Factors Influencing Soil Formation (HL) (DP IB Environmental Systems & Societies (ESS)): Revision Note

Factors influencing soil formation

• There are a number of factors that affect how soil develops, including:

  • Climate

  • Organisms

  • Geomorphology

  • Geology

  • Time

Climate factors

• Temperature variations: climate affects soil formation through temperature differences in various biomes

  • Tropical regions:

    • High temperatures and humidity promote rapid weathering and decomposition

    • This results in deep, nutrient-rich soils such as oxisols

  • Temperate regions:

    • Moderate temperatures and seasonal variations lead to well-defined soil horizons, like those found in brown earth soils

  • Polar regions:

    • Cold temperatures slow down biological processes

    • This results in thin, poorly developed soils with limited organic material

• Precipitation levels: the amount and intensity of rainfall also influence soil characteristics

  • High rainfall:

    • In areas with heavy rainfall, leaching occurs, washing away nutrients and minerals from the topsoil

    • This leads to the formation of acidic soils like podzols

  • Low rainfall:

    • In arid regions, evaporation is greater than precipitation

    • This results in soil salinisation and the development of dry, nutrient-poor soils such as aridisols

Organisms

• The presence of plants, animals, and microorganisms significantly influences soil formation

  • Plants:

    • Roots help break down parent material and contribute organic matter to the soil, enriching its nutrient content

  • Soil fauna:

    • Earthworms and other soil organisms aerate the soil and promote nutrient mixing, enhancing soil fertility

  • Microorganisms:

    • Bacteria and fungi decompose organic matter, recycling nutrients and improving soil structure

Geomorphology

• Landscape features: the physical features of the landscape affect soil formation and characteristics

  • Slope:

    • Steep slopes may lead to soil erosion

    • Gentle slopes allow for deeper soil development

  • Aspect:

    • The direction a slope faces can influence temperature and moisture levels, affecting vegetation and soil types

    • For example, south-facing slopes in the Northern Hemisphere receive more sunlight and can support more vegetation than north-facing slopes

  • Drainage:

    • Well-drained areas often have drier soils with different characteristics compared to waterlogged regions, which may produce gley soils rich in organic matter

Geology

• Types of parent material: the mineral composition of the parent material affects soil characteristics

  • Calcareous parent rock:

    • Soils derived from limestone or chalk are often rich in calcium carbonate, leading to fertile, alkaline soils like rendzinas

  • Volcanic parent rock:

    • Soils formed from volcanic ash, known as andosols, are typically very fertile due to the presence of minerals and nutrients

• Weathering processes: the type of parent rock also influences how soils develop through weathering, erosion, and deposition

  • Chemical weathering:

    • Certain rocks, like granite, undergo chemical weathering, leading to the formation of nutrient-rich soils

  • Physical weathering:

    • Other rocks, like sandstone, break down through physical processes, resulting in sandy, less fertile soils

Time

• Soil formation is a gradual process that takes time to develop

  • Soil maturity:

    • Older soils tend to have well-defined horizons and better-developed characteristics

    • This is due to long-term weathering and biological activity

  • Young soils:

    • In contrast, young soils may have less developed horizons and may be less fertile

    • They have had less time for organic matter accumulation and nutrient cycling to occur