long periods of
  soil development result in polygenetic soils

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Soil types

• Organic matter also affects soil development but
  we will deal with this later.

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Soil Forming Processes

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ReviewMature soil

• Soil is produced over time by the action of
  climate, living organisms, parent material
  influenced by relief
• Mature soils develop a vertical differentiation
  called horizons
• there are two genetically related horizons
• Soils with a vertical differentiation are called
  zonal

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Zonal soils

• A horizon
• fine grained material removed by eluviation
• B horizon
• illuvial zone enriched in fine material

• Solum

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Zonal soils

• There is little evidence for the process of
  illuviation
• organisms such as earthworms and ants also change
  the relative distribution of fine materials
• erosion by water wind and mass movements may
  reduce fines from the A horizon

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Soil Profile
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Soil Profile

• The sequence of horizons as we move from the
  surface to the bedrock is known as the soil
  profile

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Organic
Eluvial
Illuvial
Soil Profile
Partly weathered bedrock
Bedrock
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Soil classification symbols

• symbols are used as a shorthand to designate
  horizons
• more than one system of classification exists so
  it is important not to mix the symbols

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O1 O2
Organic
A1 A2
Eluvial
B1 B2 B3
Regolith
Illuvial
Partly weathered bedrock
C
D
Bedrock
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U.S O S c s
O1 O2
Organic
A1 A2
A
Eluvial
E
B1 B2 B3
Illuvial
B
Partly weathered bedrock
C
C
D
Bedrock
R
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Soil Profile Development
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Soil Profile Development

• The process of profile development continues
  throughout the life of the soil
• Involves
• humification
• weathering
• leaching
• Leaching has a marked effect on profile
  development

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Soil profile development

There are no distinct stages.
Bedrock
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Soil profile development

There are no distinct stages.
Starting point
Regolith
Bedrock
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Soil profile development

There are no distinct stages.
Starting point
Undifferentiated regolith - consists of
weathering products quartz clay soluble products
Bedrock
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Stage 2Plants grow

Bedrock
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Stage 2Plants grow

Decomposing minerals provide nutrients which
allow plant growth clay minerals hold stores of
water and nutrients
Bedrock
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Stage 3Humus accumul-ates

Bedrock
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Stage 3Humus accumulates

Bedrock
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Stage 3Humus accumulates

humic acids
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
soluble nutrients
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
soluble nutrients secondary minerals-clay iron
and aluminium oxides
Bedrock
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Stage 3Humus accumulates

humic acids basic minerals
water moves nutrients down
soluble nutrients secondary minerals-clay iron
and aluminium oxides
Bedrock
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Stage 4Layer development

dark humus layer
nutrient and clay depleted
nutrient and clay enriched
Bedrock
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Stage 5 Nutrientrecycling

decay
root uptake
nutrients
Bedrock
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PARENT
MATERIAL
acted on by
SOIL
COLOUR
WEATHERING
influenced by
CLIMATE
PROCESSES
influences
SOIL
TEXTURE
forms
determine
REGOLITH
composed of
OLD
MINERALS-
NEW
QUARTZ
MINERALS -
CLAYS
acted on by
organisms
TIME
changes over
SOIL
produces
ZONAL SOIL
contain distinct
SOIL
make up
HORIZONS
PROFILE