Lecture – 1

Fundamentals of Soil Science (2+1)

Concept and definition of soil • Branches of soil science • Pedological and edaphological
concepts • origin of the earth, atmosphere, hydrosphere and lithosphere along with their
characteristics • Earth’s crust, its composition and relative abundance of rock’s in the
earth’s crust • Soil components – mineral matter, organic matter, water and air.

1.1. Concept:
Soil is the uppermost layer of the earth in which plants grow. The soils support us all-plants,
animals and people either directly or from other life that comes from the soil. From the time man
began to think, the soil has been recognized as the mother of all - plants, animals and human. Soil
locks within its embrace the beginning of all life, and receives, at last, their discarded forms. It is
the imperishable storehouse of eternity. The thin layer of soil covering the earth’s surface
represents the difference between survival and extinction for most terrestrial life.
Land and soil are often creating confusion among students. Land includes not only soil but
also all the living organisms, air and water bodies within or on it and the rocks below. Land,
therefore is used with vast sense but soil is therefore, a part of the land and have narrowly-defined
concept.
The soils are developing as a result of chemical, biological and physical actions and
reactions within the earth’s crust. Knowledge of rocks and earth’s crust is essential in
understanding (i) the origin of different soils and (ii) causes of variations in properties of different
soils.
1.2. Definition of soil:
The word ‘soil’ derived from the Latin word ‘solum’ which means ‘floor’.
‘A soil’ means one individual soil body having a piece of landscape with depth, shape and
area. ‘The soil’ is a collective term for all the soils of the world. Therefore, ‘a soil’ is a subdivision
of ‘the soil’ having specific characteristics.
Many scientists have given the definition of soil but the most recent definition of soil by
Velayutham and Bhattacharya (2000) that seems most appropriate is as follows:
“Soil is a dynamic natural body developed as a result of pedogenic processes through
weathering or rocks, consisting of mineral and organic constituents, possessing definite chemical,
physical, mineralogical and biological properties, having a variable depth over the surface of the
earth and providing a medium for plant growth”.
1.3. Branches of Soil Science:
Soil science deals with soil as a natural body and medium for crop growth. Therefore, to gain
knowledge about soil it is necessary to consider their development, classification, physical nature,
chemical properties, biological activities, productive capacity and their tendency to erode.
Following disciplines are studied under soil science:
1. Pedology: It deals with the genesis, survey and classification and the laws of geographic
distribution of soils as a body in nature.
a. Soil genesis: It deals with the mode of origin of the soil.
b. Soil survey: It is the study of morphological description, classification and mapping of
soils.

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 c. Soil taxonomy: the grouping of soils based on their properties and characteristics.
2. Soil physics: The study on physical properties which affect the ability of soils to provide a
medium for root development and supply of nutrients for plant growth. e.g. soil colour, soil
texture, soil structure etc.
3. Soil Chemistry: The study of chemical basis of soil formation and soil fertility is soil
chemistry. It includes the role and function of individual element and their availability. Soil
pollution is also studied under soil chemistry.
4. Soil microbiology: It deals with microorganisms and their activities in soil. It includes
bacteria, fungi, actinomycetes, algae and protozoa which have direct or indirect role on soil
productivity.
5. Soil technology: It deals with the management of problem soils i.e. saline, saline sodic,
sodic and eroded soils.
6. Soil fertility: The study of nutrient status of soil with respect to the amount and availability
of nutrient to plants. Soil fertility is defined as the ability of soil to supply nutrients for plant
growth. The concept of soil fertility includes how well the nutrients are protected from
leaching and their enhanced availability to plants in addition to the quantity of nutrients the
soil contains.
7. Soil conservation: It deals with the protection of soil against physical loss by erosion or
aginst chemical deterioration. It provides management aspects for producing high yield and
at the same time, protecting it from degradation.

1.4. Pedological and Edaphological concept:

Soil is dealt with two basic concepts. The first concept considers soil as natural body, a
biochemically weathered and synthesized product of nature. The second conceives the soil as a
natural habitat for plants and justifies soil studies primarily on that basis. These conception
illustrates two approaches that can be used in studying soils - that of the pedologist and that of the
edaphologist.
Pedology (from the Greek word ‘pedon’ which means soil or earth). Pedology deals with
the origin of the soil, its classification and description. It considers the soil as a natural body and
does not focus primarily on the soil’s immediate practical utilization. A Pedologist studies,
examines and classifies soils as they occur in their natural environment. These findings may be as
useful to highway and construction engineers as well as to the agriculturists.
Edaphology (from the Greek word ‘edaphos’ which also means soil or ground) is the study
of soils in relation to growth, nutrition and yield of crop i.e. crop production. It considers the
various properties of soils as they relate to plant production. The edaphologist is practical having
the production of food and fibre as an ultimate goal.

1.5. Origin of the Earth

The universe, as we know, is composed of innumerable galaxies or star clusters. The
Galaxy in which we live, the Milky Way, is a disc-shaped revolving cluster of billions stars of
varying size and brightness. One of the stars, which are of modest size and brightness and nearer
the edge than the centre of the Galaxy, is the Sun.
The Solar System consists of the Sun, 9 Planets (8 planets excluding Pluto) with their 31
Satellites and a belt of asteroids or planetoids. The sun accounts for over 99.87 per cent of the
entire mass in our Solar System. The Sun is in the centre of the Solar System and the planets move
around.

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 The way in which our Earth was formed is based on an old theory, known as the Nebular
hypothesis. According to this hypothesis, the whole Solar System is supposed to have existed as a
nebula or matters in vapours or gases. As such it was intensely hot and supposed to be revolving.
With the lapse of time, the nebula began to cool. On cooling, the outer layers of the vapours
condensed and were thrown out in the form of rings. These rings on further condensation and
disruption became what we know as the planets.
Another theory of planet formation is Planetesimal hypothesis. It states that planets form out
of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach
sizes of approximately one kilometre, then they attract each other directly through their mutual
gravity, enormously aiding further growth into moon-sized protoplanets and later planets. The
Earth formed around 4.54 billion years ago.
The present form of earth has emerged after millions of years in which liberation of gases
from the molten masses, earthquakes, earth movements, volcanic eruptions have contributed.
Though the outside of the earth is cool, the interior is still very hot. With depth, there is a
tremendous increase in temperature. Because of the pressure from above, the interior of the earth
remains in a solid state. The surface of the earth’s crust is covered with mountains, depressions
(covered with water) and atmosphere.
Spheres of the Earth
The earth consists of three spheres – the atmosphere, the hydrosphere and the lithosphere.
Atmosphere: (Greek word ‘atmos’ means vapour and ‘sphaira’ means sphere) All round our earth,
there is a cover of air known as atmosphere. It extends to a height of 1600 km from the earth’s
surface, but has been extensively studied only up to 300 km. The atmosphere contributes 0.03% by
weight of the earth. It is made up mainly of nitrogen (78%) and oxygen (21%). Argon, carbon
dioxide, helium, ozone, neon, krypton are other gases which constitute only 1% of the total air
quantity. Besides these gases, air contains water vapour and dust particles. At the sea level the
atmosphere exerts a pressure of 1,013.25 millibars.
Average composition of atmosphere:
Name of the gas By volume (%)
Nitogen 78.084
Oxygen 20.946
Argon 0.934
Carbon dioxide 0.033
Beside this, H, NH3, H2S, SO2, O3, He, Neon, Krypton, Xenon etc are also present.

Hydrosphere: (Greek word ‘hydor’ means water and ‘sphaira’ means sphere). It is the layer of
water surrounding the lithosphere. About 71% of the earth’s surface is covered with water. The
hydrosphere makes up only 6.91% of the earth mass, but by volume it is 15 times that of the visible
land. Seawater contains highest amount of soluble salts. Average density of seawater is 1.026 due
to mineral matter dissolved in it. The average content of mineral matter in the seawater is 3.5%.

Lithosphere: (Greek word ‘Lithos’ means rocky and ‘sphaira’ means sphere). It is the inner most
body within the gaseous and watery envelops. That portion of the lithosphere, which rises above
the seawater, is visible to us and is known as land. It accounts for 93.06% of the Earth’s mass.
Visible land is more concentrated in the Northern Hemisphere. The lithosphere consists of two
portions; viz. (a) Upper or outer cool solid surface and (b) Inner hot and molten mass.
One hundred eight elements are known to occur in nature. But only few of these occur in
abundance in earth’s crust. Oxygen and silicon exist to the extent of approximately 74%,

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aluminium, iron, calcium, sodium, magnesium, potassium make up nearly 24% and all others only
nearly 1% of the earth’s crust. Oxygen, carbon and nitrogen exist in a free state in nature. Most of
the other elements are found in combination as minerals in rocks. The combination of these
minerals forms silicates, oxides, carbonates, phosphates and sulphates, which occur in the
lithosphere. These elements when combine under suitable conditions give rise to a variety of
minerals. Rocks when exposed to weathering agencies evolved into soils.
The elements are geochemically distributed into five main groups based on their bonding
characters. They are as follows:
a. Lithophile elements – which ionize readily or form stable oxyanions, viz. O, Si, Fe, Mn,
Al, K, Na, etc.
b. Chalcophile element – which tend to form covalent bonds with sulphide, viz. S, Cu, Zn,
Se, etc.
c. Siderophile elements – which readily form metallic bonds, viz. Fe, Ni, Co etc.
d. Atmosphile elements – which tend to remain in atmospheric gases, viz. N, (O), He, Ne,
Ar, Kr, Xe, etc.
e. Biophile elements – which tend to be associated with living organisms, viz. CH, O, N,
P, S, Ca, Mg, Fe, etc.
1.6. Earth crust – its composition
The earth consists of a solid part (core, mantle and the crust) and the atmosphere
surrounding it. Most of the earth’s crust is covered with sea. The crust, made of rocks is about 80
km thick. Earth’s crust covered with water is called hydrosphere. Solid crust of the earth forms
lithosphere. The atmosphere is about 272 km deep. The soil forms a very thin layer between these
two (Fig. 1).
The four major layers that make up the earth – the crust, the mantle and the outer and inner
core – vary considerably.

Fig. 1. A slice through the earth’s centre would show that it is built of four main layers.
Table 1. Chemical composition of the Earth's crust (% by weight)
Non-metals O2- 46.6 %
74.32%
Si4+ 27.72 %
Metals Al3+ 8.13 %
Fe2+ 5.00 %
Ca2+ 3.63 %
Na+ 2.83 % 25.68
K+ 2.59 %
Mg2+ 2.09 %
Others 1.41 %

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 Earth’s crust is about 1% of the total volume. Beneath the earth’s crust is the mantle, which
accounts for 82% of the earth’s total volume and descends nearly 3000 km to the molten outer core.
The central core is a ball of energy produced by radioactive elements. The temperature at the core
is 4000o C, which diminishes gradually to about 3000o C at the boundary between the outer core
and the mantle. Heat is generated from the radioactive decay and cloud of gas.

Relative abundance of Rocks in the Earth’s crust:

The composition of the upper 5km of the Earth’s crust is as follows:

 Sedimentary Rocks
- Shales 52%
- Sandstones 15% 74%
- Limestones and dolomite 7%
 Igneous Rocks
- Granite 15% 18%
- Basalt 3%
 Others 8% 8%

1.7. Soil components:

The soil consists of four major components: mineral matter, organic matter, soil air and soil
water. By volume, the air-dry soil contains about 45% mineral matter, 5% organic matter and 25%
each of water and air as given below:

Fig. 2. Volumetric composition of soil

The arrangement of solid particles and pore space is called the soil matrix. The matrix is a
three-phase system of solid (mineral matter and organic matter) liquid (water) and gas (air). The
proportion of air and water are subject to rapid and great fluctuations under natural conditions,
depending on the weather and other factors. The interactions among these components are of great
significance in providing plants with water, air and essential nutrients.

Mineral matter: Mineral matter is composed of small rock fragments and mineral of various
kinds. In general, the primary minerals are present in coarse fraction (>2mm) of the soil whereas
secondary mineral are dominant in the finer fraction (<2mm) of the soil.

Organic matter: organic matter consists of an accumulation of partially decayed and partially
synthesized plant and animal residues. The important functions of organic matter in soil are as
follows:

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 1. Supply of plant nutrients: Organic matter is a store house of nutrients. These nutrients are
released and made available to plants with the decomposition of organic matter.
Decomposing organic matter is an important source of N, P and S. Organic acid produced
during decomposition of organic matter helps in the release of plant nutrients from soil
minerals.
2. Maintenance of soil structure: Organic matter helps to bind soil particles and provides
aggregation suitable for maintenance of soil aeration.
3. Activity of soil organisms: organic matter is the main source of energy for soil organisms.
Micro-organisms derive their energy through organic matter; without which their activity
would be adversely affected. Micro-organisms also control availability of plant nutrients.

Soil water: It plays significant role in soil plant growth primarily in two ways:
Water held in soil pores: it depends on (i) amount of water present (ii) size of the soil pores. The
force of retention of water increased with the decreasing amount of water. Thus, if water is
abundant, the forces of retention will be low and vice versa.
Soil water makes up the soil solution: water containing dissolved salts in it is called soil solution.
Soil solution is n important medium for supply of nutrients to plants. The nutrients may be supplied
to plants through exchange phenomena between solid surface and soil solution and then between
soil solution and plant roots. The supply of nutrients also depends on the concentration of H+ and
OH- ion in soil solution as these ions influence solubility of nutrient elements.

Soil air: Soil air occupies pores not filled with water. Hence, content of air is determined by water
content of the soil. After a heavy rain the soil pores may be filled with water. After sometime when
water moves downward, pores are filled with air. Thus relative amount of air and water in the pore
space fluctuate continuously. Soil air contains a higher concentration of carbon dioxide and small
amount of oxygen than atmospheric air. This is due to respiration by soil organisms where large
amount of carbon dioxide is released into the soil and oxygen is consumed by organisms. Well
aggregated soil enhances root development.

Suggested books:
Brady, N.C and Weil, R. R. (2012). The Nature and Properties of Soils (14th Edition). Pearson
Education Inc., New Delhi.

Das, D.K. (2014). Introductory Soil Science – A text book. Kalyani Publishers.

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