Introductory Soil Science

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Introductory Soil Science

• Lecture 8
• 2006
• Development of a Soil Profile

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What is soil?
May depend on your view point!! Soil is a
mixture of mineral and organic matter which forms
a thin layer over the underlying bedrock. The
co-evolution of inorganic and organic components
takes place in a seral succession which begins
with exposure of bedrock and progresses towards a
relatively deep and mature soil.
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What is soil?

• Soils are composed of
• an inorganic component
• an organic component

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Soil components

• Inorganic Component
• Mineral materials
• they come from weathering of bedrock in situ and
  material that is washed or blown in.
• Organic Component
• this comes from material living in or on the soil
• these change from micro-organisms in bedrock to
  higher plants at the end of the seral succession

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What is soil?

Soils continue to develop over time and a
vertical differentiation called horizons results
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SOIL FORMATION

• Read Charman Murphy pp 3 -12

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Soil Formation

• Soil formation involves two parts
• Soil-forming factors
• Soil-forming processes
• We will discuss both-
• soil-forming factors today
• soil forming processes tomorrow

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Soil forming Factors

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Soil-forming Factors

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

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Soil-forming factors

• Parent material
• bedrock materials
• composition determines soil properties
• Transported materials
• gravity colluvial
• water alluvial
• wind aeolian
• ice glacial

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Soil-forming factors

• Bedrock is converted to the basic materials
  (minerals) which form the inorganic component of
  soils by the process of weathering

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Soil Forming FactorsParent Material

• Parent material may be
• alluvium
• loess
• bedrock

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Soil Forming FactorsParent Material

• Parent Material
• Rock Type
• Some rocks are more susceptible to weathering
  and produce
• deeper soils
• more fine particles

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Particle sizes

• Sand 2.0 - 0.02 mm (2000 20µm)
• Silt 0.02 - 0.002 mm (20-2µm)
• Clay less than 0.002 mm (2 µm)

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Soil Forming FactorsParent Material

• Parent material and particle size determine
• soil texture
• soil structure
• soil depth

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Soil Forming Factors

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

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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.