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Soil Formation

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Soil Formation Chapter 2 2.1 Parent Material soil does not simply form from bits of rock and decaying plant matter formation takes thousands of years which is why it ... – PowerPoint PPT presentation

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Title: Soil Formation


1
Soil Formation
  • Chapter 2

2
2.1 Parent Material
  • soil does not simply form from bits of rock and
    decaying plant matter
  • formation takes thousands of years which is why
    it is considered a non-renewable resource

3
2.1 Parent Materials (PM)
  • There are two major components
  • Rocks
  • Organic matter
  • Obviously, the type of rock, and the type of
    plant matter affect the final soil parent
    material greatly

4
Rock based Parent Material
  • two different types
  • residuum
  • made in situ, meaning that no movement is
    involved
  • bedrock breaks down into parent material and
    stays where it is
  • weathered
  • undergo several physical and chemical
    transformation, and end up very different to the
    parent rock
  • involves transportation and can undergo
    significant transposition

5
Organic matter
  • plants die and decompose into a variety of
    materials
  • most significant of these materials is called
    humus
  • can form directly or via material called peat
    (deposition of OM faster than decomposition)

6
Parent Material
  • So we have broken down rock PM
  • Residuum
  • Weathered
  • And we have organic material PM
  • Free humus
  • Peat bogs
  • When these two ingredients are combined, we have
    the basis of something called soil

7
2.2 Soil formation processes
  • The formation of soil is not a batch process
  • The process is continuous and dynamic due to a
    variety of factors
  • movement of new parent materials
  • leaching of soluble compounds
  • physical and chemical changes
  • And many other chemical and physical processes

8
Formation processes
  • Argillic development
  • Calcification
  • Chemical weathering
  • Desilification
  • Leaching and acidification
  • Nutrient cycling
  • Organic matter accumulation and oxidation
  • Translocation

9
2.2 Soil Formation
  • As you can see from the table, it is very complex
  • All we need to know is that soils have formed
    from a variety of parent materials
  • All the variation makes soil sampling and
    analysis very difficult
  • Soil is the hardest of all the natural matrices
    to deal with analytically

10
2.3 Major soil groups
  • Here we run into a problem!
  • Soil is very, very variable
  • Scientists need to classify things into groups in
    as part of our scientific method
  • How do we do this if soil can be anything it
    wants to be?
  • What you say, they bother classifying dirt!

11
2.3 Major soil groups
  • Australia has adopted its own soil classification
    system
  • surprisingly called the Australian Soil
    Classification
  • Exercise 2.1
  • Why develop our own classification system?
  • Australian soils are very old and very different
    to those of other countries

12
Factors affecting the rate of soil formation
  • The five factors that have been identified as
    affecting the rate and type of soil formation
    are
  • parent material
  • organisms
  • climate
  • topography
  • time

13
Parent Material
  • type of material will affect
  • the chemical and physical composition
  • the ability for certain processes to occur
  • could be
  • bedrock
  • organic material
  • an old soil surface
  • a deposit from water, wind, glaciers, volcanoes
  • material moving down a slope

14
Organisms
  • grasslands have thick organic-rich layers on the
    top of the soils because of the extended fine
    root growth,
  • forests, where the roots go much deeper, have
    much less of this type of soil
  • burrowing organisms help by mixing, aerating and
    fertilising soils through scat

15
Climate
  • warm, humid climates promote soil formation
  • dry, cool climates inhibit it
  • inhibition is observed in peat bogs, where
    temperature reduces biological activity

16
Topography
  • will determine the rate at which parent materials
    and top-layer soil is lost or gained
  • location can affect how the climatic processes
    impact it
  • soils at the bottom of a hill will get more water
  • soils on the slopes that face the sun will be
    drier than soils on slopes that do not
  • Mineral accumulations, plant nutrients, type of
    vegetation, vegetation growth, erosion, and water
    drainage are dependent on topographic relief

17
Time
  • the above factors assert themselves over time,
    often hundreds or thousands of years.
  • Soil profiles continually change over time
  • Mature soils are quite different to soils in a
    state of development

18
2.4 Soil Profiles
  • a vertical slice of earth metres deep
  • shows layers of soil some less than an
    centimetre thick, some up to a metre thick
  • a key component in the classification process
  • see ASC table

19
Soil Horizons
  • You should be able to see clearly defined regions
    of soil in the picture
  • These regions are termed horizons
  • Horizons provide information about the horizontal
    differences of various soils
  • The information they provide is of critical
    importance to soil scientists for many reasons

20
Exercise 2.1
Indicate the different soil layers shown in
Figure 2.1
21
Soil horizons
O at the top and usually less than an inch thick
E topsoil the upper soil layer abundant organisms it has more organic matter and is darker than the subsoil
A does not form in all soils a high concentration of medium-size particles such as sand and silt
B subsoil the middle soil layer it has fewer organisms and less topsoil
C the lowest layer it is less altered and weathered than the layers above and has less living matter it is made up of primarily parent material
22
Soil Horizons
  • Table 2.3 is a very simplistic model (which is
    all we need)
  • There are two other common horizon codes used
    worldwide
  • H, or Human soils (as in agriculture)
  • R, which basically stands for rock (regolith)
  • The overall code is therefore H,O,A,E,B,C R

23
A typical soil profile
24
Exercise 2.2
Return to Figure 2.1 and locate the soil horizons.
25
So how do soil horizons differ?
  • There is an obvious visible difference between
    the horizons, which leads to two obvious
    questions
  • What are the differences?
  • What causes the difference?

26
So how do soil horizons differ?
  • an example soil only

27
So how do soil horizons differ?
  • As you can see, there is significant difference
    between the horizons.
  • This obviously changes from soil to soil, and in
    some cases, season to season
  • It shows how soils vary vertically with depth, an
    indication of the complexity of the sample!

28
Is this always the case?
  • NO!
  • Soils can exhibit less or more horizons than the
    examples used here due to all of the factors that
    form soil in the first place
  • Furthermore, mot all horizons will exhibit
    significant variation from one to another!

29
The soil formation process
  • Figure 2.3 shows basic soil formation

30
The soil formation process
  • Figure 2.3 attempts to show the chronology of
    soil formation
  • It is difficult to imagine the how this process
    actually happens because the process is not
    flowing, and it is more like morphing
  • The following animation might help explain!

31
2.5 Soil Horizons in detail
  • We want to know some of the comparable attributes
    associated with each horizon
  • Age
  • Depth
  • Chemical composition
  • Physical composition
  • Your teacher will provide you with a handout to
    complete for exam study

32
The O horizon
  • O stands for Organic, as it is rich in CH2O
    from the decaying plant material
  • The O horizon is the youngest, and is always
    associated with the top level of soil (outside of
    H horizon)
  • It is usually thin, but there are many competing
    conditions that control depth
  • Usually acidic (due to organic acids) is low in
    sand silt and clay

33
The A horizon
  • Depending upon how the soil formed, can be the
    oldest part of the soil
  • Depth ranges from 0-150 cm, but again this is
    variable
  • Generally very leached (even Silicon!)
  • Exhibits a mixture of organic plus small and
    large mineral particles

34
The E horizon
  • Is formed from leaching out of the A horizon (it
    is embedded in the A horizon) in a process termed
    eluviation
  • Generally newer than most horizons as the
    leaching has to come from O E
  • Proportionally in depth with the A horizon
  • The E horizon has cations leached from the A
    horizon and is typically grey significant levels
    of large particles

35
The B horizon
  • The B horizon is a zone of illuviation where
    downward moving, especially fine material, is
    accumulated
  • Age is variable due to formation processes
  • Depth is variable due to other horizons
  • Chemistry is controlled by moisture, with clays
    being dominant creating high CEC
  • Densely packed fine particles

36
The C horizon
  • The C horizon represents the soil parent
    material, either created in situ or transported
    into its present location.
  • Beneath the C horizon lies bedrock.
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