Lectures Part 6

Lectures Part 6 - Most soils form by the weathering of rock...

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ρ = “0.00” gm/cm 3 Most soils form by the weathering of rock and minerals into smaller and chemically transformed pieces ρ = “1.00” gm/cm 3 ρ =2.65 gm/cm 3 Air Water Solid
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ρ organic matter = “0.5” gm/cm 3
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Stuff, regolith, overburden, particuLate matter etc. at the Earth surface cannot be a “soil” unless it shows some geochemical and structural changes that produce soil HORIZONS.
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Well-developed soil profile.
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Soils form by four main categories of processes. • Addition of materials (volcanoes, dust) • Chemical transformation (weathering) • Transfers (redistribution dissolved species) • Removal (removal of dissolved species)
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Each soil forming process results in an exchange of matter and energy.
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dust storms, volcanoes
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In order for the upper part of the pedosphere to be a soil it must be a natural body of weathered mineral material with some organic matter, organized into parallel layers called horizons .
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Soil Layers O1 Undecomposed litter O Horizon Organic Plant Residues O2 Partly decomposed debris A1 Zone of humus accumulation A2 Zone of strongest leaching A Horizon Zone of eluviaiton (leaching) A3 Transition to B horizon B1 Transition to A horizon B2 Zone of strongest deposition Solum, True Soil B Horizon Zone of illuviation (deposition) B3 Transition to C horizon Regolith, Weathered Material C Horizon Parent Material C Unconsolidated rock R Layer - Bedrock R Consolidated rock See “Fundamentals of Soil Physics”, D. Hillel, 1980, Academic Press.
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In real life however, soil horizons may or may not be readily apparent to the eye.
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….that is because often the horizons are characterized by chemical differences that may or may not have a visual signature. Careful visual observations however are still useful to soil scientists.
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Soils generally are less than 2 meters thick,
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and they form a discontinuous membrane around the Earth.
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Soil formed on unconsolidated river sediments.
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Sometimes recent river sediments can bury an old, previously formed soil.
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Soils form on ancient sand dunes,
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on igneous rock,
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on metamorphic rock,
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