Soil

Document Sample
Soil Powered By Docstoc
					Soil Erosion and Conservation
              “Erosion”
• a natural leveling process that wears down
  high places; fills in low places

• agents: running water, ice, wind, gravity,
  waves
       “accelerated erosion”
• Process by which soil particles are
  removed, transported and deposited;
  rate of removal of soil greater than rate
  of formation
     • 500 yrs / inch topsoil


  – Caused by removal of vegetation
     • agents: wind, water
• Deposition or sedimentation is flip side
  of erosion.

     • the soil that is removed has to go somewhere:
       wetlands, lakes, streams, atmosphere
              Minnesota
154 million tons of topsoil / year

     96% cropland

     water erosion 42%

     wind erosion 58%
                  Worldwide
75 billion metric tons soil lost / year
     (predominantly cropland)

80% cropland: moderate - severe erosion
10% cropland: slight - moderate erosion

highest rates in Asia, Africa, South America
           United States
• In past 200 yrs, 30% of US farmlands
  have been abandoned due to erosion,
  salinization and waterlogging
• wind erosion increasing
• water erosion decreasing
• 90% US cropland losing soil above
  sustainable rate
• croplands: lose 17 tons/ha/yr
• pastures: lose 6 tons/ha/yr
                In U.S….
• In past 50 yrs, average farm size
  change:
  – 90 to 190 ha (225 to 475 acres)


• to create larger fields: remove
  shelterbelts, grass strips, hedgerows

• use of heavier machinery damages soil
  Short History of Agriculture
Post WWII:
  – Increase in chemical/mechanical intensive
    production practices
    • Decrease in number of farms
    • Increase in size of farms
  – Production of commodities/export crops
    • Top 5 commodities (2003)
       – Cattle, dairy, corn, soybeans, broilers
  – Cheap food policy
    • Over-production, cost-price squeeze,
      consolidation of farms
            Farm Crisis
            Since 1980’s
• Falling prices
• Spiraling overproduction
• Bankruptcies, foreclosures
1. Water erosion
   a. rainsplash erosion
• Raindrops accelerate as fall until they
  reach speed at which friction balances
  gravity
  – for large raindrops: 30 km / hr
  – transfer kinetic energy to soil:
     • detach soil
     • destroy structure
     • transport soil (as much as 0.7 m vertically and 2
       m horizontally)
     • Only in intense rain events; soil stays local
          b. sheet erosion
Water flows smoothly in a thin film over
 surface; detached soil moves with the
 water
            c. rill erosion
• Sheet flow concentrates water into
  channels
           d. gully erosion
• Water cuts deeper into soil, rills
  coalesce into deep troughs

• cannot (easily) be removed by tillage

• most dramatic, but most soil loss is due
  to sheet and rill erosion
      Universal Soil Loss Equation
                (USLE)
A = RKLSCP
 R : rainfall erosivity (intensity, quantity)
 K : soil erodibility (erosion rate per unit of R; in Soil
  Survey)
  L : slope length
  S : slope gradient
  C : cover and management (ratio of soil loss compared
  to fallow)
  P : erosion-control practices
          2. Wind erosion
• Arid and semi-arid climates

• Dry soil; loss of structure; wind can
  remove soil particles

• Damage is on-site and off-site
• Smallest detach into suspension
  (<0.1 mm)


• medium move by saltation
  (0.1 - 0.5 mm)


• large move by rolling and sliding (creep)
  > 0.5 mm
 Wind Erosion Model (WEQ)
E = ƒ( ICKLV)
  I : soil erodibility (slope angle, soil moisture,
     structural stability)
  C : climate factor (wind speed , soil temp.,
     ppt.)
  K : roughness factor
  L : width of field factor
  V : vegetative cover
Colorado 1935
S. Dakota 1936
Rhode Island
35 mph wind
loess
 Soil Conservation Measures
US gov’t response to Great Dust Bowl:
 1. SES---SCS---NRCS

 2. 3000 Soil and Water Conservation
 Districts

 3. Shelterbelt Program
     218 million trees
 USDA’s “tolerable soil loss”
 2 - 11 metric tons / ha / yr.

 (11 = 5 tons/acre/yr)

not sustainable : soil formation rate =
          0.5 tons / acre/yr
      Prevention practices:
1. Windbreaks

Plant trees on windward side of crops
30 mph --> 13 mph
Senegal
2. Contour plowing

Cultivate with the contour of the slope
 (rather than parallel to it); lessens water
 runoff

3. Strip cropping

Plant strips of alternating crops.
(Contour strip cropping)
4. Terracing

on sloping land ; to check water flow
Inca
Bolivia
5. Reclamation of gullies

build dams (manure and straw, concrete,
 stones, sticks) to collect silt; plant gully
6. Cover crops or surface mulch
  (in orchards or vineyards)

protects ground surface between rows or
 during non-growing season
6. Conservation tillage

eliminates or restricts tilling
• In traditional tilling,
  surface soil is
  inverted,
• plant residue buried
• bare soil exposed
In conservation tillage:
  · leave plant residue on at least 30% of
    surface
No-till:

no plowing, seeds are
  planted in narrow
  slits or directly
  drilled into holes

17.5% US cropland in
  2000

increases need for
  herbicide
Conservation methods in construction:

• schedule during low rain
• work one area at a time
• cover soil immediately (vegetation,
  straw,etc)
• control runoff to prevent gullies
• trap sediment
Open-top culvert on logging road
     lead runoff water off of road
Sedimentation pond
Catch sediment
bioengineering
         Riprap channel
guide runoff, prevent gullies, reduce soil
                  loss

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:10
posted:2/2/2012
language:
pages:76