Diploma of Environmental Monitoring & Technology
Study module 1
Soil formation
Sampling & testing of
soils
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43
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STS Study module 1 – Soil formation
WHY STUDY SOIL SCIENCE?
2
THE COMPONENTS OF SOIL
2
Where do these components come from?
Rock types
3
3
THE PHYSICAL COMPONENTS OF SOILS
5
Parent Materials
Rocks
Organic matter
5
5
6
SOIL FORMATION PROCESSES
6
MAJOR SOIL GROUPS
7
Rates of soil formation
Soil Profiles
Soil Horizons
8
8
12
ASSESSMENT TASK
15
Assessment & submission rules
Problems?
References & resources
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Why Study Soil Science?
You aren’t being trained to be farmers or civil engineers, so why is this module here? Well,
first let’s look at what soil is and what it actually does?
Soil has many functions, but the three basic ones, as defined by the NSW Department of
Natural Resources, are to:
◗ support plants, make it possible for plants to grow
◗ regulate and partition water flow through the environment
◗ buffer environmental change
Soil also:
◗ provides a physical matrix, chemical environment and biological setting for water,
nutrient, air and heat exchange for living organisms
◗ controls the distribution of rainfall or irrigation water to runoff, infiltration, storage
or deep drainage.
◗ regulate water flow and affect the movement of soluble materials such as nutrients
or pesticides
◗ regulates biological activity and molecular exchanges among solid, liquid and
gaseous phases
◗ affects nutrient cycling, plant growth and decomposition of organic materials
◗ acts as a filter to protect the quality of water, air and other resources
◗ provides mechanical support for living organisms and their structures
So, based on these soil functions, it should be clearer why students of environmental
monitoring need to know something about soil and its physics and chemistry.
The Components of Soil
Soil is an extremely complex and variable material, but all soils worth the name have four
non-living components:
◗ inorganic minerals – clays, silicates, cations and anions
◗ organic materials – decomposing and decomposed organic matter
◗ water – chemically bound to various compounds, physically bound by adsorption or
as free-to-move moisture
◗ air (or at least gas) – provides channels for movement of water and organisms
There is no such thing as a typical soil, but a good, rich soil ideal for growing crops may have
less than 10% organic matter. Figure 1.1 shows a very rough indication of the typical levels
of each component.
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Figure 1.1 - Proportions of the main soil components (by volume)
A simple chemical analysis of the composition of soil is of almost no value because the
chemical form of each element is critical in determining how it will affect the properties of
the soil.
Where do these components come from?
The answer to the source of air and water in soil is rather obvious of course, but the organic
and inorganic materials are the product of breakdown of other materials: vegetation, animal
droppings and dead organisms in the case of organic matter, and rocks and fertilisers
(natural and man-added) in the case of inorganic minerals.
Rock types
A brief bit of geology is called for. The layer of material that we call soil sits on top of the
rocks that form the bulk of the earth’s outer skin – the crust. This rock that underlies the soil
is known as bedrock, and has come originally from the interior of the earth as molten
material.
Over millions of years, this cooled material breaks down through the action of weather, or is
modified by the effects of heat and pressure, into other rock types, which are physically and
chemically different to the original material.
Rocks are made of minerals, which are complex crystalline chemical compounds, based
around the silicate structure, which is a silicon surrounded by four oxygen atoms. This
structure may simply have a cation, such as Mg2+ or Fe2+, to balance the charge, or it may be
bonded to other silicate groupings in large three-dimensional structures. These will normally
have cations present, though quartz does not (it is pure silicon dioxide - silica). An example
of a more complex mineral is feldspar, with a formula of (Na,K)2O.Al2O3.6SiO2.
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The rocks that form from cooling of molten magma or lava – the former doesn’t reach the
surface – are called igneous. They are crystalline, hard and difficult to break into small
pieces. The process of crystallisation – if slow enough – will produce a number of different
igneous rock types, as different minerals form.
The first rocks to form are relatively low in silica (< 55%), and include basalt and dolerite.
These are known as ultrabasic rocks, because of their relatively high content of basic
cations, such as magnesium, iron and calcium. The silica content increases as crystallisation
proceeds. Figure 1.2 shows the crystallisation process and Table 1.1 the properties and the
common rock types in each class.
Figure 1.2 - The crystallisation process
Ultrabasic
Basic
Intermediate
Acidic
%SiO2
< 45
45-55
55-65
> 65
Colour
dark
dark
variable
light
basalt, peridotite
basalt, dolerite,
gabbro
andesite, diorite
granite
Common
rocks
Table 1.1 - Properties of different igneous rock types
The class names are important for these types of rocks, because soils which are derived
from them tend to have that type of acidity. Granite-derived soils will have a lower pH than
those derived from basalt. However, don’t think that the minerals that form these rocks, on
breakdown to become the inorganic content of soils remain unchanged. As we look more at
soil chemistry, you will see new minerals form through chemical reactions occurring in the
soil. Nevertheless, the primary rocks and minerals play a critical role in determining the
type of soils that form.
Igneous rocks break down over time, physically and chemically, a process known as
weathering. This leads to sedimentary rocks, where small rock particles settle in layers and
bind together to form a new rock material, eg sandstone, shale, limestone, dolomite. Again,
these rocks have widely different acidity, with limestone and dolomite being very basic,
while sandstone and shales are acidic.
Sedimentary rocks vary in colour, depending on the minerals that they are composed of, but
rather than being crystalline in the way that igneous rocks are, they are made up of small
(and not so small) particles, which in many cases, can be broken away.
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The Physical Components of Soils
Chemists divide soils up as you have already seen into inorganic and organic components
(plus air and water), but there is an equally important classification on the basis of physical
properties (particularly particle size). These help to define important characteristics of the
soil, especially texture, and are based on the particle size of the materials (see Figure 1.3).
Figure 1.3 - Physical classification of soils
Parent Materials
Soil formation is an exceptionally slow process, requiring hundreds or thousands of years for
rocks (particularly) and organic matter to decay into the final material we know as soil. It is
not simply a matter of bits of rock breaking off the bedrock and mixing with decayed leaves
and worm poo.
The basic building materials for soils – rocks and organic matter – go through an
intermediate stage known as the parent material (PM) before further weathering into soil.
Parent material refers to unconsolidated organic and mineral materials in which soils form.
Rocks
Most of the mineral matter in which soils form is derived in one way or another from hard
rocks. About three-quarters of the land area of the world is underlain by sedimentary rocks
and one-quarter by igneous and metamorphic rocks. Glaciers may grind the rock into
fragments and earthy material and deposit the mixture of particles as glacial till. On the
other hand, rock may be weathered with great chemical and physical changes but not
moved from its place of origin, this altered material is called residuum from rock.
The nature of the original rock affects the kinds of material produced by weathering. The
rock may have undergone various changes, including changes in volume and loss of
minerals. The point where rock weathering ends and soil formation begins is not always
clear. The processes may be consecutive and even overlapping. Quite different soils may
form from similar or even identical rocks under different weathering conditions.
Parent material may not necessarily be residuum from the bedrock that is directly below,
and the material that developed into the soil may be unrelated to the underlying bedrock.
Movement of soil material downslope is an important process and can be appreciable even
on gentle slopes, especially on very old landscapes. Other transport processes are glacier,
wind and water.
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Seldom is there certainty that a highly weathered material weathered in place. The term
"residuum" is used when the properties of the soil indicate that it has been derived from
rock like that which underlies it and when evidence is lacking that it has been modified by
movement.
Organic matter
Organic material accumulates in wet places where it is deposited more rapidly than it
decomposes. These deposits are called peat. This peat in turn may become parent material
for soils. The principal general kinds of peat, according to origin are:
◗ sedimentary peat – the remains mostly of floating aquatic plants, such as algae, and
the remains and faecal material of aquatic animals,
◗ moss peat – the remains of mosses, including sphagnum.
◗ herbaceous peat – the remains of sedges, reeds, cattails, and other herbaceous
plants.
◗ woody peat – the remains of trees, shrubs, and other woody plants.
Many deposits of organic material are mixtures of peat. Some organic soils formed in
alternating layers of different kinds of peat. In places peat is mixed with deposits of mineral
alluvium and/or volcanic ash. Some organic soils contain layers that are largely or entirely
mineral material.
In describing organic soils, the material is called peat (fibric) if virtually all of the organic
remains are sufficiently fresh and intact to permit identification of plant forms. It is called
muck (sapric) if virtually all of the material has undergone sufficient decomposition to limit
recognition of the plant parts. It is called mucky peat (hemic) if a significant part of the
material can be recognised and a significant part cannot.
The peat then further degrades to material relatively resistant to further decomposition
which is known as humus, an dark-coloured material which is an important component of
soil.
From these materials comes soil, through a sequence of further physical and chemical
changes. One of the more important changes is compaction, where the materials become
physically bonded enough to provide a stable base for vegetation. It takes 100 to 600 years
or more, to form an inch of topsoil.
Soil Formation Processes
The formation of soil is not a “batch” process: it is not a matter of a pile of ground–up rock
and peat turning into a cubic metre of soil. Movement of new parent materials and soil
vertically and horizontally, leaching of solubles, other chemical changes and other process
all contribute to a process which is continuous and dynamic. The soil in a particular location
is not fixed in composition, but continually changing. New parent materials are being added
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from above and below all the time. Chemicals are being added or subtracted by man’s
activities. Table 1.2 summarises some of the natural soil processes.
Name
Definition
Argillic development
the accumulation of fine-grained material transported into the
lower levels of the soil through translocation
Calcification
the accumulation of calcium carbonate in soils through
evaporation and degassing
Chemical weathering
the chemical breakdown of unstable minerals present in the
parent material (see Exercise 2.1 below)
Desilicification
extreme leaching, in which even the relatively insoluble
element Si is removed from the upper levels of soils
Leaching and acidification
the removal of soluble ions from the upper levels of soils;
commonly associated with a decrease in soil pH
Nutrient cycling
exchange of nutrient elements, such as nitrogen, phosphorus,
and potassium, between living biomass, dead organic matter,
and inorganic materials in soils
Organic matter
accumulation and oxidation
the addition and breakdown of organic matter in soils
Translocation
the removal of fine-grained material from the upper levels of
soils through the physical movement of particles
Table 1.2 - Soil formation processes
Different soil processes operate at different depths in soils. The relative importance of the
various soil processes in a given soil is what gives a soil its unique character. Soils with
similar characteristics--those in which the same processes are dominant--are grouped
together in soil classification schemes.
Major Soil Groups
The relative importance of these processes vary in soils developed under different
conditions of parent material, climate, organisms, topography, and time, leading to widely
varying soils, which have been classified in a number of groups, as outlined in Table 2.2,
which follow Australian Soil Classification, which is now the standard for Australian soils.
Type
Description
Anthroposols soils created by human activity
Organosols
soils not regularly inundated by marine waters and containing a specific
thickness of organic materials within the upper part of the profile
Podosols
other soils containing a Bs, Bhs, or Bh horizon according to the definition in
the Australian Soil and Land Survey Field Handbook (the 'yellow book')
Vertosols
other soils that both contain more than 35% clay throughout the solum and
possess deep cracks wider than 5mm during most years and contain
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slickensides or lenticular peds at some depth within the solum
Hydrosols
other soils that are largely saturated for at least several months in most years
Kurosols
other soils with a clear or abrupt textural B horizon and in which the upper
part of the B horizon is strongly acid
Sodosols
other soils similar to kurosols but with the upper part of the B horizon being
sodic rather than acidic
Chromosols
other soils similar to kurosols but without the strongly acidic layer
Calcarosols
other soils that contain carbonate accumulations throughout the solum, that
must have formed in situ
Ferrosols
other soils with an iron oxide content of greater than 5%
Dermosols
other soils with B2 horizons more developed than weak
Kandosols
other soils with well-developed B2 horizons with 'massive' or 'weak' structure
as defined in the 'yellow book'
Rudosols
other soils with rudimentary pedological development (little or no B horizon,
minimal A horizon development, little colour or texture change with depth)
Tenosols
all other soils
Table 1.3 - Soil groups
Classifying soils is a highly complex area, requiring specialist training, so don’t get too
bothered about trying to remember all these strange sol names.
Rates of soil formation
The five factors that have been identified as affecting the rate and type of soil formation
are:
◗ parent material – the type of material obviously will affect the chemical and physical
composition and the ability for certain processes to occur
◗ organisms – grasslands have thick organic-rich layers on the top of the soils because
of the extended fine root growth, whereas forests, where the roots go much
deeper, have much less of this type of soil; burrowing organisms help by mixing,
aerating and fertilising
◗ climate – warm, humid climates promote soil formation; dry, cool climates inhibit it
◗ topography – the general nature of the surface (flat, hilly, valley, river etc) will
determine the rate at which parent materials and top-layer soil is lost or gained
◗ time – mature soils are quite different to soils in a state of development
Soil Profiles
A soil profile is a vertical slice of earth metres deep. It shows layers of soil – some less than
an centimetre thick, some up to a metre thick. Figure 2.1 is a photograph of a typical soil
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profile. By studying soil profiles, scientists learn about the soil, its characteristics, and how
to use and protect it.
Figure 1.4 - A typical soil profile
Sand, silt, and clay are the various sized particles that make up soils. The texture of a soil is
determined by the relative amounts of these particles in the soil. For example, a sandy clay
soil may contain about 50 percent clay, 45 percent sand, and 5 percent silt. Loam soils
contain about equal amounts of all three. You will learn more about the types of soils and
their texture in a later chapter and in the laboratory.
Most soil profiles have a surface layer of organic material and two or three layers of mineral
materials. These are the horizons of a typical soil profile. There are five basic soil horizons.
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Figure 1.5 - Soil horizons
Horizon
Description
O
at the top and usually less than an inch thick; the dead plant and animal materials
decompose into nutrients that enrich the soils.
A
topsoil – the upper soil layer; plant roots, bacteria, fungi, and small animals are
abundant; plants thrive in it; it has more organic matter and is darker than the
subsoil
E
does not form in all soils; generally where the A horizon is small; it is grey in
colour, through a high concentration of medium-size particles such as sand and silt
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B
subsoil – the middle soil layer; it has fewer organisms and less topsoil; plants
don’t grow as well in it; if subsoils are clayey, they usually are harder when dry
and stickier when wet than the surrounding soil layers
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
Table 1.4 - Soil horizons. Note that there are two other horizons often used: H (for human soils
in agriculture) and R (for regolith or rock); they come before and after the listed horizons in
the table above.
It is the formation processes operating within soils that produce these distinct layers
(horizons). Different soils may show different amounts of horizon development: in some
cases horizons are visibly distinct, whereas in others horizon development and boundaries
between horizons may only be detectable by chemical and physical analysis.
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
Table 1.5 - Compositional differences between horizons (example from Swedish soil profile
data; no A horizon).
The formation of a soil profile through the various processes is summarised in Figure 1.3,
showing the gradual development of different horizons.
Figure 1.6 - Development of soil horizons. For a good animation of soil formation, see:
http://courses.soil.ncsu.edu/resources/soil_classification_genesis/soil_formation/soil_transform.
swf
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Soil Horizons
O Horizon
The O horizon is the uppermost level of any soil. Not always well-developed, the O horizon
is composed primarily of organic material, and lies above the portion of the soil profile that
is composed dominantly of weathered mineral matter. Although organic matter can be
found throughout the upper parts of many soils, it composes the bulk of the soil in the O
horizon. The organic matter in the O horizon is derived from vegetal matter that falls on the
soil surface (litter), as well as a much smaller amount of animal debris.
The O horizon, if well-developed, may contain sub-horizons that are recognisable on the
basis of the level of the decomposition of the organic matter contained. Organic matter
decomposition is brought about by the process of organic oxidation, which is mediated by
soil microbes. Three sub-horizons are commonly named.
The uppermost sub-horizon, the Oi, or fibric horizon, contains organic material that is only
slightly decomposed. Beneath this is the Oe, or hemic horizon, that contains organic matter
in an intermediate state of decomposition. The lowest recognised sub-horizon is the Oa or
sapric horizon, which contains highly decomposed organic matter.
It is in the O horizon that much of the nutrient cycling within soils occurs. Here nutrient
elements that were stored within living tissues are released back into inorganic forms,
where they are available for plant uptake. Although some nutrients are provided from the
mineral matter present in deeper soil horizons, the most active turnover of nutrient
elements commonly occurs in the O horizon.
O horizons are best developed where the rate of organic production is relatively high and
the rate of decomposition is low. They are best developed under forests, and are rare in
grasslands. The preservation of organic matter is aided by acidic or anoxic conditions, and is
hindered by humid oxidising conditions. Accordingly, soils with well-developed O horizons
are rare in the tropics, and are more common in boreal climates, and in swampy
environments where soil waters are acidic or anoxic.
A Horizon
The A horizon, which can range from 10 to 150 cm in thickness, normally lies directly
beneath the O horizon, if an O horizon is present. It is the uppermost soil horizon that is
dominated by weathered mineral matter (consisting of both secondary weathering products
and residuum) but usually contains sufficient organic matter to impart a darker colour than
that of lower horizons. Because the top of the A horizon represents the oldest part of the
soil profile, it usually contains the most weathered material in the soil. Material in the A
horizon has, at an earlier time in its history, spent time in both the C and B horizons.
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The A horizon is a zone of leaching (which is formally known as elluviation) leaching results
from rain water moving downward through pores between soil particles, removing soluble
ions and carrying them to lower levels in the soil.
The zone of maximum leaching may give rise to a distinct elluvial (or "E") horizon, which
may be located within the A horizon, at the base of the A horizon, or in place of an A
horizon. Extreme leaching removes all soluble cations, including silica, from the soil through
a process known as desilicification. Particulate matter can also move vertically through the
soil column. Clays and organic matter can be physically washed to lower positions in the soil,
a process known as translocation.
B Horizon
The B horizon, which can range from 10 cm to several meters in thickness, is the most
complex and the most highly variable of the soil horizons. Present in nearly all soils, it is a
zone of accumulation, or illuviation.
Here, salts and clays that have been washed downwards from the A horizon accumulate. It
is also in the B horizon that material from the underlying unweathered parent material and
saprolite enters the active upper soil layers (solum).
Secondary weathering products in the B horizon are generally younger than those in the A
horizon, and older than those in the C horizon. Material in the B horizon has, at an earlier
time in its history, spent time in the C horizon and will, under normal soil development, later
be transformed into the A horizon.
A dominant process in many soils is the development of argillic horizons. This process is the
logical outgrowth of translocation, and results from the accumulation of clays in the B
horizon. When extensive, distinct sub-horizons, known as argillic horizons, may be evident.
Argillic horizons are especially clay-rich portions of the soil profile. Clays usually line pore
spaces between soil grains, or form bridges between grains. Such clay coatings, termed
cutans, are one of the primary means by which soils attain structure, which can be defined
as the nature of internal coherent aggregates of soil. These internal coherent aggregates,
known as peds, are often described as blocky, prismatic or columnar.
Another process that may be important in the B horizons of some soils is calcification.
Calcification is the precipitation of calcium carbonate, usually calcite, in the pore space of
soils. Enhanced by evaporation, calcification is most important in arid settings. When
calcification is extensive, distinctive calcic sub-horizons may be formed. Such sub-horizons
are rich in nodular growths of calcium carbonate known as caliche. Caliche nodules may
coalesce to form well-cemented, nearly impermeable horizons.
In addition, sub-surface impermeable layers, called pans, can develop when minerals
precipitate in the pore spaces of soils and cement the soil into a hard, coherent mass. SubChemical, Forensic, Food & Environmental Technology [cffet.net]
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horizons within the B horizon are very useful for evaluating the history of soils. As zones of
accumulation, B horizons tend to preserve the record of past soil conditions better than
other horizons.
B Horizon development goes hand-in-hand with the development of A or E horizons.
C Horizon
The C horizon is the unconsolidated material underlying the solum layers (A and B horizons).
It consists of residual material that has been subjected to chemical weathering, but has yet
to form soil structures. In a soil derived from bedrock.
The C horizon represents an intermediate stage between bedrock and solum. In soils
derived from alluvium, wind-transported debris, or other sediments, the C horizon
represents less-weathered material without clear horizon development. In either case, the C
horizon is generally little affected by the biological processes in the upper soil horizons, is
not involved in nutrient cycling, and contains essentially no organic matter. It is the
youngest part of the soil profile, and will in time become part of the B horizon as weathering
and erosion continue.
Processes operating in the C horizon are similar to those in the B horizon, but to a much
lesser extent. Material in the C horizon generally retains the structure and character of the
unweathered parent material. The transition from C horizon to unweathered parent
material is usually gradational.
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Assessment task
This section provides formative assessment of the theory. Answer all questions by typing
the answer in the boxes provided. Speak to your teacher if you are having technical
problems with this document.
◗ Type brief answers to each of the questions posed below.
◗ All answers should come from the theory found in this document only unless the
question specifies other.
◗ Marks shown next to the question should act as a guide as to the relative length or
complexity of your answer.
1. What is meant by the term soil? [1mk]
Click here to enter text.
Assessor feedback
2. Why do you, as environmental monitoring “specialists”, need to know something about
soil science? [2mk]
Click here to enter text.
Assessor feedback
3. Use the webpage provided below to answer the following questions about weathering
[10mk]
http://www.physicalgeography.net/fundamentals/10r.html
a. What is ‘weathering’?
Click here to enter text.
Assessor feedback
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b. List the three mechanisms for weathering.
Click here to enter text.
c. List the three products of weathering that can affect rocks and minerals.
Click here to enter text.
d. Briefly describe what chemical weathering is.
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e. List the three products of weathering that can affect rocks and minerals.
Click here to enter text.
f. Briefly describe what physical weathering is.
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g. What are the six chemical reaction types involved in chemical weathering?
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h. What are the five physical weathering processes?
Click here to enter text.
i.
What is biological weathering?
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Click here to enter text.
j.
Provide three brief examples of common biological weathering events.
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Assessor feedback
4. Give three ways that soils can interact with pollutants. [3mk]
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5. Why is the exact elemental composition of soil not very important? [1mk]
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6. What is the difference between an igneous rock and a sedimentary rock? [2mk]
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7. What is the difference in composition between an acidic rock and a basic rock? [2mk]
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Assessor feedback
8. Give an example of the four types of rock (igneous/sedimentary, acidic/basic). [4mk]
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9. How do clay, silt and salt differ? [3mk]
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10. What is the difference between parent material and soil? [2mk]
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11. What is the difference between peat and humus? [2mk]
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12. List the five factors contributing to the rate of soil formation. 1[mk]
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Assessor feedback
13. Give one important difference between the following pairs of horizons [6 mk]
a. C and A;
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b. A and B;
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c. B and C.
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14. Do all soils have all horizons? [1mk]
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15. In which soil horizon do you find; [3mk]
a. the oldest, most weathered materials
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STS Study module 1 – Soil formation
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b. the greatest amount of leaching
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c. the development of argillic horizons
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STS Study module 1 – Soil formation
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References & resources
The Physical Geography website given in Question 3 is a marvellous resource for all facets of
the environment.
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