to go through this assignment you can understand the physical properties of soil .their effects on plant growth .

1. UNIVERSITY OF AGRICULTURE FAISALABAD
SUBCAMPUS DEPALPUR (OKARA)
ASSIGNMENT
On
Physical properties of soil
Subject: SES (301) soil science
SUBMITTED TO:
Dr.Tariq Aziz
SUBMITTED BY:
Group: 02
Members:
Hamood u Rehman 2019-ag-5221
Husain Abbas 2019-ag-5250
Khizra aslam 2019-ag-5260
Muqddas Zaheer 2019-ag-5258

2. Soil:
Soil is the loose surface material consisting of inorganic particles and organic matter
that covers most of the land surface. Soil provides the structural support and the source
of water and nutrients for plants used in agriculture.
Soils vary greatly in their chemical and physical properties which depend on their age
and on the conditions (parent material, climate, topography and vegetation) under which
they were formed. Processes such as leaching, weathering and microbial activity
combine to make a whole range of different soil types, each of which has particular
strengths and weaknesses for agricultural production.
Physical properties of soil

3. Physical properties play an important role in determining soil’s suitability for agricultural,
environmental and engineering uses. The supporting capability; movement, retention and
availability of water and nutrients to plants; ease in penetration of roots, and flow of heat
and air are directly associated with physical properties of the soil. Physical properties also
influence the chemical and biological properties. The most pertinent physical
properties of soil relevant to its use as a medium
for plant growth are discussed in the following
sections.
Soil Texture
Solid phase of the mineral soil mainly consists of
discrete mineral particles as the amount of
amorphous material including organic matter is
usually small. Mineral particles are not exactly
spherical but vary widely in their shape, therefore,
these particles are usually classified into three
conveniently separable groups according to certain size range based on their equivalent
diameter (diameter of a sphere that has a velocity of fall in a liquid medium equal to that
of the specific particle). The groups of different size range of mineral particles are known
as soil separates, primary particles or textural fractions, namely: sand, silt and clay. Soil
texture refers to the prominent size range of mineral particles, and is defined both
qualitatively and quantitatively. Qualitatively, it refers to the feel of soil whether coarse
and gritty or fine and smooth when rubbed between thumb and forefinger. Quantitatively,
soil texture is the relative proportion of sand, silt and clay content on weight basis. The
term soil texture is often used interchangeably with mechanical composition of soil.
It is more or less a static property affecting almost all other soil properties. Land use
capability and soil management practices largely depend on the texture.
Particle Diameter (mm) Particle Diameter (mm)
Coarse sand 2.0-0.2 Very coarse sand 2.0-1.0
Fine sand 0.2-0.02 coarse sand 1.0-0.5
Silt 0.02-0.002 Medium sand 0.5-0.25
Clay <0.002 Fine sand 0.25-0.10
Very fine sand 0.10-0.05
Silt 0.05-0.002
Clay < 0.002

4. Important properties of sand, silt and clay particles
Particle Properties Sand Visible to naked eye, generally spherical or cubical in shape, feel
gritty, low water and nutrients holding capacity, loose when dry, very low plasticity and
stickiness when wet. Silt not visible to naked eye, seen through an ordinary microscope,
generally spherical or cubical in shape, low to medium in capacity to hold water and
nutrients, feel smooth, some plasticity and stickiness when wet. Clay Visible only through
an electron microscope, platy in shape, high water and nutrients holding capacity, hard
when dry, high degree of plasticity and stickiness when wet, exhibit swelling and
shrinkage behavior.
Mechanical Analysis
Mechanical analysis or particle size analysis is
a procedure of determining the sand, silt and
clay contents in a soil sample. The primary soil
particles, which are often aggregated, are
separated and made discrete by removing the
binding agents (organic matter, calcium
carbonate, soluble salts and oxides of Fe and
Al) in soil-water suspension. The organic
matter is oxidized with H
2
O
2
while CaCO
3
and
oxides of iron and aluminum are removed by treating the sample with dilute HCl and
soluble salts by filtration with distilled water. Mechanical stirring is done to disperse the
clay particles from each other and eliminate the air. A dispersing agent (Sodium
hydroxide or sodium hexa-meta-phosphate or Calgon solution) is added to prevent the
clay particles to re-unite. Once the soil particles are dispersed into ultimate particles,
these are separated into different sized groups by sieving through graded sieves up to
approximately 0.05 mm in size. Sieves of different sized circular holes are used for
particles larger than 0.5 mm. For smaller sized particles, wire mesh screens are used. For
still smaller particles (<0.05 mm), method of sedimentation is used in which the relative
settling velocity of particles or density of the suspension from which particles are settling
is measured based on the principle of Stokes’ law.
Textural Classes
The overall textural designation of a soil as determined from the relative proportion of its
sand, silt and clay contents is called the textural class. Textural class not only conveys
the textural composition of soils but also indicates their physical properties. Soils, based
on their relative proportions of sand, silt and clay contents, are classified into twelve
textural classes as shown in Textural triangle. There are three broad primary textural
groups of soils: sandy, loamy and clayey to describe texture in relation to textural class of
the soils. To illustrate the use of textural triangle, assume that a soil contains 40% sand,

5. 45% silt and 15% clay by weight. First locate the point for 15% clay on the left side of
the triangle. Draw a line from this point across the graph parallel to the base of the
triangle. Then, locate the point for 40% sand on the base of the triangle and draw an
inward line from his point parallel to the right side of the triangle. The two lines intersect
at a point corresponding to 45% silt. The lines intersecting in the area demarcated as
‘loam’ indicates textural class of the soil sample i.e. loam.
Soil Structure
The primary soil particles do not
exist as such in natural conditions
but are bonded together into larger
units or aggregates usually termed
as secondary particles. These
aggregates formed under natural
conditions are called peds whereas
an irregular shaped coherent mass
of soil formed during tillage
operations is called a clod. Soil
structure is defined as the
arrangement of primary and
secondary soil particles in a certain
structural pattern. This arrangement
results in formation of different
sized soil pores, therefore, soil
structure may also be defined as
the arrangement of various sized soil pores in a certain structural pattern
Importance of Soil Structure
Soil structure influences almost all the plant growth factors viz. water supply, aeration,
availability of plant nutrients, heat, root penetration, microbial activity, etc. Strong
aggregation decreases detachability and transportability of soil particles by water or wind
and thus, reduces runoff and soil erosion. Soil structure is useful for classification of
soils. It is affected by tillage, cultivation and application of fertilizers, manures, lime,
gypsum and irrigation.
Classification of Soil Structure
Soil structure is described and classified based on (i) the type, as determined by shape
and arrangement; (ii) the class, as differentiated by size; and (iii) the grade, as
determined by distinctness and durability of peds
Soil Plasticity
Plasticity is the property which enables a
clay/soil to take up water to form a mass that
can be deformed into any shape and to
maintain the shape even after the deformation

6. pressure is removed. Soils with less than 15% clay do not exhibit plasticity in any moisture
range. This amount of clay, however, depends on type of the clay and the organic matter
content of the soil. The plate-like shape of clay particles and binding and lubricating
effect of adsorbed water provide plasticity to the soil.
Soil temperature
Soil temperature varies seasonally and daily which
may result from changes in radiant energy and energy
changes taking place through the soil surface. It
governs the soil physiochemical and biological
processes and also influences the interspheric
processes of gas exchange between the atmosphere
and the soil. Environmental factors affect soil
temperature by either controlling the amount of heat
supplied to the soil surface and the amount of heat
dissipated from the soil surface down the profile. Soil temperature alters the rate of organic
matter decomposition and mineralization of different organic materials. It also affect soil
water content, its conductivity and availability to plants. The paper introduced soil
temperature as a major determinant of the processes that takes place in the soil which are
necessary for plant growth.
Soil colours
Soil color has been found to be the property of soil
that most reflects its pedogenic environment and
history. Soil organic matter and iron oxides
contribute most to soil color. Organic matter darkens
soil, while iron oxides produce a range of soil colors
that are dependent on the oxidation state of the iron.
Colour is an obvious characteristic of soil. It can
provide a valuable insight into the soil environment.
Thus it can be very important in assessment and
classification. The most influential colours in a well-drained soil are white, red, brown and black.
White indicates the predominance of silica (quartz), or the presence of salts; red indicates the
accumulation of iron oxide; and brown and black indicate the level and type of organic matter.
What determines soil Colour?
Four main factors influence the Colour of a soil:
 Mineral matter derived from the constituents of the parent material
 Organic matter
 The nature and abundance of iron
 Moisture content

7. Soil porosity
Soil Porosity Information Soil porosity, or
soil pore space, are the small voids between
particles of soil. In heathy soil, these pores
are large and plentiful enough to retain the
water, oxygen and nutrients that plants need
to absorb through their roots. Soil porosity
usually falls into one of three categories:
micro-pores, macro-pores or bio-pores. These
three categories describe the size of the pores
and help us understand the soil’s permeability
and water holding capacity. For example,
water and nutrients in macro-pores will be
lost to gravity more quickly, while the very
small spaces of micro-pores are not as
affected by gravity and retain water and nutrients longer. Soil porosity is affected by soil particle
texture, soil structure, soil compaction and quantity of organic material. Soil with fine texture is
able to hold more water than soil with coarse texture. For example, silt and clay soils have a finer
texture and sub-micro porosity; therefore, they are able to retain more water than coarse, sandy
soils, which have larger macro-pores. Both finely textured soils with micro-pores and coarse soil
with macro-pores may also contain large voids known as bio-pores. Bio-pores are the spaces
between soil particles created by earthworms, other insects or decaying plant roots.