Soil Formation: Concepts, Factors, and Processes

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Soil Formation Concepts, Factors, and Processes
Lecture 1
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Subject Outline

1. What is Soil
2. Why do we study Soils?
3. Composition of Soils
4. Soil Formation- Factors
5. Soil Formation- Processes

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What is soil?

• Concepts of soil differ greatly among users of
  soil
• Clerk/Secretary Soil is dirt it may be in the
  wrong place
• Mining engineer Soil is material for mining ores
• Civil engineer Soil is material that support
  constructions
• Home owner Soil is needed to set up flower
  garden.
• Farmer Soil is medium to grow crops to make a
  living

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Soil as defined by scientists

• natural product formed from weathered rock by
  the action of climate and living organisms
  modified by topography over a period of time.

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Why Study Soils?

• Reason 1
• Soils are crucial to life
• Soil is an essential part, and some would argue,
  the most important component of the terrestrial
  ecosystem
• Soils perform vital functions

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Functions of soils
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Soil as medium for plant growth

• Physical support anchors root system so that
  plant does not fall over.
• Air Plants depend on respiration to obtain
  energy. Soils provide ventilation.
• Water soil through pores absorb water and hold
  it for plants to use.
• Temperature moderation moderates temp
  fluctuations for the roots.
• Nutrient elements soils supply mineral
  nutrients (dissolved ions) to plants .
• Protection soils protect plants from phytotoxic
  substances.

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Soil as regulator of water supplies

• Soil regulates both the quality and quantity of
  water in rivers, lakes, and underground aquifers
• Quantity of water supplies
• Some of the water may be stored in the soil and
  used by trees and other plants
• Quality of water supplies
• Water is purified and cleansed as it soaks
  through the upper layers of soil.

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Soil as recycler of raw materials

• Soils play a role in geochemical cycles
• Assimilate organic waste
• Turn it into beneficial humus
• Convert the mineral nutrients into plant and
  animal usable forms
• Returning carbon to the atmosphere to be used
  for photosynthesis again

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Soil as Habitat for soil organisms

• Handful of soil is home to billions of organisms
  in thousands of species
• How does this happen?
• Micro-environment differences
• Pore spaces
• Moisture
• Temperature
• Organic matter

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Soil as Engineering Medium

• Soil is firm and solid
• Good base to build structures
• Soils differ in stability
• Designs for structures are different for soils
• Physical properties influence engineering uses

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Why Study Soils?

• Reason 2
• Soil is an environmental interface
• Lithosphere -rocks
• Atmosphere -air
• Hydrosphere -water
• Biosphere living organisms

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Soil Profile
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Soils reflect their environments
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Soil Composition

• Soil consists of four major components in three
  major phases
• 1. Soil Air (Gas phase)
• 2. Soil Water (Liquid phase)
• 3a. Soil Mineral Matter
• 3b. Soil Organic Matter
• The relative proportions of these components
  influence the behavior and productivity of soils

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Relative Proportions of Soil Components Determine
Soil Behavior and Productivity
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Inorganic Minerals(Soild Phase)

• Inorganic minerals in soils (major solid
  framework of soil)
• Soil minerals are either primary or secondary
  minerals
• Primary minerals are the minerals that formed in
  the original rocks. They range from large
  particle stones, gravels to small particles
  sand, silt, clay.
• Secondary minerals are formed in soil by
  weathering of the primary minerals (examples are
  Kaolinite, smectites, illites etc)
• The secondary minerals normally are found in the
  clay fraction of the soil which is the fraction
  of the soil solids which is less the 2 micron or
  0.002 mm. Clay minerals are minerals which mainly
  occur in the clay sized fraction of the soil.
• Both inorganic and soil organic matter make up
  the solid fraction of soil.

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Organic Matter(Solid Phase)

• Organic matter which includes living, dead and
  synthesized matter are continuously broken down
  and incorporated.
• OM ultimately decomposes to humus last stage of
  decomposition
• Humus is the product of the decay of organic
  residues such as wood, leaves, and other
  biological materials.
• OM is also constantly lost from soil as CO2 from
  microbial respiration.

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Soil water (or soil solution) Soil Liquid Phase

• Water is vital to the ecological functioning of
  the soil.
• Soil solution contains water, dissolved ions,
  molecules and gases.
• Soil water is different from free flowing water
  in 2 ways
• Soil water is held by many types of forces within
  the pores of the soil.
• Soil water is never pure but contains hundreds of
  dissolved organic and inorganic compounds.

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Soil Air (or Soil Atmosphere)Soil Gas Phase

• contains similar gases as found in the atmosphere
  above the soil
• But often in very different proportions.
• Usually higher in carbon dioxide and lower in
  oxygen than the atmosphere.
• Is highly variable in space
• Has high relative humidity

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Interaction of the Components

• The components interact to determine the nature
  of a soil e.g.,
• soil moisture controls air and nutrient supply
• mineral particles control water movement
• Organic matter controls arrangement of minerals
  which influence pores that determine water and
  air relationships

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Soil Formation

• Five factors of soil formation
• Processes of soil formation

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Five factors of soil formation
s f(cl, o, r, p, t)

• Where
• s any soil property
• cl climate (rainfall temperature)
• o organisms (biota)
• p parent material
• r relief (slope aspect and position)
• t time (relative age of soil formation)

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1. Parent Material

• Soil parent material is the material that soil
  develops from, and may be rock that has
  decomposed in place, or material that has been
  deposited by wind, water, or ice.
• The character and composition of the parent
  material plays an important role in determining
  soil properties, especially during the early
  stages of development. E.g.,
• The texture of sandy soils is determined by
  parent material
• Movement of water is controlled by texture of
  the parent material
• Parent material influences the chemistry of the
  soil
• Parent material influences the type of clay
  minerals present in soil.

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Parent Materials form from The Weathering of
Rocks and Minerals

• What is weathering?
• .. the modification or breakdown and destruction
  of the physical and chemical characteristics of
  rocks and minerals and carrying away the soluble
  products.
• ..the nature of the breakdown (weathering)
  depends on the type of material (soft or hard?)

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Effect of rock type on weathering rates
Slate Rock
Marble Rock
Two stone markers, photographed on the same day
in the same cemetery (Photos courtesy of R.
Weil). The slate rock consists largely of
resistant silicate clay minerals, while the
marble consists mainly of calcite, which is much
more easily attacked by acids in rainwater.
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Types of Rocks

• Igneous rocks
• Form from molten magma
• Granite and diorite
• Sedimentary rocks
• Compacted or cemented weathering products from
  preexisting rocks
• Sandstone and shale
• Metamorphic rocks
• Formed by change in the form of other rocks
• Gneiss, marble, and slate

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Two main types of weathering

• a) Physical (Mechanical) weathering
• .. Causes rocks to disintegrate into smaller
  pieces without affecting their composition
• Types/Factors of Physical weathering
• Exfoliation by temperature
• Abrasion by water, ice and wind
• Disintegration by plants and animals

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• b) Chemical weathering
• .. Degradation of rocks and minerals by the
  chemical activities of water, oxygen, and
  microbial action
• Types/Factors of Chemical weathering
• Hydration
• Hydrolysis
• Dissolution
• Carbonation
• Oxidation-reduction
• Complexation

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• 1. Hydration
• Process of binding of water molecules to a
  mineral
• 2. Hydrolysis
• Splitting of water molecules into its components.
  Split components in turn attack the minerals.
• 3. Dissolution
• Process of hydrating of ions until they become
  dissociated

Hematite water
Ferrihydrite
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• Carbonation
• Carbon dioxide dissolves in water to form
  carbonic acid which accelerates chemical
  breakdown of materials
• Oxidation-Reduction
• Minerals that contain Fe, Mn,or sulfur are
  susceptible to this reaction when exposed to
  environments different from the ones in which
  they formed. This destabilizes the mineral.
• Complexation
• Biological processes produce organic acids that
  can form complexes with elements within the
  structure of a mineral thereby pooling the
  element from the mineral and destabilizing it.

Fe(II) oxide Fe(III)
oxyhydroxide Goethite
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Classification of Parent Materials

• Parent materials could be derived from the
  following
• Organic Deposits residues of plants
• Rock that weathered in place
• Rock that was deposited from elsewhere
• Parent materials are commonly classified by their
  mode of deposition at their current location.

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Types of Parent Material

• Gravity
• colluvium
• Ice transport
• glacial till, moraine, outwash
• Wind transport
• Eolian (dune sand, loess, dust)
• Water transport
• Lakes -lacustrine
• Streams alluvium (floodplain, alluvial fans,
  delta)
• Oceans marine
• Volcanic ash

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2. Climate

• May be the most influential of the four factors
  acting on the parent material
• Determines the nature and intensity of weathering
  (precipitation and temperature)
• Both affect the physical, chemical and biological
  processes
• Climate also exerts influence indirectly through
  a second soil forming factor, the living
  organisms (natural vegetation).
• Climate is so important in soil formation that
  certain evidence of climatic change could be
  found in the soil

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• Precipitation
• Water is essential for all the major chemical
  weathering reactions.
• The deeper water penetrates the parent material,
  the more effective it is in soil weathering and
  development.
• Water percolating through the soil profile
  transports soluble and suspended material from
  the upper to the lower layers.
• Thus percolating water stimulates weathering
  reactions and helps differentiate soil horizons.

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• Temperature
• Every 10 deg C, the rate of chemical reaction
  doubles
• If warm temperatures and abundant water are
  present in soil at the same time, the processes
  of weathering, leaching, and plant growth will be
  maximum and lead to deep soil profiles.
• Compare this to very modest soil profile
  development processes that are obtained in cold
  areas

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3. Organisms (Biota)
Soil organisms, both the animals (fauna) and the
plants (flora) physically churn the soil and help
stabilize the soil structure

• a) Role of natural vegetation
• Organic matter accumulation
• Cation cycling by trees
• E.g., Grassland vs. Forest
• E.g., Heterogeneous rangelands

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Heterogeneous Rangelands (Plants soil
formation)
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b) Role of animals

• Animals such as gophers, moles, prairie dogs bore
  into lower soil horizons and bring materials to
  the surface tunnels.
• Earthworms and termites
• Bring about considerable soil mixing
• Human influence
• destruction of natural vegetation
• Soil tillage for crop production
• Irrigation
• Fertilizer application

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4. Topography

• Relates to the configuration of the land surface
• It is described in terms of differences in
  elevation, slope and landscape position
• Steep slopes encourage soil loss by erosion and
  allow less rainfall to enter the soil
• Thus prevents formation of soils from getting
  ahead of soil destruction
• In the depressions where runoff tends to
  concentrate, the soil is usually more deep

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Role of Topography in Soil Formation
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5. Time

• Time that materials have been subjected to
  weathering is important because soil forming
  processes take time to show their effects.
• Clock of soil formation starts when e.g.
• Landslide exposes a new rock
• Flooding river deposits sediment on floodplain
• Glacier melts and dumps load of debris
• Bulldozer cuts and fills landscape, etc.
• Rates of weathering and soil development
• This is a function of the interaction of time and
  the other factors of soil formation.

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Role of Time in Soil Formation
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Processes of Soil formation
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Processes of Soil formation

• Processes that are involved in soil formation can
  be placed in four main groups
• Additions
• Transformations
• Transfers or Translocations
• Losses

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1. Additions

• Additions entail the inputs of materials to the
  developing soil profile from outside sources.
  E.g.
• Addition of organic matter from Plant leaves and
  sloughed-off roots
• Addition of water by precipitation
• Addition of dust particles that fall on the soil
  surface
• Addition of salts and silica that is dissolved in
  groundwater and deposited near or at soil surface

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2. Transformations

• Transformations entail disintegration and/or
  altering of composition and form of organic and
  inorganic components of soil
• Physical weathering of lager particles to
  smaller particles
• Decomposition of organic residues
• Recombination of decomposition products to form
  new minerals such as silicate clays and oxides,
  organic acids, humus and other products
• Aggregation of mineral particles

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3. Transfers or Translocations

• Translocations involve the movement of organic
  and inorganic materials laterally within a
  horizon or vertically from one horizon to the
  another.
• Movement of water
• Movement of dispersed fine clay particles
• Movement of dissolved organic substances
• The most common translocation agents are water
  and soil organisms.

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4. Losses

• Materials are lost from the soil by the
  following
• Drainage and leaching to groundwater
• Erosion of surface materials
• Evaporation
• Plant uptake
• Microbial decomposition
• Animals and humans

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Combination of Processes
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Combination of The Soil Forming Processes in
Action(Soil Horizon Development)

• A-Horizon development
• Accumulation of organic matter
• Clumping of individual soil particles
• Distinct from parent material and other layers
• B and C horizon development
• Carbonic and organic acids are carried by water
  into soil where dissolve various minerals
  (transformations)
• Soluble materials (ions Ca2, CO32-, SO42-, etc)
  are carried by water and precipitate in the soil
  from upper to lower horizons (translocation)
• Weathering of primary minerals into secondary
  minerals
• Wetting and drying cracks soils and makes
  structures.

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Soil Horizon Development
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Result of the soil forming processes in action