8.2 Soil orders and their characteristics

Soil orders are the foundation of soil classification, reflecting diverse formation processes and characteristics. From volcanic Andisols to clay-rich Vertisols, each order tells a unique story of climate, parent material, and time.

Understanding soil orders is crucial for agriculture, ecology, and land management. These classifications help predict soil behavior, guiding decisions on crop selection, irrigation, and conservation practices across different landscapes and climates.

Soil Orders and Their Characteristics

Twelve USDA soil orders

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• Alfisols feature clay-enriched subsoil with high base saturation and moderate weathering (forest soils)

• Andisols form from volcanic materials with high organic matter content and unique mineral properties (Hawaii)

• Aridisols occur in arid climates with low organic matter and accumulations of salts or carbonates (deserts)

• Entisols exhibit minimal soil development in young or highly eroded areas lacking distinct horizons (floodplains)

• Gelisols contain permafrost within 100 cm of the surface in polar regions affected by cryoturbation (Alaska)

• Histosols are organic soils with over 20-30% organic matter found in wetlands or peatlands (bogs)

• Inceptisols show moderate development with weak horizon formation in various climates (young mountain soils)

• Mollisols have dark, fertile topsoil high in organic matter typically found in grassland regions (prairies)

• Oxisols are highly weathered tropical soils with low nutrient holding capacity and high iron/aluminum oxides (rainforests)

• Spodosols are sandy soils with subsurface organic matter and aluminum accumulation in cool, humid climates (coniferous forests)

• Ultisols are highly weathered, acidic soils with low base saturation common in humid subtropical/tropical regions (southeastern US)

• Vertisols have high clay content with shrink-swell properties forming deep cracks when dry (Texas Blackland Prairie)

Soil formation processes

• Climate influences soil development through temperature and precipitation patterns affecting weathering rates and organic matter decomposition
  • Aridisols form in dry climates
  • Gelisols develop in cold regions with permafrost
• Parent material determines initial soil composition and texture
  • Andisols form from volcanic ash
  • Vertisols develop from clay-rich parent materials
• Topography impacts water movement and erosion through slope and landscape position
  • Entisols often form on steep slopes or floodplains
  • Mollisols typically develop on flat to gently sloping terrain
• Biological factors like vegetation and soil organisms contribute to organic matter accumulation and nutrient cycling
  • Spodosols often form under coniferous forests
  • Mollisols develop under grassland vegetation
• Time affects degree of weathering and horizon development
  • Oxisols are highly weathered soils forming over long periods
  • Entisols are young soils with minimal development

Global distribution of soils

• Latitudinal patterns show distinct soil order distribution
  • Gelisols dominate polar and high-altitude regions
  • Spodosols occur in boreal and cool temperate forests
  • Mollisols prevail in mid-latitude grasslands
  • Oxisols are common in tropical rainforests
• Climate-based distribution reflects soil-forming processes
  • Aridisols are found in deserts and semi-arid regions
  • Alfisols dominate temperate deciduous forests
  • Ultisols are prevalent in humid subtropical and tropical areas
• Topographic influences shape soil distribution patterns
  • Entisols are common in mountainous areas and floodplains
  • Inceptisols occur in various landscapes with moderate development
• Ecological significance of soil orders impacts ecosystem functions
  • Soils play crucial roles in nutrient cycling and storage
  • They contribute to water retention and filtration
  • Soils are important for carbon sequestration
  • They provide habitat for diverse soil organisms

Agricultural potential of soil orders

• Alfisols offer high agricultural potential with moderate fertility but may require lime application in some cases

• Andisols are highly productive when managed properly but face challenges with phosphorus fixation and erosion control

• Aridisols have limited agricultural use without irrigation and require careful salt management and soil moisture conservation

• Entisols show variable agricultural potential needing careful management to prevent erosion and frequent fertilization

• Histosols have high organic matter content but require critical drainage management to prevent subsidence with cultivation

• Inceptisols demonstrate moderate to good agricultural potential often responsive to management practices and erosion control measures

• Mollisols are excellent agricultural soils with high natural fertility but may be susceptible to wind erosion in some areas

• Oxisols have low natural fertility requiring intensive management for agriculture with lime and fertilizer applications

• Spodosols show limited agricultural potential due to acidic nature requiring liming and are often better suited for forestry

• Ultisols have moderate agricultural potential with proper management needing lime and fertilizer inputs and erosion control

• Vertisols offer high agricultural potential when moisture is adequate but are difficult to manage due to shrink-swell properties requiring critical timing of cultivation