description of soil in general . .

1. Soil physical properties

2. SOIL TEXTURE
• Refers to feel, coarseness or
fineness of soil
• -Determined by relative
proportions of sand, silt and clay
• For each textural class name is
given eg. Sandy clay or sandy clay
loam

3. Textural classes
• Sands - Soils containing70% or more sand
eg. Sang and loamy sand
• Clays –Soils containing more than 35%clay
eg. Sand clay silt clay
• Loams
• Soils containing sand and silt in almost equal
proportions. An ideal loam is composed of
approx. 40 % sand, 40%silt and 20% clay
eg .sandy loam, silt clay loam
Loams are agriculturally the most important
soils for agronomic crops

5. DETREMINATION OF SOIL TEXTURE
• Determined in the field and in lab.
• FIELD
• In the field it is determined by the sense of
feel. The soil is rubbed between the thumb
and fingers in the wet conditions.I
• Sands give a gritty feel
• Silts when dry give a feel of flour and slightly
plastic when wet
• Clays have plastic feel and exhibit stickness
when wet and hardness when dry

6. • In the laboratory
• The most accurate method. determined
through particle size analysis
• Such an analysis gives the amounts sand,Silt
and clay separates in a soil
• To determine textural name we interpolate
the data of particle size analysis in an
equilateral triangle whose base represents
sand the left hand represents clay and the
right silt
• Each side is divided into 100 equal segments

8. Exercise
• To illustrate the use of textural
triangle assume that the soil is
composed of 50% sand,
20%siltand 30% clay. Assign a
textural class

9. IMPORTANCE OF SOIL TEXTURE
• Relative resistance to root
penetration
• Soils with high silt and clay contents
usually retard root growh and extent
of branching. Root penetration is best
is best where surface soil is loose
textured. Why?

10. • ii)Soil fertility Many of the nutrient ions
which plants must extract from soils are
adsorbed by colloids
• The finer the texture of the soil the greater
is its fertility
• iii)Inflitration of water
• Inflitration rates of water entering heavy
soils are low and run off is high

11. • iv) Rate of water movement
• Fine textured soils there is a
resistance to the mass movement of
water
• Down water movement of water
through a sandy soil is so rapid

12. SOIL STRUCTURE
• Soil structure refers to the arrangement of soil
separates into units called soil aggregates.
The smallest aggregate is termed ped
• An aggregate possesses solids and pore
space.
• The arrangement of soil aggregates into
different forms gives a soil its structure.

13. Formation of aggregates
• The natural processes that aid in forming
aggregates are:
1) wetting and drying,
2) freezing and thawing,
3) microbial activity that aids in the decay of
organic matter,
4) activity of roots and soil animals, and
5) adsorbed cations.

14. • The wetting/drying and
freezing/thawing action as well as
root or animal activity push particles
back and forth to form aggregates.
Decaying plant residues and
microbial byproducts coat soil
particles and bind particles into
aggregates.
•

15. • Adsorbed cations help form
aggregates whenever a cation is
bonded to two or more particles.
• Aggregates are described by their
shape, size and stability. Aggregate
types are used most frequently when
discussing structure

16. Structure and soil horizons
• -Structure is one of the defining
characteristics of a soil horizon.
• -A soil exhibits only one structure per
soil horizon, but different horizons
within a soil may exhibit different
structures.

17. • - All of the soil-forming factors, especially
climate, influence the type of structure
that develops at each depth. Granular
and crumb structure are usually
located at the soil surface in the A
horizon. The subsoil, predominantly the
B horizon, has subangular blocky,
blocky, columnar or prismatic
structure.

18. • Platy structure can be found in
the surface or subsoil while single
grain and structureless structure
are most often associated with
the C horizon.
•

19. Soil structure types
• - The type of soil structure is
described on the basis of shape and
arrangements of the peds
• There four principal geometric forms
used to classify types of soil structure

20. – Plate- like
• The aggregates have more developed horizontal
than vertical dimensions giving a flattened
compressed appearance.
• ii)Prism –like
• The vertical axis of the aggregate is more
developed than the horizontal one with
flattened sides giving a pillar- like shape
– This type of aggregate is commonly found in sub soil
horizons of arid and semi arid regions

21. – regions
• Block -like
– All the three dimensions of the aggregate are
of equal size and the aggregate resemble a
block or cube
• - Common in heavy soils of sub humid regions
• iv)Sphere - like
• Aggregates are nearly roundish the axes being
of the same size

22. Soil structural grades
–Characteristic of many surface
soils especially those high in
Organic matter
• - They may have two types of
structures depending upon porosity
of the aggregates
–a)Granular are relatively non –
porous
• b) Crumby which are porous

23. Soil structural grades
• Soil structural grade are recognized
on the basis of stability of the
individual aggregates
• Four grades are designated
• structureless
• There is no noticeable aggregates eg.
loose sand
• Weak

24. Naming of soil structure
• Sequence followed is grade, class
and type
• For example ,If aggregates are
graded as strong classed as fine and
are granular type the soil structure is
designates as strong fine granular

26. Aggregate Stability
– . Stable soil aggregation is a very valuable
property of productive soils. Yet, the stability
of soil aggregation is very reliant on the type
of minerals present in the soil.
– Certain clay minerals form very stable
aggregates, while other clay minerals form
weak aggregates that fall apart very easily

27. • Highly weathered silicate clays,
oxides, and amorphous volcanic
materials tend to form the most
stable aggregates. The presence
of organic matter with these
materials improves stable
aggregate formation.

28. • In nutrient management, the
aggregate stability is important
because well-aggregated
minerals are well drained and
quite workable.

29. • In contrast, less weathered silicate clays,
such as montmorillonite, form weak
aggregates. Some silicate clays are said
to have a shrink-swell potential. This
means that the soil minerals expand, or
swell, when wet, causing the soil to
become sticky and drain poorly. When dry,
these soils shrink and form cracks. The
make-up of the lattice structure of silicate
clays determines the shrink-swell
potential.
•

30. Importance of soil structure
–The soil structure per se is not a
plant growth factor but it influences
all most all plant growth factors
–Water supply, aeration availabity of
plant nutrients, microbial activity
and root penetration are affected by
soil structure

31. – Aggregates are important in a soil because
they influence bulk density, porosity and
pore size.
– The soil structure modifies the influence of
soil texture with regard to moisture and air
relationships. For instance platy structure
normally hinders free drainage. The best
structure for favourable physical properties
soil is either cramby or granular

32. –Pores within an aggregate are
quite small as compared to the
pores between aggregates and
between single soil particles.
This balance of large and small
pores provides for good soil
aeration, permeability and
water-holding capacity.

33. Destruction of aggregates
• -Tillage, falling raindrops and compaction are
primarily responsible for destroying
aggregates.
• As the cutting edge of a tillage implement is
pulled through the soil, the shearing action at
the point of contact breaks apart aggregates.
• The amount of aggregate destruction that
results from tillage depends on the amount of
energy the tillage implement places in the
soil.

34. • The field cultivator has little down
pressure and destroys few
aggregates. The disk, however,
has both cutting action because
of the rotation of the disk and
shearing action. Together there is
substantial down pressure and
destruction.

35. • Aggregates on the soil surface can be
broken down by the beating action of
raindrops. The single particles that were
once part of the aggregate can easily form
a crust when the soil dries. The crust looks
very similar to the crust formed on a
puddle after it rains. It is very difficult for
water to infiltrate a crust and for seedlings
to push up through a crust.

36. • Thus, field operations that lead to
aggregate destruction at the soil surface
have detrimental secondary effects. The
particles also can be eroded if they
become detached by rainfall.
• -Compaction can lead to the breakdown of
aggregates in the surface soil and subsoil
if the applied force from wheel traffic,
animal traffic or human traffic is greater
than the force holding an aggregate

37. Promotion of aggregates
• - Aggregation is promoted by root growth
and the addition of organic material.
Roots excrete compounds that are used
as food by microorganisms. Also, as roots
absorb water and dry the soil, cracks form
along planes of weakness. - Lastly, when
roots decay, root channels serve as
conduits for water that facilitate
wetting/drying and freezing/thawing.
.

38. • - Organic material may be added in the
form of crop residue, animal manure,
sludge, and green manure. These
additions are usually made to the surface
soil and are critical to the development of
granular and crumb structure. As
organic material is incorporated by tillage,
soil animals and microorganisms, it aids in
subsoil structure development

39. SOIL COLOUR
• Most obvious characteristics of soil
• Soil color does not affect the behavior
and use of soil; however, it can indicate
the composition of the soil and give clues
to the conditions that the soil is subjected
to.
• Has no direct effect to plant growth
but it has an indirect one through its
effect in temperature and moisture

40. • Most obvious characteristics of soil
• Soil color does not affect the behavior
and use of soil; however, it can indicate
the composition of the soil and give clues
to the conditions that the soil is subjected
to.
• Has no direct effect to plant growth
but it has an indirect one through its
effect in temperature and moisture

41. • Soil can exhibit a wide range of
colours; gray, black, white, reds,
browns, yellows and under the right
conditions green.
• Predominant soil colours are grey
brown and rust
• Varying horizontal bands of colour in
the soil often identify a specific soil
horizon.

42. Causes of soil colour
• clays have the greatest effect on soil colour
• Humus is black or dark brown iron oxides may
be red or rust brown. Quartz mostly grey or
greyish white
• Soil colour changes with the moisture content.
The more moist the soil is the darker is the
colour because the refractive properties of the
solid components and of the air are very
different and that the light that falls on a dry soil
is largely reflected The refractive properties of
water and soil particles are almost similar

43. • It is therefore desirable to
examine the soil colour when the
sky is clear because the
wavelenghts of the lights are in
constant proportion

44. Significance of soil colour
• The darker the soil the higher is its relative
productivity due to amount of OM present
• Light colour frequently results from
amounts of quatz which has no nutritional
value
• -Practically all soil profiles reveal a change
in colour from one horizon to the next
• In classification of soils colour is very
helpful (soil taxanomy system eg
charnozem (black)

45. • Due to its effect on radiant energy
soil colour also influences other
soil properties Black and dark
colours absorb more heat than
white colours dark soil tend to be
much warmer Implications?

46. Study and classification of soil colour
• Often described by using general
terms, such as dark brown,
yellowish brown, etc., soil colors
are also described more
technically by using Munsell soil
color charts,

47. • which separate color into components of hue
(relation to red, yellow and blue), value
(lightness or darkness) and chroma
(paleness or strength)
• Hue refers to the dominant spectral colour or
quality which distinguishes red from yellow
• Value or brightness expresses apparent
lightness as compared to absolute white
• Chroma defines gradations of purity of color
•

48. Management of soil colour
• Under some conditions it may be
desirable to lower or raise soil temperature
• This may be accomplished by changing
soil colour Effect of crop residues following
decomposition renderes the soil dark in
colour so that during dry season it can
adsorb sunshine energy

49. SOIL TEMPERATURE
• -Is an extremely important property of soil
• - Soil temperature plays an important role
in many processes, which take place in
the soil such as chemical reactions and
biological interactions.
• -It affects plant growth directly and also
influences moisture aeration structure
microbial, enzymic activities,
decomposition of plant residues and the
availability of nutrients

50. • - In temperate climate it is temperature that
limits crop production due to decreased
metabolic activities, Retardation of root
elongation, reduced seed germination
• - Development and activity of microflorareacts
very markedly to temperature variations. Eg.
Below5C nitrification ceases
• How about high temperatures?

51. Factors affecting soil temperature
• -Both external ( environmental) and
internal ( soil) factors affects soil
temperature
• i) Solar radiation
• -The amount of radiation that is
received by soil surface depends on:
• -The insulation by air water vapour ,
clouds snow plants or mulch

52. • -The angle with which the soil faces the
sun which is governed by factors such as
time of the day, latitude and season
•
• ii)Conduction of heat from the atmosphere
• Conduction of heat through air is small. It
can only affect soil temperature only by
contact Therefore air convection or wind is
in the heating up of soil by conduction
from the atmosphere

53. • up of soil by conduction from the atmosphere
• ii)Rainfall-Depending on its temperature rainfall
can cool or warm the soil
• iv) Insulation
• -The soil can be insulated from environmental
temperature by a plant cover, mulch, snow and
fog
• -Insulation serves to maintain a more uniform
soil temperature

54. • v) Biological activity
• - Involves heat. The higher is the activity the
higher the temperature. Eg during composting
• vi)structure, texture and moisture
• Soils in a compacted condition have a more
thermal conductivity than the loose condition
• This is because This is beacusemineral particles
conducts heat better than air

55. • conducts heat better than air
• - A soil in a natural structure has
higher conductivity than recently
disturbed one
• Moisture at the soil surface cools
the soil through evaporation

56. • Soil temperature varies in response to
exchange processes that take place
primarily through the soil surface.
These effects are propagated into the
soil profile by transport processes and
are influenced by such things as the
specific heat capacity, thermal
conductivity and thermal diffusivity.

57. Management of soil Temperature
• Some of the factors influencing soil
temperature can be modified
• An Adjustment of roughness, soil
moisture and soil colour can alter thermal
conductivity
• Mulch and crop residues, plastic paper
have been utilized
• Vegetation keeps the soil relatively cool
by shading the ground

59. Consistency of soils
• Soil consistency is the strength with which
soil materials are held together or the
resistance of soils to deformation and
rupture. Soil consistency is measured for wet,
moist and dry soil samples. For wet soils, it is
expressed as both stickiness and plasticity,

60. • Consistency is the term used to describe the ability of
the soil to resist rupture and deformation. It is
commonly describe as soft, stiff or firm, and hard.
• Water content greatly affects the engineering behavior
of fine-grained soils. In the order of increasing
moisture content (see Figure 2 below), a dry soil will
exist into four distinct states: from solid state, to
semisolid state, to plastic state, and to liquid state. The
water contents at the boundary of these states are
known as Atterberg limits. Between the solid and
semisolid states is shrinkage limit, between semisolid
and plastic states is plastic limit, and between plastic
and liquid states is liquid limit.

61. • Consistency is the term used to describe the ability of
the soil to resist rupture and deformation. It is
commonly describe as soft, stiff or firm, and hard.
• Water content greatly affects the engineering behavior
of fine-grained soils. In the order of increasing
moisture content (see Figure 2 below), a dry soil will
exist into four distinct states: from solid state, to
semisolid state, to plastic state, and to liquid state. The
water contents at the boundary of these states are
known as Atterberg limits. Between the solid and
semisolid states is shrinkage limit, between semisolid
and plastic states is plastic limit, and between plastic
and liquid states is liquid limit.

62. • Consistency is the term used to describe the ability of
the soil to resist rupture and deformation. It is
commonly describe as soft, stiff or firm, and hard.
• Water content greatly affects the engineering behavior
of fine-grained soils. In the order of increasing
moisture content (see Figure 2 below), a dry soil will
exist into four distinct states: from solid state, to
semisolid state, to plastic state, and to liquid state. The
water contents at the boundary of these states are
known as Atterberg limits. Between the solid and
semisolid states is shrinkage limit, between semisolid
and plastic states is plastic limit, and between plastic
and liquid states is liquid limit.

64. • Atterberg limits, then, are water contents at
critical stages of soil behavior. They, together
with natural water content, are essential
descriptions of fine-grained soils.
• Liquid Limit, LL
Liquid limit is the water content of soil in
which

65. • soil grains are separated by water just enough for
the soil mass to loss shear strength. A little higher
than this water content will tend the soil to flow
like viscous fluid while a little lower will cause the
soil to behave as plastic.
• Plastic Limit, PL
Plastic limit is the water content in which the soil
will pass from plastic state to semi-solid state.
Soil can no longer behave as plastic; any change
in shape will cause the soil to show visible cracks.

66. • Shrinkage Limit, SL
Shrinkage limit is the water content in which
the soil no longer changes in volume
regardless of further drying. It is the lowest
water content possible for the soil to be
completely saturated. Any lower than the
shrinkage limit will cause the water to be
partially saturated. This is the point in which
soil will pass from semi-solid to solid state.

67. SOIL TILTH
• Tilth, Physical condition of soil, especially in
relation to its suitability for planting or
growing a crop.

68. • Factors that determine tilth include the
formation and stability of aggregated soil
particles, moisture content, degree of
aeration, rate of water infiltration, and
drainage. The tilth of a soil can change rapidly,
depending on environmental factors such as
changes in moisture

69. • The objective of tillage (mechanical
manipulation of the soil) is to improve tilth,
thereby increasing crop production; in the
long term, however, conventional tillage,
especially plowing, often has the opposite
effect, causing the soil to break down and
become compactedJump to: navigation,
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70. • TILTH Refers to soil general suitability to support plant
growth
• Technically It combines the Physical properties of
particle size, moisture content, degree of aeration,
rate of water infiltration, and drainage into
abbreviated terms in order to more easily present the
agricultural prospects of a piece of land.
• Soils with good tilth has large pore spaces for
adequate water inflitration and water movement

71. Management of soil Tilth
• On Coarse textured soils
-Major limitation in sandy soils is low water and
nutrient holding capacity
-Nutrient leaching particularly N is a problem
-The best management practice is gradual
application of OM

72.
• Fine textured soils, Clayey soils
- Major limitation on such soils is lack of large
pores therefore restricting water and air
movement
- Soils easily waterlog when water cannot move
down the profile

73. - Soils easily waterlog when water cannot move
down the profile
- During irrigation or rainy season the limited
large pore space quickly fills with water
reducing roots oxygen supply
-The best management practice is routine
application of OM which will encourage
favorable microbial decomposition, hence
enhanced pore space

74. Soil Air
• In nutrient management, soil aeration
influences the availability of many nutrients.
Particularly, soil air is needed by many of the
microorganisms that release plant nutrients to
the soil. An appropriate balance between soil
air and soil water must be maintained since
soil air is displaced by soil water.

75. • Air can fill soil pores as water drains or is
removed from a soil pore by evaporation or
root absorption. The network of pores within
the soil aerates, or ventilates, the soil. This
aeration network becomes blocked when
water enters soil pores. Not only are both soil
air and soil water very dynamic parts of soil,
but both are often inversely related:

76. • An increase in soil water content often causes
a reduction in soil aeration.
• Likewise, reducing soil water content may
mean an increase in soil aeration.
• Since plant roots require water and oxygen
(from the air in pore spaces), maintaining the
balance between root and aeration and soil
water availability is a critical aspect of
managing crop plants.

77. • An increase in soil water content often causes
a reduction in soil aeration.
• Likewise, reducing soil water content may
mean an increase in soil aeration.
• Since plant roots require water and oxygen
(from the air in pore spaces), maintaining the
balance between root and aeration and soil
water availability is a critical aspect of
managing crop plants.

78. Composition of soil air
• By comparison to atmospheric air, the soil air
contains more carbon dioxide, less oxygen
and an almost equal amount of nitrogen.
Several other gases are available but in traces
• Variation in composition of soil air exists
depending on place, time and soil itself

79. Table 3. Comparison between soil air and
atmospheric composition

80. Factors affecting soil air
• i)Soil properties
• Sol texture, structure and OM have effects on soil
air because they affect air capacity and
permeability in the soil
• Granular soils contain less than one half as much
CO2 as structureless soils . Loams contain more
CO2 than sands Usually the amount of CO2 and
that of O2 decreases with depth

81. • The presence of growing plants tend to reduce
O2 of the soil and to increase the amount of CO2
due to respiration of roots The CO2 content of
the soil tend to increase by manuring
iii) Biological activities
• Addition of organic material in the soil enhances
biological activities
• In decomposion of organic molecules, O2 is
utilized and CO2 is evolved

82. iv) Seasonal variations
• Variations in temperature and moisture causes
variations in soil air
• Renewal of soil air
• For proper aeration the gases should freely
interchange between the soil and the
atmosphere
• Inadequate interchange may have effects in root
respiration
• Exessive moisture conditions restrict aeration

83. Factors affecting renewal of soil air
• These are: rainfall, temperature, pressure and
wind
• Entry of Entry of rain water in the soil causes
displacement of the soil air from the pores

84. • How do other factors affect renewal of soil air?
Importance of soil air
• O2 is required for respiration of plant roots
microbes and soil fauna
• CO2 helps to dissolve the nutrients from rocks
and minerals
• The N2 Of soil air may be fixed by symbiotic and
non symbiotic organisms and utilized by higher
plants

85. Management of soil air
• Proper aeration is needed for proper crop
production
• Physical properties eg. stricter, moister and
temperature should be made favorable for good
aeration
• Generally, open structure, low moisture content
and high temperatures increases diffusion rates.
Addition of OM, tillage practices and mulch
increases aeration

86. Table 4. Soil Oxygen and Carbon Dioxide
Content at Various Depths (Trinidad)

87. Soil Atmosphere
• The soil atmosphere is not uniform
throughout the soil because there can be
localized pockets of air.
• The relative humAir in the soil often contains
several hundred times more carbon dioxide.
• Humidity of soil air is close to 100%, unlike
most atmospheric humidity.

88. SOIL DENSITY
DEFINITION
• Density, as applied to any kind of
homogeneous monophasic material of mass
M and volume V, is expressed as the ratio of
M to V. Under specified conditions, this
definition leads to unique values that
represent a well-defined property of the
material.

89. • The density of the soil can be expressed
either by particle (true) density or as bulk
density (apparent)
• -Particle density is related to the solid portion
of the soil mass only
• It is defined as the weight per unit volume of
the solids. Ranges from 2.4 to 2.75 g/cc

90. • Particle density is the volumetric mass of the solid soil.
It differs from bulk density because the volume used
does not include pore spaces.
• Particle density = oven-dry soil weight / volume of soil
solids
• Particle density represents the average density of all
the minerals composing the soil. For most soils, this
value is very near 2.65 g/cm3 because quartz has a
density of 2.65 g/cm3 and quartz is usually the
dominant mineral. Particle density varies little
between minerals and has little practical significance
except in the calculation of pore space.

91. Bulk density
• It is total volume of soil occupied by both
solids and pores
• Measured by determining the oven dry weight
of an undisturbed core of soil of known
volume
• The BD of soil is always smaller than its
particle density ( Why)

92. • The magnitude decreases with finess in
texture
• Loose porous soils have low BD. BD is altered
by cropping practices. Intensive cultivation
makes the soil more compact and increases
weight per unit volume

93. Porosity
• Porosity or pore space refers to the volume of
soil voids that can be filled by water and/or
air. It is inversely related to bulk density.
Porosity is calculated as a percentage of the
soil volume:
• Bulk density x 100 = % solid space
Particle density
• 100% – % Solid Space = Percent Pore Space

94. Sample Calculation 1
• A 260 cm3 cylindrical container was used to
collect an undisturbed soil sample. The
container and soil weighed 413 g when dried.
When empty the container weighed 75 g.
What is the bulk density and porosity of the
soil?

95. • A. To determine bulk density:
• Sample Volume = 260 cm3; Sample Weight =
413 - 75 = 338 g ; Bulk density = 338 g/260
cm3= 1.3 g /cm3
• B. To determine porosity:
• Bulk density = 1.3 g /cm3; Particle density =
2.65 g /cm3; Porosity = 100 - (1.3/2.65 x 100)
= 51%

96. • Sample calculation 2
• Given the following information:
• -Total volume of core = 98.2g/cc
• -Saturated weight =185g
• -Oven dry weight =150g

97. • a) Calculate porosity
• Porosity = Total pore volume/Total volume
• Pore volume water volume at saturation – water
volume at oven dry weight
• Volume of water = weight of soil
• Therefore, 185g – 150g =35g
• Wight of water is therefore 35g and pore volume
is 35g/cc
• Porosity is therefore 35/98.2 = 0.36

98. • b) Calculate Bulk density and particle density
( take home assignment)

99. SOIL WATER CONTENT
( WETNESS)
• Water content or moisture content is the
quantity of water contained in a material,
such as soil (called soil moisture), is expressed
as a ratio, which can range from 0 (completely
dry) to the value of the materials' porosity at
saturation. It can be given on a volumetric or
mass (gravimetric) basis.

100. • ENERGY STATE OF SOIL WATER
•
• -Soil water can contain energy in different quantities
and forms.
• -There are two forms of energy, kinetic and potential
• Movement of water in the soil is quite slow, its KE
which is proportional to proportional to the velocity
squared is considered to be negligible
• -The potential energy which is due to position or
internal condition is important in determining the state
and movement of water in the soil

101. • -The potential energy of soil water varies over a very wide
range
• -Differences in PE of water between one point and another
give rise to the to the tendency of water to flow with
• Movement is from where the PE is higher to where it is
lower
• -The rate of decrease of PE with distanceis infact the
moving force causing flow in the soil
• -The force acting on soil water directed from a zone of
higher to the lower potential is equal to the negative
potential gradient (-dq/dx) which is the change of energy
potential with distance x. The negative sign indicates that
the force acts in the direction of decreasing potential

102. • Water Potential (Ψ)
• Most of the issues about soil water relate to
its energy state and its movement (e.g. Evapo
transpiration and Deep Drainage).
• -Therefore, water moves constantly in
direction of potential energy (i.e. wet to dry
soil), where the gradient of potential energy
with distance is the moving force causing flow.

103. • In soil, the reference state is the energy level of water in
the soil at saturation. That is, when all pores are filled with
water. At this point soil water potential (Ψ) is nominally
zero (~0). In most cases, however, soil water potential (Ψ)is
less than zero. This is indicated by giving soil water
potential (Ψ) a negative sign (-ve).
• In practical terms, and as the soil dries out soil water
potential (Ψ)decreases and becomes increasingly more -ve.
So that when soil water potential (Ψ) is “high” it means Ψ
is less –ve and is therefore very close to 0. When soil water
potential (Ψ)is high it means soil water is
held loosely,
highly available and
ready to move somewhere else.

104. SOIL MOISTURE CHARACTERISTIC
CURVE

105. • Water retention curve is the relationship
between the water content, θ, and the soil water
potential, ψ. This curve is characteristic for
different types of soil, and is also called the soil
moisture characteristic.
• It is used to predict the soil water storage, water
supply to the plants (field capacity) and soil
aggregate stability. Due to the hysteretic effect of
water filling and draining the pores, different
wetting and drying curves may be distinguished.

106. • The general features of a water retention curve
can be seen in the figure, in which the volume
water content, θ, is plotted against the matric
potential, . At potentials close to zero, a soil is
close to saturation, and water is held in the soil
primarily by capillary forces. As θ decreases,
binding of the water becomes stronger, and at
small potentials (more negative, approaching
wilting point) water is strongly bound in the
smallest of pores, at contact points between
grains and as films bound by adsorptive forces
around particles

107. • Sandy soils will involve mainly capillary binding,
and will therefore release most of the water at
higher potentials, while clayey soils, with
adhesive and osmotic binding, will release water
at lower (more negative) potentials. At any given
potential, peaty soils will usually display much
higher moisture contents than clayey soils, which
would be expected to hold more water than
sandy soils. The water holding capacity of any soil
is due to the porosity and the nature of the
bonding in the soil.

108. • Soil moisture characteristic curve is strongly affected by soil
texture. The graeter the clay content, in general, the greater the
water retension at any particular sunction and the more gradual
the slope of the curve
• -In a sandy soil most of the pores are relatively large and once
these pores are emptied at a given suction only a small amount of
water remains
• -In a clay soil the pore size distribution is is more uniform and more
water is adsorbed so that increasing increasing the matric suction
causes a more gradual decrease in water content ( See figure
below)
•
• -Soil structure also influences the shape of the curve particularly in
the suction range-Effect of compaction is to reduce total porosity

109. Water flow
• Soil contains a large distribution of pore sizes
and channels through which water flows. The
flow of water (i.e. liquid) in soil is important
for calculations of water balance and
redistribution of solutes and energy within
soil-plant-atmosphere system.

110. • Flow rate also depends on hindrances; friction
between water and particle surfaces as well as
pore constrictions and other interruptions in
flow path.
• It is generally recognized that three types of
water movement occur in soil;
• Saturated,
• Unsaturated, and
• Vapour

112. • Saturated Hydraulic Conductivity
• Saturated flow occurs when soil pores are
completely filled with water Saturated flow in
soil with large continuous pores can be rapid if
driven by large differences in gravity and
pressure.

113. • Unsaturated flow is generally rapid through
fine sand or well aggregated loams (medium
sized pores) and slower through very fine and
poorly aggregated clayey soil (very small
pores).
• Vapour flow occurs only in relatively dry soil
where vapour pressure differences are
significant.

114. • Field capacity
• Field Capacity is the amount of soil moisture or
water content held in the soil after excess water
has drained away and the rate of downward
movement has decreased. This usually takes
place 2–3 days after rain or irrigation in pervious
soils of uniform structure and texture. The
physical definition of field capacity (expressed
symbolically as θfc) is the bulk water content
retained in soil at −33 J/kg (or −0.33 bar) of
hydraulic head or suction pressure.

115. • Permanent wilting point (PWP) or wilting point (WP)
is defined as the minimal point of soil moisture the
plant requires not to wilt. If moisture decreases to this
or any lower point a plant wilts and can no longer
recover its turgidity when placed in a saturated
atmosphere for 12 hours. The physical definition of the
wilting point (symbolically expressed as θpwp or θwp) is
defined as the water content at −1500 J/kg of suction
pressure, or negative hydraulic head.
• However, it is noted that the PWP values under field
conditions are not constant for any given soil but are
determined by the integrated effects of plant, soil, and
atmospheric conditions.