Reading Question
1. What are the horizons of a typical soil profile and what are some of the
characteristics of each horizon?
Soils
-> the regolith or pedosphere (unconsolidated sediments):
soil = upper part of the regolith or pedosphere that supports life
• Forms in situ
• Influenced by micro- and macro-organisms
A. Components = what is in soil…..
1. Sand, silt and clay
2. Organics (litter & humus)
3. H2O & nutrients
Soil Forming Factors
1. Substrate/parent material : residual vs. transported soils
->controls texture and amount of nutrients
2. Duration/Time: controls soil maturity-> amount and quality
Soil Forming Factors
3. Climate: most important factor
– Soil type
– Soil character - horizons
Soil Forming Factors
4. Topography: slope angle => gravity
aspect = amount of sunlight.......
5. Vegetation - plant types
Soil Horizons
Soil profile: * soil horizons.......
1. O-horizon => organics (litter and humus)
2. A1& A2-horizons => zone of leaching:
O+A = topsoil
3. B-horizon => zone of accumulation: subsoil
4. C-horizon => broken up parent material (bedrock)
Soil Forming Processes
• Processes that give horizons their characteristics
• Humification - formation of humus and organic acids
• Gleization - reduction of iron - waterlogged soils
• Podzolization - downward chemical migration - Al, Fe
Soil Forming Processes
• Lessivage - downward (mechanical) migration of clay
• Ferralization - intense weathering
• Calcification - accumulation of calcium in subsurface horizons
Soil Classification
• Factors - parent, climate, time, topography, and organisms
• Oficial USDA system - 12 orders in the taxonomy
Soil Classification
• Alfisols (Al + Fe) - moderate leaching, subsurface clay, hardwood forests
(climate, organisms)
• Andisols (volcanic-ash) - vocanic ash parent (parent material)
• Aridosols (arid) - few organics - subsurface salts and caliche (climate)
• Entisols (recent) - poorly developed horizons, recent acumulation or
constant erosion, floodplains, mountains, badlands (time, topography)
Soil Classification
• Gelisols (permafrost) - weakly weathered (climate)
• Histosols (organic) - abundant organics (>25%), few minerals (topography)
• Inceptisols (young) - weakly developed horizons, little clay (time,
climate)
• Mollisols (soft) - moderate leaching, organic topsoil, prairie soils
(climate, organisms)
Soil Classification
• Oxisols (oxide) - very old, extremely leached, tropics (time, climate)
• Spodosols (ashy) - cold, moist climates, well developed B, pine forests
(parent, organisms, climate)
• Ultisols (leached) - subsurface clay, strongly leached, humid subtropical
(climate, time, organisms)
• Vertisols (inverted) - high clay content (>35%), shrink and swell,
not strongly leached (parent material)
Question
• What type of soil occurs in the Red River Valley?
• How can you explain this geologically?
Paleosols
• Fossil soils
• Relict versus buried soils
• Quaternary to Precambrian
• Oldest well developed = Ordovician - why?
• Indicators of paleo- climates and environments
Paleosols
• Have only been widely recognized in the last 15 years - Why?
• Diagenesis - structures and components altered
• Keys to recognition
– Stratigraphic context
Paleosols
• Keys to recognition
• Evidence of life
– Root traces and trace fossils
– bioturbation
• Traces of horizons and structures
– Peds (cutans), blocky texture
– Color, caliche, glaebules