8.1 Soil taxonomy and classification systems

Soil classification systems provide a standardized framework for describing and categorizing soils. These systems help scientists, farmers, and land managers communicate effectively about soil properties and behavior, enabling better decision-making for agriculture and environmental conservation.

The USDA Soil Taxonomy and FAO World Reference Base are two major global classification systems. They use hierarchical structures based on soil-forming processes, diagnostic horizons, and key properties. Proper classification involves field observations, lab data, and understanding local soil-forming factors.

Soil Classification Systems

Purpose of soil classification

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• Standardization of soil description facilitates communication among soil scientists enables consistent soil management practices (USDA Soil Survey Manual)
• Soil mapping and inventory aids in land use planning supports agricultural and environmental decision-making (GIS-based soil maps)
• Prediction of soil behavior helps anticipate soil responses to management practices guides soil conservation efforts (erosion control measures)
• Research and education provides framework for studying soil properties and processes enhances understanding of soil-landscape relationships (catena concept)

Global soil classification systems

• USDA Soil Taxonomy developed by United States Department of Agriculture widely used in US and many other countries (12 soil orders)
• FAO World Reference Base (WRB) international system developed by Food and Agriculture Organization designed for global application and correlation between national systems (32 reference soil groups)
• Other significant systems include Canadian System of Soil Classification Australian Soil Classification Russian soil classification system

Structure of soil classification

• USDA Soil Taxonomy hierarchy organized in six levels:
  1. Order: Highest level based on major soil-forming processes (Alfisols, Vertisols)
  2. Suborder: Reflects moisture and temperature regimes (Udalf, Ustalf)
  3. Great Group: Considers specific soil properties and horizons (Hapludalf)
  4. Subgroup: Indicates intergrades or extragrades (Typic Hapludalf)
  5. Family: Focuses on properties important for plant growth (Fine-loamy, mixed, active, mesic Typic Hapludalf)
  6. Series: Lowest level represents locally significant differences (Miami series)
• Criteria for differentiation include diagnostic horizons soil temperature and moisture regimes soil depth and parent material soil chemical properties (pH, base saturation, organic matter content) soil physical properties (texture, structure, color)

Application of soil taxonomy

• Steps for classifying a soil:
  1. Identify diagnostic horizons and features (argillic horizon, calcic horizon)
  2. Determine soil moisture and temperature regimes (udic, thermic)
  3. Assess key soil properties (organic matter content, clay mineralogy)
  4. Use classification keys to progress through hierarchical levels
• Important considerations: field observations and laboratory data crucial for accurate classification familiarity with local soil-forming factors aids in classification practice with multiple soil profiles improves classification skills
• Tools for classification include soil survey manuals and field guides online resources and databases (Web Soil Survey) soil classification software and apps (SoilWeb)