Natural Resources and Water
Managing Queensland's natural resources...for today and tomorrow
land series
Understanding soil
Soil is the thin outer layer covering the land surface
of the earth. It is made up mainly of mineral
particles, organic materials, air, water and living
organisms, all of which slowly yet constantly interact.
A Horizon
Most plants get their nutrients and water from the
soil. Plants in turn are the main source of food for
animals and birds. Most living things on land
therefore depend on soil for their very existence.
The surface soil where
nutrient, organic matter and
biological activity levels are
highest
B Horizon
However, soil is more fragile than it appears and is
easily damaged, washed or blown away. If we
understand soil and manage it properly, we will
avoid destroying the basis of our environment.
Generally has a lighter
colour, lower fertility and
less biological activity than
A horizon. Texture may be
heavier than the A horizon
How soil forms
C Horizon
Soil forms continuously from the gradual breaking
up of rocks through physical, chemical and
biological processes known as weathering. The
accumulation of material through the action of water,
wind and gravity also contributes to soil formation.
These processes can be very slow, taking many
thousands of years. This makes soil a scarce
commodity—one that should be used with care.
Five main interacting factors affect the formation of
soil—parent material, living organisms, climate,
topography and time. Interactions between these
factors cause an infinite variety of soils across the
earth’s land surface.
Parent materials
Soil minerals form the basis of soil. They are
produced from rocks (parent material) through the
processes of weathering and natural erosion. Water,
wind, temperature change, gravity, chemical
interaction, living organisms and pressure
differences all help break down rocks.
The types of parent materials and the conditions
under which they break down will influence the
properties of the soil formed eg. soils formed from
granite are often sandy and infertile. On the other
hand, basalt under moist conditions breaks down to
form fertile, clay soils.
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March 2006
Queensland the Smart State
Weathering rock material
from which soil forms
Parent material
A typical soil profile
Organisms
Soil formation is influenced by organisms such as
plants, micro-organisms such as bacteria or fungi,
burrowing insects, animals and humans. As soil
forms, plants begin to grow in it; they mature, die
and regrow. Their leaves and roots are added to the
soil. Animals eat plants; their wastes and eventually
their bodies are added to the soil. This begins to
change the soil. Bacteria, fungi, worms and other
burrowers break down plant litter and animal wastes
and remains, to eventually become organic matter.
Climate
Climate (rainfall, temperature and wind) influences
the rate of weathering and also affects plant growth.
Temperature affects the rate of weathering and
organic decomposition. With a colder and drier
climate, these processes can be slow, but with heat
and moisture they are relatively rapid.
Produced by: Natural Resource Sciences
Authors: Bruce Carey and Ben Harms
QNRM05352
Natural Resources and Water
Rainfall dissolves some of the soil materials and
holds others in suspension. The water carries these
materials down through the soil. This is known as
leaching. Over time this process can change the
soil, making it less fertile.
Topography
The shape, length and grade of slope affects
drainage. Aspect determines the type of vegetation
on a slope and the amount of rainfall received.
These factors cause variation in soil formation.
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when it rains. Plants rely on this stored water till it
rains again. The deeper the A and B horizons of he
soil, the more water it can store and the better plants
will grow.
Texture
Soil texture (eg. loam, sandy loam or clay) refers to
the proportion of sand, silt and clay sized particles
that make up the mineral fraction of the soil. ‘Light’
soil refers to a soil high in sand relative to clay.
‘Heavy’ soils are made up largely of clay.
Natural erosion
Texture is important because it influences the
amount of water that the soil can hold, the rate of
water movement through the soil as well as its
workability and fertility. For example, sand is well
aerated, but does not hold water and is low in
nutrients. Clay soils generally hold more water, and
are better at supplying nutrients.
Soil materials are continuously moved within the
natural landscape by the action of water, gravity and
wind eg. heavy rains erode soils from the hills and
deposit it in lower areas, forming deep soils. The
soils left on steep hills are usually shallower.
To determine the texture of a sample of soil, work
the soil between your fingers until it breaks down
into individual particles. Then add water to the soil
and continue working it in your hands until all the
aggregates in the soil are no longer present.
Transported soils include alluvial (water
transported), colluvial (gravity transported) and
aeolian (wind transported).
Evaluate how the soil feels in your hands. Attempt to
roll it into a ball and then into a rod with a diameter
of 7 to 10mm. Can the rod be shaped into a ring?
Table 1 helps you interpret these tests to determine
the texture of a soil.
Time
The length of time that soil materials have been
weathered, influences soil properties. Minerals
weathered from rocks are furthered weathered to
form materials such as clays and oxides.
The properties of soils
Soils vary in their suitability for specific purposes eg.
a deep, fertile clay soil is suitable for intensive
agriculture, but a shallow, sandy soil is most suited
to grazing and growing trees. The suitability of a soil
for a particular purpose can be determined by
looking at some of the easily recognisable features
and carrying out simple tests. The soil properties to
recognise are soil depth, texture, structure, colour,
pH level and nutrient status. These properties may
change with soil depth.
The soil profile
An important feature of a soil is that it changes with
depth. To properly analyse a soil, it should be
examined from the surface to the parent material. To
do this you may need to dig a pit, use a soil auger,
or inspect a road cutting or an eroded gully that may
have exposed the underlying soil.
As soil develops over time, layers or horizons form.
Collectively, the soil horizons constitute a soil profile.
Most soils exhibit three horizons:
• the dark topsoil or A horizon
• the more clayey subsoil or B horizon
• the underlying weathering rock material or
C horizon.
The A horizon is the most fertile part of the soil
profile. Soil acts like a sponge that soaks up water
TABLE 1. PROPERTIES USED TO DETERMINE
SOIL TEXTURE
Texture
classification
Feel test
Sand
Not coherent,
has gritty feel
Coheres and is
friable
Has gritty feel
Friable and
coherent
Has gritty feel
Sand grains
cannot be felt
Sandy loam
Loam
Sandy clay loam
Clay loam
Light clay
Medium clay
Heavy clay
Easy to mould
Fairly stiff and
plastic to mould
Very stiff and
plastic to mould
Rod and ring
test
Will not roll into rods
and breaks easily
Rolls into a rod, but
cannot be turned into
a ring without cracking
Rolls into a rod easily.
Can be turned into a
ring without a lot of
cracking
Texture often changes with depth so that roots have
to cope with different conditions as they penetrate
the soil. A soil can be classified according to the
manner in which the texture changes with depth.
The three profile types are uniform, texture-contrast
and gradational.
Uniform soils have the same texture throughout the
profile. Soils formed from basalt on the Darling
Downs have a clay texture throughout their profile
while soils on sandhills in western Queensland have
a sandy texture throughout the profile.
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Texture-contrast soils have an abrupt texture
change between the surface soil (A horizon) and the
subsoil (B horizon) below. They are sometimes
called ‘duplex’ soils. A good example of a texture
contrast soil is a Sodosol. Typically, a Sodosol has
up to 200 mm of sandy loam overlying a dense,
sodic and infertile, clay subsoil.
In gradational soils, the texture changes gradually
from lighter to heavier down the profile, eg. in a
Kandosol. Typically, a Kandosol has a sandy loam
surface gradually changing to a sandy clay loam and
then to a medium clay.
Structure
Soil structure refers to the way soil particles group
together to form aggregates (or peds). These
aggregates vary in size and shape from small
crumbs through to large blocks.
Very sandy soils are structureless because sand
grains do not cling together. Some soils resemble a
large solid, featureless mass. They are said to be
‘massive’ and have little or no structure. ‘Good’ soils
fit in between the two extremes. A well-structured
soil breaks up easily into aggregates or peds with a
definite shape (for example, granular or blocky), and
size (1 to 60 mm). Organic matter helps give a soil
good structure by binding soil particles together.
Good structure is important, as it allows water to
soak into the soil and excess water to drain away. It
also allows air movement through the soil. Both air
and water are vital for healthy plant growth and
continued nutrient supply.
Colour
Soil colour is strongly influenced by humic (organic)
materials which are brown or black, iron oxides (red
or yellow ) and features of the parent material (eg.
silicates present as sand). Poorly drained soils may
contain blue, grey and green colours.
Soil pH
Soil pH is the measure of the acidity or alkalinity of
the soil. It affects plant growth, as it determines the
availability of nutrients in the soil. Soil pH is
measured on a scale from 0 to 14 with 7 being
neutral. A highly acid soil can have a pH as low as 3,
while a highly alkaline soil can be close to pH l0.
Most soils have a pH of between 6 to 8. Plant growth
is usually best in a slightly acid soil (pH 6-7). A field
kit to measure pH can be purchased from shops
specialising in gardening supplies.
Nutrients
For plants to be healthy, they need a steady supply
of nutrients from the soil. Nitrogen (N), phosphorus
(P), potassium (K), sulphur (S), calcium (Ca) and
magnesium (Mg), are required in relatively large
quantities (macronutrients). Others are required in
small quantities (micronutrients or trace elements),
eg. copper (Cu), zinc (Zn) and manganese (Mn).
A shortage or absence of any one of these essential
nutrients can severely retard plant growth. Too many
nutrients can be as bad as too few. The availability
of nutrients is affected by the pH of the soil eg.in
very acid soils, manganese and aluminium may be
present in toxic concentrations. The nutrient status
of a soil can be determined by a laboratory analysis
of the soil or the plants that grow in it.
Other soil features
Texture, structure, colour, pH and nutrient status are
the properties most commonly used to compare
soils. Some other important soil properties are:
Dispersibility
This is characteristic of some soils that have a high
concentration of sodium or magnesium in the clay
fraction. A ‘sodic’ soil has a sodium ion
concentration of > 6%. When these soils come into
contact with water, they may become unstable and
‘disperse’. Dispersion in the surface soil leads to
crusting and surface sealing, dispersion in the
subsoil accelerates erosion and may lead to the
formation of gullies and ‘tunnels’.
Organic matter and soil carbon
sequestration
Soil organic matter is the component of soil derived
from all biological sources—whether living or nonliving. Soil organic matter is a vital indicator of soil
‘health’, because of it’s impact on a variety of soil
functions and properties. It provides the energy
source for micro-organisms in the soil, is a reservoir
of nutrients (especially N,P & S) and improves the
structural stability, water holding capacity and pH
buffering capacity of the soil.
Soil organic matter content is difficult to measure
directly. Laboratory tests actually measure soil
organic carbon (SOC), which makes up about 58%
of total soil organic matter. Soil organic matter is
made up of several ‘pools’ that vary in their
contribution to soil functions and their longevity in
soil systems. Organic residue deposited in or on the
soil is the most ‘active’ pool, but may be rapidly lost
(has low stability). Humus (made up of resistant
compounds derived from decayed organic residues)
is a ‘slow’, more stable pool. Charcoal is very stable,
but is not biologically active, and therefore is an
‘inert’ or ‘passive’ pool.
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Organic carbon, present in soil organic matter, is an
important global carbon pool, approximately three
times larger than the amount of carbon (C) stored in
vegetation and twice the amount stored in the
atmosphere. However, mainly because of our dry
climate, organic C stocks in Australian soils are
much lower than the global average.
Soil cultivation and soil degradation result in losses
of organic carbon which is released as CO2 into the
atmosphere. Land clearing and overgrazing also
contribute to the loss of soil carbon.
Improved soil management strategies such as crop
stubble retention on the soil surface and reduced
grazing pressure have the potential to increase the
store of soil C, thereby acting as sinks for
atmospheric C.
Permeability and porosity
Permeability is a measure of how easily water
moves through a soil. At the surface, it affects the
rate at which water can enter a soil, called the
‘infiltration rate’. It is affected by soil structure and
texture. Porosity is the amount of space around
mineral grains that can be filled by water or air.
Salinity
Salts occur naturally in soils. However through
changes in land use, salt levels can be concentrated
on the soil surface by evaporation from a shallow
watertable. Salt levels can become so high that only
salt tolerant plant species can survive.
Water holding capacity
Field capacity (upper limit of available water) is a
term used to describe the amount of water held in
the soil after free water drains from the macropores.
Permanent wilting point is the stage when soil water
is held too strongly for the plant to extract it and the
plant wilts beyond recovery (also known as the
lower limit of available water).
Plant available water capacity (PAWC) is the
quantity of water between field capacity and
permanent wilting point which is available for
plant use.
An important concept is that land should be used in
accordance with its capability.
The following fact sheets produced by the
Department of Natural Resources and Water (NRW)
provide information on various forms of
soil degradation:
• L13 Erosion control in cropping lands
• L40 Soil limitations to water entry
• L91 Erosion control in grazing lands
• L81 Gully erosion
• L51 Salinity in Queensland
• L45 Soil acidification
• L84 Soil compaction in cropping land
• L60 Acid sulfate soils in Queensland.
Soils and land use
A good soil is hard to define. It depends on what it is
to be used for. A productive soil, that will support
healthy plants, has several characteristics. It needs
to be deep, well structured, fertile, well drained,
biologically active and of medium pH. The
combination of these features provides an
environment that is ideal for plant growth.
A soil that is prized for its agricultural value may be
less suitable for other uses. For example, some of
our best cropping soils are cracking clays, but their
capacity to shrink when dry and expand when wet
creates problems for buildings and roads.
To assist land managers in determining land use
options for their soil, a series of Soil Maps, Land
Resource Bulletins and Land Management Manuals
have been prepared for most parts of Queensland.
In addition to information on soil types and soil
properties, these resources may provide a regional
overview of the land resources, climate, vegetation,
and land management issues.
Further information
The following references contain useful information
about soils:
•
Soil degradation
•
Soils can be subjected to various forms of
degradation that affect their productive potential.
Some forms of degradation can be reversed while
others can be difficult or uneconomic to treat.
•
Prevention is far better than cure and it is necessary
for land managers to know what forms of
degradation are most likely to affect individual soils.
Understanding Soil Ecosystem Relationships (USER)
- Information about soil properties, soil water,
nutrition, soil organisms, weed control (phone the
NRW Service Centre at 07 3896 3216)
The Land Managers Monitoring Guide is an on-line
publication providing information on the monitoring of
a range of indicators related to soils and their
management (type monitoring guide in the search box
on the home page of <www.nrw.qld.gov.au>)
Land Management Manuals available for a number or
districts (type land management manuals in the
search box on the home page of
<www.nrw.qld.gov.au> or phone 07 3896 9502).
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Fact sheets are available from NRW service centres and the NRW Information Centre phone (07 3237 1435). Check our web site
<www.nrw.qld.gov.au> to ensure you have the latest version of this fact sheet. While every care is taken to ensure the accuracy of this
information, the Department of Natural Resources and Water does not invite reliance upon it, nor accept responsibility for any loss or
damage caused by actions based on it.
© The State of Queensland (Department of Natural Resources and Water) 2006