Title: Soil Formation
1Soil Formation
22.1 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
32.1 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
4Rock 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
5Organic 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)
6Parent 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
72.2 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
8Formation processes
- Argillic development
- Calcification
- Chemical weathering
- Desilification
- Leaching and acidification
- Nutrient cycling
- Organic matter accumulation and oxidation
- Translocation
92.2 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
102.3 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!
112.3 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
12Factors 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
13Parent 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
14Organisms
- 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
15Climate
- warm, humid climates promote soil formation
- dry, cool climates inhibit it
- inhibition is observed in peat bogs, where
temperature reduces biological activity
16Topography
- 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
17Time
- 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
182.4 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
19Soil 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
20Exercise 2.1
Indicate the different soil layers shown in
Figure 2.1
21Soil 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
22Soil 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
23A typical soil profile
24Exercise 2.2
Return to Figure 2.1 and locate the soil horizons.
25So 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?
26So how do soil horizons differ?
27So 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!
28Is 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!
29The soil formation process
- Figure 2.3 shows basic soil formation
30The 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!
312.5 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
32The 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
33The 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
34The 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
35The 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
36The 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.