Physical Properties
Chapter 3
3.1 Introduction

Physical properties are those aspects of the soil that are
related to the soil’s 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 > 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
Soil Separate
Particle size range (mm)
Very coarse sand
2-1
Coarse sand
1.0-0.5
Medium sand
0.5-0.25
Fine sand
0.25-0.1
Very find sand
0.1-0.05
Silt
0.05-0.002
Clay
< 0.002
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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
•
•
•
•
•
•
sands
sandy loams
sandy clay loam
silt
silty clay loam
sandy clay
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•
•
•
•
•
loamy sands
loam
clay loam
silt loam
silty clay
clay
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Example 3.1
72% sand
25% clay
3% silt
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Exercise 3.1
%sand
%silt
%clay
Texture Class
(a)
45
16
39
??? Be consistent
(b)
16
39
45
Clay
(c)
39
45
16
Loam
(d)
33
33
34
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]
Bulk1.04
Density
Porosity  1 
600.6
%
0.4
Particle
Density
2.65
soil volume
wet weight of soil
dry weight of soil
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135 cm3
160 g
130 g
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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 don’t 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 (>100°C)
“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 50%C, ~5%N
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
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