Soil-forming Factors

Soils form in response to five key factors: Climate, Organisms, Relief, Parent Material, and Time.

Learning Goals

• Learn the five factors of soil formation
• Learn a bit about the processes by which soils develop

Acknowledgment

Some of this material is from and/or inspired by Prof. Phil Larson’s Physical Geography course.

Reading

Five Factors of Soil Formation (From University of Minnesota Extension)

Five Factors: Hans Jenny (1941)

[Adapted from P.H. Larson]

Cl,O,R,P,T

• Climate – Temperature and moisture (precipitation, humidity)
  • Temperature speeds up or slows down chemical reactions that break down or weather rocks and minerals.
  • Moisture content determines degree of leaching through soil
• Organisms – soils chemically altered and physically mixed by:
  • Burrowing animals
  • Growing plant roots
  • Enzyme-secreting bacteria and fungi
• Relief – Topography. The slope and direction a landscape faces influences:
  • Sunlight hours
  • Temperature
  • Water runoff
  • Erosion
  • Organic and mineral matter build-up.
• Parent Material
  • The chemical composition of original unweathered rock (or till, or wind-blown sediments, or…) influences the mineral content of the soil.
• Time & More time provides more opportunity for the above four factors to form a soil
  • Older soils are more weathered
  • Soil residence time can be reflected in the degree of soil development

Soil-development processes

Soils form following the above factors via these processes

• Addition
  • Organic matter accumulation: Things die on the surface (plants, animals) and below the surface (plant roots, burrowing animals)
  • Inorganic matter accumulation
    • Dust
    • Regolith production
    • Sediment deposition
• Loss
  • Erosion – stripping of organic and mineral content from the surface (e.g., by wind, water, ice)
  • Dissolution – removal of minerals within the soil in solution
• Transformation
  • Decomposition of primary minerals into secondary minerals
  • Translocation (movement of material within soils)
• Leaching = movement of dissolved matter via groundwater in a soil. May move it within the soil or carry dissolved matter out of the soil completely.
  • Leaching from upper soil units into deeper soil units (can carry materials down via water that changes pH, gas content, and redox as the water evaporates).
  • Leaching from parent material or organic matter.

Video

Jerry Delsol has an excellent introduction to these processes that I’ve used to help myself learn! It’s far better than anything that I could make, so here you are:

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