5

• Map scale


  How many inches on the map represents inches on the ground
• Soil maps differ in their map scale
• Orders of Soil Survey

  • First order (very intensive, detailed, min. size 1 ha)
  • Second order (intensive, detailed, min. size 0.6-4 ha)
  • Third order (extensive, min. size 1.6-16 ha)
  • Fourth order (extensive, reconnaissance, min. size 16-252 ha)
  • Fifth order (exploratory, regional/national planning, min. size 252-4000 ha)
• Soil Taxonomy


  System of soil classification developed by USDA Soil Survey
• Soil Taxonomy guidelines

  • Classify soils on basis of properties
  • Soil properties should be readily observable and/or measurable
  • Soil properties should either affect soil genesis or result from soil genesis
• Reasons for classifying soils

  • Organize knowledge about soils
  • Understand relationships among different soils
  • Establish groups for practical purposes (predicting behavior, identifying best uses, estimating productivity, extending research results)
• Levels of soil classification

  • Order
  • Suborder
  • Great group
  • Subgroup
  • Family
  • Series
• Required knowledge in classifying soils

  • Diagnostic Horizons
  • Mineralogy
  • Particle size distribution
  • Temperature Regimes
  • Moisture Regimes
• Diagnostic horizons


  Distinct types of horizons that reflect nature of soil formation
• Diagnostic surface horizons

  • Mollic
  • Anthropic
  • Umbric
  • Histic
  • Ochric
  • Plaggen
• Mollic horizon is dark, soft, surface layer with high base saturation, formed under prairie vegetation
• Anthropic horizon is like mollic but contains more than 250 ppm of citric acid soluble P2O5
• Umbric horizon is like mollic but has low base saturation
• Histic horizon is organic soil (20-30% organic matter) that is saturated with water
• Ochric horizon is thin, light colored surface layer that does not fit any of the other diagnostic surface horizons
• Plaggen horizon is a man-made, surface horizon greater than 50 cm thick created by many years of addition of manure
• Diagnostic subsurface horizons

  • Argilic
  • Agric
  • Natric
  • Spodic
  • Oxic
  • Sombric
  • Placic
  • Duripan
  • Fragipan
  • Albic
  • Calcic
  • Gypsic
  • Petrocalcic
  • Cambic
• Argilic horizon is an illuvial horizon of clay accumulation
• Agric horizon has an accumulation of clay and humus to the extent of 15% of the soil volume
• Natric horizon is the same as argilic but with > 15% exchangeable sodium
• Spodic horizon is an illuvial accumulation of Al and Fe oxides and organic matter, found in acid sandy soils
• Oxic horizon is a very weathered layer of only Fe and Al oxides and 1:1 clay minerals, found in tropical soils
• Sombric horizon is light-colored, low % base-saturation and well-drained
• Placic horizon is a thin, black to dark reddish pan cemented by iron, iron and manganese or by iron-organic matter complex
• Duripan is a subsoil cemented by silica
• Fragipan is a subsoil that is hard when dry but brittle when moist
• Albic horizon is a light colored subsoil from where clay and free iron oxides have been leached out
• Calcic horizon is an accumulation of CaCO3 or Ca Mg(CO3)2
• Gypsic horizon is an accumulation of gypsum
• Petrocalcic horizon is cemented by CaCO3
• Cambic horizon is a slightly altered layer, not weathered enough to be argilic
• Soil Temperature Regimes

  • Pergelic (MAT<0°C)
  • Cryic (MAT 0-8°C)
  • Frigid (MAT <8°C)
  • Mesic (MAT 8-15°C)
  • Thermic (MAT 15-22°C)
  • Hyperthermic (MAT >22°C)
• Soil Moisture Regimes

  • Aquic (saturated, no dissolved oxygen)
  • Aridic/Torric (dry >half the time when soil temp >5°C, moist <3 months)
  • Ustic (dry >3 months, continuously moist ≥3 months)
  • Udic (dry <3 months)
  • Xeric (continuously dry 45 days after summer, continuously moist 45 days after winter)
• The 12 Soil Orders

  • Entisol
  • Inceptisol
  • Aridisol
  • Gelisol
  • Mollisol
  • Andisol
  • Spodosol
  • Alfisol
  • Ultisol
  • Oxisol
  • Vertisol
  • Histosol
• Entisols are very young soils showing very limited profile development
• Inceptisols are young soils with moderate profile development, showing the beginning of horizon development
• Aridisols have limited change in parent material due to dryness, found in arid regions with <10 inches of rainfall
• Gelisols are young soils with little profile development, with presence of permafrost layer
• Mollisols have thick, dark, soft surface horizons, formed under grassland
• Andisols are soils from volcanic ash and cinders, with very light, low bulk density, and high P fixing capacity