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
those of soil moisture content. Soil strength decreases with
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
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
Presence of pore water
Shear Strength in Soils :
The shear strength of a soil is its resistance to shearing
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
Shear failure occurs when the stresses between the particles
are such that they slide or roll past each other
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
Internal Friction :
Internal Friction angle (f), is the measure of the shear strength of soils due
Gravity generates stresses (force per unit area) in the ground at
Stress on a plane at a given point is viewed in terms of two
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
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
is the tangential force acting on area A.
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
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
Dr. P.K. Mani
Department of Agril. Chemistry and Soil Science
Bidhan Chandra Krishi Viswavidyalaya,
Mohanpur, Nadia, West Bengal, India
E-mail: email@example.com, Website: www.bckv.edu.in
The term puddling was defined by
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
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
• 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.
σ = Fn / A
τ = Fs / A
ε = δz / zo
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
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
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
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
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
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
Similarly, Na–saturated clays puddle more easily than
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
(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
(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.
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
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).
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
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
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
• 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
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
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
Ferrolysis (4) is another long–term effect of puddling
that may lower soil productivity.
The puddling index is the ratio of the volume of settled soil to
the total volume of soil sample and is expressed as a
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
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
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 ,