Soil physics atterberg limit,compaction, shear strength,crusting and puddling

Soil Physics
ACSS-501

Dr. P.K. Mani
Department of Agril. Chemistry and Soil Science
Bidhan Chandra Krishi Viswavidyalaya,
Mohanpur, Nadia, West Bengal, India
E-mail: pabitramani@gmail.com, Website: www.bckv.edu.in

Copyright© Markus Tuller and Dani Or2002-2004

Syllabus:
Unit-III:
Soil consistence,
Dispersion and workability of soils;
Soil Compaction and consolidation;
Soil strength, swelling and shrinkage-basic concepts

Unit-IV:
Soil tilth, characteristics of good soil tilth;
Soil crusting-mechanism, factors affecting and evaluation;
Soil conditioners,
Puddling, its effect on soil physical properties;
Clod formation.

Daniel Hillel has written the most
widely used textbooks in soil physics.
Soil physics, as a scientific endeavor, deals with the state and
movement of matter and with the fluxes and transformations of
energy in the soil and related porous media. (SSSA)

Soil Physics 2010


Soil Physics 2010


Soil Structure

• Soil structure is the arrangement of
particles in the soil

• Structure affects aeration, water
movement, heat transfer, and root growth

• Granular is ideal for agriculture, allows for
maximum pore space


Impact of decline in soil structure on soil physical quality

Effects of soil structure on ecosystem functions.


prismatic

granular

blocky

platy

single grained

columnar

massive

What is Soil Consistency ?
•Soil consistence provides a means of describing the degree and kind of
cohesion and adhesion between the soil particles as related to the
resistance of the soil to deform or rupture.
•Since the consistence varies with moisture content, the consistence can
be described as dry consistence, moist consistence, and wet
consistence.
•Consistence evaluation includes rupture resistance and stickiness.
The rupture resistance is a field measure of the ability of the soil to
withstand an applied stress or pressure as applied using the
thumb and forefinger.
•Soil consistency is defined as the relative ease with which a soil can be
deformed use the terms of soft, firm, or hard.
•Consistency largely depends on soil minerals and the water content.


little or no soil adheres to fingers after
release of pressure

soil adheres to both
fingers after
release of pressure
with little stretching
on separation of
fingers
soil adheres firmly to both fingers after release
of pressure with stretches greatly on separation
of fingers

Soil Consistency - Atterberg Limits
Atterberg, a Swedish agriculturist, proposed a concept dividing the entire cohesive
range of the soil into five stages and six divisions of soil wetness. These limits,
corresponding with soil moisture content from harsh consistency to viscous flow, are
called Atterberg constants. Shrinkage Limit, Lower Plastic Limit, Cohesion Limit.
Sticky Limit, Upper Plastic Limit , Upper Limit of Viscous Flow.

Depending on Moisture Content soil can be divided into:
Moisture Content (w)

(-)

1.

Solid
Shrinkage Limit (SL)

2.

Semi-Solid
LI = 0

3.

Plastic

4.

Liquid

(+)

Liquidity
Index (LI)

Plastic Limit (PL)
Plasticity Index
(PI) = PL - LL
Liquid Limit (LL)

LI = 1

LL: The lowest water content
above which soil behaves like
liquid, normally below 100.
PL: The lowest water content at
which soil behaves like a plastic
material, normally below 40.
PI: The range between LL and PL.
Shrinkage limit:
the water content below which
soils do not decrease their volume
anymore as they continue dry out.
–needed in producing bricks and
ceramics .

Lower Plastic limit: This refers to the
moisture content corresponding with
the lower limit of the plastic range
Upper Plastic Limit. This is also
called the liquid limit or the lower limit
of viscous flow.

Liquid Limit - Definition
Liquid Limit (LL) is defined as the moisture content at
which soil begins to behave as a liquid material and
begins to flow
(Liquid limit of a fine-grained soil gives the moisture content at
which the shear strength of the soil is approximately
2.5 kN / m2)

Plastic Limit - Defn
The moisture content (%) at
which the soil when rolled
into threads of 3.2mm (1/8
in) in diameter, will
crumble.

PL = w% at d 3.2 mm (1/8 in.)

•Casagrande (1932) studied the relationship of the plasticity index to the
liquid limit of a wide variety of natural soils.
• He proposed a plasticity chart


Plasticity Index - Definition
Plasticity Index is the difference between the
liquid limit and plastic limit of a soil.
PI = LL – PL


Shrinkage Limit - Definition
The moisture content, in percent, at which the volume of the soil mass
ceases to change


A complex and interactive
relationship between Atterberg’s
limits, soil tilth, and soil moisture
content is shown in Fig.

Soil produces a good tilth when
cultivated at a moisture content
corresponding to a friable consistency or
in the vicinity of the lower plastic limit.
Soil does not produce clod when
plowed at this moisture content. Soils are
highly susceptible to compaction and
puddling when cultivated within the
plastic range.
If the lower plastic limit is smaller than
field capacity, soil structure may be
adversely affected if soil is cultivated at
moisture content between the lower
plastic limit and the field capacity.
 If the lower plastic limit is greater than
the field capacity, good soil tilth is
produced when it is cultivated at
moisture content between the lower
plastic limit and field capacity.

Shrinkage Limit - Measurement


The Atterberg limits are a basic measure of the nature of a fine-grained soil..
Thus, the boundary between each state can be defined based on a change in
the soil's behavior. The Atterberg limits can be used to distinguish between
silt and clay, and it can distinguish between different types of silts and
clays.

Why Atterberg Limits ?
•Atterberg limits are important to describe the consistency of fine-grained

soils
•A fine-grained soil usually exists with its particles surrounded by water.
•The amount of water in the soil determines its state or consistency
•Four states are used to describe the soil consistency;
solid, semi-solid, plastic and liquid
•The knowledge of the soil consistency is important in defining or
classifying a soil type or predicting soil performance when used a
construction material.
The soil consistency is a practical and an inexpensive way to distinguish
between silts and clays

Schematic variation of soil bulk density in non swelling
(rigid), moderately swelling and extensively swelling soils.
Fig. 2, rigid or non
swelling soils do not
change their
specific volume, ν,
and hence, their bulk
density ρ during
their water content θ
variation range.
b

Rigid or non swelling soils are usually
coarse – textured, organic matter – poor, and hard to till.
They also have low aggregate stability, high module of rupture, and
low resilience after a given damage (e.g. compaction by agricultural
traffic). They are considered to have hard-set behavior.


Schematic variation of soil bulk density in non swelling (rigid),
moderately swelling and extensively swelling soils.
In contrast,
extensively swelling
soils undergo
significant bulk
density, ρb,
variations during
their water content,
θ, variation range.

They are usually fine –textured, with smectitic type of clays. They develop
desiccation cracks on drying, which confers them high resilience, and little
tillage requirement. They are considered to have self-mulch behavior.

Fig. 3. A hard plow pan developed in the subsoil of a non swelling sandy
loam (Haplic Phaeozem) of the Argentine Pampas. After several years of
disc plowing, a hard plow pan is developed in the subsoil.

Fig. 4. Extensively swelling
Vertisol of the Argentine
Mesopotamia, cropped to
soybean using zero tillage

The process of swelling is mainly caused by the intercalation of water
molecules entering to the inter-plane space of smectitic clay minerals
( Parker et al.1982). An schematic visualization of this process is depicted by Fig. 5.


Consequences of soil swelling
Unfavorable effects are
 the destruction of buildings, roads and pipelines in uncropped soils,
the leaching of fertilizers and chemicals below the root zone through
desiccation cracks (by pass flow). In these soils horizontal cracks break
capillary flux of water.
Favourable effects,
swelling clays can be used to seal landfills storing hazardous wastes.
This sealing avoids the downward migration of contaminants to ground
water .
In cropped soils, the development of a dense pattern of cracks on
drying improves water drainage and soil aeration, and
decreases surface runoff in sloped areas.
Soil cracking is closely related to the recovery of porosity damages by
compaction.

Methods for assessing soil swell-shrink potential
a) Coefficient of linear extensibility, COLE
It characterizes the variation of soil volume from 1/3 atm water retention
(i.e. field capacity) to oven dry conditions:
where Lm is length of moist
sample, Ld is length of dry
sample, is wet bulk
densit y
(measured on
plastic coated clods at 0.3
or 0.1 bar suction) and ρb is
dry bulk density.

A range of soil swell-shrink
potential can be
distinguished


SOIL VISCOSITY

As soil moisture content increases, its consistency changes from plastic,
to sticky, to viscous. When viscous, soil flows under stress and the flow
is proportional to the force applied. When plastic, a certain amount of
force must be applied before any flow is produced. The flow behavior of a
soil is explained by the Bingham equation
where V is the volume of flow,
μ is the coefficient of mobility,
F is the force applied,
F′ is the force necessary to
overcome the cohesive forces
(also called the yield value), or
F′ is zero and the volume of flow is
proportional to the force.
The constant of proportionality k in
viscous flow is the coefficient of
viscosity of the liquid

V=kμ(F−F′)


SOIL SHRINKAGE

Atterberg limits also have an important application to soil shrinkage. Atterberg
defined “shrinkage limit” as the soil moisture content below which the soil
ceases to shrink, and represents the lower moisture limit of the semisolid
state or soft-friable consistency. The process of shrinkage is due to the
manifestations of the diffused double layer, and due to the forces of surface
tension at the air-water interface.
The magnitude of volume change
depends of
soil structure,
Aggregate shape, porosity and
pore size distribution, nature,
and amount of clay.
Therefore, the shrinkage
process is related to the change in
total volume (Vt) in relation to the
change in volume of water (θ) in
Normal (upper solid line) and residual (lower solid line)
the soil .
−3
shrinkage curves for a soil bulk density of 1.1 Mg m .

Figure which shows two distinct types of shrinkage.
The normal shrinkage (curve segment labelled AB) refers to the process
in which decrease in total soil volume (Vt) is proportional to the volume of
water (θ) withdraw from the soil.
The slope of the normal line is an important indicator of the kind of
shrinkage. If the angle is 45°, the soil displays a normal shrinkage. If the
angle is <45°, the soil displays less than normal shrinkage. The angle of
the line of normal shrinkage is an important soil characteristic(Mitchell,
1992) and is influenced by managt.
The normal shrinkage continues
until the point when there is a strong
interaction between particles,
and further shrinkage is
caused by compression and
orientation of particles rather than
due to decrease in Vt.
This shrinkage is called the residual
shrinkage (curve segment labeled BC).

Application of Shrinkage
Soil shrinkage is a rapid process compared with swelling which can
continue for several years under confined environments.
In agricultural soils, shrinkage is evidenced by formation of cracks.
 Soil cracks are large if the soil is cohesive (e.g., Vertisols) and
small but numerous when soil is well structured with little cohesion
between aggregates.
 When soils develop large cracks, there is a considerable damage to
plant roots.
Roots in a severely cracked soil are confined to the small and dense
soil mass between the cracks, thereby decreasing water and nutrient use
cracks
efficiencies. Roots also affect soil shrinkage
(Mitchell and Van Genuchten, 1992).

SOIL STRENGTH
It refers to the capacity of a soil to resist, withstand, or endure an applied
stress (σ) without experiencing failure (e.g., rupture, fragmentation, or
flow).
It is soil’s resistance that must be overcome to cause physical
deformation (ε) of a soil mass.
It implies the maximal stress which may be induced in soil without causing
it to fail.
soil strength has applications to
root growth,
seedling emergence,
aggregate stability,
erodibility and erosion,
compaction and compactability, and
draft requirements for plowing

Factors Affecting Soil Strength
Soil Structure. Aggregate size is an important determinant of soil strength.
Stress at fracture decreases exponentially with increase in aggregate (clod) diameter.
Soil Bulk Density. It determines the magnitude of particle-to-particle
contacts. Effects of soil bulk density on soil strength are confounded
with
those of soil moisture content. Soil strength decreases with
increase in
total soil volume
ln S= − F ln V + A
S - soil strength, V - soil volume, A -adjustment factor, and F - soil constant
Properties of Soil Solids. Soil constitution (i.e., particle size distribution,
clay mineralogy, and soil organic matter concentration) affects soil
strength through changes in aggregation, soil bulk density and specific
volume, moisture content, and types of pores.
Soil Moisture Content. Soil strength increases with decrease in soil
moisture content or moisture potential. Soil drying increases strength by
increasing capillary cohesion as it increases the
effective stress, and
compactness by shrinkage
For a given bulk density, soil strength decreases with increasing soil moisture content. For
39
a given soil moisture content, soil strength increases with increase in soil bulk density. In
general, fine-textured soils at low moisture content exhibit high strength.

Strength of different materials

Steel

Tensile
strength

Concrete

Soil

Compressive
strength

Shear
strength

Complex
behavior

Presence of pore water
40

Shear Strength of Soils
Soil strength may be of two
types:
(i)resistant to volumetric
compression, and
(ii)resistant to linear

deformation or shear
strength.

Dr. Attaullah Shah

Shear strength of a soil is the resistance to deformation by continuous
shear displacement of soil particles due to tangential (shear) stress.
41


SHEAR STRENGTH OF SOILS:
Necessity of studying Shear Strength of soils :
• Soil failure usually occurs in the form of “shearing” along internal
surface within the soil.
• Thus, structural strength is primarily a function of shear strength.

Shear Strength:
• The strength of a material is the greatest stress it can sustain.
• The safety of any geotechnical structure is dependent on the
strength of the soil.
• If the soil fails, the structure founded on it can collapse.

Mass Wasting: Shear Failure

45

Shear Strength in Soils :
 The shear strength of a soil is its resistance to shearing
stresses.
 It is a measure of the soil resistance to deformation by
continuous displacement of its individual soil particles.
 Shear strength in soils depends primarily on interactions
between particles.
 Shear failure occurs when the stresses between the particles
are such that they slide or roll past each other

46

Components of shear strength of soils
Soil derives its shear strength from two sources:
– Cohesion between particles (stress independent component)
• Cementation between sand grains
• Electrostatic attraction between clay particles
– Frictional resistance and interlocking between particles
(stress dependent component)
Cohesion (C), is a measure of the forces that cement
particles of soils

47

Internal Friction :
Internal Friction angle (f), is the measure of the shear strength of soils due
to friction

48

Stresses:
Gravity generates stresses (force per unit area) in the ground at
different points.
Stress on a plane at a given point is viewed in terms of two
components:
Normal stress (σ) : acts normal to the plane and tends to
compress soil grains towards each other (volume change)
Shear stress (t ): acts tangential to the plane and tends to
slide grains relative to each other (distortion and ultimately
sliding failure)

49

Stress refers to the force per unit area.
For a given plane at a point, the resultant stress vector may be
divided into two components: normal and tangential stress.
Normal Stress (σ). Normal stress is caused by a force vector
perpendicular to the area of action σ =Fn/A
where Fn is the force acting normal to the area A.
The transmitted normal stress generally decreases with distance from
the applied load and with distance from its line of action.
Tangential Stress (τ) or Shearing Stress.
This stress is caused by a force vector parallel to the area of
action
τ =Ft/A
where Ft

is the tangential force acting on area A.
50

Definition of stress and strain
The reaction of a solid body to a force F or a combination of forces
acting upon or within it can be characterized in terms of its relative
deformation or strain. The ratio of force to area where it acts is called
stress.

normal stress
σ = Fn / A
shear stress
τ = Fs / A

normal strain
ε = δz / zo
shear strain
γ = δh / zo

Note that compressive stresses and strains are positive and
counter-clockwise shear stresses and strains are positive.


Shear strength of soil is

“The capacity of a soil to resist the internal and external
forces which slide past each other”

52

Factors Influencing Shear Strength:
The shearing strength, is affected by:
– Soil composition: mineralogy, grain size and grain size distribution,
shape of particles, pore fluid type and content, ions on grain and in pore
fluid.
– Initial state: State can be describe by terms such as: loose, dense, over
consolidated, normally consolidated, stiff, soft, etc.
– Structure: Refers to the arrangement of particles within the soil mass; the
manner in which the particles are packed or distributed.
Features such as layers, voids, pockets, cementation, etc, are part of the
structure.

Total

vs.

effective stresses

When a load is applied to soil, it is carried by the water in the
pores as well as the solid grains. The increase in pressure
within the pore water causes drainage (flow out of the soil), and
the load is transferred to the solid grains.
The rate of drainage depends on the permeability of the soil.
The strength and compressibility of the soil depend on the
stresses within the solid granular fabric. These are called
effective stresses.


Formulation of Shear Strength of Soil:
• In reality, a complete shear strength formulation would account for all
previously stated factors.
• Soil behavior is quite complex due to the possible variables stated.
Coulomb Failure Criterion :
Coulomb stated that
“the shear stress at failure is a function of
normal stress”
and is given by:

Charles Augustin de Coulomb
(1736 - 1806)


Charles Mohr

The Mohr Failure Criterion:
Mohr presented in 1900 a theory of rupture of materials, that was the result
of a combination of both normal and shear stresses.
The shear stress at failure is thus a function of normal stress and the Mohr
circle is tangential to the functional relationship given by Coulomb

The Mohr-Coulomb Failure Criterion:
This theory states that: “a material fails because of a critical
combination of normal stress and shear stress, and not from
their either maximum normal or shear stress alone”


Soil strength – undrained shear
The maximum value of stress that may be sustained by a material is termed strength.

The strength is independent of the normal stress since the response to
loading simple increases the pore water pressure and not the effective
stress.
The shear strength τ f is a material parameter which is known as the
undrained shear strength su.

τ f = (σ a - σ r) = constant


Soil strength – the angle of friction

The strength increases linearly with increasing normal stress and is zero
when the normal stress is zero.
τ'f = σ'n tanφ‘ where,
φ' is the angle of friction
In the Mohr-Coulomb criterion the material parameter is the angle of
friction f’ and materials which meet this criterion are known as frictional.
 In soils, the Mohr-Coulomb criterion applies when the normal stress is
an effective normal stress.

Soil strength

-

cohesion
•..

The strength increases linearly with increasing normal stress
and is positive when the normal stress is zero.

τ'f = c' + σ'n tanφ'

φ' is the angle of friction
c' is the 'cohesion' intercept
In soils, the Mohr-Coulomb criterion applies when the normal stress is an
effective normal stress.

Typical values of φ‘, c’ and su
Undrained shear strength
Hard soil
Stiff soil
Firm soil
Soft soil
Very soft soil
Drained shear strength
Sands
Clays
Precompression stress Pv
soft
firm
stiff

su > 150 kPa
su = 75 ~ 150 kPa
su = 40 ~ 75 kPa
su = 20 ~ 40kPa
su < 20 kPa
c´ (kPa)
0
0 - 30 kPa

φ´ (deg)
30° - 45°
0 - 20°

0-50 kPa
50-150 kPa
> 150 kPa

Mohr Theory of Soil Strength
This theory is based on the functional relationship between normal stress (σ)
and tangential or shearing stress (τ). The envelope of the family of circles
is used as a criterion of shearing strength of soil.
When a series of stress states just sufficient to cause failure is imposed on the
same soil material, these states can be plotted as a set or family of Mohr
circles.
The line tangent of these circles, called the envelope of the family of
circles, is used as a criterion of shear strength.
When this envelope is a straight line, it can be described mathematically by Eq.
τ = τo + bσ
The intercept (τ o) is the shear stress needed to
cause failure when normal stress (σ) is zero,
and is called soil cohesion (C) or
cohesiveness. Substituting these terms in Eq.
yields following Eq. used to express soil shear
strength.

65

The functional relationship between shearing stress (τ)
and normal stress (σ) is given by Mohr’s circle (a or τo
is the intercept and constant b is the tangent of angle Φ

TRIAXIAL COMPRESSION TEST

Failure Surface

By Kamal Tawfiq, Ph.D., P.E.

Triaxial Compression Test

1- Applying
Confining
Stresses

2- Loading

)F

F2

Each Circle
=
One Test

F2

F2

F2
F2

3- At Failure

Deviator stress = ) Ff
+

F2

Axial
= F1 stress

Confining stress = F2

F2

F2

J

F2

2
F2

F2
+

F1 = Major Principle Stress
F2 = Minor Prencipal Stress

+

Failure
Surface

+

)F

) Ff
)F = deviator stress

= F1

Failure Surface

Cohesion =

c
F2

2

F2

F2 F 1

F1

F1

Fn
By Kamal Tawfiq, Ph.D., P.E.

Soil Compaction
Dr. P.K. Mani


Soil Compaction – desired or not?
• In agriculture and
forestry soil
compaction is
undesirable.

• For many engineering
applications a well
compacted soil is
crucial for safe
foundations (the
Leaning Tower of
Pisa is an example of
building on soft soil).

Image: Opera Primaziale Pisana


Soil Compaction in the Field:

5- Sheep foot Roller

1- Rammers

2- Vibratory Plates
6- Dynamic Compaction
3- Smooth Rollers

4- Rubber-Tire

Porosity value generally
ranges from 0.3 to 0.6
(30–60%).
In clayey soils, the porosity
is highly variable because
the soil alternately swells,
shrinks, aggregates,
disperses, compacts, and
cracks

it ranges between 0.3 and 2.


Definition:

Soil compaction is defined as the method of mechanically
increasing the density of soil by reducing volume of air.

γsoil (2) > γsoil (1)

Load

Air

Air
Water

Water

Soil
Matrix

Compressed
soil
Solids

Solids

γsoil (1) = WT1
VT1

γsoil (2) = WT1
VT2


Why Soil Compaction:
1- Increase Soil Strength
2- Reduce Soil Settlement
3- Reduce Soil Permeability
4- Reduce Frost Damage
5- Reduce Erosion Damage

Water is added to
lubricate the contact
Factor Affecting Soil Compaction:
surfaces of soil
1- Soil Type
particles and improve
2- Water Content (w c)
the compressibility of
3- Compaction Effort Required (Energy)
Types of Compaction : (Static or Dynamic) the soil matrix
1- Vibration
2- Impact
3- Kneading
4- Pressure

Water is added to lubricate the contact surfaces of soil
particles and improve the compressibility of the soil matrix

Soil Compaction in the Lab:
1- Standard

Proctor Test
2- Modified Proctor Test
3- Gyratory Compaction

Standard Proctor Test Modified Proctor Test

Gyratory Compaction


Effect of Soil Type:
The soil type - that is,
grain-size distribution,
shape of the soil grains,
speciific gravity of soil
solids,
and amount and type of
clay minerals presenthas a great influence on
the maximum dry unit
weight and optimum
moisture content


4 types of compacn curves encountered in soils

Type A compaction curves are
those that have a single peak.
This type of curve is generally
found for soils that have a liquid
limit between 30 and 70.
Curve type B is a one-and-onehalf-peak curve,
and curve type C is a doublepeak curve. Compaction curves
of types B and C can be found
for soils that have a liquid limit
less than about 30.
Compaction curves of type D do
not have a definite peak. They
are termed odd shaped. Soils
with a liquid limit greater than
about 70 may exhibit
compaction curves of type C or
D. Such soils are uncommon.

Agricultural Soil Compaction – Causes

Operation of
heavy vehicles
(e.g. harvesters,
construction
machines) on
agricultural land
can cause soil
compaction.


SOIL COMPACTION
Soil compaction can be conceptually viewed in a dynamic or a static
situation, and in practical applications.
•
Dynamic situation, it is a physical deformation or a volumetric strain.
•

Static situation, it is the characteristic related to soil resistance to
increase its bulk density.

•

In practice, soil compaction is a process leading to compression of a
mass of soil into a smaller volume and
deformation resulting in decrease in total porosity and
macroporosity and reduction in water transmission and gaseous
exchange.

•

The degree or severity of soil compaction is expressed in terms of soil bulk
density (ρb), total porosity(ft), aeration porosity (fa), and void ratio (e).
The volume decrease is primarily at the cost of soil air, which may be
expelled or compressed. The compression of soil solids (i.e.,change in
ρs) and water (i.e., change in ρw) is evidently not possible.

Compression of a moist soil due to external load may displace the liquid
and increase the contact area between two particles (Fig.). The
magnitude of increase in contact area depends on the degree of
rearrangement or deformation of the particles. The menisci formed by
the liquid may also change due to differences in the contact area. The shape
of the meniscus depends on surface tension forces, which are usually small
compared with the external load. The deformation may be elastic and soil
particles may regain their original shape when the applied load is released.

The degree of deformation and rearrangement depends on soil
structure and aggregation, and on the extent to which soil particles can
change position by rolling or sliding.
For partly saturated clayey soils, the volume change depends on
reorientation of the particles and displacement of water between
particles. The particle rearrangement may lead to closed packing with
attendant decrease in void ratio.

e=eo−c log P/Po
where eo = void ratio at the initial pressure Po, c = slope of the
curve on semilogrithmic plot, and P = applied pressure that
changed
the final void ratio to e.
Degree of soil compaction may also be expressed in terms of total
porosity in relation to the external load (Soehne, 1958) .

ft = −A lnP+f10
where ft is total porosity, f10 is the porosity obtained by compacting loose
soil at a pressure of 10 PSI, A is the slope of the curve, and P is the applied
pressure.

Soil compaction is extremely relevant to
• Agriculture because of its usually adverse impact on root development
and crop yields;
• Civil engineering because of its relation to settlement, stability, and
groundwater flow; and to
• Environments because of its effects on erosion, anaerobiosis,
transport of pollutants in surface and sub-surface flow, and nature and rate
of gaseous flow from soil to the atmosphere.
From an agricultural perspective especially in relation to plant root
growth, there is an optimal range of soil bulk density, which for most
soils is <1.4 Mg/m3.
However, the optimum range of soil bulk density may differ among
soils and crops (Kyombo and Lal, 1994). For some soils (e.g., Andisols or
soils of volcanic origin) the optimal density may be as low as 1.0.
A similar case may be in soils containing a high level of soil organic matter
content. It is precisely because of these differences in response to bulk
density that effects of compaction on crop yield are highly soil-dependent

Soil Compactibility
Soil compaction or densification happens due to external load or force
applied to the soil.
The force applied per unit area is defined as stress, which may be
normal stress when it is perpendicular to the soil ;
shear stress when it has a tangential component.
Compression is the process of increase in soil mass per unit volume due to
external load. The load may be static or dynamic. The latter is applied in the
form of vibration, rolling, or trampling.
While compression in unsaturated soils is called “compaction,”
that in saturated soils is termed “consolidation.”
Soil compressibility is the “resistance of a soil against volume decrease
by external load.”
In comparison, soil compactability is the difference between the initial
bulk density and the maximum bulk density to which a soil can be
compacted by a given amount of energy at a defined moisture content.


WHAT ARE THE CONSEQUENCES OF SOIL COMPACTION FOR
PLANT GROWTH?
Soil compaction can have both desirable and undesirable effects on
plant growth.
Desirable Effects
•Slightly compacted soil can speed up the rate of seed germination
because it promotes good contact between the seed and soil. In
addition, moderate compaction may reduce water loss from the soil due
to evaporation and, therefore, prevent the soil around the growing seed
from drying out. Corn planters have been designed specifically to provide
moderate compaction with planter mounted packer wheels that follow seed
placement.
•A medium-textured soil, having a bulk density of 1.2 g/cc , is generally
favorable for root growth.
• However, roots growing through a medium-textured soil with a bulk density
near 1.2 g/cc will probably not have a high degree of branching or
secondary root formation.
•In this case, a moderate amount of compaction can increase root
branching and secondary root formation, allowing roots to more
thoroughly explore the soil for nutrients. This is especially important for plant
uptake of non-mobile nutrients such as phosphorus.

Undesirable Effects
• Excessive soil compaction impedes root growth and
therefore limits the amount of soil explored by roots.
This, in turn, can decrease the plant's ability to take
up nutrients and water.
•From the standpoint of crop production, the adverse
effect of soil compaction on water flow and storage
may be more serious than the direct effect of soil
compaction on root growth.
•In dry years, soil compaction can lead to stunted,
drought stressed plants due to decreased root growth.

Fig.2. Nitrogen
deficiency
symptoms in corn.

•Soil compaction in wet years decreases soil
aeration. This results in increased denitrification .
• There can also be a soil compaction induced
nitrogen and potassium deficiency (see Fig. 2 & 3).
Plants need to spend energy to take up potassium.
• Reduced soil aeration affects root metabolism.
All of these factors result in added stress to the crop
and, ultimately, yield loss.

Fig. 3. Potassium
deficiency
symptoms in corn.

Research from North America and
Europe indicates that crops respond
to soil compaction as shown in
Figure 4.

• In a dry year, at very low
bulk densities, yields
gradually increase with an
increase in soil compaction.
Soon, yields reach a maximum level at
which soil compaction is also optimal
for the specific soil, crop, and climatic
conditions. However, as soil
compaction continues to increase
beyond optimum, yields begin to
decline.

•With wet weather, yields are
decreased with any increase
in compaction.

Fig. 4. Effects of weather on crop
yield response to compaction
level
(Soane et al., 1994).

Due to the increase in bulk density, the porosity of soil decreases.
Large pores (called macropores), essential for water and air movement
in soil, are primarily affected by soil compaction. Research has
suggested that most plant roots need more than 10 percent airfilled
porosity to thrive.

Total porosity and macroporosity were greatly reduced in an original
and a subsoiled but subsequently recompacted plow pan compared
to an uncompacted pasture.
( Kooistra, and Boersma. 1994. Soil Tillage Research 29:237–247.)

The surface of long-term, no-till soil cannot be
compacted to as great a density as conventionally
tilled soil due to higher organic matter contents.
(Thomas, et al.,1996)

No-till has a lot of advantages
over tillage—reduced labor
requirements, reduced
equipment costs, less runoff
and erosion, increased
drought resistance of crops,
and higher O.M. content
and biological activity.
•The higher O.M. content
and biological activity in
no-till makes the soil more
resilient to soil compaction.
Topsoil from long-term no-till and
conventional till fields were subject
to a standard compaction treatment
at different moisture contents.

The “Proctor Density Test” is used to determine what the maximum compactability of
soil is. The conventional till soil could be compacted to a maximum density of 1.65
g/cm3, which is considered root limiting for this soil. The no-till soil could only be
compacted to 1.40 g/cm3, which is not considered root limiting. Thus, topsoil
compaction would be less of a concern in no-till fields.

this vicious compaction/
tillage spiral is an
environmental threat with
impacts beyond the
individual field.(Duiker,
2004)

More tillage operations and more power
are needed to prepare a seedbed in
compacted soil. This leads to increased
pulverization of the soil and a general
deterioration of soil structure, which
makes the soil more sensitive to
recompaction.
Therefore, compaction can enforce a
vicious tillage spiral that degrades soil
and results in increased emissions of the
greenhouse gases CO2, CH4, NO2 due to
increased fuel consumption and slower
water percolation. Ammonia losses also
increase because of decreased infiltration
in compacted soil. More runoff will cause
increased erosion and nutrient losses to
surface waters. At the same time, reduced
percolation through the soil profile restricts
the potential for gw (ground water) recharge
from compacted soils.

The most direct effect of
soil compaction is an
increase in the bulk density
of soil.
Optimum bulk densities for
soils depend on the soil
texture .
Whenever the b.d . exceeds
a certain level, root growth
is restricted..
A note of caution must be made here in respect to the effects of tillage on bulk
density . No-till soils often have a higher bulk density than recently tilled soils.
However, because of higher organic matter content in the topsoil and
greater biological activity, the structure of a no-till soil may be more favorable
for root growth than that of a cultivated soil, despite the higher bulk density.

•Soil compaction causes a decrease in large pores (called macropores), resulting
in a much lower water infiltration rate into soil, as well as a decrease in saturated
hydraulic conductivity.
•Unsaturated hydraulic conductivity sometimes increases due to compaction.
Unsaturated hydraulic conductivity is important when water has to move to roots.
Thus, compacted soils are sometimes not as drought sensitive as uncompacted soils—
assuming the root system is of equal size in both cases, which is usually not the case.
Typically, the net effect of compaction is that crops
become more easily damaged by drought because of a small root system.

Management of Soil Compaction in Agricultural Lands
There are two strategies of soil compaction management:
(i) minimizing risks of soil compaction or compaction prevention, and
(ii) compaction alleviation .
Preventive strategies are economic and have less adverse impacts on crop yields and
environments than the curative measures of compaction alleviation (Larson et al., 1994).
•A useful strategy to prevent soil compaction is to minimize the vehicular traffic to the
absolutely essential by reducing the number and frequency of operations, and performing
farm operations only when the soil moisture content is below the optimal range for the
maximum proctor density.
• Mulch farming and conservation tillage (Lal, 1989; Carter, 1994) reduce the risk of soil
compaction for some soils and environments.
• Guided traffic system, low ground pressure tires (Vermeulen and Perdok, 1994),
adoption of dual tires, and wide tires are other innovative ideas of decreasing pressure on
soil.
• The guided traffic system involves confining vehicular traffic to permanent narrow lanes
and reducing the fractional area affected by traffic wheels to as little as possible
(Taylor,1994).

MANAGING SOIL COMPACTION
Avoid trafficking wet soil. Only wet soil can be compacted. Fields should not be trafficked
if they are at or wetter than the plastic limit
 Keep axle loads below 10 tons. Subsoil compaction is caused by axle load and is
basically permanent.
Decrease contact pressure by using flotation tires, doubles, or tracks. Topsoil
compaction is caused by high contact pressure. To reduce contact pressure, a load needs
to be spread out over a larger area. This can be done by reducing inflation pressure. A rule of
thumb is that tire pressure is the same as contact pressure. Tires inflated to 100 psi such as
truck road tires should be kept out of the field
Decrease trafficked area by increasing swath and vehicle width or by decreasing
number of trips.
Reduce the area of a field that is subject to traffic by increasing swath width of manure
spreaders or the spacing between wheels so individual wheel tracks are more widely spaced.
Increase soil organic matter content and soil life.
Soil that has high organic matter content and thrives with soil organisms is more
resistant to compaction and can better recuperate from slight compaction damage.

Use tillage sparingly
•Soil tillage should be used sparingly to alleviate compaction when
no other means can be used.
•Growers should avoid falling into the vicious compaction/tillage spiral
as explained earlier.
•If any tillage is done, try to leave as much crop residue as possible
at the soil surface to protect against erosion and to use as a food
source for certain soil organisms such as earthworms.
•Noninversion tillage is preferable.
•If possible, perform tillage only in the seed zone.


The vehicle load can be distributed over a large area by using dual tires or wide
tires. The soil compaction hazard is less when the load is distributed over a large
area.

Subsoiling by chisel plow can decrease
bulk density and reduce soil strength
temporarily. The long-term goal is to create
stable biochannels in the subsoil.

Crop residue
mulch in a
no-till farming
system
buffers the
impact of
heavy
vehicles and
minimizes the
risk of soil
compaction


Soil Crusting
Dr. P. K. Mani

E-mail: pabitramani@gmail.com, Website:

Soil crust or surface seal, refers to the thin dense layer on the soil
surface characterized by low porosity, high density, and low
permeability to air and water.
Crusting is a soil surface phenomena caused by susceptibility of aggregates
at the soil-air interface to disruptive forces of climatic elements and
perturbations caused by agricultural practices (e.g., tillage and traffic).
Slaking, deflocculation, or dispersion of aggregates on rapid wetting or
submersion in water, is attributed to
 numerous factors including the effect of entrapped air, predominance of
Na+ on the exchange complex, and weak aggregate strength caused by low
level of soil organic matter content and weak ionic bonds. (Sumner and
Stewart ,1992).
Dispersion, reorientation of dispersed particles, drying, and
desiccation, lead to formation of a thin crust on the soil surface.

Crusted condition with high surface roughness due to raindrop


•A physical crust is a thin layer with reduced porosity and increased density at the sur face
of the soil.
• A biological crust is a living community of lichen, cyanobacter ia, algae, and moss
growing on the soil sur face and binding it together .
• A chemical crust or precipitate is white or pale colored and forms in soils with a high
content of salts. Both chemical and biological crusts can form on and extend into a physical
crust.
Properties of Crust
The crusted layer is more dense but may be of similar textural makeup than the unaffected
soil beneath it. The crust is primarily characterized by reduction in total volume, size,
shape, and continuity of pores.
Thickness of the crust may range from <1mm to 10 mm (Norton, 1987).
Very thin crusts are called “skin seal.” These microlayers are usually <0.1 mm
thick, extremely dense with no visible pores (McIntyre, 1958).
 Skin seals may be formed by reorientation of fine dispersed particles and/or washedin-fine material that plug the larger pores.
The magnitude of reduction in porosity of the crust may range from 30 to 90%, with
corresponding decrease in pore size.
The pore diameter in the crust may be as small as 0.075 mm (Valentin and Figueroa, 1987).

Soil structure at extreme scales.
Left: textural porosity in the clay
mineral kaolinite. The scale bar is
0.004 mm. Many of the pores visible
are small enough to retain water that
is not available to plants.

Right: structural porosity in a “selfmulching” clay soil with well-defined
aggregates and large pores that play
a role in drainage, aeration, root
growth and the activities of soil fauna.


Soil Crusts : General Aspects
A. Types

The two main types of soil crusts generally recognized are
structural and depositional crusts (Shainberg, 1985).
•The structural crusts are formed due to the shattering effect of raindrops
on an otherwise structurally more stable soil surface, followed by a
reorientation of soil particles,
• depositional crust is formed by sedimentation of particles from standing
or slowly flowing water.
several other types have been recognized and described, for example,
(i)White saline crusts resulting from salt accumulation near the soil surface (e.g. Brabant
and Gavaud, 1985);
(ii) Black saline crusts due to an accumulation of sodium carbonate (Aubert, 1976); and
(iii) Yellow saline crusts on very acid sulphate soils (Le Brusq et al., 1987) composed
mainly of aluminium, iron and magnesium sulphates.


Diagram showing processes involved and sequence of formation of the
various types of crusts (after Bresson and Valentin, 1990).


Mode of formation
Casenave and Valentin (1989) distinguished four main processes : (i) wetting,
(ii) raindrop impact, (iii) runoff and (iv) drying.
Wetting is important in loamy and clayey soils. When dry soil is rapidly wetted, air
entrapment occurs and pressure differences disrupt soil aggregates. Swelling occurring
concurrently, further aids the disruption process (Valentin,1991). Oversaturation of the
uppermost few millimeters of soil results in suspended, dispersed clay which fills the particle
interstices, thus forming a structural type surface crust. Slaking of dry aggregates due to
rapid wetting can occur independently of impact forces.
Raindrop impact
is the main cause of crusting on sandy soils. Sandy aggregates are quite fragile and
therefore readily break down under raindrop impact. Crater-like features develop at the
surface, with coarser particles on top of finer ones. This sorting process, together with
the accumulation of finer particles above the zone compacted by raindrops or above the
zone of a compressed air layer (Collinet, 1988), are considered to be the most significant
processes in the formation of Structural 1 and Structural 2 types of surface crusts .


During a light rainfall (intensity 0.1 mm h-1) drops of median diameter
1.25 mm, velocity 4.8 ms-1 falling at a rate of 280 m-2 s-1 were recorded
(Lull, 1959, Morin, 1993). The associated kinetic energy measured per unit
area and time was 12 J m-2 h-1. (Kinetic energy, E= ½ mv2, where m is the
mass of rain per unit area and v is the impact velocity of rain drop)
 Heavy rainfall of intensity 15 mm h-1 was associated with larger drops,
2.05 mm median diameter, greater fall velocity 6.7 m s-1, fell at a rate of 495
drops m-2 s-1 with a kinetic energy of 340 J m-2 h-1.
 During a cloudburst intensities of 1100 mm h-1 may occur which,
depending on the drop diameter can give rise to a kinetic energy of 3300 J
m-2 h-1 or greater.

Runoff
results in lateral movement of particles, combined with sorting. It favors the
formation of a dense, laminated crust referred to as a "runoff depositional"
crust.
When surface roughness decreases, runoff velocity (or wind velocity)
increases.
The one or two sandy microlayers of the Structural 2 or Structural 3 crust
types are washed away, exposing the denser seal composed of fine
particles, or the Structural 1 crust may be smoothed.
If the process proceeds leaving coarser gravel behind, a gravel crust
develops.
Drying
increases the strength of a surface crust exponentially as a
function of decreasing water content (Valentin, 1986). Cracks and a curling
up of platy structures can occur due to differences in shrinkage forces
among the microlayers. This phenomenon is pronounced in the
case of sedimentary crusts.

III. Methods Employed in the Study of Soil Crusts
A. Microscopy1. Micromorphological studies

The microscopic study of thin sections provides a powerful method for
examining soil surface crusts. It helps in characterizing the size and nature of
the components and the arrangement and porosity of the microlayers
forming the crusts.
Micromorphology was used to study surface crusts of saline soils in Egypt
2. Electronmicroscopy

Back-scattered electron scanning imagery (BESI) of thin sections is more
satisfactory, making clear delineation of the microlayers possible (Bresson
and Valentin, 1990). A further advantage of this technique is that information
on porosity is gained for a region just a few micrometers below the crust
surface.

Crust Strength Measurements
Crust strength was measured using the modulus of rupture technique
(Richards, 1953). It was found that the modulus of rupture is strongly
dependent on the bulk density of the bricquettes, irrespective of the
method used to increase bulk density.

S = 3FL/2bd2

Where S is the modulus of rupture in dyne/cm2,
F is the breaking force in dyne [the
breaking force in grams weight (the
mass of water needed for breaking the
briquettes placed on two supports on a
one-handle scale) × 980),
L is the distance between the two lower
supports, (cm)
b is the width of the briquettes, (cm) and
d is the depth or thickness of the
briquettes (cm).
The bar is a CGS unit of pressure and equal to one million
2

2


The methodology used for measuring modulus of rupture (MR) as an index
of crusting was that proposed by Reeve (1965).
Soil particles were collected after passing through 2.00 mm sieve and
poured in an aluminum rectangle mold (7 cm×3.5 cm×1 cm) which was
alighted on a screen covered with a filter paper. The internal surface of mold
was carefully covered with a thin layer of petroleum jelly to prevent the soil
particles from adhering to the mold. The excess soil was striked off by a
leveler. By adding distilled water from the beneath of the screen, soils
soaked well for 30 min. Then the samples were dried at 50oC in oven.
The briquettes were carefully removed to measure the pressure needed for
breaking them using the equation:
2

S = 3FL/2bd


Factors Affecting Crust Formation
Some soils are undoubtedly more prone to crusting than others. The permanence of a crust
formed under conditions of water application also varies greatly - e.g. self-mulching clay
soils have a discontinuous crust, but when wet and other conditions favoring crusting
prevail, a very dense crust with low hydraulic conductivity may form.
The factors involved in soil crusting may be grouped into two classes :
(A) those intrinsic to the soil and those due to (B)external influences

A. Intrinsic Soil Properties
1. Soil texture
Soil particle size distribution, particularly clay and gravel/cobble contents
and relative proportions of the various soil separates, affect soil crusting. High
clay contents generally favor aggregation and reduce the rate of crust
formation, although clay mineralogy and exchangeable cation composition will
modify this generalization. Medium-textured soils (< 20% clay) are
usually very susceptible to crusting. It is probable that in extremely sandy soils
the amount of clay, once dispersed, is not sufficient to clog the conducting
pores at the soil surface.


Several studies have shown that the texture most prone to crusting
consists of approximately 90% sand and 10% silt or clay
Surface gravel and cobbles may either increase or decrease soil crusting. In
the arid zone of west Africa, soils containing coarse fragments are usually
severely crusted (Casenave and Valentin, 1989). Conversely, in wet
savannah and in the rainforest zone, gravel originating from disintegrated
Iron pans
usually remains free on an uncrusted topsoil .
(Collinet and Valentin,1979).
In other studies,
coarse fragments protected the smaller
surface aggregates from raindrop
impact - in the same way as a mulch –
thus increasing infiltration and reducing
erosion(Collinet and Valentin, 1984)


2. -Clay mineralogy
Miller(1987), concluded that if the dominant clay mineral of the clay fraction is kaolinite,
crusting should be less serious, although the presence of even small amounts of
smectite and/or micaceous minerals could drastically increase the soil's crusting
tendency. Furthermore, the presence of free iron in soils from the humid to subkumid
tropics would also have a stabilizing effect.

3. Carbon content
organic matter is one of the most important aggregate stabilizing agents in soil.
The positive effect of organic matter on structural stability is more pronounced on sandy than
on the more finer textured soils. If the ratio R = organic matter (%)/(silt + clay)(%) is
considered, then 4 classes of soil with regard' to crusting hazard were distinguished. Crusting
hazard is greatest when R <5% and least when R > 9%. The threshold between low and high
crusting susceptibility occurred when R =7%.
R <5
: severe physical degradation
5<R<7 : high hazards of physical degradation
7 < R< 9 : low hazards of physical degradation,
9 < R : no physical degradation


Organic carbon percentage in the soil very clearly determined the MWD of
water-stable aggregates. The MWD increased from about 0.3 mm at 1.1 %
organic carbon to 3 mm when the organic carbon increased to about 2.4 %
4. Sesquioxide content

The stabilising effect of Fe and Al hydrous oxides and oxides are commonly
regarded as an important factor in aggregate formation. Farres (1987) tested
the aggregate stability of 20 soils from Mozambique and found that higher
amounts of iron were associated with greater soil stability to the effects of
raindrop impact.
The positive effect of a high Fe203 +
Al2O3 content on maintaining a good
infiltration rate under simulated rain
was shown by Smith (1990).
With increasing degree of
weathering (climatic factors), the
silica: sesquioxide ratio
increased and resistance to crusting
decreased


5. Exchangeable cations
It is well known that a high percentage of exchangeable sodium (high ESP) and in some
cases of exchangeable Mg, favors clay dispersion ( van der Merwe and Burger, 1969).
This in turn would increase soil crusting. In this regard the critical ESP (or critical EMgP),
that is, the ESP below which crusting is not affected by ESP, is of the utmost importance,
particularly in the case of irrigated soils.
Levy and van der Watt (1988) studied
the effects of clay mineralogy and soil
sodicity on crusting of four South African
soils. The effect of ESP on crusting
differed widely : some soils were hardly
affected, others affected at high ESP only
and others were affected at all ESP levels.
In all cases, a crust did form on the soil
surface; the ease and rate of crust
formation are apparent from the
infiltration rate versus cumulative
infiltration curves.

6. Topography and microtopography
Crusting processes, especially those related to kinetic energy of rain, are
most pronounced on very low gradients.
On steeper slopes, runoff and surface layer removal can be sufficient to
remove the crust as it forms, thus preventing the sharp decrease in
infiltration rate usually observed.
Microtopography and surface roughness affect crust formation and runoff in that the depth of
water films at the soil surface, microslopes and water flow velocity are all influenced on a
meso-scale.
The result is that a number of crust types and the severity of crusting varies from the
depressions to the tops of "mounds" (Levy et al., 1988).
Thus runoff depositional or sedimentary crusts develop in the depressions whereas
structural and erosional crusts are located on the more elevated zones.


B. External Factors
1. Kinetic energy of rain
Since the energy with which a falling water drop strikes the soil surface is
clearly related to its shattering effect on the aggregates, measurement or
calculation of the kinetic energy of water drops striking the soil surface is
very relevant. Both drop size (mass) and impact velocity, determine
kinetic energy, and the latter depends on the former.
Valentin and Ruiz
Figueroa (1987), using
rains with different kinetic
energies and various
types of soil cover
(sugarcane residue
mulch, shading gauze
and mosquito gauze)
showed that, for a sandy
loam soil in the Ivory
Coast, soil crusting was
directly related to rain
kinetic energy.
Crust and microlayer development under various
combinations of kinetic energy

The kinetic energy of the rain before runoff occurs is of greatest
importance for crusting, since once the surface is covered by a water film
the effect of raindrop impact is reduced
2. Irrigation
The composition of irrigation water, particularly in respect of its sodicity and
electrolyte concentration, determines the ease of chemical dispersion of the
soil aggregates.
3. Wind action
Wind acts as an agent of erosion. Particles are sorted, transported and
deposited by wind. The weakest microlayers of surface crusts, that is, the
sandy microlayers of structural type 1 and 2 crusts, are removed by wind
and the surface seal exposed, forming an erosion crust.


V. Consequences of Soil Crusting
A. Physical Consequences
1. Infiltration and runoff

Fig. Infiltration intensity function of
time in a uniform and porous soil as
well as in a soil covered with a crust
(Musy & Soutter, 1991)

Infiltration curves vs. cumulative
rainfall during seal formation ...


The crust rapidly becomes the dominant control on infiltration on most bare
soils. The loss in porosity in the surface layer decreases its hydraulic
conductivity, so there is a sharp contrast in the hydraulic conductivity of the
crust and sub-crust layers.
Surface crusting affects 2 characteristics of infiltration: the rate at which
infiltration decreases and the final infiltration rate.

On soils with good aggregate stability,
infiltration decreases more slowly and
remains at a greater final infiltration
rate.
It should be noted that the negative
impact of crusting on infiltration can be
used positively for water harvesting

60

Infiltration rate (mm/h)

Figure shows theoretical curves
relating infiltration to aggregate
stability.
Crusting proceeds more quickly on
soils with low aggregate stability, so
infiltration rate decreases more rapidly,
and the final infiltration rate is lower.

40

Good aggregate stability

20
Intermediate aggregate stability
Poor aggregate stability

0
0

20
40
60
80
Cumulative rainfall (mm)

100

Expected changes in infiltration
with cumulative rainfall for soils
with different aggregate stabilities.

B. Biological consequences
1. Seedling emergence
Hutson (1971) reported on experiments in which the effect of crust strength
of a Hutton Shorrocks (Rhodic Paleustalf) soil on the emergence of wheat
seedlings was studied. Emergence occurred only when the modulus of
rupture was below 400 millibar
2. Agricultural productivity
It is usually difficult to assess the contribution of a single soil physical
factor to an increase or decrease in crop yields. Surface crusts may
adversely affect seedling emergence and water storage in soil and thus
influence plant density and crop yield.
5
100

4

Biomass (g)

Emergence rate (%)

80
60
40

2
1

20
0

3

1

2
3
WEEK

4

Crust
No crust

Crusting inhibits emergence of Chloris
guyana grass over a 4 week period.

0

Crust

No crust

Crusting inhibits growth of Chloris
guyana grass.


C. Beneficial Effects of Soil Crusting
In arid and semi-arid regions, surface crusting is not invariably detrimental to
crop growth since in some areas overland flow can be managed more
effectively for agricultural production. Reij et al. (1988) reviewed water
harvesting techniques, mainly in Africa's drought-prone zones.
•Some of them are very old, such as the terraced wadi systems in southern
Tunisia, called "jessour" and described by Bonvallot (1986).
•Water is collected from small channels across mountain slopes into earth
dams which reduce the flow velocity, increase storage and permit the
accumulation of sediments. Thus cereal and tree crops are possible in zones
with as little as 100-200 mm annual rainfall.
VI. Soil Management for the Prevention Control of Crusting
A. -Physical Soil Management
1. Tillage practices
Many experiments have shown that conservation tillage practices such as
minimum tillage, surface mulching, strip-cropping, contour ploughing
etc. will reduce runoff and soil loss (Mallett et al, 1981). For arable
agricultural production, tillage is an essential input.

2. Mulching
It has been shown that mulching results in a rapid regeneration of surface
structure (Scholte, 1989), the prevention of surface crusting (Kooistra et
al., 1990), and an increase in faunal activity and hence favorable effect on
surface crusting.
3. Improving vegetative cover
Avoidance of bare soil surfaces will combat crusting. In animal husbandry, it is
essential to maintain the environmentally dictated stocking rates. In many
areas, a grass ley farming system, among others directed at improving soil
aggregation, has been advocated.
Several trees or shrubs are thought to improve soil surface structure,
e.g. Acacia albida in the Sahel (Dancette and Poulain,1969) and pigeon pea
(Cajanus cajan) in the wet savannah zone (Hulugalle and Lal, 1986).
4. Irrigation management
On soils highly susceptible to crusting, irrigation systems delivering high
kinetic energy drops should be avoided. In the case of overhead irrigation,
Valentin and Ruiz Figueroa (1987) insisted that the appropriate sprinkler
systems be selected and used at suitable intensities so as to achieve low
kinetic energy drops

B. Chemical Soil Management
1. Use of gypsum/phosphogypsum
Gypsum/phosphogypsum is a much vaunted ameliorant to use when sodicity
is high or electrolyte concentration is very low
2. Use of soil conditioners:
There is no doubt about the ability of synthetic soil conditioners to create
stable aggregates. Polyacrylamide (PAM), bitumen and urea
formaldehyde were shown to be effective to reduce evaporation from the
soil surface.
Index of Crusting
FAO (1979) proposed an index of crusting (Ic) based on textural
composition and soil organic matter content
Sf is % fine silt, Sc is % coarse silt, Cl is % clay, and SOM is %
soil organic matter content.
Obviously, Ic is inversely related to clay and soil organic matter
content, and directly to fine and coarse silt content.


Soil and crop management
options for reducing crust
formation and minimizing
adverse effects on crops


Soil Puddling
Dr. P.K. Mani


The term puddling was defined by
as

Buehrer and Rose (1943)

“the destruction of the aggregated condition of the soil by
mechanical manipulation within a narrow range of moisture
contents above and below field capacity (0.3 bars), so that soil
aggregates lose their identity and the soil is converted into a
structurally more or less homogeneous mass of ultimate
particles.”
After puddling, a soil is called a puddled soil,
defined as a
“dense soil with a degraded soil structure; dominated by
massive or single-grain structure, resulting from handling the
soil when it is in a wet, plastic condition so that when it dries it
becomes hard and cloddy.” (Gregorich et al., 2001).

Advantages of Puddling:
Although puddling, as practiced in much of tropical Asia, involves
a great amount of labor, the method has been widely adopted
primarily because of its compatibility with other components of
production technology and economic conditions, which include:
• Improved weed control by primary and secondary tillage
through puddling
• Ease of transplanting.
• Establishment of a reduced soil condition, which improves
soil fertility and fertilizer management.
• Reduced draft requirements for primary and secondary tillage.
• Reduced percolation losses resulting in conservation of
water from rainfall action and irrigation completed.
• Reliability of monsoon rains by the time puddling operations
have been completed (De Datta et al. 1978).

During puddling, soils undergo two deforming stresses:
normal (load) stress, associated with compression, and
tangential stress causing shear.
Compression is most effective below the plastic limit, and
shearing effects dominate above the upper plastic limit. The
work done during puddling can be expressed by

Since, puddling is done under saturated condition of soil; it is
shear stress which causes dispersion of soil particles in water.
Rotary puddler due to rotary motion of its blades matches the
weakest fracture plane of soil mass disintegrating it into fine
particles (Sharma and De Datta, 1985).

Definition of stress and strain
The reaction of a solid body to a force F or a combination
of forces acting upon or within it can be characterized in
terms of its relative deformation or strain. The ratio of force
to area where it acts is called stress.

normal stress
σ = Fn / A
shear stress
τ = Fs / A

normal strain
ε = δz / zo
shear strain
γ = δh / zo

Note that compressive stresses and strains are positive and
counter-clockwise shear stresses and strains are positive.

Apparent Specific Volume of Soil.
Change in the apparent specific volume of soil reflects the
susceptibility of a soil to puddling. Puddlability is the change in
apparent specific volume per unit of work expended. The
change in the apparent specific volume of soil is the difference
between apparent specific volume after and before puddling
(Bodman and Rubin 1948):

where ap = after puddling, bp = before puddling. The data are
expressed as cm3/g.
If the density of water is considered equal to 1 g/cm 3 , which is
usual in engineering works
(McCarthy 1977), the equation will be
where m = mass of water per unit mass of oven-dry soil, or
gravimetric moisture content, and Dp = particle density
(Bodman and Rubin 1948).

 Process of puddling
The process of puddling in rice culture is accomplished by a
series of tillage operations beginning at soil moisture contents
above saturation (i.e.,flooded) and ending at moisture contents
closer to field capacity (see Field water cycle).
This process is best understood by considering the
changes in soil strength within aggregates and between
aggregates.
According to Koenigs (1961), the cohesion within soil
aggregates decreases with increasing soil moisture contents.
The individual aggregates become soft and may or may not
disintegrate depending on their stability. The cohesion between
aggregates is very low at low moisture contents but
increases rapidly with increasing moisture, peaking at about field
capacity, and decreasing sharply as moisture contents
approach saturation.
.

Maximum puddling occurs at moisture contents between
 field capacity and saturation.
At such moisture contents, the cohesion within soil
aggregates is minimum, so shear planes may easily form.
Moreover, when aggregates of dry soil are wetted, uneven
swelling and explosion of trapped air also helps form shear
planes.
At moisture content below saturation, cohesion between
the aggregates and clods is maximum, and movement of
aggregates along each other and along the implement is
therefore restricted.
Consequently, the energy of the puddling implement is
effectively transferred to shear and destroy the aggregates.

The cohesion between aggregates depends primarily on the
number of contact points between aggregates.
The number of contact points is minimal in a dry soil and
approaches a maximum at about field capacity because of
the increased thickness of water films and the swelling of the
aggregates themselves.
At higher moisture contents, the thick moisture films act
as lubricants and decrease the number of contact points
between aggregates. At approximately field capacity the
cohesion within the aggregates is very low and the
cohesion between the aggregates is maximum.
When force is applied by a plow or a foot, the aggregates are
easily destroyed because of the combined effects of high
friction and low internal aggregate strength

Soils with high cohesion within aggregates, caused by
stabilizing agents such as:
 Fe and Al hydrous oxides, calcium carbonates, and
organic matter, need a larger energy input for puddling.
 High clay content favors puddling,
but kaolinitic clays are more difficult to puddle than
montmorillonite clays.
Similarly, Na–saturated clays puddle more easily than
Ca–saturated clays.
Andepts and Oxisols are extremely difficult to puddle, and the
degree of aggregate breakdown seems lower than in other soils.

Consequences of puddling
(i) Aggregate destruction
The primary consequence of puddling is the destruction of soil
aggregates (Sharma and De Datta, 1985). A puddled soil
consists essentially of a two-phase or solid-liquid system.
Individual clay particles or clusters thereof are oriented in
parallel rows and are surrounded by capillary pores
saturated with water.
Sand and silt particles and some remaining aggregates are also
part of the matrix.
The degree of aggregate destruction is difficult to quantify
because drying is necessary to measure aggregation.
Kawaguchi et al. (1956) and others provide evidence of
aggregate destruction after puddling and subsequent drying.

(ii) Changes in porosity
Non capillary pores are essentially eliminated in the process
of puddling. Bodman and Rubin (1948) found that 91–100% of
the volume occupied by such pores was destroyed by
puddling a silt loam.
Capillary porosity increases drastically. Because most
of these pores are smaller than 0.2 mm in effective radii, water
may move through pores as a liquid but can be lost only was
vapor.
(iii) Bulk density
Immediately after puddling a saturated soil, the apparent
specific gravity or bulk density is less than that of the original
soil because of the larger total pore volume occupied by water.
With time, however, the bulk density of the flooded soils
increases probably because of a slow settling of the clays.
When dried, puddled soils shrink dramatically with resultant
large increases in bulk density ( Sanchez, 1968).

(iv) Increased soil moisture retention
As a consequence of the destruction of noncapillary pores,
the increase in water-saturated capillary pores, and the
decrease in initial bulk density, puddled soils hold more water
than unpuddled soils at a given moisture tension.
The effect is measurable within a range of 0 to 10 bars of soil
moisture tension.
(V) Decreased moisture losses
The changes in porosity and water retention result in sharply
reduced soil moisture loss patterns in puddled soils (Sharma
and De Datta, 1985).
Puddling decreased percolation losses by a factor of 1000
regardless of soil properties.

Reducing conditions
Due to the absence of air, reduction processes can take place
as soon as the soil is puddled (Breazeale and McGeorge,1937).
Puddled soils remain reduced regardless of whether they
are flooded until cracks begin to form. Lack of oxygen in the
soil pores inhibits the growth of most crops except rice and other
anaerobic species.
Nitrates are lost through denitrification (Aggarwal, 1995).
Organic matter decomposition
Puddling, like any other aggregate disruption process,
temporarily hastens organic matter decomposition due to
increased accessibility of the substrate by soil microorganisms.
Puddling increases the mineralization of soil organic nitrogen
during the first month after puddling and flooding (Harada et
al., 1964), but the effect disappears at later stages (Briones, 1966).

Nutrient availability
Puddling flooded soils does not directly increase the
availability of nutrients to the rice plant (Sanchez,1973)
 Small increases in iron and manganese availability have
been recorded (Naphade and Ghildyal, 1971) but are not large
enough to be of practical significance.
Puddling often indirectly increases the availability of nutrients
by decreasing leaching losses of cations such as NH4+
Effects of puddling on crop growth:
The effects of puddling on crops other than rice are clearly
detrimental (McGeorge and Breazeale, 1938).
For rice, puddling is considered advantageous because it
facilitates land leveling, permits the farmers to work the soil
regardless of moisture status, reduces initial weed infestations,
and, most important, decreases water and leaching losses.

Regeneration of structure
Puddling is not an irreversible process. The original
structure can be regenerated through the processes of
alternate wetting and drying or freezing and thawing.
The puddled soil must be dried first, after which aggregates
are reformed by these processes.
Tillage at the appropriate moisture content facilitates
regeneration of structure.
This is accomplished most readily in soils high in organic
matter or iron and aluminum oxides (Koenigs, 1961).
Briones (1977) concluded that montmorillonitic clay soils with
low organic matter and iron oxide contents are more difficult
to convert from lowland to dryland use than kaolinitic clay with
higher organic matter and iron oxide contents. This indicates
that incorporation of crop residues aids regeneration of soil
structure.

Puddling, however, is a double-edged sword in rainfed paddy
systems. In most cases puddling attenuates the increases in
soil moisture tension during temporary droughts and
increases yields.
But when intense droughts take place shortly after
transplanting, the puddled soil may shrink, crack, and impede
rice root development to a degree from which plants cannot
recover afterwards (Sanchez, 1973; De Datta and Kerim, 1974).
Another potential detrimental effect of puddling is the time
required for the soil to dry and be prepared for aerobic crops
grown in rotation with rice.
This time interval may be several months in clayey
montmorillonitic soils but only several days in clayey
kaolinitic, allophanic, or oxidic soils.
In continuous paddy rice systems, this effect is irrelevant.

Drying of the puddle soil leads to formation of cracks,
which may have an adverse impact on root growth of rice
seedlings.

• Puddling and subsequent flooding differentiate lowland rice
soils chemically and pedologically from other arable soils.
• An important difference between a dryland and a puddled
lowland soil is the presence of the reduced soil layer in the
puddled soil system.
•The puddled layer is divided into several subhorizons.
• The formation of relatively impermeable layers, or plow
pans, is attributed to physical compaction (at the same
depth) during puddling, and to eluviation of clays and
reduced iron and manganese.
• The plow pan is found in loamy soils that have grown rice
for many years and in well drained Latosols, but is absent in
clayey soils, Vertisols, young alluvial, and calcareous
soils (Moormann and Dudal 1964).

Long-term effects of puddling
Long–term puddling forms a hardpan in the subsoil below
the puddled layer. It may take 3 to 200 yr for a hardpan to
form, depending on soil type, climate, hydrology, and puddling
frequency (24).
Subsurface hardpans develop from physical compaction
and precipitation of Fe, Mn, and Si.
Compact, 5– to 10–cm thick layers which occur in low land
rice soils between 10 and 40 cm depth and that have higher
(dry) bulk density and lower total porosity and water
permeability than the over- and underlying soil horizons were
called plow pans by Koenigs (in 24) and traffic pans by
Moormann and van Breemen

 Chemically cemented pans are formed in the oxidized
subsoil, usually 15 to 20–cm deep, by precipitation of Fe, Mn,
and Si from upper, reduced soil layers.

Soils with slowly permeable subsurface horizons, oxidized
subsoils, low pH, high concentrations of easily reducible Fe
and Mn, and easily decomposable organic matter favor
chemical precipitation.
Because continuously submerged soils have excessively
reduced conditions, Fe– and Mn–pans form very slowly.
Under favorable conditions, Mn– and Fe–pans may develop in
8–40 yr.
Ferrolysis (4) is another long–term effect of puddling
that may lower soil productivity.

Puddling index:
The puddling index is the ratio of the volume of settled soil to
the total volume of soil sample and is expressed as a
percentage.

Where,
PI = puddling index in per cent,
Vs = volume of settled soil and
Vt = total volume of soil sample.
A higher value of puddling index indicates the better quality
of puddling

Soil Tilth

• Soil tilth is the

workability of the soil based on texture,
structure and some other factors

• Soil moisture and compaction play a role
• Often due to over-working the soil and preventing root
penetration

Soil tilth is defined as
“ the physical condition of a soil as related to its ease of tillage, fitness
as a seedbed, and its importance to seedling emergence and root
penetration ” (SSSA, 1979).

Tilth index
Tilth index was calculated using the model developed by Singh
et al. (1992) and a new model developed from the regression of
crop yields on soil physical properties. The model proposed by
Singh et al. (1992) utilizes bulk density, cone index, organic
matter content, aggregate uniformity coefficient, and
plasticity index as parameters for deriving tilth index.
Since in puddled soil, aggregates are broken down, the
aggregate uniformity coefficient in rice plots does not carry
any significance, and the cone index could not be reliably
obtained during the wheat season in unsaturated soils due to
high clay content (>30%).
Therefore, only bulk density, organic matter content, and
plasticity index were included in the model for determining the
tilth index for rice and wheat.
Singh et al. (1992) indicated that the number of properties used
in calculating tilth index could be varied.

According to the model of Singh et al. (1992), the tilth index is a
multiplicative combination of tilth coefficients expressed as

where TI is the tilth index (0.0 ≤ TI ≤ 1.0), CF the tilth coefficient,
and n the number of soil properties used for calculation of the tilth
index.
The limiting, critical and non-limiting values of tilth coefficients
assigned by Singh et al. (1992) for the soil properties, to simulate the
Neill’s sufficiency curve are given in Table 1.

The proposed regression model is based on multiple linear
regression of crop yield on pertinent soil physical properties as
Y = a + b1X1 + b2X2 +· · ·+bnXn (2)
where Y is the grain yield of the crop,
X1, X2, . . . ,Xn are the different soil properties,
and a, b1, b2, . . . , bn are constants.
Those physical properties whose coefficients (b1, b2, . . . , bn) in
Eq. (2) were found significant by t-test were selected for
calculating the tilth index.

These selected physical properties were then individually
subjected to linear regression with yields of rice and wheat
and their coefficients of determination (R2) were obtained.
The proportionate variation of R2, obtained from the linear
regression of the selected properties on yield, was then
expressed as Ai ,

The soil tilth index (TI), as originally developed by Singh et al

where TI is the soil tilth index (0.0 ≤ TI ≤ 1.0), CF1 to CF5 are
the tilth coefficients of bulk density, cone index, plasticity
index, aggregate uniformity coefficient,and organic matter
content, respectively.
Singh et al.proposed a quadratic relationship for the tilth
coefficients for each soil factor. The proposed general form of
equation was:


where CFx is the tilth coefficient for the soil property (X) and A0, A1,
A2 are empirical constants. Singh et al.3 derived this relationship
simply by examining each soil factor separately according to defined
criteria. The
defined criteria in each case involved setting three important levels
for each soil property that were critical in the growth of a crop. These
were non-limiting (sufficient level), critical and limiting points. The
nonlimiting condition is the optimal level for maximum plant growth,
while the limiting level is the level above which the plants will not
normally survive8. These values were then plotted on a graph and the
best fitting polynomial curve determined to define a regression
equation to establish other values within the range. The tilth
coefficients were normalized to range between 0and 1, so that a tilth
index of 0 indicated an absolutely limiting level of a soil property and
a value of 1 indicated the optimum level.

MECHANICAL IMPEDANCE
Mechanical impedance occurs where
soil is lacking in pores of appropriate
size for roots or shoots to grow
through, and/or is too hard for the
growing root or shoot to push out of
the way.
A root must be able to enlarge existing
pores, or create new pores, to elongate
through the soil. It seems probable that
root hairs (which are involved in nutrient
uptake) can only grow into pre-existing
pores which are of the same or greater
diameter than they, i.e. >= 10 μm
dia. growth in a soil with no
a. Root
mechanical impedance problems;
b. Root growth in a soil with prismatic structured subhorizons. Vertical
root extension is restricted to the cracks between the clay structures;
c. Root growth above a compacted subsoil. Vertical extension is hindered
but restricted drainage causing aeration problems may also be a factor

Importance of soil
conditioners/Amendments
under INM


Importance of soil conditioners/Amendments under INM

Introduction
• A soil conditioner, also called a soil amendment, is a
material added to soil to improve plant growth and
health.
• The type of conditioner added depends on the current
soil composition, climate and the type of plant.
•
A conditioner or a combination of conditioners
corrects the soil's deficiencies.
• Fertilizers, such as peat, manure, anaerobic digestate
or compost, add depleted plant nutrients.
•
Gypsum releases nutrients and improves soil
structure.


Characteristics of soil conditioners

•
•
•

Soil conditioners are natural and earthy.
Absorb water rapidly.
Compost is “Synthetic manure made from decomposing
materials, fertilizer and soil.

•

Leaves and manures are also natural products.



Functions of soil conditioners

•
•
•
•

They help to improve the amount of minerals in the soil.
Soil that is rich in minerals will produce much healthier vegetation.
Leaves work by attracting earthworms which create a healthy soil .
Soil improved by

•
•
•

Physical
Chemical
Biological



Importance of soil conditioners

•

Soil conditioner is a product which is added to soil to improve the soil
quality.

•

Soil conditioners can be used to rebuild soils which have been damaged
by improper management, to make poor soils more usable, and to
maintain soils in peak condition.

•

A wide variety of products can be used to manage soil quality, with most
being readily available from nurseries and garden supply stores.

•

People can also generate their own soil conditioner with materials from
home.



Importance of soil conditioners

•

Many soil conditioners are designed to improve soil structure in some
way.

•

Soils tend to become compacted over time, which is bad for plants,
and soil conditioners can add more loft and texture to keep the soil
loose.

•

They also add nutrients , enriching the soil and allowing plants to
grow bigger and stronger.

•

Soil conditioners improve the

water retention in dry, coarse soils

which are not holding water well, and they can be added to adjust the
PH of the soil to meet the needs of specific plants or to make highly
acidic or alkaline soils more usable.

Soil physical conditions and soil conditioners

•

Soil physical condition is one factor that can limit
crop production.

•

Poor soil physical condition can restrict water
intake into the soil and subsequent movement,
plant root development, and aeration of the soil.

•

These goals can be accomplished in part through
the use of good management techniques.

•

Producers and researchers alike are interested in
improving the physical condition of the soil and,
thus, enhance crop production.


Vital role of soil conditioners

•
•
•
•
•
•
•
•
•
•

Improved soil structure and aeration
Increased water-holding capacity.
Increased availability of water to plants
Reduced compaction and hardpan conditions.
Improved tile drainage effectiveness
Alkali soil reclamation
Release of “locked” nutrients
Better chemical incorporation
Better root development
Higher yields and quality

Role of Soil Conditioner

•

Soil conditioners may be used to improve water retention in
dry, coarse soils which are not holding water well, and they
can be added to adjust the pH of the soil to meet the needs
of specific plants or to make highly acidic or alkaline soils
more usable.

Examples of soil conditioners
• Peat
• Compost
• Coir
• Manure
• Straw
• Vermiculite etc.,


Types and use of soil conditioners/amendments under INM

Types

Organic soil conditioners

Inorganic
(Synthetic) soil conditioners



Types of Organic soil conditioners
Organic
Green Manure
Compost
Peat
Crop Resides
Coconut shell mulch



Types of Inorganic soil conditioners

Inorganic

Synthetic Binding Agents

Mineral Conditioners

Gypsum


Soil physics atterberg limit,compaction, shear strength,crusting and puddling

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