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Soil Formation: Concepts, Factors, and Processes

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Title: Soil Formation: Concepts, Factors, and Processes


1
Soil Formation Concepts, Factors, and Processes
Lecture 1
2
Subject Outline
  1. What is Soil
  2. Why do we study Soils?
  3. Composition of Soils
  4. Soil Formation- Factors
  5. Soil Formation- Processes

3
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

4
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.

5
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

6
Functions of soils
7
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.

8
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.

9
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

10
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

11
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

12
Why Study Soils?
  • Reason 2
  • Soil is an environmental interface
  • Lithosphere -rocks
  • Atmosphere -air
  • Hydrosphere -water
  • Biosphere living organisms

13
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14
Soil Profile
15
Soils reflect their environments
16
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

17
Relative Proportions of Soil Components Determine
Soil Behavior and Productivity
18
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.

19
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.

20
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.

21
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

22
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

23
Soil Formation
  • Five factors of soil formation
  • Processes of soil formation

24
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)

25
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.

26
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?)

27
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.
28
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

29
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

30
  • 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

31
  • 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
32
  • 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
33
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34
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.

35
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36
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

37
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

38
  • 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.

39
  • 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

40
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

41
Heterogeneous Rangelands (Plants soil
formation)
42
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

43
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

44
Role of Topography in Soil Formation
45
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.

46
Role of Time in Soil Formation
47
Processes of Soil formation
48
Processes of Soil formation
  • Processes that are involved in soil formation can
    be placed in four main groups
  • Additions
  • Transformations
  • Transfers or Translocations
  • Losses

49
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

50
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

51
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.

52
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

53
Combination of Processes
54
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.

55
Soil Horizon Development
56
Result of the soil forming processes in action
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