Introductory Soil Science

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Introductory Soil Science

• Australian Soils
• Lecture 16

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References

• Charman Murphy
• Chapter 8.

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Soils an Australian viewpoint

• The soils of Australia
• mirror the continents great age,
• are in many cases unique

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Australian Soil map
From Digital Atlas of Australian Soils
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Soil-forming Factors

• Factors determining soil type
• parent material
• climate
• topography
• time

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Soil-forming Factors

• Parent Material
• Granites and acidic crystalline rocks
• Areas of younger basic volcanics
• Sedimentary basins

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Soil forming Factors

• Climate
• Australias latitudinal position dictates much of
  its climate character
• 40 in tropics
• Relatively dry, 50 lt 300mm 80 lt 600mm. (form
  rainfall, little snow)
• High evaporation (ave) over much of the continent
  gt 2000 mm

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Soil-forming Factors

• Climate
• Present climate of much of Oz has little
  relevance dynamically to the character of the
  soil e.g.
• Extensive lateritic remnants from early -middle
  Tertiary time
• Great alluvial plains (cent Qld to Vic) were laid
  down in the late Cainozoic

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Soil forming factors

• Topography
• Relatively flat
• No high mountains
• Undulating hills and extensive plains

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Soil forming factors

• Time
• Much of the mallee landscape of SA accumulated by
  aeolian deposition in the Quaternary
• This is in great contrast to conditions in
  N-hemisphere, where much of the landscape (soils)
  has been the outcome of relatively recent glacial
  periods.

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Oz

• Time
• Australian inland has been dominated by arid
  desert conditions since the end of the Tertiary
• Due to unreliable (annual) rainfall, the current
  changes occurring in most soils are much more
  anthropogenic than climatic, considering
  evaporation

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Australian Soils

• Differ significantly from those of other
  countries.
• Surface Horizons
• Subsurface Horizons

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Some Typical Features of Surface Horizons

• O horizons are rarely present
• sparse, hard woody plant litter with relatively
  low rainfall prevent O formation.
• May be found with some heaths wet sclerophyll
  forests.

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O Horizon Development in Oz

• Most have only a thin scatter of plant debris
  (dry leaves, twigs).
• Very few O1 or O2 horizons
• If found generally less than 5 cm thick
• Found higher rainfall zones under rain forest,
  wet sclerophyll forest, some coniferous forests,
  vine thicket heath vegetation

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O horizon (cont.)

• Clearing natural vegetation rapid loss
• Regular burning leads to almost complete
  destruction of accumulated litter
• Soil fauna, microflora nutrient status
  influence the O horizon

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Surface Horizons

• A1 horizons usually have low organic matter
  levels
• related to the quantity quality of litter.
• Organosols (Podzols) are an exception.
• Note that black earths rendzinas don't have
  high organic matter

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Surface Horizons

• A1 horizons often have poor structural
  development
• due to lack of mesofaunal activity.
• Apedal (massive single-grain) and weakly pedal
  At horizons cover large areas of Australia
• A1 horizons are often hard-setting
• the surface becomes compact hard during
  seasonal drying-out.

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Some Typical Features of Subsurface Horizons

• Strong texture contrast between A B horizons is
  common.
• Particularly when B horizon is sodic.
• Debate exists about the cause of texture
  contrast.
• Eluviation/illuviation of clay -rel. high
  rainfall.
• Formation of clay in the B horizon.
• Different depositional events.
• Bioturbation

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Subsurface Horizons

• Bleached A2 horizons are common
• often in association with texture contrast and
  sodic B horizons.
• They indicate that organic matter, iron compounds
  and possibly clays have been eluviated.
• B horizons often have poor structural development
• lack of biological activity
• long history of weathering-

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Subsurface Horizons

• Accumulation of salts is common.
• Arid to sub-humid climates particularly in the
  southern third of the continent.
• Due to inadequacy of rainfall.
• carbonates of calcium sometimes magnesium
• calcium sulphate -arid to semi-arid usually
  below a CaCO3 concentration
• sodium chloride
• sulphides -associated with waterlogged/permanentl
  y moist soils, eg. NE coast. Oxidise to sulphuric
  acid on drying contact with the atmosphere.

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Subsurface Horizons

• Deep profile development (gt lm).
• Either relicts or forming under current climatic
  conditions
• Presence of laterite in the profile.
• Laterite is a generic term used to describe the
  presence of ironstone or ironstone concretions.
• Various iron (and aluminium) compounds may be
  involved.
• Widespread across Oz
• often is derived from the fossil illuvial horizon
  of an ancient soil.

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Soil Types Vegetation Types

• Association between soils and vegetation
• Soil type usually has a profound influence on
  vegetation type and generalisations can be made.
• What is meant by "vegetation type"?
• refers to the structure (appearance)of the
  vegetation, classified according to the height
  and foliage cover of the upper canopy.

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Oz native veg

• Due to limited water, vegetation is dictated,
  both in form distribution by climatic changes
  e.g.
• Hummock grassland of the extreme arid regions
  through low shrubland or tussock grassland,
• To tall shrubland or scrub of the semi-arid
  regions

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Oz native vegetation

• To the woodland or low open-forest in the
  subhumid regions
• To open forests in the humid regions
• And closed forest in the wettest sites
• However much divergence, both floristically and
  structurally, from this climatic zonation arises
  from changes in soil and topography

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Oz native vegetation

• Trees and shrubs are predominantly evergreen, and
  their leaves are (generally) hard and tough
  (sclerophyllous)
• A few distinctively Oz genera, notably
  Eucalyptus, Acacia and Triodia (plectrachne)
  dominate
• Contrast, other arid regions low proportion of
  woody species, Oz no large succulents

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Oz native veg

• Over the better-watered areas of southern
  eastern Oz much of the veg has been modified, or
  cleared replaced by exotics (pastures, crops or
  forests)
• Considerable areas of various types of vegetation
  have been preserved in a relatively natural
  condition

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Oz

• Is a land of low relief
• With very poor external drainage system
• Only two large drainage systems only 1 makes it
  to sea
• The Murray Basin, one key to agric wealth of S-E
  Oz, tributaries from S Qld, eastern NSW, and Vic
  mountains (all 1 mill km2)

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Oz

• The other drainage system is the GAB (Great
  Artesian Basin), one of the worlds largest
  centres of internal drainage, catchment of 1
  million km2

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Age ?

• Soils from Oz are said to be quite old (Tertiary)
  yet they are in constant development
• laterite silcrete were formed along with
  several invasions of the sea during the Tertiary
  (the effects are clear today)

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Soil (limitations/opportunity)

• Soil salinity the widespread problem of low
  soil fertility.
• Effects of salinity greater issue to Oz than any
  other continent
• Fertility is emphasized in the ecology, both
  plant animal (raw materials may have been
  through many cycles of weathering)

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Oz vs the World

• Few, if any, prominent features of Oz are unique
  to Oz
• Have similarities and differences with other
  parts of the world (soil cover)

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Oz vs the World

• Two features stand out
• Oz soils compared to N hemisphere (affected by
  Pleistocene glaciations)
• The amount of soil similar to Oz are of minor
  aspects of the landscape, compared to Oz

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Oz vs the World

• Oz lacks any frozen soils limited podzols with
  Organic horizon
• Limited areas of various humic soils derived from
  loess (feature of much of Eu N-A)
• Missing soils derived from volcanic ash
• Anthropogenic soils are rare
• Yet it is still possible (many areas) to find
  soils with little (if any) modification by Human
  activities

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(figure 6.1, p.92 in Charman Murphy).

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