soil tillage ppt.pdf

SOIL SCIENCE AND TECHNOLOGY
Dr. SOMSUBHRA CHAKRABORTY
AGRICULTURAL AND FOOD ENGINEERING
IIT KHARAGPUR
Topic
Soil Tillage and Soil Density

Tillage
The preparation of soil for planting and the cultivation of soil after planting.
TNAU Agritech Portal

Tillage and soil tilth
Tilth refers to the physical condition of the soil in relation to plant growth.
It depends on-
• Aggregate formation
• Stability
• Bulk density
• Soil moisture content
• Degree of aeration
• Rate of water infiltration
• Drainage
• Capillary water capacity

Conventional tillage
Farmers use machines like a plow or disc to turn over and loosen the soil
after harvest (a process called tillage). This can leave the soil exposed to
rain and wind, which can sometimes lead to erosion of the topsoil that
is needed to grow a crop
http://allaboutfood.aitc.ca

Types of conventional tillage
• Primary tillage: Primary tillage is the first soil tillage after the last
harvest. It is normally conducted when the soil is wet enough to allow
plowing and strong enough to give reasonable levels of traction. This
can be immediately after the crop harvest or at the beginning of the
next wet season. When there is sufficient power available some soil
types are ploughed dry.
• Objectives
1. To attain a reasonable depth (10-15 cm) of soft soil with varying clod sizes
2. Kill weeds by burying or cutting and exposing the roots
3. Soil aeration and water accumulation
4. chop and incorporate crop residues
knowledgebank.irri.org

Types of conventional tillage
• Secondary tillage: Secondary tillage is any working completed after
primary tillage and is undertaken for
1. Reducing clod size
2. Weed control
3. Incorporation of fertilizers
4. Puddling
5. Leveling soil surface
knowledgebank.irri.org

Conservation tillage and soil tilth
In recent decades, agricultural land-management systems have been developed
that minimize the need for soil tillage and leave the soil surface largely covered
by plant residues, thereby maintaining
• Soil biological habitat
• Stabilizing soil structure
• Conserving soil organic matter
• Physically protecting the soil from drying sun, scouring wind, and beating rain
These systems are called conservation tillage.
The U.S. Department of Agriculture defines conservation tillage as that
which leaves at least 30% of the soil surface covered by residues

Conservation tillage
This is a technique for planting seed that minimizes the disruption of
soil and therefore helps prevent soil erosion. Farmers use special
equipment to plant seeds, leaving most of the residues (e.g. stalks) of
the previous crop intact. Planting in this way allows the crop residue to
break down, which adds organic matter (like composting) while
protecting the soil from erosion
Morning Ag Clips

No till
One crop is planted in the residue of another, with virtually no tillage.

Soil crusting
• Falling drops of water during heavy rain or
sprinkler irrigation can beat apart aggregates
exposed at the soil surface
• Once the aggregates become dispersed, small
particles and dispersed clay tend to wash into
and clog the soil pores
• Soon the soil surface is covered with a thin,
partially cemented, low permeability layer
material called a surface seal
• As the surface seal dries, it forms a hard crust
Integrated Crop Management - Iowa State University

Problems of soil crusting
• Inhibits water infiltration
• Increases erosion losses
• Inhibits emergence of seedlings
• In arid and semiarid regions, soil sealing and crusting can have
disastrous consequences because high runoff losses leave little water
available to support plant growth

Soil Crust Struggles to break a soil crust

Management of soil crusting
• Keeping some vegetative or mulch cover on
the land to reduce the impact of raindrops.
• Once a crust has formed, it may be necessary
to rescue a newly planted crop by breaking up
the crust with light tillage (as with a rotary
hoe), preferably while the soil is still moist.
• It can be minimized by using Soil Conditions.
Farmlink

Soil conditioners
• Improved management of soil organic matter and use of certain soil
amendments can “condition” the soil and help prevent clay dispersion and crust
formation.
1. Gypsum
2. Organic Polymers

• Gypsum-
1. Used for collecting the soil physical condition
2. Used in low salinity to sodium rich soil
3. Improve the flocculation of the soil
• Organic Conditioners –
Polyacrylamide (PAM) is effective in stabilizing surface aggregates when applied
at rates as low as 1–15 mg/L of irrigation water or sprayed on at rates as low as
1–4 kg/ha.

Soil density
1. Particle density:
• Soil particle density Dp is defined as the mass per unit
volume of soil solids
• Thus, if 1 m3 of soil solids weighs 2.6 megagrams (Mg), the
particle density is 2.6 Mg/ m3 (which can also be expressed
as 2.6 grams per cubic centimeter)
• Particle density is essentially the same as the specific
gravity of a solid substance
• Particle densities for most mineral soils vary between the
narrow limits of 2.60 and 2.75 Mg/ m3

2. Bulk density:
• Bulk density Db, which is defined as the mass of a unit
volume of dry soil. This volume includes both solids and
pores
• The units are same as particle density
• But the value of bulk density is changeable unlike particle
density
• Generally coring instruments are used to determine the soil
bulk density

Determination of soil bulk density
Cylindrical core:
• The sampler head contains an
inner cylinder and is driven into
the soil with blows from a drop
hammer
• The inner core containing an
undisturbed soil core. Trimmed
on the end with a knife. The
volume can easily be calculated
from its length and diameter.
• The weight of soil is calculated
after drying

Factors affecting Db
• Effect of Soil Texture- Fine-textured soils have lower bulk density
than the coarse-textured soil

• Effect of soil structure- The well aggregated soils have lower Db than
poorly aggregated soils.

• Effect of Organic matter- Organic matter helps to form a good
aggregation. So, it reduces the bulk density of the soil
• Depth of Soil Profile- Pore space reduces with the increase in soil
depth. So, Db generally increased

Agricultural Land:
• The long-term intense tillage increases soil bulk density by depleting soil
organic matter and weakening soil structure
• In mechanized agriculture, the wheels of heavy machines used to pull
implements, apply amendments, or harvest crops can create yield-limiting soil
compaction (plow pans or traffic pans)

• Vehicle tires (750 kg load per
tire) compact soil to about 50 cm
• The more narrow the tire, the
deeper it sinks and the deeper its
compaction effect

Influence of Db on soil strength and root
growth
Effect of Soil Water Content:
Soil strength is increased when a soil
is compacted to a higher bulk density,
and also when finer-textured soils dry
out and harden. So, it can easily
restrict the growth of the root

• Effect of Soil Texture:
The more clay present in a soil, the smaller the average pore size,
and the greater the resistance to penetration at a given bulk
density than the sandy soil
• Effect of Land Use and Management:
Land uses as row crop agriculture often markedly and
simultaneously affect soil bulk density and strength in ways that
restrict or enhance root growth and water movement

IIT KHARAGPUR
Topic
Soil Porosity and Consistency

Concepts Covered:
 What is soil porosity
Factors affecting soil porosity
Soil consistence and consistency

Porosity
Soil porosity refers to percent of soil volume occupied by pore
spaces
Total pore space includes both air and water filled pores
An ideal soil would have a total porosity of 50% with equal amount
of air and water filled pores

Calculation of porosity
𝑩𝒖𝒍𝒌 𝒅𝒆𝒏𝒔𝒊𝒕𝒚, 𝑫𝒃 =
𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒊𝒍 𝒔𝒐𝒍𝒊𝒅𝒔(𝑾𝒔)
[𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅𝒔 𝑽𝒔 + 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒑𝒐𝒓𝒆𝒔 𝑽𝒑 ]
𝑷𝒂𝒓𝒕𝒊𝒄𝒍𝒆 𝒅𝒆𝒏𝒔𝒊𝒕𝒚, 𝑫𝒑 =
𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒔𝒐𝒊𝒍 𝒔𝒐𝒍𝒊𝒅𝒔(𝑾𝒔)
𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅𝒔 𝑽𝒔
From equations 1 and 2, equating for Ws,
𝑫𝒑 ∗ 𝑽𝒔 = 𝑫𝒃 ∗ 𝑽𝒔 + 𝑽𝒑 which implies
𝑽𝒔
𝑽𝒔
+𝑽𝒑
=
𝑫𝒃
𝑫𝒑
We know that 𝑽𝒔
𝑽𝒔+𝑽𝒑
∗ 𝟏𝟎𝟎 = % 𝒐𝒇 𝒔𝒐𝒍𝒊𝒅 𝒔𝒑𝒂𝒄𝒆 and
%𝒑𝒐𝒓𝒆 𝒔𝒑𝒂𝒄𝒆 + %𝒔𝒐𝒍𝒊𝒅 𝒔𝒑𝒂𝒄𝒆 = 𝟏𝟎𝟎%
Hence, %𝒑𝒐𝒓𝒆 𝒔𝒑𝒂𝒄𝒆 = 𝟏𝟎𝟎% −
𝑫𝒃
𝑫𝒑
∗ 𝟏𝟎𝟎
1
2

Factors affecting total pore space
1. Management
Highly compacted soils have less porosity
Well granulated, organic matter rich soil have higher porosity
Intense cultivation reduces the porosity due to reduction in organic
matter content
Organic matter content increases the porosity

Compaction and porosity
www.motherearthnews.com

2. Size of pores
Soil pores occur in wide variety of size and shape
This determines the role they play in the soil
Two major types of pores are
Macropores
Micropores

Classification and function of pores

Macropores
• Effective diameter>0.08 mm
• Macropores readily allow the movement of air and water
• Accommodates the plant root system and animals

Types of macropores
Macropores in between individual soil grains
Responsible for movement of air and water in sandy soils
Interped pores in well structured soils
Occurs between tightly packed blocky peds
and also prismatic peds
Ray R. Weil

Types of macropores
Bio pores are a type of macropore created by roots,
earthworms and other animals
Tubular shaped and continuous
In clayey soils, bio pores are major pores for facilitating
plant root growth
 Both soil structure and texture influence the
distribution of micro and macropores
Root growing in Interped zone of a prismatic ped
Ray R. Weil

Micropores
Effective diameter<0.08 mm
They retain water in field conditions but too
small for air movement
However, most of the water is not readily
available for plants
Small micropores, known as nanopores, act as
hiding place for adsorbed pollutants and organic
materials
 Size, shape and interconnection of pores is
important rather than volume of pores

Volume distribution of soil separates and
pores in good structured soil
Ray R. Weil

3. Cultivation and pore size
Continuous cropping reduces the soil organic matter
Consequently, macropore reduces
Conservation tillage promotes long-lived network of biopores
They increase the macroporosity of surface layers

Soil consistence
• Consistence: ease with which soil can be
reshaped or ruptured
• 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
KissPNG

Rupture resistance
• A measure of the strength of the soil to withstand an applied
stress
• Moisture content is also considered
• – Dry
• – Moist (field capacity)

Wet consistency
• Stickiness
The capacity of soil to adhere to other objects
Estimated at moisture content that displays maximum
adherence between thumb and fore finger
• Plasticity
Degree a soil can be molded or reworked causing permanent
deformation without rupturing

Stickiness classes
• Non-Sticky – little or no soil adheres to fingers after
release of pressure
• Slightly Sticky – soil adheres to both fingers after
release of pressure with little stretching on separation
of fingers
• Moderately Sticky – soil adheres to both fingers after
release of pressure with some stretching on separation
of fingers
• Very Sticky - soil adheres firmly to both fingers after
release of pressure with stretches greatly on separation
of fingers

Stickiness classes
Non-Sticky Slightly Sticky Very Sticky

Plasticity
• The degree to which puddled or reworked soil can be permanently
deformed without rupturing
• Evaluation done by forming a 4 cm long wire of soil at a water
content where maximum plasticity is expressed

Atterberg limits
http://www.engr.uconn.edu
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 .

Plasticity classes
• Non-Plastic– will not form a 6 mm dia, 4 cm long wire, or if formed ,
can not support itself if held on end
• Slightly Plastic–6 mm dia, 4 cm long wire supports itself, 4 mm dia,
4 cm long wire does not
• Moderately Plastic– 4 mm dia, 4 cm long wire supports itself, 2 mm
dia, 4 cm long wire does not
• Very Plastic– 2 mm dia, 4 cm long wire
http://www.engr.uconn.edu

IIT KHARAGPUR
Topic
Soil Water Energy Concepts

Soil water: importance
oExtremely important for soil physical, chemical and biological
processes
o Weathering of minerals to decomposition of organic matter
oIn the soil
o Water can flow up as well as down
o Plants may wilt and die in a soil whose profile contains a million kilograms
of water in a hectare
o A layer of sand or gravel in a soil profile may actually inhibit drainage,
rather than enhance it

Soil water: importance
o Soil–water interactions influences
Water loss by leaching
Surface runoff
ET
Air and water balance in soil pores
Rate of change in soil temperature
Rate and kind of metabolism of soil organisms, and
Capacity of soil to store and provide water for plant growth

Structure and properties of water
Two-dimensional representation of a
water molecule showing a large oxygen
atom and two much smaller hydrogen
atoms.
The H¬O¬H angle of 105° results in an
asymmetrical arrangement.
One side of the water molecule (that
with the two hydrogens) is
electropositive; the other is
electronegative. This accounts for the
polarity of water.

1. Polarity
2. H-bonding
a) hydrogen atom of one
water molecule is
attracted to the oxygen
end of a neighboring
water molecule, thereby
forming a low-energy
bond between the two
molecule
b) accounts for the
polymerization of water

1. Polarity
2. H-bonding
3. Hydration
a) Cations such as H+, Na+, K+, and Ca2+
become hydrated through their
attraction to the oxygen (negative)
end
b) Negatively charged clay surfaces
attract water, this time through the
hydrogen (positive) end of the
molecule
c) Dissolution of salts in water

1. Polarity
2. H-bonding
3. Hydration
4. Cohesion vs. Adhesion
5. Surface tension
a) Evident at liquid–air interfaces
b) Results from the greater
attraction of water molecules
for each other (cohesion) than
for the air above
P.C: Ray R. Weil

Capillary mechanism
h =0.15/r
D.C: Ray R. Weil
1. Capillarity can be demonstrated by placing one end of a
fine (< 1 mm diameter), clean glass tube in water
2. The water rises in the tube; the smaller the tube bore,
the higher the water rises.
3. The water molecules are attracted to the sides of the
tube (adhesion) and start to spread out along the glass
in response to this attraction.
4. The cohesive forces hold the water molecules together
and create surface tension, causing a curved surface
(called a meniscus).
5. Lower pressure under the meniscus in the glass tube
(P2) allows the higher pressure (P1) on the free water
to push water up the tube. The process continues until
the water in the tube has risen high enough that its
weight just balances the pressure differential across the
meniscus

Capillary mechanism in soil
D.C: Ray R. Weil
1. The height of rise h doubles when the tube
inside radius is halved
2. The same relationship using glass tubes of
different bore size
3. The same principle also relates pore sizes in a
soil and height of capillary rise, but the rise of
water in a soil is rather jerky and irregular
because of the tortuous shape and variability in
size of the soil pores (as well as because of
pockets of trapped air)
4. The finer the soil texture, the greater the
proportion of small-sized pores and, hence, the
higher the ultimate rise of water above a free-
water table.
5. However, because of the much greater frictional
forces in the smaller pores, the capillary rise is
much slower in the finer-textured soil than in
the sand

Capillary movement of soil water in both direction
P.C: Ray R. Weil
Surface runoff collection basin Stream bank

Soil water energy concepts
1. The retention and movement of water in soils, its uptake and
translocation in plants, and its loss to the atmosphere are all
energy-related phenomena
2. Kinetic energy is certainly an important factor in the rapid,
turbulent flow of water in a river, but the movement of water in
soil is so slow that the kinetic energy component is usually
negligible.
3. Potential energy is most important in determining the status
and movement of soil water.
4. Higher energy state lower energy state

Soil water energy concepts
The total energy state of soil water is defined
by its equivalent potential energy, as
determined by the various forces acting on the
water per unit quantity.
Forces acting on soil water (in the vadose zone) are:
1. Capillary forces
2. Adsorptive forces (adhesion of water to solid soil
surfaces): Capillary and adsorptive forces together
result in soil matric potential
3. Gravitational forces
4. Drag or shear forces (at soil surface-water interface)

3 models of water distribution

Soil water potential
1. To quantify potential energy state of soil water, a reference
state is needed.
2. Reference state: potential energy of pure water, with no
external forces acting on it, at a reference pressure
(atmospheric), reference temperature, and reference
elevation.
3. Soil water potential is then determined as potential energy per
unit quantity of water, RELATIVE to the reference potential of
zero.

DC: N. C. Brady

Formal definition: Total soil water potential is defined as the amount of
work per unit quantity of pure water that must be done by external
forces to transfer reversibly and isothermally an infinitesimal amount of
water from the standard state to the soil at the point under
consideration.
Since water in soil has various forces acting upon it, potential energy
usually differs from point to point, and hence its potential energy is
variable as well.
REMEMBER: Potential = Force x Distance = mgl =ρwVgl (Nm)

Total Soil water potential
YT= Yg + Ym+ Yo + Yh +…………(N/m2)
Yg = gravitation potential
Ym = matric potential
Yo = osmotic potential
Yh = hydrostatic potential

Gravitation potential
The force of gravity acts on
soil water the same as it does
on any other body , the
attraction being toward the
Earth’s center. The
gravitational potential Yg of
soil water may be expressed
mathematically as:
Yg = gh
P.C: Ray R. Weil

Pressure potential (Hydrostatic + Matric)
Includes:
(1) The positive hydrostatic pressure due to the weight of
overlying water in saturated soils and aquifers
(2) The negative pressure due to the attractive forces between
the water and the soil solids or the soil matrix

Hydrostatic potential
(1)The hydrostatic pressures give rise to what is often
termed the hydrostatic potential Yh, a component
that is operational only for water in saturated zones
below the water table
(2)Anyone who has dived to the bottom of a
swimming pool has felt hydrostatic pressure on his
or her eardrums

Matric potential
(1)The attraction of water to solid surfaces gives rise
to the matric potential Ym
(2)Always negative because the water attracted by the
soil matrix has an energy state lower than that of
pure water

Pressure potential
 The top of the saturated zone is termed the water
table.
 Above the water table, the soil is unsaturated and its
water subject to the influence of matric potentials
 Water below the water table in saturated soil is
subject to hydrostatic potentials

Osmotic potential
 The osmotic potential Yo is attributable to the presence of
both inorganic and organic substances dissolved in water.
 As water molecules cluster around solute ions or
molecules, the freedom of movement (and therefore the
potential energy) of the water is reduced.
 The greater the concentration of solutes, the more osmotic
potential is lowered. As always, water will tend to move to
where its energy level will be lower, in this case to the
zone of higher solute concentration.

Potential per unit mass (m) : m = potential/mass = gl (Nm/kg)= gl (joules/kg)
Potential per unit weight (h) : h = potential/weight = mgl / mg = l (m, head unit)= equivalent height of water
Potential per unit volume (y) : y = potential/volume = rwVgl / V = rwgl (N/m2, water pressure units)
= rwgl Pa(SI Unit)
Consequently, we do not need to compute the soil-water potential directly by
computing the amount of work needed, but measure the soil-water potential indirectly
from pressure or water height measurements !!!!

Expression of soil water potential

Hysteresis
 The relationship between soil
water content and matric
potential of a soil upon being
dried and then rewetted
 Due to factors such as the
nonuniformity of soil pores,
entrapped air, and swelling
and shrinking
 As soils are wetted, some of
the smaller pores are
bypassed, leaving entrapped
air that prevents water
penetration.
Diagram courtesy of N Brady and Ray R. Weil

IIT KHARAGPUR
Topic
MEASUREMENT OF SOIL WATER

Methods for measuring soil
water content
Direct method
(Gravimetric)
Indirect methods
(need to calibrate)
Electrical
properties
Radiation
technique
Acoustic
method
Thermal
properties
Chemical
methods
Electrical
Conductance
Dielectric constant
-Neutron scattering
-g- ray attenuation
- Gypsum blocks
- Nylon blocks
- Change in
conductance
TDR

Measuring water content
Water that may be evaporated from soil by heating at 1050C to a
constant weight
Gravimetric moisture content (w) =
mass of water evaporated (g)
mass of dry soil (g)
Volumetric moisture content (q) =
volume of water evaporated (cm3)
volume of soil (cm3)
q = w *
bulk density of soil
density of water
Bulk density of soil (Db) =
mass of dry soil (g)
volume of soil (cm3)

Example: A soil is sampled by a cylinder measuring 7.6 cm in diameter and 7.6 cm
length. Calculate gravimetric and volumetric water contents, and dry bulk density
using the following data:
1. Weight of empty cylinder = 300 g
2. Weight of cylinder + wet soil = 1000 g
3. Weight of cylinder + oven dry (1050C) soil = 860 g
Volume of cylinder = p*r2*h = 3.14*(7.6/2)2*7.6 = 345 cm3
Weight of wet soil = 1000 – 300 = 700 g
Weight of dry soil = 860 – 300 = 560 g
Dry bulk density = 560/345 = 1.62 g cm-3
Gravimetric moisture content = (700-560)/560 = 0.25 or 25%
Volumetric moisture content = Db *w = 1.62*0.25 = 0.41 or 41%

Neutron scattering
Neutron Moisture Probe
 Lowered into the soil via a previously
installed access tube
 Contains a source of fast neutrons and
a detector for slow neutrons
 When fast neutrons collide with
hydrogen atoms (most of which are
part of water molecules), the
neutrons slow down and scatter
 The number of slow neutrons counted
by a detector corresponds to the soil
water content
aces.nmsu.edu

Neutron scattering: drawbacks
1. Radiation permit needed
2. Expensive equipment
3. Not good in high OM soils
4. Requires access tube.
aces.nmsu.edu

TDR (time domain reflectometer)
o A dielectric material is poor at conducting an electric current,
but can support an electrostatic field (something like a
magnetic field)
o Instruments that measure the dielectric properties of soil can
be used to determine the proportion of the soil volume
comprised of water because the dielectric constant for water
(81) is far greater than for mineral particles (3–5) or for air (1).
o Therefore, the dielectric constant for the whole soil is nearly
proportional to the volume of water in the soil in the
immediate vicinity (3–4 cm) of the sensor

TDR (measures both soil moisture content and salinity)
P.C: Ray R. Weil aces.nmsu.edu

Measuring soil water (matric) potentials
• The tenacity with which water is attracted
to soil particles is an expression of matric
water potential Ym.
• Field tensiometers measure this attraction
or tension
• The tensiometer is basically a water-filled
tube closed at the bottom with a porous
ceramic cup and at the top with an airtight
seal
• Useful between 0 to -85 kPa

• Electrical Resistance Blocks (cheap but calibration required)
Iowa State University
• Blocks are made of porous gypsum (CaSO4
・ 2H2O), embedded with electrodes
• When placed in moist soil, the fine pores
in the block absorb water in proportion to
the soil water potential
• The more tightly the water is being held in
the soil, the less water the block will be
able to absorb.
• The resistance to electricity flow between
the electrodes embedded in the block
decreases in proportion to how much
water has been absorbed in the block

• Thermocouple Psychrometer
• Since plant roots must overcome both
matric and osmotic forces when they draw
water from the soil, there is sometimes a
need for an instrument that measures
both
• In a thermocouple psychrometer, a
voltage generated by the evaporation of a
water drop is converted into a readout of
soil water potential (Ym + Y0)
• Most useful in relatively dry soils
Decagon Devices

• Pressure Membrane Apparatus
• Used to subject soils to matric
potentials as low as -10,000 kPa.
• After application of a specific matric
potential to a set of soil samples, their
soil water contents are determined
gravimetrically
• This important laboratory tool makes
possible accurate measurement of
water content over a wide range of
matric potentials in a relatively short
time
(Photos and diagram courtesy of Ray R. Weil)

http://moralesmi.faculty.mjc.edu

IIT KHARAGPUR
Topic
TUTORIAL

We will cover numerical problems and
solutions for
 Soil BD and PD
Soil Porosity
Soil Water Content

BD
1. Calculate the bulk density of a 400 cm3 soil sample that weighs 575 g (oven dry weight).
Solution: ρb = Ms/Vs
= 575g/400cm3
= 1.44g/ cm3
2. Calculate the bulk density of a 400 cm3 soil sample that weighs 600 g and
that is 10% moisture.
Solution: Oven dry wt. = 600g/1.1 = 545.5g
r b = 545.5g/400cm3 = 1.36 g/cm3

BD
3. Calculate the volume of a soil sample that is 12% moisture, weighs 650 g and has a bulk
density of 1.3 g/cm3.
Solution: Oven dry wt. = 650g/1.12 = 580.4g
1.3 g/cm3 = 580.4g/vol.
vol. = 446.4cm3
4. Calculate the bulk density of a rectangular soil sample with dimensions 12 cm by
6 cm by 4 cm, that is 15% moisture content and weighs 320 g.
Solution: Vol. of soil = 12cm x 6cm x 4cm = 288cm3
Oven dry wt. = 320/1.15 = 272g
r b = 272/288 = 0.97g/cm3

BD and Porosity
5. Calculate the oven dry weight of a 350 cm3 soil sample with a bulk density of 1.42
g/cm3.
Solution: 1.42g/cm3 = Ms/350cm3 Ms = oven dry wt. = 497g
6. Calculate the porosity of a soil sample that has a bulk density of 1.35 g/cm3. Assume the
particle density is 2.65 g/cm3.
Solution: Porosity = (1-(r b/r d)) x 100 = (1-(1.35/2.65)) x 100 = 49%

Porosity
7. Calculate the porosity (n) of a 250 cm3 clod that contains 140 cm3 water when
saturated.
Solution: Porosity = Vair + Vwater/Vtotal = 140cm3/250cm3 = 56%
8. Calculate the bulk density of a soil sample that has a porosity of 45%.
Solution: for 1cm3 soil, assume r d of 2.65 g/cm3
1cm3-.45cm3 = .55 x 2.65g/cm3 = 1.46g/cm3

Porosity and PD
9. Calculate the porosity of a 250 g sample that contains 65 g of water when 55% of
the pores are full of water.
Solution:
Oven dry wt. = 250g-65g = 185g soil
Vol. of soil solids = 185g/2.65g/cm3 = 69.8cm3 soil
Saturated water content = 65cm3/0.55 = 118.2cm3 water
Total vol. of soil = 118.2cm3 + 69.8cm3 = 188cm3
Porosity = Vair + Vwater/Vtotal = 118.2cm3/188cm3 x 100 = 63%
10. What is the particle density of a soil sample that has a bulk density of 1.55 g/cm3 and a
porosity of 40%?
Solution: Porosity = (1-(r b/r d)) x 100
40 = ((1-1.55/r d) )x 100
1.55/r d = 0.6
r d = 2.58g/cm3

Acknowledgement
Department of land and water Resources, UCA Davis, USA : class notes

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