Soil Science Review
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Soil Science
Soil Horizons
Soil Horizons
Soils form from bedrock residue or material deposited by ice, wind, water, or gravity
Layers = horizons
Form over time
Where soil-forming factors favorable, 5-6 master horizons may be in a soil profile
Each master horizon is subdivided into specific layers with unique identity
Thickness of each layer varies with location
Disturbed conditions (intensive agriculture, severe erosion) not all horizons present
Young soils have fewer major horizons
Soil Horizons
Uppermost layer in undisturbed soil may be organic horizon = O horizon
Consists of fresh and decaying plant residue (leaves, needles, twigs, moss, lichens, other organic material)
Oa, Oe, and Oi used to identify levels of decomposition
Dark color because of large amounts of accumulated humus
Below O horizon is A horizon
Mainly mineral material
Generally darker than lower horizons because of varying amounts of humified organic material
Where most root activity occurs and generally most productive layer of soil
A horizon buried beneath more recent deposits is called Ab horizon
Soil Horizons
E horizon – generally bleached or whitish
as water moves down this horizon, soluble minerals and nutrients dissolve and some dissolved materials washed (leached) out
Main feature – loss of silicate clay, iron, aluminum, humus, or some combination, leaving concentration of silica sand and silt particles
Below A or E horizon – B horizon(subsoil)
Generally lighter colored, denser, and lower in content of organic matter than A horizon
Commonly is the zone where leached materials accumulate
Characterized by materials that make up accumulation; Bt horizon = clay accumulation
Other accumulations: iron, aluminum, humus, carbonates, gypsum, and silica
B horizon that does not have recognizable concentrations, but has color or structure different from adjacent horizons = Bw horizon
Soil Horizons
Deeper is C horizon (substratum)
May consist of material with less clay than overlying horizons or may consist of other less weathered sediments
Partially disintegrated parent material and mineral particles found here
Some soils have a soft bedrock horizon – called Cr
C horizon described as 2C consists of different material, generally of older age than horizons above it
Lowest horizon R horizon = bedrock
Can be within a few inches of surface or many feet below
Where bedrock is very deep and below the normal depths of observation, an R horizon is not described
Soil = LIFE
Soil Orders
Twelve soil orders:
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Alfisols
Moderately leached and have relatively high native fertility
Mainly formed under forest and have subsurface horizon where clays have accumulated
Found in temperate humid and subhumid regions
Very productive for agriculture and tree farms
Supports about 17% of world’s population
Andisols
Ando in Japanese = “black soil”
Formed in volcanic ash or other volcanic ejecta
Typically dominated by glass and other weathered products
Have unique chemical and physical properties including high waterholding capacity and ability to “fix” (and make unavailable to plants) large quantities of phosphorus
Least extensive soil order in world
Aridisols
Latin aridus = dry
CaCO3-containing soils of arid regions that exhibit subsurface horizon development
Dry most of the year and limited leaching
Clays, calcium carbonate, silica, salts, and/or gypsum accumulate in horizons of this soil order
Soluble salts, gypsum, and CaCO3 tend to be leached from soils of moister climates
Mainly used for range, wildlife, and recreation
Not used in agricultural production unless irrigation water available
Entisols
Recent origin
Developed in unconsolidated parent material and lack discernible horizons and features associated with the other soil orders
All soils that do not fit into one of the other 11 orders are Entisols
Characteristics vary greatly, both in environmental setting and land use
Many found in steep, rocky settings; Entisols of large river valleys and associated shore deposits provide cropland habitat for million of people worldwide
Most extensive of soil orders; US – 12.3% of land area
Gelisols
Latin gelare = to freeze
Very cold climates that contain permafrost within two meters of surface
Limited geographically to high-latitude polar regions and localized high mountain elevations
Support only 0.4% of world’s popularion – lowest of orders
Gelisol landscapes makes them sensitive to human activities
Occur on very old land surfaces, but show relatively little morphological development
Decomposition is very slow; store large quantities of organic carbon (only wetland soils contain more organic matter)
Histosols
Greek histos = tissue
Composed mainly of organic material
Contain 20-30% organic matter by weight and more than 40 cm thick
Often referred to as peats and mucks and have physical properties that restrict use for engineering purposes: low weight-bearing capacity, subsidence when drained
Form in wetlands where restricted drainage inhibits decomposition of plant and animal remains, allowing organic material to accumulate over time
Ecologically important because of large quantities carbon contained
Inceptisols
Latin inceptum = beginning
Exhibit minimal horizon development; more developed that Entisols, but lack features characteristics of other soil orders
Widely distributed and occur across wide range of ecological settings; found on fairly steep slopes, young geomorphic surfaces, and on resistant parent materials
Most found in mountainous areas and used for forestry, recreation, and watershed
Support approximately 20% of world’s population, largest percentage of any of soil orders
Mollisols
Latin mollis = soft
Grassland ecosystems
Characterized by thick, dark surface horizon (fertile); known as mollic epipedon – result from long-term addition of organic materials derived from plant roots
Some of most important and productive agricultural soils in world and extensively used for this purpose
Common in middle latitudes and extensive in prairie regions (Great Plains of the US)
Most extensive soil order in the US – 21.5% of land area
Oxisols
French oxide = oxide
Very highly weather soils found primarily in intertropical regions
Contain few weatherable minerals and often rich in iron and aluminum oxide minerals
Characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorous retention by oxide minerals and low cation exchange capacity (CEC)
Most nutrients contained in the standing vegetation and decomposing plant material
Can be quite productive with inputs of lime and fertilizers
Spodosols
Greek spodos = wood ash
Acid soils characterized by subsurface accumulation of humus with aluminum and iron
Typically form in coarse-textured parent material and have light-colored E horizon overlying reddish-brown spodic horizon
Process forming these horizons – podzolization
Many support forest; naturally infertile and require additions of lime to be productive agriculturally
Often occur under coniferous forest in cool, moist climates
Ultisols
Latin ultimus = last
Strongly leached, acid forest soils with relatively low native fertility
Found primarily in humid temperate and tropical areas, typically on older stable landscapes
Intense weathering of primary minerals and much of calcium, magnesium, and potassium leached
Have subsurface horizon where clay accumulate, often strong yellowish or reddish colors resulting from presence of iron oxides
“red clay” soils of southeastern US
Often support productive forests
High acidity and relatively low quantities of plan-available calcium, magnesium, and potassium make them poorly suited for continuous agriculture without use of fertilizer and lime
Can be very productive with inputs
Vertisols
Latin verto = turn
Clay-rich soils that shrink and swell with changes in moisture content
Dry periods – soil volume shrink and deep, wide cracks form; soil volume expands when wetted
Shrink/swell action creates serious engineering problems and generally prevents formation of distinct, welldeveloped horizons
Primarily in Texas
Soil Horizons
Soil Horizons
Soils form from bedrock residue or material deposited by ice, wind, water, or gravity
Layers = horizons
Form over time
Where soil-forming factors favorable, 5-6 master horizons may be in a soil profile
Each master horizon is subdivided into specific layers with unique identity
Thickness of each layer varies with location
Disturbed conditions (intensive agriculture, severe erosion) not all horizons present
Young soils have fewer major horizons
Soil Horizons
Uppermost layer in undisturbed soil may be organic horizon = O horizon
Consists of fresh and decaying plant residue (leaves, needles, twigs, moss, lichens, other organic material)
Oa, Oe, and Oi used to identify levels of decomposition
Dark color because of large amounts of accumulated humus
Below O horizon is A horizon
Mainly mineral material
Generally darker than lower horizons because of varying amounts of humified organic material
Where most root activity occurs and generally most productive layer of soil
A horizon buried beneath more recent deposits is called Ab horizon
Soil Horizons
E horizon – generally bleached or whitish
as water moves down this horizon, soluble minerals and nutrients dissolve and some dissolved materials washed (leached) out
Main feature – loss of silicate clay, iron, aluminum, humus, or some combination, leaving concentration of silica sand and silt particles
Below A or E horizon – B horizon(subsoil)
Generally lighter colored, denser, and lower in content of organic matter than A horizon
Commonly is the zone where leached materials accumulate
Characterized by materials that make up accumulation; Bt horizon = clay accumulation
Other accumulations: iron, aluminum, humus, carbonates, gypsum, and silica
B horizon that does not have recognizable concentrations, but has color or structure different from adjacent horizons = Bw horizon
Soil Horizons
Deeper is C horizon (substratum)
May consist of material with less clay than overlying horizons or may consist of other less weathered sediments
Partially disintegrated parent material and mineral particles found here
Some soils have a soft bedrock horizon – called Cr
C horizon described as 2C consists of different material, generally of older age than horizons above it
Lowest horizon R horizon = bedrock
Can be within a few inches of surface or many feet below
Where bedrock is very deep and below the normal depths of observation, an R horizon is not described
Soil = LIFE
Soil Orders
Twelve soil orders:
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
Alfisols
Moderately leached and have relatively high native fertility
Mainly formed under forest and have subsurface horizon where clays have accumulated
Found in temperate humid and subhumid regions
Very productive for agriculture and tree farms
Supports about 17% of world’s population
Andisols
Ando in Japanese = “black soil”
Formed in volcanic ash or other volcanic ejecta
Typically dominated by glass and other weathered products
Have unique chemical and physical properties including high waterholding capacity and ability to “fix” (and make unavailable to plants) large quantities of phosphorus
Least extensive soil order in world
Aridisols
Latin aridus = dry
CaCO3-containing soils of arid regions that exhibit subsurface horizon development
Dry most of the year and limited leaching
Clays, calcium carbonate, silica, salts, and/or gypsum accumulate in horizons of this soil order
Soluble salts, gypsum, and CaCO3 tend to be leached from soils of moister climates
Mainly used for range, wildlife, and recreation
Not used in agricultural production unless irrigation water available
Entisols
Recent origin
Developed in unconsolidated parent material and lack discernible horizons and features associated with the other soil orders
All soils that do not fit into one of the other 11 orders are Entisols
Characteristics vary greatly, both in environmental setting and land use
Many found in steep, rocky settings; Entisols of large river valleys and associated shore deposits provide cropland habitat for million of people worldwide
Most extensive of soil orders; US – 12.3% of land area
Gelisols
Latin gelare = to freeze
Very cold climates that contain permafrost within two meters of surface
Limited geographically to high-latitude polar regions and localized high mountain elevations
Support only 0.4% of world’s popularion – lowest of orders
Gelisol landscapes makes them sensitive to human activities
Occur on very old land surfaces, but show relatively little morphological development
Decomposition is very slow; store large quantities of organic carbon (only wetland soils contain more organic matter)
Histosols
Greek histos = tissue
Composed mainly of organic material
Contain 20-30% organic matter by weight and more than 40 cm thick
Often referred to as peats and mucks and have physical properties that restrict use for engineering purposes: low weight-bearing capacity, subsidence when drained
Form in wetlands where restricted drainage inhibits decomposition of plant and animal remains, allowing organic material to accumulate over time
Ecologically important because of large quantities carbon contained
Inceptisols
Latin inceptum = beginning
Exhibit minimal horizon development; more developed that Entisols, but lack features characteristics of other soil orders
Widely distributed and occur across wide range of ecological settings; found on fairly steep slopes, young geomorphic surfaces, and on resistant parent materials
Most found in mountainous areas and used for forestry, recreation, and watershed
Support approximately 20% of world’s population, largest percentage of any of soil orders
Mollisols
Latin mollis = soft
Grassland ecosystems
Characterized by thick, dark surface horizon (fertile); known as mollic epipedon – result from long-term addition of organic materials derived from plant roots
Some of most important and productive agricultural soils in world and extensively used for this purpose
Common in middle latitudes and extensive in prairie regions (Great Plains of the US)
Most extensive soil order in the US – 21.5% of land area
Oxisols
French oxide = oxide
Very highly weather soils found primarily in intertropical regions
Contain few weatherable minerals and often rich in iron and aluminum oxide minerals
Characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorous retention by oxide minerals and low cation exchange capacity (CEC)
Most nutrients contained in the standing vegetation and decomposing plant material
Can be quite productive with inputs of lime and fertilizers
Spodosols
Greek spodos = wood ash
Acid soils characterized by subsurface accumulation of humus with aluminum and iron
Typically form in coarse-textured parent material and have light-colored E horizon overlying reddish-brown spodic horizon
Process forming these horizons – podzolization
Many support forest; naturally infertile and require additions of lime to be productive agriculturally
Often occur under coniferous forest in cool, moist climates
Ultisols
Latin ultimus = last
Strongly leached, acid forest soils with relatively low native fertility
Found primarily in humid temperate and tropical areas, typically on older stable landscapes
Intense weathering of primary minerals and much of calcium, magnesium, and potassium leached
Have subsurface horizon where clay accumulate, often strong yellowish or reddish colors resulting from presence of iron oxides
“red clay” soils of southeastern US
Often support productive forests
High acidity and relatively low quantities of plan-available calcium, magnesium, and potassium make them poorly suited for continuous agriculture without use of fertilizer and lime
Can be very productive with inputs
Vertisols
Latin verto = turn
Clay-rich soils that shrink and swell with changes in moisture content
Dry periods – soil volume shrink and deep, wide cracks form; soil volume expands when wetted
Shrink/swell action creates serious engineering problems and generally prevents formation of distinct, welldeveloped horizons
Primarily in Texas
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