Soil

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Chapter 14

• Soil
• Soil Formation
• Parent Material è subsoil è topsoil
• Parent material goes through process of
  weathering
• - mechanical weathering 
• freeze-thaw
• roots
• Friction
• wind and water move small particles, exposing new
  material.

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Chapter 14

• - chemical weathering
• oxidation
• hydrolysis
• acid content
• - primary succession 
• Lichens
• increase humus content (and lowers pH)
• Enhance chemical/physical weathering 

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Chapter 14

• - earthworms
• 500,000 per 2.4 acre (1 hectare)
• 10 tons per year
• Mix soil allow pore space for water and air
• - fungi and bacteria
• è decompose organic material and reduce size of
  organic particles.
•  
• These physical, chemical and biological process
  allow soil formation 
• 1 centimeter per 15 years at best
• More often 1 cm hundreds of years

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Chapter 14

• Soil Properties
•  Texture
• - gravel (gt 2 mm)
• - sand (0.05 to 2 mm)
• - silt (0.002 to 0.05 mm)
• - clay (lt 0.002 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
  nutrients20 clay, 40 each of silt/sand

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Chapter 14

• Structure
• - clumpiness and friability
• - Sands dont clump, but clays do.
• - Soil texture and moisture content determine
  friability.
•  
• Good soils for agriculture
• moderate friability
• good drainage
• enough air after drainage
• enough water for plants

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Chapter 14

• Soil Profile
• Distinct layers known as horizons. 
• A horizon
• thickness varies, most life forms, nutrients and
  organic matter in upper A zones - less in lower A
  zones.
• E Horizon
• B/w A and B in heavily leached soils/few
  nutrients. Dense forests typically
• B horizon
• less organic material, fewer organisms, and
  nutrients leached out of A horizon.

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Chapter 14

• C horizon
• weathered parent material, no organic material,
  little or no nutrients.
• influences soil pH and texture.
• R Horizon
• Unaltered bedrock below C

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Chapter 14

• Two basic soil types
• 1. Grassland soils
• - deep A horizon
• - low rainfall, little leaching
• - thin B horizon, which has little mineral and
  organic material
•  
• 2. Forest soils
• - Much thinner A horizon
• - more rainfall, leaching more material to B
  horizon
• - much leaching of clay particles can lead to
  hardpan layer.

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Chapter 14

• Desert
• - very little rain results in poorly developed
  horizons.
• - little plant growth results in low organic
  matter.
• Tropical
•  - high temperatures and humidity allows rapid
  decomposition of organics.
• - much leaching
• - rapid erosion or baking

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Chapter 14

•  
• Topography
• - in flat areas, as soil is built, it stays in
  place.
• - on steeper slopes, soil is transported downhill
  until it reaches a flat area (typically a
  floodplain).
• Can categorize many soils within the two basic
  types, based on numerous factors
• 15,000 soil types in North America

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Chapter 14

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

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• Costs
•  - most productive layers (A horizons) lost
  first
•  - farmers must add more fertilizers
•  - streams bottoms become covered with silt,
  destroying much habitat
•  - sediments must be dredged if in shipping
  lanes
• - Carries away more soil than produced each
  year
• Bottom linemining soilfinite resources if used
  this way

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Chapter 14

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

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Chapter 14

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

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Chapter 14

• Terracing level areas carved out of steeply
  sloped lands. Flat areas used for crops, while
  walls of terrace are protected. 
• Quite costly, high maintenance, and not very
  feasible for highly mechanized farms.
•  
• Waterways channels designed to move water
  with little or no gully erosion.
• Windbreaks
• (1) layers of vegetation left on the soil when
  not cultivated.
• (2) trees, shrubs or fences placed perpendicular
  to prevailing wind direction.

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Chapter 14

• Reduced and Conservation Tillage
•  (1) soil is plowed buries weeds, adds
  nutrients, lifts deep nutrients, exposes dark
  soils which will warm more quickly.
•  (2) field is disked or harrowed breaks up
  clods, kills remaining weeds, prepares soil for
  seeds.
•  (3) a temporary plant is often grown to reduce
  erosion.
•  è every pass costs money and leaves soil
  exposed.

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Chapter 14

• Variations
• - Mulch tillage (tilling entire surface just
  prior to planting)
• - strip tillage (only till narrow strip where
  seeds will be planted)
• - no-till (special planters that inject seeds
  into soil).
•  
• è less time, but more herbicides

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Chapter 14

• Other benefits of reduced tillage
• Less erosion (saves valuable soil, clearer
  streams and rivers, less dredging).
•  
• Winter food and cover for wildlife.
•  
• Allows row crops to be grown on hillsides where
  previously they could not be grown.
•  
• Fuel is saved, soil less compacted with fewer
  trips.
•  
• Can often plant a second crop immediately after
  first crop.