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Soils. We know more about the movement of celestial bodies than about the soil underfoot. - Leonardo da Vinci. Soil: Definition. Solid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life.

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We know more about the movement of celestial bodies than about the soil underfoot.

- Leonardo da Vinci

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Soil: Definition

  • Solid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life.

  • Engineering definition: Anything that can be removed without blasting

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Soil Production

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Soil Production: Inputs

Conversion of rock to soil

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Soil Production: Outputs

Downslope movement of soil

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Soil Thickness: Storage

Soil thickness reflects the balance between rates of soil production and rates of downslope soil movement.

  • Slope

  • Weathering Rate

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Factors of Soil Formation

  • Climate

  • Organisms

  • Parental Material

  • Topography

  • Time

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Factors of Soil Formation

  • Climate

    • Temperature and precipitation

    • Indirect controls (e.g., types of plants)

    • Weathering rates

    • The greater the rainfall amount, the more rapid the rate of both weathering and erosion.

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Factors of Soil Formation

  • Organisms

    • Types of native vegetation

    • Weathering is dependent of plant growth

    • Plant and animal activity produces humic acids that are powerful weathering agents.

    • Plants can physically as well as chemically break down rocks.

    • Plants stabilize soil profiles, Animals (including humans) tend to increase erosion.

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Factors of Soil Formation

  • Parent Material:

    • Chemistry

    • Mineralogy

    • Grain size

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Factors of Soil Formation

  • Topography:

    • Ground slope

    • Elevation

    • Aspect (e.g., north facing vs. south facing slopes)

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Factors of Soil Formation

  • Downslope transport of soil is a function of slope:

    • Erosion rate = f(S)

The steeper the surface slope, the more likely any eroded material is to be transported out of the system.

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Factors of Soil Formation

Soils on hillslopes reach an equilibrium thickness, often about 1 m.

Soils on flat surfaces, such as floodplains or plateaus, tend to thicken through time due to weathering rates being greater than sediment transport rates.

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Factors of Soil Formation

  • Time

    • Development and destruction of soil profiles

    • Typical reaction rates are slow, the longer a rock unit has been exposed, the more likely it is to be weathered.

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Soil Development

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Additions to Soils

  • Inputs from outside ecosystem

    • Atmospheric inputs

      • Precipitation, dust, deposition

    • Horizontal inputs

      • Floods, tidal exchange, erosion, land-water movement

  • Inputs from within ecosystem

    • Litterfall and root turnover

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  • Decomposition of organic matter

  • Humification to form complex organic matter

  • Weathering of rocks

    • Physical weathering

      • Fragmentation of rock

        • Freeze-thaw; drying-wetting; fire

      • Physical abrasion

        • Abrasion by glaciers

    • Chemical weathering

      • Dissolves primary minerals

      • Forms secondary minerals

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  • Breakdown of soil organic matter to form soluble compounds that can be absorbed or leached

  • Depends on

    • Quantity of input

    • Location of input (roots vs. leaves)

    • Environment

      • Temperature

      • Moisture

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Soil Horizons and Profiles

Soil Horizons

  • Layers in Soil

  • Not Deposited, but Zones of Chemical Action

    Soil Profile

  • Suite of Horizons at a Given Locality

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Soil Profiles

  • Over time different levels of a soil can differentiate into distinct horizons that create soil profiles.

  • Chemical reactions and formation of secondary minerals (clays).

  • Leaching by infiltrating water.

  • Deposition and accumulation of material leached from higher levels in the soil.

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Soil Profiles

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A Horizon

Cookport soil, Pennsylvania

B Horizon

C Horizon

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Physical weathering breaks rocks into small mineral particles.

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Chemical weathering dissolves and changes minerals at the Earth’s surface.

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Decomposing organic material from plants and animals mixes with accumulated soil minerals.

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Parent material (bedrock) undergoes weathering to become regolith (soil + saprolite).

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Soil is a mixture of mineral and organic matter lacking any inherited rock structure.


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Saprolite is weathered rock that retains remnant rock structure.


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Limits of Soil Development

Balance Between:

  • Downward Lowering of Ground Surface

  • Downward Migration of Soil Horizons

    If erosion rapid or soil evolution slow, soils may never mature beyond a certain point.

    Extremely ancient soils may have lost everything movable

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Rates of Soil Development

U.S. Department of Agriculture estimates that it takes 500 years to form an inch of topsoil.

That’s less than 0.01 mm yr-1

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Erosion of Natural Capital

Modern rates of soil loss are 100 to 1000 times rates of soil formation (typically mm yr-1 to cm yr -1 in agricultural settings).

Sets up a fundamental problem due to the erosion of natural capital!

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Soil and the Life-Cycle of Civilizations

How long would it take to erode 1 m thick soil?

Thickness of soil divided by the difference between Rate of soil production and erosion.

1 m ≈ 1000 years

1mm - .01 mm

This is about the life-span of most major civilizations...

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Man—despite his artistic pretensions, his sophistication, and his many accomplishments—owes his existence to a six-inch layer of topsoil and the fact that it rains.

- Author Unknown

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A nation that destroys its soils, destroys itself.

– President Franklin D. Roosevelt, Feb. 26, 1937.

National Archives: 114 SC 5089

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