Syllabus Edition
First teaching 2024
First exams 2026
Factors Influencing Soil Formation (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Jacque Cartwright
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
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