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Rocks to SoilAP Env Sci 2011-2012
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Earth’s Surface is Constantly Changing
• Internal Processes: elevating parts of Earth’s surface– Mountain Building– Volcanic Activity
• External Processes: breaking rock and moving it to lower elevation– Weathering– Mass Wasting– Erosion
• Weathering Processes Make Soil
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Soil Formation
• Soil – several ways to define:– Unconsolidated material overlying bedrock– * Material capable of supporting plant
growth *
• Soil is produced by weathering – Physical, Chemical, Biological Processes– Climate, topography, source material
composition, and time are factors
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Mechanical Weathering
• Large pieces are broken up into smaller pieces– Frost Wedging– Biological Activity
• Processes do not work alone!
• Chemical processes attack exposed surface, so:– smaller pieces -> more surface area exposed– more exposure -> more chemical attack
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Frost wedging of Diabase Boulders at Sourlands
Michael Hambrey
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2. Mech. weathering – Biological activity
Biological activity does mechanical work and introduces chemicals to rock
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Chemical Weathering
• What does the Weathering?– Water, with small quantities of dissolved
components– ex. CO2 (forms carbonic acid) -> H2CO3– ex. O2 (oxidizer)
• Crust is made up primarily of silicate minerals
• Fate of Most Abundant Elements:– Sodium, Calcium, Potassium, Magnesium –
form ions– Iron – oxidizes to iron oxides (red/yellow)– Aluminum, Silicon, Oxygen – form clays
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Example: Granite
• Granite: mainly quartz & potassium feldspar (KAlSi3O8)
• Potassium Feldspar reacts with Carbonic Acid:2(KAlSi3O8) + 2(H2CO3) + H2O ->
Al2Si2O5(OH)4 + 2K+ + 2HCO3- +4SiO2
• Product is clay mineral and ions in solution
• Quartz is resistant to chemical weathering
• Quartz (sand) is transported to sea or elsewhere
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3. Chemical weathering Breakdown by Acids
Natural sources: Decaying organic matter makes CO2. CO2 + water carbonic acid
Anthropogenic source (human impact): SO2 and NOx create acids when combined with water acid rain
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Factors Affecting Rate of Weathering
• Exposed Surface Area
• Rock Characteristics– Mineral composition– Solubility– Cracks, etc
• Climate– Temperature – Moisture– Warm temperature/high moisture favors
chemical weathering
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Weathering rates of gravestones (Kingston)
Slate
18171837
Marble
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What is Soil?• Soil is a combination of mineral and organic matter,
water, and air.
• It is the part of the regolith (rock and mineral fragments produced by weathering) that supports the growth of plants
25%
25%
45%
5%water
air
mineral
humus (organic)
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Characterizing Soil Texture: The Soil Triangle
Source: EID
Let’s Practice….
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Soil: Big to Small
• Sand: 2 to 0.2 mm• Silt: 0.02 to 0.002 mm• Clay: 0.002 mm and smaller
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You Need to Know the Soil Triangle: Why?
• Texture impacts soil’s ability to retain and transmit air:– Larger particles have larger spaces between them– Packing -> loose = easily aerated (sand)
• Texture impacts soil’s ability to retain and transmit water:– Surface area -> small particles have more SA/volume– Nutrients and water retained by surfaces
• Sandy soils drain fast, dry out rapidly• Clay drains slowly
• Texture affects “workability” (agriculture)
• Loam soils supports plant life best
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Control of Soil Formation
• Parent Material: Source of weathered mineral matter– Parent = Bedrock – residual soils– Parent = Sediment – transported soils
• Time: How Long?• Climate: Temperature and Precipitation• Plants and Animals: Source of organic
matter• Topography:
– Steep slopes often poor soil– Bottomlands often poor drainage, much organic– Best: flat to undulating uplands
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Soil Profile
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Soil Profile
• O Horizon: Loose and partly decayed organic matter
• A Horizon: Mineral matter mixed with some humus
• E Horizon: Light colored mineral particles. Zone of eluviation and leaching
• B Horizon: Accumulation of clay transported from above
• C Horizon: Partially altered parent material
• Solum: O, A, E, and B layers. The “true soil” where soil formation processes are underway and plant and animal life exist.
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Soil and Plant Growth: Minerals, Water, Air
• Minerals: – From Weathering– Lost by Leaching– Replaced by Fertilizer
•Organic•Inorganic
– Can you identify the three most important?
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Soil and Plant Growth: Minerals, Water, Air
• Water:– Absorbed through roots– Lost through transpiration– Soil gains water through Infiltration– Soils retain water to varying degrees:
Water-Holding Capacity
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Soil and Plant Growth: Minerals, Water, Air
• Air:– Roots need air– Aeration of soil creates space for air to
enter– Compaction has opposite effect
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Soil Gains and Losses
• Gain Organic Matter:– Ample organic matter (humus)– Nutrients, infiltration and water-holding
capacity, aeration all increase
• Lose Organic Matter:– Humus loss (how?)– Nutrients, infiltration and water-holding
capacity, aeration all decrease
• Mineralization: Near Complete Loss of Organic Component!
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