Soils and weathering
• Soil: important resource, food, wood, etc.
• Origin: weathering of regolith (fragmented bedrock)
• 2 types weathering: physical and chemical
• 4 ingredients: water, organic matter (humus), air (pores), minerals
• 2 types: transported and no transport
Factors controlling soil formation
• Climate (humid or dry)- most important
• Organic activity- e.g. worms
• Relief of land
• Parent rock (ign, met, sed)
• Time (reaction rate)
Weathering
• Physical and chemical processes breakdown parent rock; operate together
• Erosion: transportation of weathering products (ice, water, wind)
• Residual soil: no transportation
• Transported soils: water, ice
Physical weathering
• Important in dry climates
• Breakdown of parent rock
• Frost wedging (high latitudes)
• Plant roots
• Animals (worms): process 10 tons/acre/year)
Chemical weathering
• Important in humid climates (tropics)
• 3 types of important chemical reaction (Table 6.1)
• Solution: i) acid forms; ii) mineral dissolves
• e.g. Limestone caves
• Hydration and oxidation: addition of O and water. “rust”
• Hydrolysis: breakdown of feldspar to clays
Importance of clays in soils
• Small size- large surface area
• Adsorb water for plant roots
• Attract nutrient ions: K+, Ca++, Mg++ etc
• Too much clay: poor drainage
• Too sandy: few nutrients, good drainage
Rate of weathering
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Temperature (climate)
Grain size- smaller faster rates
Environment (humid or dry)
Mineral stability:
Quartz (very stable),
Feldspars (breakdown to clays)
amphibole, pyroxene (Fe-Mg silicates)
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Geologic features
Spheroidal weathering: rectangular blocks become spherical (chemical weathering)
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Exfoliation domes: uplift, erosion (physical weathering)
Soils
• Residual soils: develop from fragments of bedrock in place (regolith)
• Transported soils: water, ice, wind
• Glacial soils: glacial drift (water)
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Loess (glacial origin; wind transport)
• Mid-west breadbasket: Loess
Soil profile
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Soil horizons: O, A, B, C
O- organic rich (humus)
A- leached zone
B- accumulation zone
C- weathered bedrock (regolith)
Horizon thickness may vary (e.g. desert, no A zone, thick B zone)