پنجشنبه ۲۲ شهریور ۰۳ | ۲۳:۰۸ ۸ بازديد
The soil is the lifeblood of your crop or pasture. This important asset supplies nutrients, water
and oxygen to plants, supports machinery and animal traffic, and provides a medium for the
decomposition of crop and pasture residues.
Soil management will affect land productivity and environmental sustainability. Maintaining the
health of your soil to maximise productivity will require an awareness of soil characteristics and
how they should be managed.
Farms may have a variety of soil types that require specialised management to optimise
productivity and prevent land degradation. Different soil types occur for a variety of reasons, such
as different geology, position in the landscape and drainage.
This booklet is the first in a series of three booklets called ‘Know your Soils’. The three booklets
provide an easy and informative way for you, the farmer, to assess and understand your soil.
All three booklets are inter-linked and should be used together to achieve an understanding of
the way your soil behaves and how it should be managed.
Part 1: Introduction to Soils - This booklet provides detailed background
information on soil, including soil forming processes, soil profile descriptions,
physical characteristics of soil, and soil management and land degradation
issues.
Part 2: assessing Your Soils - This booklet details
eight exercises for you to carry out to assess some
characteristics of your soil. The topics covered include:
soil classing, the hole, soil colour and depth, stone
size and percentage, soil texture, friability, soil slaking/
dispersion and soil pH.
Part 3: Managing Your Soils - This booklet allows you
to use the information you have collected in Part 2, to
interpret the behaviour and physical characteristics of
your soil. Part 3 also provides management options for
some limitations such as poor structure, poor internal drainage and acid
soils.
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P R e f a C e
and oxygen to plants, supports machinery and animal traffic, and provides a medium for the
decomposition of crop and pasture residues.
Soil management will affect land productivity and environmental sustainability. Maintaining the
health of your soil to maximise productivity will require an awareness of soil characteristics and
how they should be managed.
Farms may have a variety of soil types that require specialised management to optimise
productivity and prevent land degradation. Different soil types occur for a variety of reasons, such
as different geology, position in the landscape and drainage.
This booklet is the first in a series of three booklets called ‘Know your Soils’. The three booklets
provide an easy and informative way for you, the farmer, to assess and understand your soil.
All three booklets are inter-linked and should be used together to achieve an understanding of
the way your soil behaves and how it should be managed.
Part 1: Introduction to Soils - This booklet provides detailed background
information on soil, including soil forming processes, soil profile descriptions,
physical characteristics of soil, and soil management and land degradation
issues.
Part 2: assessing Your Soils - This booklet details
eight exercises for you to carry out to assess some
characteristics of your soil. The topics covered include:
soil classing, the hole, soil colour and depth, stone
size and percentage, soil texture, friability, soil slaking/
dispersion and soil pH.
Part 3: Managing Your Soils - This booklet allows you
to use the information you have collected in Part 2, to
interpret the behaviour and physical characteristics of
your soil. Part 3 also provides management options for
some limitations such as poor structure, poor internal drainage and acid
soils.
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P R e f a C e
your soil, and most importantly, enable you to gain an understanding of your soil and how to manage it
better.
you can use the ‘Know your Soils’ booklets to monitor changes in the condition of your soil over time.
Part 2 will help you describe your soil and Part 3 will assist you in understanding and managing your soil.
it is recommended that you do not use this series as a ‘one off’. The routine use of ‘Know your Soils’
every three to five years will allow you to observe patterns and changes.
By assessing two or three paddocks each year you can develop baseline information for each paddock
and then re-sample to start the monitoring process. if you intend to monitor paddocks, it is important to
re-sample as close as possible to the original sampling point. Early detection of changes through regular
monitoring will enable problems to be managed and even prevented. This will reduce the detrimental
effects of soil degradation on productivity and the environment.
it is also a good idea to independently monitor the chemical conditions of your soil in conjunction with
this series. This will help you manage the application of fertilisers and allow you to work out if you have
any chemical problems.
in Part 3 you will identify limitations that can be managed. Limitations, such as subsoil acidification, are
likely to change over time and are costly and difficult to treat. Monitoring pH, particularly at depth, will
enable the early detection of subsoil acidification and provide an opportunity for prevention.
Recording sheets are located at the back of Part 2. These sheets will allow you to record the information
you collect so you can refer to them when you re-sample. you can independently do the same with your
chemical analysis.
Cross referencing between booklets
Each of the three booklets in the ‘Know your Soils’ Series is numbered for ease of reference. All
sections found in Part 1 have the prefix ‘1’. For example, 1.1 introduction to Soil and 1.5.5 wind
Erosion. whereas 2.7 Soil Slaking/Dispersion Exercise is a practical activity found in Part 2; and
Section 3.6 Determining Soil Drainage can be found in Part 3.
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1.1.1 SoIl anD aGRICUlTURe
Agricultural viability depends upon a healthy soil. A healthy soil is obtained through knowledge
of the soil and the application of appropriate management practices. Poor management
practices can lead to soil degradation, which can reduce productivity. Soil is not a renewable
resource. The loss of 1 mm of topsoil can take 3000 years to replace.
Soil has a number of functions important for agricultural production, these being:
● A medium for plant growth supplying and retaining nutrients, oxygen and water
● A medium for the decomposition of crop and pasture residues
● An anchor for plants
● A medium that should support machinery and animal traffic under most soil conditions
without degradation.
Soil is essentially made up of four main fractions, but the fractions vary greatly in arrangement
and form, hence soils can be physically and chemically quite different. understanding why soil
can be so varied, and how different soils require different management is important for an
effective farming enterprise.
1.1.2 WHaT IS SoIl?
Soil is the unconsolidated outer layer of the Earth’s crust. it appears in a variety of forms
(different textures, colour, structure and nutrient content) depending on a number of factors
outlined further in this booklet.
Although soil is highly variable, there are
four main ingredients (fractions) that are
consistent with all types of soil: mineral,
organic matter, water and air. These four
fractions fall into two categories: solid
(mineral and organic matter) and non-
solid (water and air).
The solid category
The solid category makes up
approximately 50-70% by volume of the
total soil and consists of the mineral and
organic matter fractions. Combined, the
mineral and organic matter fractions give
the soil a characteristic known as texture.
i) Mineral fraction
The mineral fraction makes up about 95-99% of the solid category. it is comprised of the
weathered remains of parent rocks that are broken down over many thousands of years.
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Non-Solid
Solid
Organic
Water
Air
Mineral
Figure 1 Approximate proportions of the
four soil fractions in a typical soil.
characteristics of soil are generally determined by the type of parent rock (geology) from
which it was formed.
The mineral fraction is made up of four main particles that are defined by the diameter of
the particle:
Gravel > 2 mm
Sand 0.02-2 mm
Silt 0.002-0.02 mm
Clay < 0.002 mm
The proportion of these particles plays an important role in the fertility of the soil and its
response to management.
Clay particles are the smallest particles of the mineral fraction. They are an important
component of soil as they have a negative electrical charge. This enables them to hold and
exchange nutrients (which also have an electrical charge). Clay particles in soil provide an
exchange site for plant nutrients. Clay soils are normally more fertile than sandy soils.
The type and quantity of clay in the soil can affect the amount of nutrients held for plant
use and the ease at which these nutrients are released to the plant. The electrical charge
on clay particles enables them to stick to other clay particles and also to sand and silt
particles (this is the reason why clays are so sticky). This can be good and bad in terms of
soil management as will be explained in Section 1.5 of this booklet: Soil Management and
Land Degradation.
Gravel, sand and silt particles do not have an electrical charge and have minimal ability to
hold and exchange plant nutrients. They do, however, have other important roles such as
aeration of the soil. Gravel and coarse sand particles are larger than clay, silt and fine sand
particles, and they have bigger gaps between them. Therefore more space is available for
movement of water and air through the soil.
A good mix of sand, silt and clay particles will allow the soil to hold sufficient nutrients as
well as allowing adequate exchange of air and water essential for plant growth.
ii) organic matter fraction
organic matter is the other part of the solid soil category. it consists of the remains of
living organisms in various stages of decomposition as well as living micro-organisms. in
agricultural soils in Victoria, organic matter can comprise up to 6% of the solid soil fraction
however it is more common for agricultural soils to have between 0.5 and 3% organic
matter. in forest soils, organic matter can comprise up to 20%.
organic matter occurs in various forms from undecomposed to completely decomposed
(humus), and all forms provide benefits for the soil. Like clay particles, humus is also
electrically charged and is able to store and release nutrients. Humus is also able to
increase the water holding capacity of the soil. This is particularly important for sandy soils
which have a low water holding capacity.
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organic matter decomposing it to humus. Micro-organisms also live on products of living
plants, some providing benefits to the plant, others creating disease. Most soil organisms
decompose organic matter and release nutrients for plant growth as well as improving soil
structure. Micro-organisms in the soil are important for soil health.
The non-solid category
The non-solid category can be divided into two fractions: water and air.
water and air should comprise around 30-50% of an agricultural soil. This depends on the
time of year and the soil type. water and air are essential for plant growth. in particular,
oxygen and carbon dioxide need to be exchanged between the soil and the atmosphere for
root respiration to occur.
1.2 CHaRaCTeRISTICS of SoIl
There are three characteristics of soil that are important for plant growth and productivity:
physical, biological, and chemical. A change in one characteristic is likely to affect another.
Physical
The aspects of the soil that you can see and touch. Physical charactersitics include: soil
texture, soil colour, soil depth, soil structure, porosity (spaces between particles) and stone
content. The role physical properties play in soil health are:
● To supply water and air to plant roots and allow adequate water and air movement into
and through the soil profile.
● To store water for plant growth.
● To support machine and animal traffic.
Chemical
The ability to supply nutrients for plant growth and store nutrients in the profile without loss by
leaching. The ability to keep clay aggregates chemically stable which impacts on soil structure.
biological
The ability to support a healthy microbial population for organic matter breakdown (faeces
and crop and pasture residues), nutrient cycling and the growth of nitrogen fixing bacteria.
1.2.1 PHYSICal CHaRaCTeRISTICS
Soil texture
As mentioned above, soils are partially made up of a mineral fraction (gravel, sand, silt and
clay particles) and an organic matter fraction. it is these two fractions combined (minus the
gravel particles) that give the soil a characteristic known as texture. There are many different
texture grades depending on which particles (sand, silt, clay or organic matter) are dominant
(if any). Soil texture provides a good way of characterising soils and can be described through
the behaviour of a moist lump of soil when rubbed between your thumb and forefinger (refer
to 2.5 Soil Texture Exercise). Some examples of texture and their characteristics are as follows.
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Sand - Particles are visible to the eye, feels gritty and sand grains can be heard when
rubbed. Sticks together only when moist e.g. beach sand.
Silt - Particles not visible to the eye, feels silky (like talcum powder) and can be
rolled when moist to form short rods. Sticks together when moist or dry.
Clay - Particles not visible to the eye, feels smooth and plastic and can be rolled to
form long rods when moist. Sticks together when moist or dry e.g. pottery clay.
Soils having a range of particle sizes and types
Loam - About 20 -25% clay with the remaining fraction composed of sand, silt and
organic matter.
Sandy loam - Loam with sand dominating.
Silt loam - Loam with silt dominating.
Clay loam- Loam with clay dominating.
Many other texture grades can be described and all differ due to the proportion of each
primary particle size and the amount of organic matter. Soil textures are often referred to as
light textured (sand dominant) or heavy textured (clay dominant).
The texture of the soil can change with depth. obvious changes in texture through the soil
profile can provide an easy way of assessing the different horizons (layers). Determining how
the soil texture changes (if at all) at depth is important in understanding what plant roots have
to contend with. in some cases the soil texture can differ markedly through the soil, ranging
from a light texture (e.g. sandy loam) and changing abruptly into a heavy texture (e.g. clay).
This is known as a texture contrast or duplex soil (refer to 1.3 Soil Profile). This could be a
problem for some plants. The ideal situation for plants is gradational soils, that is a soil texture
that gradually becomes heavier down the profile. Some soils do not change textures, that is,
they are the same texture throughout the soil (uniform soil). if the soil is clay all the way down,
it will often drain quite slowly, so plants that can cope with wet roots will grow best in this soil
type. Soil that is sandy all the way down will drain very quickly and plants that can deal with
dry conditions will be suited to this soil type.
Refer to 2.5 Soil Texture Exercise.
Water and nutrient holding cap
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