Revision

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Introductory Soil Science

• Revision
• Weeks 1-4

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The Earth as a System

• There are 4 principal systems within the larger
  Earth System
• Each of these can be further subdivided
• To isolate a system it must have definable
  boundaries
• The nature of the boundaries is one of the most
  important defining characteristics of a system

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Earth Systems Cycles

• Inter-actions between the spheres are powered by
  internal and external energy sources that
  continue to modify and renew the Earth system
• The Earth is a closed system
• The Earth is in more or less a steady state

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System Concept

• This is a helpful way to break down a large
  complex problem into smaller more easily studied
  pieces
• A system is any portion of the Universe that can
  be isolated from the rest of the universe for the
  purpose of studying change
• Small systems are parts of larger systems

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System Concept

• Systems may be
• isolated,
• closed or
• open.
• This is determined by the nature of the
  boundaries that set a system apart from its
  surroundings

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Earth System - Important features

• The Earth as a system
• Closed
• Steady state
• Resources are finite
• Changes to one part affect other parts

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Living in a Closed System

• The amount of matter in a closed system is fixed
  and finite
• Mineral resources are finite
• Material wastes remain and accumulate
• There is no away to throw things to
• When changes are made to one part of a closed
  system, the results of those changes will
  eventually affect other parts of the system

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Living in a Closed System

• What do we need?
• food, water, and air
• Where do these things come from?
• From nature
• They are part of the Earths Natural Capital
• They rely on ecosystems for their renewal

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Cycles

• The changes and balancing take place via cycles
• Chemical elements such as oxygen and carbon
  circulate through the Earth by photosynthesis and
  respiration
• These are called geochemical cycles or
  biogeochemical cycles
• The systems can be viewed as reservoirs and the
  cycles trace the flux or flow between them

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Cycles In the Earth System

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Couplings Feedback Loops

• These maintain balance in the system
• Eg. An electric blanket

Electric blanket
Body temperature
Positive coupling
Electric blanket
Body temperature
Negative coupling
Electric blanket
Body temperature
(-)
Feedback loop
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Cycles in the Earth System

• Q? How is balance maintained?
• A By cycling materials from one part of the
  system to another.
• The recycling of elements among the components of
  the Earth system is the key to the continued
  functioning of Earth as a living planet

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Cycles

• The changes and balancing take place via cycles
• Chemical elements such as oxygen and carbon
  circulate through the Earth by photosynthesis and
  respiration
• These are called geochemical cycles or
  biogeochemical cycles
• The systems can be viewed as reservoirs and the
  cycles trace the flux or flow between them

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Earths systems and cycles

• atmosphere, hydrosphere, biosphere and other
  systems act as reservoirs for holding the Earths
  materials
• Reservoirs are linked by pathways along which
  materials are transported.
• These processes are called biogeochemical cycles
• Together they form part of the giant system of
  the Earth

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Hydrological Cycle
Atmosphere
transpiration
evaporation
precipitation
evaporation
Ocean
Surface water
infiltration
Groundwater
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Energy sources

• Two energy sources drive the cycles in the Earth
  System
• Sun
• Drives surface processes like weather
• Internal Energy
• Drives processes like volcanic eruptions,
  earthquakes etc

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The Rock Cycle

• Driven by internal and external energy sources
• Leads to the continuing differentiation of the
  solid Earth materials

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What do you need to know about the rock cycle

• Recognise that earth materials cycle between
  reservoirs
• Reservoirs are rocks
• Cycle is driven by internal and external energy
  sources
• Rock cycle is unique to Earth in our Solar System
  and explains the constantly changing morphology
  of the Earths surface

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The Rock Cycle

• Driven by both energy sources
• Cycles materials which make-up solid Earth
• Material cycled between three main reservoirs
• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks
• Other minor reservoirs are assoc. with these
• Soil is one important minor reservoir that we
  will deal with in detail.

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Rock Cycle
Igneous Rock
magma
Metamorphic Rock
sediment
Sedimentary Rock
soil
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Minerals

• rocks are composed of minerals
• Composition of minerals is related to the
  relative abundance of the elements that make-up
  Earth
• Only a small number of minerals make-up rocks
• Different groups of minerals characterise the
  three main groups of rocks
• Properties of minerals are related to their
  chemistry

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Minerals

• Minerals are
• Naturally occurring
• Solid
• Inorganic
• and must have
• a specific chemical composition
• a regular atomic arrangement (crystalline)
• These last two characteristics determine the
  physical properties that we use to identify
  minerals.

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Chemical composition of minerals

• Most minerals are composed of compounds of
  silicon and oxygen (called silicates)
• Why?
• Because silicon oxygen make-up 74 of the crust
• These with 6 other elements make-up more than 99
  of the crust

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Silicate Minerals

• Silicon fits neatly into the hole made when 4
  oxygens arrange themselves as a tetrahedron

because size (charge) is important atoms of
similar size can substitute for silicon eg. Al
Fe.
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The Silicate Tetrahedron

• Complex anion of silicon (4) and oxygen (2-)
• (Si04)4-
• Regular tetrahedron

O
Si
O
O
O
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Rock Weathering
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Rock Weathering

• Changes in the composition and texture of rocks
  as a result of exposure at the Earths surface
• Changes takes place in situ
• affects the outermost, exposed parts of the rock
  mass
• causes rock to break-up and become more
  susceptible to erosion

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Rock Weathering

• The outer part of the rock mass which is affected
  by weathering is called the regolith
• the regolith may extend 100 metres or more below
  the surface

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Rock Weathering

• Weathering involves reaction with the physical
  and chemical components of the surface
  environment
• Physical components (mechanical weathering)
• Temperature - expansion and contraction
• Freezing thawing
• Salt crystallisation
• Off-loading exfoliation
• Abrasion
• Plant and animal influences

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Rock Weathering

• Chemical components
• Water - solutions
• Dissolution, oxidation, hydrolysis, hydration
  acidification
• oxygen
• carbon dioxide
• Pollutants
• NOXs SOXs

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Rock Weathering

• Factors which affect weathering
• rock composition
• rock grain size
• rock structure
• climate - temp rainfall
• industrial pollution

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Rock Weathering

• Products of weathering
• solutes
• detritus
• rock fragments
• minerals
• new minerals - clay
• old minerals - quartz

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Soil Minerals

• Primary Minerals
• Unchanged except for physical break-up
• Secondary minerals
• Result from weathering
• Are usually smaller
• Result from the recombination of original
  silicate minerals that have been broken into
  constituent parts

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Clay Minerals

• produced by weathering reactions which break down
  the silicate structure. Weathering produces
• cations
• anions
• broken fragments of silicate minerals
• broken fragments consist of groups of cations
  enclosed by 4 or 6 oxygen atoms
• one of these small fragments may act as the
  nucleus attracting to it charged bodies suspended
  in the soil solution
• in this way a clay mineral is born

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A clay mineral is born
o-

H
H
Ca
Na
Micelle
H
NH4
H
H
H
o-
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A clay is born

• silicate clays build little by little about the
  nucleus
• most clays are crystalline with sheet structures
  like the micas
• the cations are
• silicon in the tetrahedral position
• aluminium in the octahedral position
• hence they are called aluminosilicates

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The clay minerals

• because of the excess oxygen in the sheets their
  charge is negative
• so they act like large anions
• they attract cations and the positive ends of
  water molecules
• these cations may be easily removed and replaced
  by others
• this is called cation exchange

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Clay structure
silica sheet
silica sheet

alumina sheet
alumina sheet
silica sheet
- O-OH -
fixed distance
variable distance
- O - O -
silica sheet
silica sheet
alumina sheet
alumina sheet
silica sheet
11 lattice clay
21 lattice clay
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Soil Chemistry
The influence of the chemical nature of soil on
its overall appearance and behaviour is
considerable.

• For example
• pH
• Fertility
• Swelling and shrinking
• Water retention

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Major Ideas
Silicon oxygen are the dominant elements in the
earths crust and soil. Si O combine to form
silicate tetrahedra. Tetrahedra share 1 or more
oxygens to form different silicate minerals
families Clays are sheet-type silicates. Micelles
are clay particles with negative charge - they
attract positively charged particles. The
properties of micelles vary with their
composition.
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Major Ideas

• Soil chemistry has significant effect on soil
  properties
• pH
• water holding capacity
• nutrient status
• structure
• physical movement of the soil mass

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