2. Soil???
Soil can be defined as the organic and inorganic
materials on the surface of the earth that provide the
medium for plant growth.
Word soil derived from the latin word that is solum
which means floor.
3. Soil develops slowly over time and is composed of many
different materials. Inorganic materials, or those materials that
are not living, include weathered rocks and minerals.
Weathering is the mechanical or chemical process by which
rocks are broken down into smaller pieces. As rocks are broken
down, they mix with organic materials, which are those materials
that originate from living organisms. For example, plants and
animals die and decompose, releasing nutrients back into the soil.
4. Importance of Soil
Soil is an important element essential for the survival of living organisms. The
importance of soil is mentioned below:
• The fertile soil helps in the growth and development of the plants. The plants thus
produced are healthy and provide food, clothing, furniture, and medicines.
• It supports many life forms including bacteria, fungi, algae, etc. These microbes, in
turn, maintain environmental balance by retaining the moisture and decaying the
dead organisms.
• The topsoil supports certain life activities such as reproduction, hatching, nesting,
breeding, etc. of a few organisms.
• The organic matter present in the soil increases the fertility of the soil which is
responsible for the growth of the plants. It also contains certain minerals and
elements that are necessary for the plants to carry out their cellular activities.
• Soil is used for making cups, utensils, tiles, etc. The contents in the soil such as
gravel, clay and sand are used in the construction of homes, roads, buildings, etc.
• Useful mineral medicines such as calcium, iron, and other substances such as
petroleum jelly for cosmetics are extracted from the soil.
• The soil absorbs the rainwater. This water is evaporated and released into the air
during sunny days, making the atmosphere cooler.
5. How soil is formed???
soil is formed by weathering of rocks. Solid rock can weather away in one of
the three ways into the soil, namely:
• Mechanical Weathering
• Chemical Weathering
• Biological Weathering
6. Mechanical Weathering
This is commonly observed near the surface of the
earth. Also called physical weathering, as this process is
influenced by physical forces such as wind, water and
temperature.
7. Chemical Weathering
As the name suggests, chemical weathering occurs
when rocks are broken down by chemical reactions.
Often, such types of weathering can change the
chemical composition of the soil.
8. Biological Weathering
Though not an actual weathering process, living organisms
weaken and subsequently disintegrate rocks, often by initiating
mechanical or chemical weathering. For instance, tree roots can
grow into cracks in the rock, prying them apart and causing
mechanical fractures.
Microorganisms can secrete chemicals that can increase the
rock’s susceptibility to weathering.
9. Soil profile??
Now, if you look down at the soil under your feet,
you cannot tell very much about that soil. So, when
you study soil, it's helpful to grab a shovel and dig a
hole that is big enough to reveal a vertical section of
soil that ranges from the surface to the underlying
rock, referred to as a soil profile.
The soil profile is somewhat like the soil's fingerprint,
and it will differ from other soil samples based on
factors like its color, texture, structure and thickness,
as well as its chemical composition.
11. Soil horizon???
• Each layer of a soil profile is referred to as a soil horizon.
These horizons are identified by letters. Horizon A is the upper
layer, closest to the surface. You can think of this horizon as
the topsoil.
• In fact, you can use this as a memory jogger to help remember
the order of the horizons. The letter A is at the top of the
alphabet and refers to the topsoil layer. As you move deeper
into the layers of the soil profile, you have horizons B and C,
giving us the three main horizons.
12.
13. Three basic physical properties of
soil
1. Soil separates
2. Soil texture
3. Soil structure
14. Soil texture
Soil texture refers to the relative percentage of sand, silt and clay in a soil.
Natural soils are comprised of soil particles of varying sizes. Texture is an
important soil characteristic because it will partly determine water intake
rates (absorption), water storage in the soil, and the ease of tillage
operation, aeration status etc. and combinedly influence soil fertility.
As for an example, a coarse sandy soil is easy to cultivate or till, has sufficient
aeration for good root growth and is easily wetted, but it also dries rapidly
and easily loses plant nutrients through leaching. Whereas in case of high-
clay soils (> 35% clay) have very small particles that fit tightly together,
leaving very little pore spaces which permits very little room for water to flow
into the soil. This condition makes soils difficult to wet, drain and till.
15. Classes of soil texture
Texture names are given to soils based upon the relative
proportion of each of the three soil separates such as sand, silt
and clay.
Soil that are preponderantly clay, are called clay (textural class),
those with high silt content are silt (textural class) those with high
sand percentage are sand (textural class).
Three broad and fundamental groups of soil texture classes are
recognised:
1. Sand
2. Loam
3. clay
16. Sand??
• The sand group includes all soils of which the sand
separates make up 70 per cent or more of the
material by weight. Two specific classes are
recognised as sand and loamy sand.
17. Loam??
• A loamy soil containing many sub-divisions does not exhibit
the dominant physical properties of any of these three soils
separates sand, silt and clay.
• An ideal loam soil may be defined as a mixture of sand, silt
and clay particles which exhibits light and heavy properties
in about equal proportions.
• Note that loam does not contain equal percentages of sand,
silt and clay. It does, however, exhibit approximately equal
properties of sand, silt and clay.
18. Clay??
• A clay soil must carry at least 35 per cent of the clay
separate and in most cases not less than 40 per
cent.
• For an example, sandy clay soils contain more sand
than clay. Similarly silty clay soils contain more silt
than that of the clay.
19. Based on these three broad and fundamental groups, the
different textural class names developed by U.S.
Department of Agriculture
24. Soil structure??
• The arrangement of soil particles and their
aggregate into certain defined patterns is called
structure.
• The primary soil particles such as sand, silt
and clay usually occur grouped together in the
form of aggregates.
• Natural aggregates are called peds, whereas
clod is an artificially formed soil mass.
25. Structure is studied in the field under natural
conditions and it is described under three
categories:
1. Type: Shape or form and arrangement pattern of
peds.
2. Class: Size of peds.
3. Grade: Degree of distinctness of peds.
26. Types of Soil Structure:
There are four principal forms of soil structure:
a. Plate-like:
In this structural type of aggregates are arranged in relatively
thin horizontal plates. The horizontal dimensions are much
more developed than the vertical. When the units are thick,
they are called platy, and when thin, laminar.
Platy structure is most noticeable in the surface layers of virgin
soils but may be present in the sub-soil. Although most
structural features are usually a product of soil forming forces,
the platy type is often inherited from the parent material,
especially those laid down by water.
27. b. Prism like:
• The vertical axis is more developed than
horizontal, giving a pillar-like shape. When the top
of such a ped is rounded, the structure is termed as
columnar, and when flat, prismatic.
• They commonly occur in sub-soil horizons in arid
and semi-arid regions.
28. Block like
• All these dimensions are about the same size and the
peds are cube-like with flat or rounded faces. When
the faces are flat and the edges sharp angular, the
structure is named as angular blocky.
• When the faces and edges are mainly rounded it is
called sub angular blocky. These types usually are
confined to the sub-soil and characteristics have much
to do with soil drainage, aeration and root
penetration.
29. c. Spheroidal (Sphere-like):
All rounded aggregates (peds) may be placed in this category,
although the term more properly refers to those not over 0.5
inch in diameter.
Those rounded complexes usually lie loosely and separately
When wetted, the intervening spaces generally are not closed
so readily by swelling as may be the case with a blocky
structural condition.
Therefore in sphere-like structure infiltration, percolation and
aeration are not affected by wetting of soil. The aggregates of
this group are usually termed as granular which are relatively
less porous; when the granules are very porous, the term used
is crumby.
30.
31.
32. Classes of Soil Structure:
Each primary structural type of soil is differentiated into 5 size-classes
depending upon the size of the individual peds.
The terms commonly used for the size classes are:
1. Very fine or very thin
2. Fine or thin
3. Medium
4. Coarse or thick
5. Very coarse or very thick.
The terms thin and thick are used for platy types, while the
terms fine and coarse are used for other structural types.
33. Grades of Soil Structure:
Grades indicate the degree of distinctness of
the individual peds.
It is determined by the stability of the
aggregates.
Grade of structure is influenced by the
moisture content of the soil. Grade also
depends on organic matter, texture etc.
34. Four terms commonly used to describe the grade of
soil structure are:
1. Structure-less:
There are no noticeable peds, such as conditions exhibited by
loose sand or a cement-like condition of some clay soils.
2. Weak structure:
Indistinct formation of peds which are not durable.
3. Moderate structure:
Moderately well-developed peds which are fairly distinct.
4. Strong structure:
Very well-formed peds which are quite durable and distinct.
For naming a soil structure the sequence followed is grade,
class and type; for example, strong coarse angular blocky (soil
structure).
36. Factors Affecting Soil Structure:
The development of structure in arable soil depends on the
following factors:
1. Climate
2. Organic matter
3. Tillage
4. Plant roots
5. Soil organism
6. Fertilizers
7. Wetting and drying
37. Composition of Soil
The soil is composed of different components:
5% organic matter, 45% minerals, 20-30%
different gases and 20-30% water.
Therefore, the soil is known as a heterogeneous
body. Given below is the composition of soil in
detail:
38. Organic Matter
Organic substance is found in very small amounts in the
soil.
Plants and animals are the main sources of organic
matter. Depending upon the decomposition stage, the
organic matter is of the following three types:
• Completely decomposed organic matter
• Partially decomposed organic matter
• Undecomposed organic matter
39. Minerals
• Minerals are an important element of the soil.
These are solid components composed of
atoms.
• These occur naturally and have a fixed
chemical composition.
• Olivine and feldspar are the main minerals
present in the soil.
40. Gaseous Components
The air-filled pores of the soil contain the
gaseous components.
Nitrogen and oxygen present in the pores is
generally the atmospheric air fixed by the
microorganisms.
However, the composition of carbon dioxide
is higher due to the gas produced by
microorganisms present in the soil.
41. Water
• The soil dissolves the minerals and nutrients in
the water and transports it to different parts of
the plants.
• These are essential for the growth and
development of the plant.
43. Importance of soil water
• Water, an excellent solvent for most of the plant
nutrients, is a primary requisite for plant growth. Water
serves four functions in plants: it is the major
constituent of plant protoplasm (85-95%); it is
essential for photosynthesis and conversion of
starches to sugars
• it is the solvent in which nutrients move into and
through plant parts; and it provides plant turgidity,
which maintains the proper form and position of plant
parts to capture sunlight.
• In fact, the soil water is a great regulator of physical,
chemical and biological activities in the soil.
44. • Plants absorb some water through leaf stomata (openings), but most
of the water used by plants is absorbed by the roots from the soil.
For optimum water used, it is vital to know how water moves into
and through the soil, how the soil stores water, how the plant
absorbs it, how nutrients are lost from the soil by percolation, and
how to measure soil water content and losses.
• Soils also serve as a regulated reservoir for water because it receives
precipitation and irrigation water. A representative cultivated loam
soil contains approximately 50% solid particles (sand, silt, clay
and organic matter), 25% air and the rest 25% water. Only half
of this water is available to plants because of the mechanics of
water storage in the soil.
45. Classification of Soil Water:
There are generally two types of soil water
classification based on drying of wet soils and
growing plants there in
(A) physical
(B) biological
46. A. Physical Classification:
Under physical classification soil water is grouped into three on the
basis of retention: gravitational, capillary and hygroscopic water.
Gravitational water
Gravitational water may be defined as the water that is held at a
potential greater than -1/3 bar and that portion of the soil water that
will drain freely from the soil by the force of gravity.
In-spite of having low energy of retention, gravitational water is of
little use to plants water occupies the larger pores resulting poor
aeration.
Therefore, the removal of excess water is a must for the growth of
most plants.
47. Capillary water
Capillary water is held in the micro-pores of soils
(capillary pores). Capillary water is retained on
the soil particle by force of attraction between soil
particles and water molecules
48. Hygroscopic water
• Hygroscopic water is defined as the water that is held
by the soil particles at a suction of more than -31
bars.
• It is essentially non-liquid and moves primarily in the
vapour form.
• This water is held so tenaciously that plants are not
able to absorb it and thereby unavailable to plants.
• Some micro-organisms can utilize such form of
water.
49. Biological Classification:
There is a definite relationship between moisture retention and its utilization
by plants. Biological classification is based on the availability of soil moisture
to the plant. Soil water under this system of classification can be divided into
three categories.
1. Available water
Available water is defined as that portion of water which is retained in the soil
between field capacity (-1/3 bar) and the permanent wilting coefficients (-15
bars). This water is easily usable by plants and therefore, it is called plant
available water. Plant available water is equal to the difference of water
percentage at field capacity and a permanent wilting point.
50. 2. Unavailable water
Unavailable water is defined as the water which is held at soil water
potential greater than -15 bars. It is unavailable to plants. It includes the
whole of the hygroscopic water plus a part of the capillary water below
the wilting point.
3. Superfluous water
Superfluous water is defined as the water which is retained in the soil
beyond the field capacity soil moisture tension. This water includes
gravitational water plus a portion of capillary water removed from large
interstices. Such type of water is unavailable to plants and rather
presence of such water in the soil for a long period causes harmful
effect for plant growth because of lack of air.
51.
52. Soil porosity
• Pore-spaces (also called voids) in a soil consist of that portion
of the soil volume not occupied by solids, either mineral or
organic.
• The pore-space under field conditions, are occupied at all
times by air and water.
• Pore-spaces directly control the amount of water and air in the
soil and indirectly influence the plant growth and crop
production.
