Chapter 15 Soil Resources

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Chapter 15Soil Resources
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Overview of Chapter 15

• What is soil?
• Soil Properties
• Major Soil Orders
• Soil Problems
• Soil Conservation
• Soil Reclamation

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Soil

• Uppermost layer of earths crust that supports
  plants, animals and microbes
• Soil Forming Factors
• Parent Material
• Rock that is slowing broken down
• Time
• Climate
• Organisms
• Carbon dioxide from plants helps form carbonic
  acid to break down rock. Lichens produce acids
  too.
• Topography
• Regions surface features

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Soil Composition

• Mineral Particles (45)
• Weathered rock
• Provides essential nutrients for plants
• Varies from soil to soil
• Older soil mineral deficient
• Younger soil mineral rich
• Organic Material (5)
• Litter, animal dung, dead remains of plants and
  animals, humus (picture)
• Humus black or dark brown material that remains
  after decomposition
• Helps to bind mineral ions
• Holds water in soil
• Water (25)
• Air (25)

Humus black or dark brown material that remains
after decomposition
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Soil Composition

• Pore space
• 50 of soil
• Soil air- good for aeration
• Soil water- provides water to roots

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Soil Composition

• Soil water
• Originates as precipitation
• Water not bound to soil particles or absorbed by
  roots leaches through various layers and carries
  minerals
• Deposition of leached material is illuviation
• Leached materials can reach groundwater

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Soil Composition

• Soil Air
• Contains many of the same gases found in
  atmospheric air
• Gases present in different proportions
• More carbon dioxide than oxygen
• result of cellular respiration of soil organisms
• Carbon dioxide forms carbonic acid in water
• Important weathering processes

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Soil Horizons (distinctive horizontal layers)

• Soil Profile is a vertical section showing
  horizon layers
• O-horizon
• Rich in organic material
• May be absent in deserts
• A-horizon
• Topsoil
• E-horizon
• Not present in all soils
• B-horizon
• Lighter colored subsoil
• nutrients
• C-horizon
• Weathered parent material
• Below extent of most roots
• Saturated with groundwater

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Soil Organisms

• There are millions of microorganisms in 1 tsp of
  fertile agricultural soil
• Bacteria are most numerous

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Soil Organisms

• Soil organisms provide ecosystem services
• Important environmental benefits that ecosystems
  provide
• Examples
• Decaying and cycling organic material
• Breaking down toxic materials
• Cleansing water
• Soil aeration (especially done by earthworms)

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Soil Organisms

• Earthworms
• Transport soil components via castings (bits of
  soil passed through the gut)
• Tunnels aerate soil
• Ants
• Tunnels aerate soil too
• Some food collected decomposes increasing organic
  matter
• Carry seeds
• Fungi and roots of vascular plants
• Mycorrhizae
• Enhannce plant growth

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Nutrient Cycling

• Nutrients are cycled between plants, organisms
  and soil
• Example
• Bacteria and fungi decompose plant and animal
  wastes
• They are transformed into CO2, soil nutrients and
  water

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Soil Properties

• Soil Texture refers to relative proportion of
  different sized particles
• Particles bigger than 2 mm in diameter not
  considered (why?)
• Relative proportion of sand, silt and clay
• Sand 2mm-0.05mm (can see with naked eye)
• Silt 0.05mm-0.002mm (need EM to see)
• Clay gt0.002mm

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Soil Properties

• Soil texture affects soil properties
• Coarse textured soil (sandy)
• Will not hold water well- flows through easily
• Fine textured soil (high in clay)
• Poor drainage
• Low oxygen levels in soil

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Soil Properties

• Minerals
• Often present in charged forms (ions)
• Magnesium and potassium are positively charged
  ions needed for plant growth
• Clay has negative charge
• Attracts magnesium and potassium

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Soil Properties
What is a loam?
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Soil Properties

• Soil Acidity
• Measured using pH scale
• 0-7 acidic
• 7 neutral
• 7-14 basic
• pH of most soils range from 4-8
• Affects solubility of certain plant nutrients
• Optimum soil pH is 6-7
• This is the pH where plant nutrients are most
  available to plants
• Soil amendments (ex lime) can be used to achieve
  this pH

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Soil Properties

• Acidic Soil (low pH)
• Example Pygmy Forest, in Mendocino County, CA
  (pH 2.8-3.9)
• Aluminum and manganese more soluble and may be
  toxic
• Potassium leaches more readily
• Basic Soil
• Calcium phosphate less available (needed for
  plants)
• Decomposition of organic matter generally
  decreases pH

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Major Soil Groups

• Variations in soil forming factors cause
  variation in soils around globe
• Soil Taxonomy
• Separates soils into 12 orders
• Subdivided into more than 19,000 soil series that
  vary by locality
• Five common soil orders
• Spodosols, alfisols, mollisols, aridosols, oxisols

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Major Soil Groups

• Spodosols
• Form under coniferous forests (cooler, good
  precip, good drainage)
• O-horizon composed of needles (acidic)
• E-horizon is ash-gray under A-horizon
• Not good farmland- too acidic

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Major Soil Groups

• Alfisols
• Temperate deciduous forests
• Brown to gray-brown A-horizon
• Precipitation high enough to leach most organics
  and nutrients out of A-, E- and B-horizons
• Soil fertility maintained by leaf litter

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Major Soil Groups

• Mollisols
• Found in temperate, semi-arid grassland
• Very fertile soil
• Thick, dark brown/ black A-horizon (humus)
• Soluble nutrients stay in A-horizon due to low
  leaching (rain does not wash out)
• Grain crops are grown

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Major Soil Groups

• Aridosols
• Found in arid regions of all continents
• Low precipitation preclude leaching and growth of
  lush vegetation
• Development of salty horizon possible
• Need water to grow crops

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Major Soil Groups

• Oxisols
• Found in tropical and subtropical areas with high
  precipitation
• Very little organic material accumulation due to
  fast decay rate
• B-horizon is highly leached and nutrient poor
• As plants and animals decay nutrients are taken
  up quickly

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Soil Problems

• Sustainable soil use wise use of soil resources,
  without reduction in fertility, so that it is
  useful for future generations
• Soil Erosion
• Defined as wearing away or removal of soil from
  the land
• Caused primarily by water and wind
• Why a problem?
• Causes a loss in soil fertility as organic
  material and nutrients are eroded
• More fertilizers must be used to replace
  nutrients lost to erosion
• Sediments can cause problems (How?)
• Plant cover can reduce erosion
• Accelerated by poor soil management practices

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Soil Problems

• Problems in USA and world
• Every 5 years Natural Resources Conservation
  Service measure soil erosion in USA
• Soil erosion of croplands has declined (between
  1982-1992)
• Most serious in S. Iowa, N. Missouri, Texas, E.
  Tennessee
• Water erosion is bad in midwestern grain belt and
  parts of California
• Worldwide annual loss of as much as 75 billion
  metric tons of topsoil

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Case in Point American Dust Bowl

• Great Plains have low precipitation and subject
  to drought
• 1930-1937 severe drought
• Area was overused (farming/grazing)
• No natural vegetation roots to hold soil in place
  (removed)
• Replaced by annual crops
• Winds blew soil as far east as NYC and DC.
• Rain caused severe erosion
• Farmers went bankrupt
• US Soil Conservation Service addressed the issues
  (how?)

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Soil Problems

• Nutrient Mineral Depletion
• When organisms die and microorganisms decompose
  them, the essential nutrient minerals are
  released into the soil
• Farming and clearcutting disrupt cycle

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Soil Problems

• Mineral depletion in tropical rainforest soils
• Soils are somewhat nutrient-poor because the
  nutrient minerals are stored primarily in
  vegetation
• When the forest is cleared, its efficient
  nutrient cycling is disrupted
• Can recover but takes a long time

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Soil Problems

• Soil Salinization
• Def gradual accumulation of salt in the soil,
  usually due to improper irrigation techniques
• Often in arid and semi-arid areas (natural
  mineral salt conc. high)
• The little precipitation that falls is quickly
  evaporated
• Leaves behind salts
• Salt concentrations get to levels toxic to plants

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Soil Problems

• Desertification
• Def degradation of once-fertile rangeland,
  agricultural land, or tropical dry forest into
  nonproductive desert
• Typically a human-induced condition
• Change in vegetation changes climate, further
  decreasing precipitation levels
• Big problem in Asia and Africa (soil is
  overexploited)

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Soil Conservation

• Conservation Tillage
• Residues from previous years crops are left in
  place to prevent soil erosion
• Includes no tillage
• Leave soil undisturbed

• Crop Rotation
• Planting a series of different crops in the same
  field over a period of years
• lessens insect damage and disease, helps retain
  essential nutrient minerals, decrease erosion,
  and helps maintains soil fertility

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Soil Conservation
Strip Cropping

• Contour Plowing
• Plowing around hill instead of up-down
• Decreases soil erosion
• Strip Cropping (type of Contour Plowing)
• Alternating strips of different crops along
  natural contours
• Terracing
• Creating terraces on steep slopes to prevent
  erosion

Terracing
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Preserving Soil Fertility

• Organic fertilizers
• Animal manure, crop residue, bone meal and
  compost
• Nutrient available to plants only as material
  decomposes
• Slow acting and long lasting
• Inorganic fertilizers
• Manufactured from chemical compounds
• Soluble
• Fast acting, short lasting
• Environmentally sound to limit use
• Mobile- easily leach and pollute groundwater
• No water holding capacity (like organic)

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Compare Organic Fertilizers to Inorganic
Fertilizers

• What are the pros and cons of both?
• Which is better for the environment and why?

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Soil Reclamation

• It is possible to fix soil erosion
• EX Dust Bowl in USA in 1930s and badly eroded
  land in N. China
• Two steps
• Stabilize land to prevent further erosion
  (planting of native grasses during the dustbowl)
• Restoring soil to former fertility (use
  fertilizers)
• Best way to reduce effects of wind on soil is
  shelterbelts
• Row of trees planted to reduce wind erosion of
  soil

• Another way is Agroforestry
• uses both forestry and agricultural techniques to
  improve degraded areas and offer economic benefits

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Soil Conservation Policies in US

• Soil Conservation Act 1935
• Authorized formation of Soil Conservation
  Service, now called Natural Resource Conservation
  Service (NRCS)
• Assess soil damage and develop policies to
  improve soil
• Food Security Act (Farm Bill) 1985
• Required farmers with highly erodible soil had to
  change their farming practices
• Instituted Conservation Reserve Program (CRP)
• Pays farmers to stop farming highly erodible land