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Ch 8Earth SystemsModules 25Weathering and Soil Science
11/13/17
Weathering vs. Erosion
• Weathering – Breakdown of rocks
• Erosion – Moving of broken-down
material
Weathering
• Physical weathering The mechanical breakdown of rocks and minerals.
o Water
o Frost Wedging
o Roots
o Animals
Weathering• Chemical weathering The breakdown of
rocks and minerals by chemical reactions, the dissolving of chemical elements from rocks, or both.
o Oxidation (rust)
o Acids
o Water
Soil Formation
Five factors determine the properties of soil:
• Parent material (rock material from which the inorganic components of a soil are derived)
• Climate
• Topography
• Organisms
• Time
Organics
Topsoil
Subsoil
Parent Rock
Caliche
• Generally in arid or semiarid regions in aridisol soil
• Hardened natural cement of calcium carbonate that binds gravel, sand, clay, and silt
• Forms when minerals leach from the upper layer of the soil (the A horizon) and accumulate in the next layer (the B horizon)
• Occurs at depths of approximately 3 to 10 feet under the surface
Properties of Soil
Particle Size
Properties of Soil
Permeability
• The permeability of soil depends on its texture.•Sand, with its large, loosely packed particles, drains quickly.
•Clay drains much more slowly.
Permeability
Texture
• Important for Cation Exchange Capacity
• Clay has (-) charge
• Soils with high CEC desirable to agriculture
• 20%+ clay, too much water retention
Soil Conservation
Erosion
Compaction
Salinization
Soil is a nonrenewable resource
Entisols
• Little to no soil development
• Occur in areas of recently deposited sediments
• Entisols make up the second largest group of soils, occupying about 16% of the Earth’s surface
Aridisols
• The climate in which Aridisols occur also restricts soil weathering processes
• Often contain accumulations of salt, gypsum, or carbonates, and are found in hot and cold deserts worldwide
• Occupy about 12% of the Earth’s glacier-free land area
• Majority of soil around the Phoenix metro area
