Factors Influencing Soil Formation

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LECTURE 3

• Factors Influencing Soil Formation
• Biota Time
• Processes of soil formation

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Definitions

• Percolation
• (J.B. Thompson)
• Truncated soil
• (A.I. Bennett)

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The 5 factors influencing soil formation

• Parent materials
• Climate
• Biota
• Topography
• Time

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Sequences that develop when one of these factors
is dominant in determining differences

• Parent material lithosequence
• Climate climosequence
• Biota biosequence
• Topography toposequence
• Time - chronosequence

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Biota (living organisms)

• Contribute towards
• Organic matter accumulation
• Biochemical weathering
• Profile mixing
• Nutrient cycling
• Aggregate stability
• Can be divided into

Natural vegetation
Animals
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Natural Vegetation

• Reduces natural soil erosion rates.
• Slows down rate of mineral surface soil removal.
• Certain leaves produce organic acids that bring
  iron and aluminium into solution.
• Accelerate the downward movement and accumulation
  of these metals in the B-horizon.

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• Development of the A horizon
• Comparison of soils under grassland and forest
  vegetation.
• Thickness of A-horizon
• Structural stability of mineral soil aggregates
• Cation cycling by trees
• Ability of vegetation to take up mineral elements
  from the soil influences the characteristics of
  soils that develop
• NB acidity!
• Comparison between coniferous forest and
  deciduous forest

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• Heterogeneity in rangelands
• In arid and semi-arid landscapes, there is
  competition for limited soil water
• Does not permit vegetation dense enough to cover
  soil surface
• This alters soil properties in several ways
• Trapping of windblown dust
• Scavenging of nutrients form interplant areas
• Deposition of nutrients under plant canopies
• Addition of organic acids
• Development of islands of enhanced fertility,
  thicker A-horizons etc.

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Animals

• Role of large animals, e.g. moles, rabbits,
  gophers
• Bring materials from deep in the profile to the
  surface
• Open the profile up to air and water
• Role of smaller creatures, e.g. earthworms, ants,
  termites
• Soil mixing (bioturbation)
• Aeration of soil
• Increase in stability of soil aggregates
  (assuring infiltration of water)
• Activities of animals decrease the differences
  among soil horizons.

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• Human influences?
• It is believed that Native Americans regularly
  set fires to maintain prairie grasslands in
  Indiana and Michigan.
• Human destruction of natural vegetation.
• Tillage of soils.
• Irrigation, addition of fertilizers.
• Mining, urbanization.

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Time

• Stopwatch starts ticking when parent materials
  are deposited or exposed.
• Rates of weathering
• young vs. mature soils?
• Time spans generally in the range of hundreds to
  hundreds of thousands of years.
• Very rare that soil formation can be observed in
  a human lifetime.

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• Chronosequences
• Methods such as carbon dating, analysis of
  fossils and human artifacts etc. used as evidence
  regarding the time spans involved in soil
  formation, but another tool is to analyse a
  chronosequence.
• E.g. on alluvial terraces of differing age.
• Interaction with parent materials
• Residual parent materials vs. transported parent
  materials.
• Interaction with other soil forming factors.

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• S ƒ(c, p, r, v, o, t)

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Processes of soil formation
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• Transformations
• Translocations
• Additions
• Losses

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Transformations

• Mineral weathering.
• Organic matter breakdown.
• Some soil constituents are modified or destroyed,
  others are synthesized.

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Translocations

• Movements of organic and inorganic materials from
  one horizon up or down to another.
• Material moved by water and soil organisms.

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Additions

• Organic matter from leaves
• Dust from atmosphere
• Minerals from groundwater
• Others?

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Losses

• Leaching to groundwater.
• Erosion.
• Others?

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