3.3 Soil classification systems and mapping

Soil classification systems organize soils based on properties and formation processes. They use hierarchical structures with diagnostic horizons and measurable properties to categorize soils, facilitating communication among scientists and supporting land management decisions.

The USDA Soil Taxonomy and FAO World Reference Base are two major systems. They differ in structure and nomenclature but both use diagnostic features. Soil orders, suborders, and great groups reflect dominant soil-forming processes and specific properties.

Soil classification systems

Purpose and principles

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• Provide standardized framework for organizing and describing soils based on properties, characteristics, and formation processes
• Facilitate communication among soil scientists, predict soil behavior, and support land use planning and management decisions
• Use hierarchical structure with broader categories at higher levels and more specific categories at lower levels
• Employ diagnostic horizons, soil properties, and environmental factors to differentiate soil types
• Cover all known soil types comprehensively while remaining flexible to accommodate new discoveries and variations
• Involve extensive field observations, laboratory analyses, and data compilation to establish consistent criteria for categorization
• Aim to be quantitative and objective, relying on measurable soil properties rather than subjective assessments
  • Examples of measurable properties:
    • Soil pH
    • Organic matter content
    • Clay content
    • Cation exchange capacity (CEC)

Development and application

• Developed through systematic examination of soil profiles, including:
  • Horizon identification
  • Texture analysis
  • Chemical testing
• Utilize key diagnostic features for classification:
  • Presence of argillic horizons
  • Organic matter content
  • Base saturation
  • Occurrence of specific minerals or compounds (calcite, gypsum)
• Incorporate soil color determination using Munsell color charts
  • Indicates organic matter content, drainage conditions, or mineral composition
• Require understanding of soil formation processes:
  • Weathering
  • Leaching
  • Illuviation
  • Organic matter accumulation

USDA vs FAO systems

System characteristics and structure

• USDA Soil Taxonomy primarily used in the United States
• FAO World Reference Base (WRB) designed for global application
• USDA system uses six hierarchical categories:
  • Order
  • Suborder
  • Great Group
  • Subgroup
  • Family
  • Series
• WRB uses two levels:
  • Reference Soil Groups
  • Qualifiers
• USDA Soil Taxonomy recognizes 12 soil orders
• WRB identifies 32 Reference Soil Groups

Classification criteria and nomenclature

• USDA system emphasizes soil temperature and moisture regimes
• WRB focuses more on soil-forming factors and processes
• USDA uses primarily Latin-derived terms (Mollisols, Alfisols)
• WRB incorporates more common language descriptors (Chernozems, Luvisols)
• Both systems use diagnostic horizons and properties for classification
  • May define and prioritize these differently
• WRB designed to be more adaptable to local conditions and easier to use internationally
• USDA system provides more detailed classifications within the United States

Soil orders, suborders, and great groups

Soil order classification

• Determined by presence or absence of diagnostic horizons and specific soil properties
• Reflect dominant soil-forming processes
• Examples of soil orders:
  • Alfisols: Forest soils with clay accumulation in subsoil
  • Aridisols: Dry soils of desert regions
  • Histosols: Organic-rich soils (peat, muck)

Suborder and great group differentiation

• Suborders differentiated based on:
  • Soil moisture regimes (Aquic, Udic, Ustic)
  • Temperature regimes (Frigid, Mesic, Thermic)
  • Other properties influencing soil genesis and behavior
• Great groups identified by considering:
  • Additional diagnostic horizons
  • Chemical properties (base saturation, pH)
  • Physical characteristics (soil depth, texture)
• Examples of suborders and great groups:
  • Alfisols → Udalfs (moist Alfisols) → Hapludalfs (typical moist Alfisols)
  • Aridisols → Argids (Aridisols with clay accumulation) → Haplargids (typical Argids)

Soil maps for land management

Map components and interpretation

• Visually represent spatial distribution of soil types within given area
• Use color-coded polygons or units to depict different soil types
• Map units may represent single soil series or complex of multiple soil types
• Include legend providing information on:
  • Soil types
  • Soil characteristics
  • Classification according to relevant soil taxonomy
• Vary in scale:
  • Detailed (1:12,000 or larger) for site-specific planning
  • General (1:250,000 or smaller) for regional assessments
• Require understanding of:
  • Map symbols
  • Contour lines
  • Relationship between topography and soil distribution

Applications and technological advancements

• Crucial tools in:
  • Land use planning
  • Agricultural management (crop selection, irrigation planning)
  • Construction projects (foundation design, drainage systems)
  • Environmental assessments (wetland delineation, erosion control)
• Geographic Information Systems (GIS) revolutionized soil mapping:
  • Allow digital storage, analysis, and updating of soil information
  • Enable integration with other spatial data (topography, land use)
  • Facilitate creation of interactive, multi-layered soil maps
• Remote sensing technologies enhance soil mapping:
  • Satellite imagery for large-scale soil type identification
  • Drone-based sensors for detailed soil property mapping