Introductory Soil Science

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Introductory Soil Science

• Land Degradation
• Erosion

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Soil Degradation

• Erosion

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References

• Charman Murphy
• Chapter 2 pp 13-38
• Chapter 3 pp 44-58
• Chapter 10.6 pp 185-187
• Chapter 11.5.3 p 203
• Chapter 12 pp 205-220
• Chapter 16.3.3,17.5,20.1.1,20.1.2,

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Who cares? And why should we?

• National view costing 2.5 billion /yr
• 150 per person per year
• Prevention will cost more
• More than half of Australia in need of help
• Most away from fertile coastal fringe
• Society demanded production

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Sobering facts

• No other environmental issue so plainly affects
  every single Australian
• Ravages the economy, fouls rivers, harbours
  drinking water, it decimates native plant
  animal species, it threatens livelihoods,
  destabilises whole communities casts shadows
  over future generations

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sobering

• 1 or more kg of soil are lost with every bottle
  of wine produced
• 7 kg may be lost for each loaf of bread we
  produce
• And it is estimated that we have removed 20
  billion trees during land clearing

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Erosion

• Erosion is the process of removal of the products
  of weathering
• It can occur by
• Gravity
• Water
• Wind
• Ice (very limited in Australia)

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Soil Erosion

• The removal of soil by wind and water.
• Hastened by loss of protective vegetation and
  repeated disturbance by cultivation or
  overgrazing.
• It removes the uppermost layers of the soil where
  nutrients are concentrated.

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Gravity Erosion

• mass movements
• landslides and landslips
• avalanches
• soil creep
• Scree slopes
• Colluvium

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Water Erosion

• Responsible for the majority of erosion
• In rivers, streams and the sea
• Transport in
• solution
• Saltation (limited)
• suspension
• Produces alluvium in fans and channel fill
  deposits

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Water erosion

• When water is applied to the surface of the soil
  there are a number of options seep in
  (infiltration), set (pond) or flow.
• Seal formation what happens when water (rain
  etc.) hits the ground

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Rainfall

• Are all rainfall events equal?
• Runoff occurs anytime rain intensity exceeds the
  infiltration capacity of the soil (all other
  things being equal)

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Soil characteristics

• Soil erosion results from the combined influence
  of several parameters, the most important of
  which are soil characteristics as well as
  topographic, climatic, or land-use parameters.

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Soil erodibility

• Defined as the inherent soil property to react to
  water action in
• Reducing infiltration rate decreasing soil
  surface roughness due to aggregate breakdown,
  i.e., increasing the risk of runoff
• Being detached transported by the resulting
  runoff

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Soil characteristics

• Aggregate stability soil structural response to
  rainfall
• Slaking dispersion
• Closely related to primary soil characteristics,
  but
• No single relationship between aggregate
  stability and primary soil characteristics

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Organic matter

• One of the most important well-known aggregate
  stabilising agents in soil.
• Could be thought of second only to texture (but
  remember OM wide range of types)

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Slope, aspect and surface storage

• Gradient
• Slope length
• Moisture
• Surface roughness

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Surface cover

• Plant canopies permeable mulches are important
  in reducing soil erosion by water why?
• Three classes
• Canopy effects
• ground cover effects
• within-soil effects

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Types of erosion

• sheet
• rill
• gully

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Sheet

• The uniform removal of soil from the surface
• Text pg 17-19
• Assumed to be the first step
• Assumed to be low

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Rill erosion

• Begins when sheet erosion reaches 15 t / ha /
  annually
• Small channels - easily destroyed
• Less than 300mm deep

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Gully erosion

• The removal of soil by running water
• Deeper than 300mm
• Gully head
• Gully side

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Water to wind

• Like water erosion, wind erosion is a natural
  process
• shaping Earths surface
• changing land forms
• transporting geological materials from one place
  to another

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Wind Erosion

• An effective agent of erosion
• Material is transported in suspension and by
  saltation
• Produces
• loess
• dunes

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Wind erosion

• Although vital along the coast
• Greatest threat is dryland farming areas in
  Australia

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How does it happen?

• When forces from the wind exceed the gravity and
  cohesion forces of the soil grains at the surface
  the soil particle moves

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Key findings from readings

• Sands loamy sands more susceptible to wind
  erosion than sandy loams loams, generally
  clay loams clay are lowest
• Ground cover is essential
• Rainfall conditions before tillage vital
• Particles are enriched with nutrients N(16),
  (CEC,WHC(1/2)), P()

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Processes of erosion

• The creep fraction (particle sizegt0.5mm)
• The saltation fraction (0.1 0.5mm)
• The suspension fraction (lt0.1mm)

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Generalities

• Soil loss rates increase from S to N from
  inland to coastal regions (rainfall)
• Land use major effect on soil loss
• Soil loss per unit area tends to decrease with
  increased catchment size
• Event dominated losses
• Bushfires etc. can increase losses by 50x

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Generalities

• Soil formation is assumed to be less than 1 t
  /ha/ yr
• Losses can be as high as 500 t/ha/yr in the
  tropic to subtropic sugar cane areas

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Control of erosion

• Proper practices need to be followed to reduce
  sediment movement 12 basic principles follow
• Maintain vegetative cover (short water, tall
  wind)
• Maintain ground cover (e.g. litter, mulch)
• If area is being utilised cover during peaks
• Incorporate biomass into the soil
• Minimise soil disturbance (leave rough)
• Add soil amendments (PAMs water)

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Control of erosion

• Add supporting practices (e.g. ridges
  orientation)
• Prevent excessive rill erosion (terraces etc)
• Where possible, modify topography
• Avoid long field lengths (wind)
• Use barriers (e.g. windbreaks)
• If limited control, set up barriers to trap
  moving sediment.