Soil Formation

Chapter 2

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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

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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

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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

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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)

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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

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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

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

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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

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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!

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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

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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

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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

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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

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Climate warm, humid climates promote soil formation

dry, cool climates inhibit it

inhibition is observed in peat bogs, where temperature reduces biological activity

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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

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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

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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

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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

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Exercise 2.1

1st layer

3rd layer

2nd layer

5th layer

4th layer

Indicate the different soil layers shown in Figure 2.1

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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

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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

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A typical soil profile

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Exercise 2.2

O Horizon ?

E Horizon ?

A Horizon ?

C Horizon ?

B Horizon ?

Return to Figure 2.1 and locate the soil horizons.

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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?

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So how do soil horizons differ? an example soil only

Variable O E B C

pH 3.4 4.6 4.9 5.3

% sand 0 84 72 68

% silt 0 15 28 31

% organic 40 1 6 0.5

Cation exchange capacity 133 4 55 8

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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!

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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!

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The soil formation process Figure 2.3 shows basic soil formation

PM

O O

R

B

C

A

E

O O

C

B

C

A

O O

C

PM

C

A

O O

R

C

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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!

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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

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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

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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

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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

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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

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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.