Soil and Its Uses

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Chapter 14

• Soil and Its Uses

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Soil Formation

• Parent Material gt subsoil gt topsoil
• Parent material goes through process of
  weathering.
• Two main types of weathering mechanical and
  chemical.

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Mechanical Weathering

• Freeze-thaw
• Roots
• Friction (glaciers, water)
• Wind and water move small particles and expose
  new materials.

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Chemical Weathering

• Oxidation
• Hydrolysis
• Acid content of rain

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Primary Succession

• Lichens - Increase humus content and lower pH
• Earthworms - average 500,000 per hectare
  process 9 M tons per yr.
• Fungi and bacteria - decompose organic matter and
  reduce the size of organic particles

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Soils

• These physical, chemical, and biological
  processes work together to allow soil formation
• Very slow process 1 centimeter per 15 years
  under ideal conditions. Can take MUCH longer.

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Soil Properties - Texture

• Gravel gt2 mm
• Sand 0.5 to 2 mm
• Silt (0.002 to 0.05 mm)
• Clay lt0.002 mm
• Texture (combination of sand, silt, and clay)
  determines how much air and water the soil can
  contain.
• Loam good drainage but enough retention of
  nutrients

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Structure of soils

• Clumpiness and friability
• Sands dont clump clays do
• Soil texture and moisture content determine the
  friability of the soil.

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Good Soils

• Moderately friable
• Good drainage
• enough air after drainage
• enough water retained for plants
• too much water - plants die from no O2
• too much air - plants may die from lack of water

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Soil Composition

• 45 minerals
• 25 air
• 25 water
• 5 organics
• 5 organisms
• 10 roots
• 85 humus

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Soil Profile - Horizons

• A-horizon - thickness varies, most life forms,
  nutrients, and organic matter in upper A zones -
  less in lower A zones
• B-horizon - less organic material, fewer
  organisms, and nutrients leached out of A horizon.

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Soil Horizons

• C-horizon - weathered parent material, little or
  no nutrients. Influences soil pH and texture
• others as well
• O-horizon organic material / leaf litter on
  topsoil usually found in forests
• E-horizon light layer under A horizon formed by
  leaching usually found in areas of high rains

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Two basic soil types

• Grassland soils
• deep A horizons
• low rainfall, little leaching
• thin B horizon, little organics and minerals
• Forest soils
• thinner A horizons
• More rain, leaches more material to B
• Much leaching of clay can lead to an impermeable
  hardpan

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Other Soil Issues - Deserts

• Very little rain - poorly developed horizons
• Little plant growth - little organic matter

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Other Soil Issues - Tropical

• High temperature and humidity leads to rapid
  decomposition of organics
• Very large amounts of leaching render soil
  infertile due to lack of nutrients

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Topography

• In flat areas, as soil is built it stays in
  place.
• Steeper slopes, soil is transported until it
  reaches a flat area usually a floodplain
• As a result, floodplains typically have deep soil
  horizons and generally good soils.

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Soils Categorizing

• Can categorize many soils within the two basic
  types (grassland or forest) based on numerous
  factors
• However, there are 15,000 distinct soil types
  just in North America.

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Erosion

• Movement of soils by wind or water
• In the north central Texas area, more than 15
  tons per acre per year is considered critical
• Most of these soils end up in streams and rivers
  to be deposited in lakes and oceans
• Mississippi River - moves 325,000,000 mTons per
  yr.

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Costs

• Most productive layers (A) lost first
• Farmers must add fertilizers
• Stream bottoms become covered with silt,
  destroying much habitat
• sediments must be dredged if in shipping lanes -
  paid for by taxes.

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Erosion Issues

• In the U.S., about 50 of lands are capable of
  raising crops (21 currently, 26 in pastures)
• Only about 2 of this land is not susceptible to
  excessive erosion.
• Worldwide, only about 35 of lands are capable of
  raising crops (11 currently, 24 in pastures)

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Soil Conservation Efforts

• Contour Farming tilling at right angles to the
  slope of the land.
• Can reduce erosion by 50 and adds more water to
  soils for crops.
• Strip Farming Intertwining plots of contour
  tilled row crops with sown grain crops.
• Under the right conditions, can be even more
  effective than contour farming alone.

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Soil Conservation (cont)

• Terracing level areas carved out of steeply
  sloped lands. Flat areas used for crops, while
  walls of terrace are protected.
• Quite costly, high maintenance, not feasible for
  highly mechanized farms
• Waterways channels designed to move water with
  little or no gully erosion

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Windbreaks

• Layers of vegetation left on the soil when not
  cultivated
• Trees, shrubs, or fences placed perpendicular to
  the prevailing wind direction breaks up energy,
  prevents movement of soils, good in flat areas.

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Normal Tillage

• 1. Soil is plowed buries weeds, adds nutrients,
  lifts deep nutrients, exposes dark soils which
  warm fast
• 2. Field is disked or harrowed breaks up clods,
  kills remaining weeds, prepares soils for seed.
• 3. Temporary crop is often grown to prevent
  erosion
• Each pass costs money and exposes soils

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Variations

• Reduce secondary tillage (plant seeds after first
  plowing)
• Strip tillage (only till narrow strips where
  seeds will be planted)
• no-till (special planters that inject seeds
  into soils)
• Requires less time and fuel, but more herbicides

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Benefits of Reduced Tillage

• Less erosion - saves valuable soil, clearer
  streams, less dredging
• Winter food and cover for wildlife
• Allows row crops to be grown on hillsides where
  previously impossible
• Fuel is saved, soil less compacted
• Can often plant a second crop immediately after
  the first harvest