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Chapter 12: Soils and Soil Development
Physical Physical GeographyGeographyNinth EditionNinth Edition
Robert E. Gabler
James. F. Petersen
L. Michael Trapasso
Dorothy Sack
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Soils
• Soil: dynamic natural body capable of supporting a vegetative cover• It contains chemical solutions, gases, organic refuse, flora, and fauna
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Soils
• Soil integrates:– Atmosphere– Hydrosphere– Lithologic– Biotic
• Soil is home to organisms, forming environments in which they live
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12.1 Major Soil Components
• Four major components of Soil:– Inorganic materials– Soil water– Soil air– Organic matter
• Interaction and proportion of each are important factors
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12.1 Major Soil Components
• Inorganic Materials– Insoluble materials– Rock fragments and
minerals that will not readily dissolve in water
– Chemical composition of soil result from:
• Weathering• Old deposits• Dissolved minerals
– Soil Fertilization
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12.1 Major Soil Components
• Soil Water– Original source is precipitation– Open system– Capillary water– Hygroscopic water– Gravitational water– Eluviation– Illuviation– Hardpan– Stratification
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12.1 Major Soil Components
Q: What are some examples of energy and matter that flow into and out of the soil system?
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12.1 Major Soil Components
Q: How does deposition by capillary water differ from deposition (illuviation) by gravitational water?
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12.1 Major Soil Components
• Soil Air– As much as 50% of soil may consist of spaces
between soil particles and clumps– Supplies microorganisms with oxygen and
carbon dioxide
• Organic Matter– Humus: decayed remains of plant and animal
material– Humus supplies nutrients and minerals to soil
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12.2 Characteristics of Soil
• Readily Testable properties:– Color– Texture– Structure– Acidity– Alkalinity– Capacity to hold and
transmit water
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12.2 Characteristics of Soil
• Color– Red or yellow (iron)– Black (decomposed)
• Texture– Soil texture: particle size– Clay (< 0.002 mm)– Silty (0.002 to 0.05 mm)– Sandy (0.05 to 2.0 mm)– Rocks (> 2.0 mm)
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12.2 Characteristics of Soil
• Texture and proportion of particle size– Soil grade (% sand,
silt, and clay)– Loams– Soils with a higher
proportion of large particles tend to be well aerated and allow for infiltration
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12.2 Characteristics of Soil
• Structure– Soil peds– Porosity– Permeability– Classified by form:
• Columns• Prisms• Angular blocks• Nutlike spheroids• Laminated plates
• Crumbs• Granules
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12.2 Characteristics of Soil
• Acidity and Alkalinity– pH scale (0-14)– Lower pH (higher
acidity)– Higher pH (alkaline
conditions)– Soil acidity or alkalinity
helps determine available nutrients
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12.2 Characteristics of Soil
• Acidity and Alkalinity– Optimum pH varies
by plant species– Leaching– Soils in the east tend
to be acidic– Soils in the west tend
to be alkaline
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12.3 Development of Soil Horizons
• Parent material
• Soil profiles show:– Color– Structure– Composition– Other characteristics
with depth
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12.3 Development of Soil Horizons
• Soil Horizons: – Distinct layers
distinguished by their physical and chemical properties
– Designated by set of letters that refer to:
• Composition• Dominant process• Position in the soil profile
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12.3 Development of Soil Horizons
• Soil Horizons: – O horizon– A horizon– E horizon– B horizon– C horizon– R horizon
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12.4 Factors Affecting Soil Formation
• Weathering
• Chemical reactions
• Physical Processes
• Soil development is a function of:– Climate (Cl)– Organic matter (O)– Relief (R)– Parent material (P)– Time (T)
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12.4 Factors Affecting Soil Formation
• Parent Material– Residual parent material
(e.g. physical or chemical breakdown)
– Transported parent material (e.g. carried by water, wind, etc.)
– Sandstone– Chemicals & nutrients
reflect composition
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12.4 Factors Affecting Soil Formation
• Organic Material– Plant die and
decompose– Leaves fall to the
ground– Most fertile soil are
typically grasslands– Microorganisms (e.g.
bacteria)– Earthworms, ants…
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12.4 Factors Affecting Soil Formation
• Climate– Global vs. Local scale– Equatorial:
• Higher temps increase soil microorganisms, preclude thick accumulations of humus
– Middle Latitudes:• Cooler temps slow decay and produce rich humus
– Polar Latitudes• Cold temperatures and limited plant growth result in
thin humus
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12.4 Factors Affecting Soil Formation
• Climate, Temperature, and organic materialQ: What range of mean annual temperature is
most favorable for the accumulation of humans?
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12.4 Factors Affecting Soil Formation
• Climate– Moisture Conditions
• Ample precipitation supports plant growth which increases organic content
• Too much precipitation will cause leaching
– Evaporation rate
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12.4 Factors Affecting Soil Formation
• Land Surface Configuration– Slope– Aspect
(direction of its faces)
• Time– Young &
mature soils– Alluvium
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12.5 Soil-Forming Regimes
• Self-forming regimes: vary mainly due to climate and vegetation
• Climate differences produce 3 primary soil-forming regimes:– Laterization– Podzolization– Calcification
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12.5 Soil-Forming Regimes
• Laterization– Humid and subtropical
climates– Result of high temp
and abundant precip.– Laterite: soil type
(brick-like)– No O horizon,
absence of organic acids
– Topsoil reddish
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12.5 Soil-Forming Regimes
• Podzolization– High middle latitudes– Moist with short, cool
summers and severe winters
– Podzol– East Coast of U.S.
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12.5 Soil-Forming Regimes
• Calcification– Evapotranspiration
exceeds precipitation– Often thick calcium
carbonate (alkali dusts)
– Deserts of American west
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12.5 Soil-Forming Regimes
• Regimes of Local Importance– Salinization:
concentration of salts– Occurs most often:
• in dry areas• Intensive irrigation
– Gleization: poorly drained soils in cold, wet climates
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12.6 Soil Classification
• Soil Taxonomy– Soil classification system that is based on their
characteristics– Natural Resources Conservation Service
(NRCS)– Mapped by their spatial distribution
• Soil Surveys– Books that outline and describe soils in a region– Useful for factors such as fertility, irrigation, &
drainage
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12.6 Soil Classification
• NRCS Soil Classification System– Based on development and composition– Soil order: largest division– Subdivisions– Horizons below the surface:
• Subsurface horizon• Epipedons
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12.6 Soil Classification
• Common Soil Horizons (NRCS Soil System)
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12.6 Soil Classification
• NRCS Soil Orders: based on a variety of characteristics and processes
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12.6 Soil Classification
• Map of dominant soil orders in U.S.
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12.6 Soil Classification
• World Map of dominant soil orders (NRCS system)
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12.6 Soil Classification
• World Map of dominant soil orders (NRCS system)
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12.6 Soil Classification
– Entisols
– Inceptisols
– Histosols
– Andisols
– Gelisols
– Aridisols
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12.6 Soil Classification
– Vertisols
– Mollisols
– Alfisols
– Spodosols
– Ultisols
– Oxisols
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12.7 Soil as a Critical Natural Resource
• Soil Fertility
Q: What could have been done to prevent the kind of soil loss shown in this example?
Q: What other soil conservation practices are often used to preserve the soil resource?
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Physical Geography
End of Chapter 12: Soils and Soil Development