Physical Properties

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Physical Properties

• Chapter 3

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3.1 Introduction

• Physical properties are those aspects of the soil
  that are related to the soils bulk properties
• important physical properties of soil
• texture
• structure
• moisture

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3.2 Soil Texture

• a term commonly used to designate the
  proportionate distribution of the different sizes
  of mineral particles in a soil
• does not include any organic matter or mineral
  particles gt 2 mm

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3.2 Soil Texture

• According to their size, these mineral particles
  are grouped into separates.
• A soil separate is a group of mineral particles
  that fits within definite size limits expressed
  as diameter in millimetres
• Sizes of the separates used in the USDA system of
  nomenclature for soil texture are shown in Table
  3.1

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3.2 Soil Texture
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3.2 Soil Texture

• all mineral particles discussed in this section
  are less than 2 mm
• an analysis of soil texture must include removal
  of large particles
• done by sieving
• analytical procedure by which the percentages of
  the various soil separates are obtained is called
  a mechanical analysis

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3.2 Soil Texture

• mineral soils (mainly rock) are a mixture of soil
  separates
• on the basis of the proportion of these various
  separates that the textural class names of soils
  are determined

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3.2 Soil Texture
The 12 soil texture classes
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Example 3.1 72 sand 25 clay 3 silt
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Exercise 3.1
??? Be consistent
Clay
Loam
Clay Loam
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Texture Classes

• The notes explain in detail the differences
  between the twelve texture classes
• You are required to be able to tell the
  difference between the classes
• But as it is so boring to read, you can do this
  as study for the exam!
• So start highlighting!

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Significance

• texture is one of the most important soil
  characteristics
• influences many other properties of great
  significance to land use and management
• terms used to describe soils based on their
  texture
• sandy or coarse-textured soils
• loamy or medium-textured soils
• clayey or fine textured soils

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Significance of sandy soils

• Sandy soils tend to be
• low in organic matter content
• Low in native fertility
• low in ability to retain moisture and nutrients
• low in cation exchange and buffer capacities
• rapidly permeable

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Significance of sandy soils

• Consequences of sandy soils
• thick, upland deposits of such soil materials are
  often quite droughty
• need irrigation at times during dry seasons
• are best adapted to deep-rooted crops (such as
  citrus where temperatures permit)

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Significance of sandy soils

• Consequences (continued)
• have high bulk densities and are well-suited for
  road foundations and building sites
• total amounts of fertiliser per crop are usually
  quite high
• require good water management, including more
  frequent irrigations and/or artificial drainage

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Significance of loamy/clayey soils

• Loamy and finer soils tend to be
• more fertile
• contain more organic matter
• have higher cation exchange and buffer capacities
• are better able to retain moisture and nutrients
• permit less rapid movement of air and water

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Significance of loamy soils

• All of this is good up to a point
• too sticky when wet
• too hard when dry to cultivate
• may have shrink-swell characteristics that affect
  their suitability adversely for use as building
  sites and for road construction

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What is the best soil?

• "Best for what?"
• sandy loams soils generally
• better suited for a wider variety of purposes
• yield better agricultural yields

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3.3 Soil Structure

• individual particles of sand, silt, and clay tend
  to become clustered together in soil
• clustering into aggregates gives structure to the
  soil
• eg the granules of soil clinging to dug up grass
  roots

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3.3 Soil Structure

• a structural unit is called a ped
• the surfaces of peds persist through cycles of
  wetting and drying in place
• clods and fragments are different to peds
• they form as a consequence of factors other than
  soil formation, eg digging
• some soils lack structure and are referred to as
  structureless or massive

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3.3 Soil Structure

• soils structure described by
• shape
• size
• grade
• of the units
• special set of terms used for classification (as
  with texture)

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Shape

• The following terms describe the basic shapes and
  related arrangements
• platy
• prismatic
• columnar
• blocky
• granular

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Size

• Five classes are employed
• very fine
• fine
• medium
• coarse
• very coarse
• size limit classes vary from one shape to another
  (see Table 3.1)

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Grade

• describes the distinctness of units
• criteria are
• the ease of separation into discrete units
• the proportion of units that hold together when
  handled
• classes used
• weak - the units are barely observable in place
• moderate - the units are well formed and evident
  in undisturbed soil
• strong - the units are distinct in undisturbed
  soil

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How does all of this fit?

• The three terms for soil structure are combined
  in the order (1) grade, (2) size, (3) shape.
• Example
• strong fine granular
• used to describe a soil that separates almost
  entirely into discrete units that are
• loosely packed
• mostly between 1 and 2 mm in diameter
• roughly spherical

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3.4 Soil Porosity

• water is only able to travel through soil because
  of the spaces between particles pores
• pore size and distribution important in
  determining the movement of water in soil
• large pores can conduct more water, more rapidly
  than small pores

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3.4 Soil Porosity

• Suction is a measure of the energy required to
  remove water from a given pore
• It is easier to remove water from a large pore
  than from a fine pore

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Exercises 3.3

• density mass value

135 130
1.04
60
0.4
0.6
2.65
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3.4 Soil Porosity

• porosity of sandy soils is less than that of
  clayey soils
• larger particles in sands cannot pack together as
  efficiently as the small ones in clays
• water will drain very rapidly from large pores,
  such as those found in sands, but very slowly
  from the smaller pores in clays
• topsoil (the A horizon) has a greater porosity
  than the subsoil (B horizons).
• Why should this be?
• less sand in lower horizon

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3.4 Soil Porosity

• air molecules are able to move equally well
  through any empty pores, regardless of size
• if gas encounters a pore filled with water,
  movement is very slow
• clays, which retain water in the small pores, are
  not well aerated and can suffer oxygen depletion
  to the roots

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3.4 Soil Porosity

• an ideal soil has a porosity of around 50,
• an even division between small and large pores.
• a balance between water storage and transport,
  and oxygen diffusion

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3.5 Soil Colour

• Soil colour is important because it is an
  indirect measure of other important
  characteristics
• water drainage and aeration
• organic matter content
• certain inorganic components
• measured by a Munsell soil-colour book

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3.6 Soil Moisture

• With regards to water, you have probably thought
  that water was either available or unavailable
• Commonly associated with flood or drought
  mentality
• But there is so much that you dont know!

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Exercise 3.4

• List reasons why the water-soil relationship is
  important
• storage
• transportation
• availability to plants, micro-organisms

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3.6 Soil Moisture

• There are three ways that water interacts with
  soil
• Hygroscopic interaction
• Capillary rise
• Gravitational fall
• first water taken in is hygroscopic, then
  capillary then gravitational

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Hygroscopic (adhesion) water

• very tightly bound to the soil particle by
  positive-negative interactions due to the
  polarity of water and the soil compound
• not available to plants
• can only be lost by oven drying (gt100C)
• air dry soils still have this water

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Capillary (cohesion) water

• this is adsorbed onto the hygroscopic water
• can be accessed by plants
• it is the most important water for plants because
  it does not drain away

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Drainage (gravitational) water

• this water occupies the pores between particles
• will drain away over time through the force of
  gravity
• always on the move
• not considered as available water
• it can top up the capillary water if it has
  become depleted

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3.6 Soil Moisture

• saturated soil the large pores are filled with
  gravitational water and not air
• permanent wilting point (PWP) soil that has
  been depleted of its capillary water
• field capacity (FC) soil with no gravitational
  water but maximum capillary water
• difference between FC and PWP is the amount of
  water available to plants
• varies between soils

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Figure 3.5
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3.7 Organic matter

• an important role in
• aggregation
• water-holding capacity
• infiltration capacity
• closely related to soil fertility
• contributes considerably to the cation exchange
  capacity of soils
• nutrients such as N, S B are almost totally
  derived from organic matter

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Organic matter

• typical levels are 0.5-6
• more than 50C, 5N
• decomposition of organic matter releases N that
  growing plants can use
• high release rates result from
• high soil temperatures
• good aeration
• moist soil
• low clay contents