2.1 Soil formation processes

Soil formation is a complex process influenced by various factors and processes. Understanding these elements is crucial for grasping how different soils develop and their properties.

This section explores the CLORPT factors (Climate, Organisms, Relief, Parent material, and Time) that shape soil development. We'll also dive into weathering processes, soil horizons, and key soil-forming mechanisms that create diverse soil types worldwide.

Factors in Soil Formation

CLORPT Factors

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• Five main factors influence soil formation known as CLORPT factors
  • Parent material
  • Climate
  • Topography
  • Organisms
  • Time
• Parent material constitutes original geologic or organic matter soil forms from (bedrock, sediments, organic deposits)
• Climate affects soil formation through temperature and precipitation patterns
  • Influences weathering rates
  • Impacts chemical reactions in soil
• Topography impacts soil formation by affecting:
  • Drainage
  • Erosion
  • Deposition processes
• Organisms contribute to soil formation through:
  • Decomposition
  • Nutrient cycling
  • Physical alteration of soil structure
  • Includes both macro and microorganisms (earthworms, bacteria)
• Time plays crucial role in soil development
  • Older soils generally exhibit more developed profiles and characteristics
  • Can take thousands of years for mature soil profiles to form

Parent Material and Climate Influences

• Parent material determines initial soil composition and texture
  • Igneous rocks (granite) produce coarse-textured soils
  • Sedimentary rocks (limestone) create fine-textured soils
• Climate strongly influences rate of weathering and soil formation
  • Warm, humid climates accelerate chemical weathering (tropical regions)
  • Cold, dry climates slow soil formation processes (arctic tundra)
• Precipitation affects leaching and mineral translocation
  • High rainfall leads to more nutrient leaching and acidic soils
  • Low rainfall results in less leaching and potentially alkaline soils
• Temperature impacts organic matter decomposition rates
  • Higher temperatures increase microbial activity and organic matter breakdown
  • Lower temperatures preserve organic matter, leading to accumulation (peat bogs)

Topography, Organisms, and Time Effects

• Slope angle influences soil depth and erosion rates
  • Steep slopes have thinner soils due to increased erosion
  • Gentle slopes allow for deeper soil development
• Aspect affects soil moisture and temperature regimes
  • North-facing slopes in Northern Hemisphere tend to be cooler and moister
  • South-facing slopes receive more direct sunlight, often warmer and drier
• Vegetation types shape soil characteristics
  • Forests contribute more organic matter to soil surface (leaf litter)
  • Grasslands develop deep, organic-rich topsoil layers
• Soil fauna impact soil structure and nutrient cycling
  • Earthworms improve soil aeration and mixing
  • Termites in tropical soils create complex tunnel systems
• Time allows for development of distinct soil horizons
  • Young soils (alluvial deposits) may lack clear horizon differentiation
  • Ancient soils (parts of Australia) can have highly weathered, deep profiles

Weathering's Role in Soil

Physical Weathering Processes

• Physical weathering breaks down rocks without changing chemical composition
• Temperature fluctuations cause thermal expansion and contraction
  • Leads to rock fracturing and exfoliation
• Frost action in cold climates breaks apart rocks
  • Water expands when freezing, widening cracks
• Plant root growth exerts pressure on rocks
  • Roots penetrate cracks and gradually widen them
• Abrasion by wind-blown particles or moving water erodes rock surfaces
• Salt crystallization in arid environments can fracture rocks
  • Salt wedging occurs as salts expand in rock pores

Chemical and Biological Weathering

• Chemical weathering alters rock composition through various reactions
• Hydrolysis breaks down minerals in presence of water
  • Feldspar in granite decomposes to form clay minerals
• Oxidation occurs when minerals react with oxygen
  • Iron-bearing minerals rust, weakening rock structure
• Carbonation dissolves carbonate rocks
  • Limestone dissolves in carbonic acid formed by CO2 in rainwater
• Biological weathering involves living organisms
  • Lichens secrete acids that dissolve rock surfaces
  • Bacteria accelerate mineral breakdown through metabolic processes
• Plant roots release organic acids that enhance chemical weathering
• Burrowing animals expose fresh rock surfaces to weathering agents

Weathering Factors and Soil Development

• Climate strongly influences weathering intensity
  • Tropical climates promote rapid chemical weathering
  • Arid climates favor physical weathering processes
• Rock type affects susceptibility to different weathering processes
  • Granite resists chemical weathering but is vulnerable to physical processes
  • Limestone easily dissolves through chemical weathering
• Topography impacts exposure to weathering agents
  • Steep slopes experience more intense physical weathering
  • Depressions may accumulate water, enhancing chemical weathering
• Weathering produces smaller particles incorporated into developing soil
  • Sand-sized particles result from physical weathering of quartz
  • Clay minerals form through chemical weathering of feldspars
• Weathering releases nutrients essential for plant growth
  • Potassium from feldspar weathering
  • Calcium and magnesium from carbonate rock dissolution
• Rate of weathering influences soil texture and fertility
  • Rapid weathering in tropics can lead to nutrient-poor, clay-rich soils
  • Slow weathering in temperate regions often results in fertile loam soils

Soil Profiles and Horizons

Major Soil Horizons

• Soil profile reveals distinct layers called horizons from surface to bedrock
• O horizon forms topmost layer
  • Consists primarily of organic matter from plant and animal residues
  • Commonly found in forest soils, may be absent in grasslands
• A horizon, or topsoil, lies below O horizon
  • Rich in organic matter
  • Zone of maximum biological activity and nutrient cycling
  • Often dark in color due to humus content
• E horizon, when present, occurs below A horizon
  • Zone of maximum leaching
  • Light-colored due to loss of clay, iron, and aluminum compounds
• B horizon, or subsoil, characterized by accumulation
  • Receives clay, iron oxides, and other materials leached from upper horizons
  • Often reddish or yellowish due to iron oxide accumulation
• C horizon consists of partially weathered parent material
  • Transition between soil and bedrock
  • Retains some characteristics of original parent material
• R horizon represents underlying bedrock
  • Unweathered parent material from which soil has developed

Horizon Development and Characteristics

• Horizon formation results from soil-forming processes over time
• A horizon development:
  • Organic matter accumulation from plant roots and leaf litter
  • Mixing by soil organisms (bioturbation)
  • Typically has granular or crumb structure
• E horizon formation:
  • Intense leaching in humid climates
  • Common in forest soils, particularly under coniferous vegetation
  • May be absent in young or dry soils
• B horizon characteristics:
  • Clay accumulation creates blocky or prismatic structure
  • Iron oxide coatings give distinct color (rubification)
  • May contain lime accumulations in arid climates (calcic horizon)
• C horizon features:
  • Lacks soil structure found in upper horizons
  • May contain rock fragments or saprolite (chemically weathered bedrock)
• Horizon boundaries vary in distinctness and shape
  • Abrupt boundaries indicate rapid changes in soil properties
  • Gradual boundaries suggest more uniform soil development
• Horizon thickness varies with soil age and formation factors
  • Young soils may have thin or absent B horizons
  • Mature soils in stable landscapes can have very thick B horizons

Special Horizon Types and Variations

• Buried horizons indicate past soil surfaces covered by new material
  • Denoted by adding "b" to horizon symbol (Ab, Bb)
  • Common in alluvial or volcanic ash deposits
• Calcic horizons form in arid and semi-arid climates
  • Accumulation of calcium carbonate
  • May form a hard, cemented layer called caliche
• Argillic horizons result from clay illuviation
  • Significant increase in clay content compared to overlying horizons
  • Indicates advanced soil development
• Spodic horizons form in humid, acidic environments
  • Accumulation of organic matter, aluminum, and iron compounds
  • Typical of Spodosols in coniferous forests
• Fragipans are dense, brittle subsurface layers
  • Restrict root growth and water movement
  • Common in some temperate region soils
• Plinthite forms in tropical and subtropical soils
  • Iron-rich, humus-poor mixture that hardens irreversibly when exposed
  • Indicator of seasonal waterlogging and intense weathering

Soil-Forming Processes

Additions and Losses

• Additions to soil profile enhance soil volume and nutrient content
  • Organic matter accumulation from plant and animal residues
  • Atmospheric deposition of dust and dissolved substances in precipitation
  • Sediment deposition through erosion and flooding (alluvial soils)
• Losses from soil profile reduce soil volume or alter composition
  • Leaching removes soluble materials, moving them to lower horizons or groundwater
  • Erosion by wind or water removes surface particles
  • Volatilization releases gaseous compounds (ammonia from fertilizers)
• Balance between additions and losses influences soil development
  • Net accumulation leads to soil thickening over time
  • Net loss results in soil thinning or complete removal (badlands topography)

Translocation and Transformation

• Translocation moves materials within soil profile without chemical change
  • Clay particles move downward through eluviation and illuviation
  • Organic matter transported by water or soil fauna
  • Dissolved substances move with soil water flow
• Transformation alters physical and chemical properties of soil components
  • Organic matter decomposition by microorganisms
  • Mineral weathering produces secondary clay minerals
  • Oxidation-reduction reactions in waterlogged soils
• Pedoturbation mixes soil materials through various processes
  • Freeze-thaw cycles in cold climates
  • Animal burrowing (gophers, earthworms)
  • Tree uprooting creates pit and mound topography
• Soil structure formation involves aggregation of soil particles
  • Influenced by clay content, organic matter, and biological activity
  • Creates peds of various shapes and sizes (granular, blocky, prismatic)

Specific Soil-Forming Processes

• Gleization occurs in waterlogged environments
  • Reduction of iron compounds creates characteristic gray colors
  • Mottling patterns form due to fluctuating water tables
  • Common in wetland soils and poorly drained areas
• Laterization dominates in tropical environments
  • Intense weathering removes silica and bases
  • Accumulation of iron and aluminum oxides
  • Results in deep, red soils (Oxisols)
• Podzolization occurs in cool, humid climates under acidic vegetation
  • Organic acids leach iron and aluminum from surface horizons
  • Accumulation of these elements in B horizon creates spodic horizons
  • Typical of coniferous forest soils
• Calcification characterizes soil formation in arid and semi-arid regions
  • Accumulation of calcium carbonate in subsoil
  • Can form hardpans that impede drainage and root growth
• Salinization results from salt accumulation in soil profile
  • Common in arid regions with high evaporation rates
  • Can severely limit plant growth and soil productivity
• Argillation involves clay formation and movement within the soil
  • Weathering of primary minerals produces clay particles
  • Illuviation creates clay-enriched B horizons (argillic horizons)