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Introduction of Soil

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BASIC INTRODUCTION OF

SOIL SCIENCE

i
BASIC INTRODUCTION OF

SOIL SCIENCE

Guru Dayal
District Coordinator – SRIJAN,
(Self – Reliant Initiatives through Joint Action)

Dr. Ram Bharose

Assistant Professor
Department of Soil Science & Agricultural Chemistry,
NAI, SHUATS, Prayagraj

Raghvendra Kumar Kushwaha

M.Sc. (Agri.) Soil Science,
Sam Higginbottom University of Agriculture,
Technology and Sciences Allahabad-211007,U.P

iii
 1. INTRODUCTION OF SOIL

Satya Ranjan Mohanta

Ph.D. in Soil Science
Sam Higginbottom University of Agricultural,
Technology and Sciences, Allahabad- 211007, U.P.

Taniya Mistri
Ph.D. in Soil Science
Sam Higginbottom University of Agricultural,
Technology and Sciences, Allahabad- 211007, U.P.

ABSTRACT
Soil, often regarded as the skin of the Earth, is a complex and dynamic natural resource
that plays a vital role in sustaining life on our planet. This abstract provides a succinct
overview of the essential aspects of soil, emphasizing its formation, composition,
ecological significance, and the multifaceted challenges it faces in the modern world. Soil
formation is a gradual and intricate process influenced by geological, climatic, biological,
and topographical factors. Over time, weathering of rocks and organic matter
decomposition contribute to the development of distinct soil horizons, each with unique
characteristics. The composition of soil is diverse and encompasses mineral particles,
organic matter, water, air, and a rich community of microorganisms. These components
interact synergistically to support terrestrial ecosystems, providing essential nutrients to
plants, acting as a water reservoir, and facilitating critical biogeochemical cycles.
Introduction
Word soil is derived from a Latin word “Solum” which means ground or dirt. Soil is a
dynamic natural body and act as an important medium for crop growth and natural habitat
for living organisms. Soil is more or less loose and crumby part of the outer earth crust.
Soil is our, plants, animals and all living beings support, thus it can be said that its mother
of all. It provides nutrients, water, medium to the plant for its proper growth, it is the
medium for growing food, providing raw materials for construction, and it purifies water
for humans whereas for many animals it’s their home (habitat). Soils are developed by
natural forces acting on natural materials (weathering of rocks and minerals).
Definition of soil by various known scientist:
“Soils are applied solely to those superficial or nearly superficial horizons of rocks, that
have been more or less modified naturally by the interaction of water, air and various kinds

Basic Introduction of Soil Science | 1

of organisms, either living or dead; this being reflected in a certain manner in composition,
structure and colour of such formations. Where these conditions are absent, there are no
natural soils, but either artificial mixtures or rocks.”
- Dokuchaev
“Soil is a dynamic natural body on the surface of the earth in which plants grow, composed
of mineral and organic materials and living forms.”
-Buckman and Brady
“Soil is more or less loose and crumby part of the outer earth crust in which, by means of
their roots, plants may or do find foot hold and nourishment as well as all other conditions,
essential to their growth.”
- Hilgard
“Soil is a natural body developed by natural forces acting on natural materials. It is usually
differentiated into horizons from mineral and organic constituents of variable depth which
differ from the parent material below in morphology, physical properties and constituents,
chemical properties and composition and biological characteristics.”
- Joffe and Marbut
Soil Science
Soil science encompasses the investigation of soil, a natural resource found on the surface
of the earth, including the formation, categorization and mapping of soil; the physical
characteristics, chemical characteristics, biological characteristics and fertility
characteristics of soil; and these characteristics in the context of soil use and management.
Subjects under Soil Science
(i) Soil chemistry- the study of the chemical composition, properties, and reactions of
substances within the soil environment. It encompasses the investigation of various
chemical processes and interactions that occur in soil.
(ii) Soil physics- that focuses on the physical properties and behavior of soil. It primarily
deals with the study of how soil particles and their interactions influence the physical
characteristics and processes within the soil environment.
(iii) Soil geology- the study of the geological aspects and factors that influence the
formation and properties of soils. It involves the examination of how geological materials,
processes, and history interact with soils. Soil geology encompasses various geological
factors, including the parent material (rock or sediment) from which soils are derived, the
influence of geological structures, and the impact of past geological events on soil
development.

2 | Guru Dayal, Dr. Ram Bharose & Raghvendra Kumar Kushwaha

(iv) Soil biology- the study of the living organisms and biological processes that occur
within the soil environment. It encompasses the examination of a wide range of organisms,
from microorganisms like bacteria, fungi, and protozoa to larger organisms like
earthworms, insects, and plant roots. Soil biology explores how these organisms interact
with each other and with the soil matrix, and it plays a crucial role in soil health, nutrient
cycling, and ecosystem functioning.
Branches of Soil Science
• Pedology: In Pedology, the origins of soil, its classification, and its
description are discussed. Derive from the Greek word pedon which
means soil or earth.
• Edaphology: Study of soil from the standpoint of higher plants is called
Edaphology. (Derivefrom Greek word edaphoswhich means soil or
ground). Edaphologists take into account the various characteristics and
properties of soil in connection with the plant production.
Components of Soil on volume basis
• Mineral matter (45%)
• Organic matter (5%)
• Soil water (25%)
• Soil air (25%)

• Mineral Matter in Soils

Mineral matter is the inorganic components of soil that are not derived from living
organisms. It is the portion of soil that consists of various mineral particles, such as sand,

Basic Introduction of Soil Science | 3

silt, and clay, as well as other inorganic materials like rocks, stones, and minerals. There
are two types of minerals are there, i.e.,

(i) Primary minerals- In primary minerals soil made up of coarser fractions of the soil
namely sand and silt. They are formed after the crystallization of molten magma. They are
further divided into silicate minerals and non-silicate minerals.
Silicate minerals are those which are made up of silicon- oxygen tetrahedron (SiO4). For
about >90% of silicate minerals are present in the earth’s crust.Ex: -mica, olivine (Fe, Mg)2
SiO4, hornblende, brucite, quartz SiO2, etc.
Whereas non-silicate minerals consist of oxides, hydroxides, carbonate and sulphate
groups. Ex: - Hematite Fe2O3, Gibbsite Al2O3.H2O, Calcite CaCO3, Gypsum CaSO4. 2H2O.
(ii) Secondary minerals- In secondary minerals soil made up of finer fractions of the soil
namely clay. The are form after weathering of primary minerals under various temperature
and pressure. They are mainly formed due to chemical weathering. Ex:- silicate clays,
hydrous oxides of Fe & Al.

• Organic Matter in Soils

Organic matter in soil consists of decomposed plant and animal residues, such as dead
leaves, roots, and other organic materials. They are in continuous breaking state and 3-5%
by weight in top soil. It is the store house of nutrients. Some characteristics of organic
matter in soils are discussed below;

➢ Gives desirable surface soil structure

➢ Provides large pore size
➢ Improve water holding capacity
➢ Improves aeration
➢ Main source of energy for micro-organisms
➢ Major source of nitrogen, phosphorus (5-60%), potassium (80%)
➢ Act as chelate
➢ Maintain soils temperature (cool in summer and warm during winter)
➢ Increase CEC
➢ Reduce soil erosion

• Soil Water
Soil water is the amount of water that occupies the pore spaces between soil particles within
a specific volume of soil. Soil water is essential for plant growth and is a critical component
of the soil ecosystem.
It influences the soil texture, structure, nature of inorganic and organic colloids, number of
exchangeable cations, size and amount of pore space.

4 | Guru Dayal, Dr. Ram Bharose & Raghvendra Kumar Kushwaha

Soil water can be classified into several categories based on its availability and movement
within the soil profile. These classifications are important in agriculture, hydrology, and
environmental science. The primary classifications of soil water are:

PHYSICAL CLASSIFICATION OF WATER

1) Gravitational Water (Free Water):
▪ Gravitational water is the water that drains through the soil due to gravity. It is the
excess water that soil cannot hold and is typically found in the larger pores between
soil particles.
▪ This water is not available for plant use as it drains quickly from the root zone,
causing waterlogging and potentially leaching nutrients.
▪ Potential by which the water is held is – 1/3 bar.
2) Capillary Water (Available Water):
▪ Capillary water is the water held in the soil against the force of gravity in the smaller
pores between soil particles. It is available for plant uptake.
▪ This is the water that plants can access, and its availability depends on factors such
as soil texture, structure, and moisture content.
▪ Water is held within the soil particles by potential of -1/3 to -31 bars.
3) Hygroscopic Water:
▪ Hygroscopic water is the water held tightly to soil particles by molecular forces. It
is not available for plant use because plants cannot extract it.
▪ This water is essentially unavailable and exists as a thin film surrounding soil
particles.
▪ Water is held by the soil particles with the suction of more than -31 bars.

Basic Introduction of Soil Science | 5

BIOLOGICAL CLASSIFICATION OF WATER
1) Field Capacity:
▪ Field capacity is the maximum amount of water the soil can hold against gravity
after excess water has drained away. It represents the amount of water available for
plant roots.
▪ It is considered the optimal moisture level for most crops.
▪ In field capacity water potential is less than – 1/ 3 bar.
2) Wilting Point:
▪ Wilting point is the minimum amount of water in the soil below which plants cannot
extract enough water to maintain their turgidity, leading to wilting.
▪ It is a critical threshold for plant survival and growth.
▪ In wilting point water potential is less than – 15 bars.
3) Saturation:
▪ Saturation occurs when all pore spaces in the soil are filled with water, leaving no
air spaces.
▪ It is a temporary condition and is usually followed by drainage due to gravity.
▪ At saturation water potential is 0 bar.
4) Available Water Capacity:
▪ Available water capacity is the range of soil moisture between field capacity and
the wilting point.
▪ This range represents the water that is available to plants for growth and is often
used to determine irrigation needs.
▪ Water available to plants at the potential between -1/3 to -15 bars.

• Soil Air
Soil air refers to the air that occupies the pore spaces or voids within the soil matrix. It is
the air found in the spaces between soil particles, including the space between soil

6 | Guru Dayal, Dr. Ram Bharose & Raghvendra Kumar Kushwaha

aggregates and the spaces between individual mineral and organic particles in the soil. Soil
air is an essential component of the soil environment and plays a crucial role in supporting
plant growth and soil ecosystem functions.
The composition of soil air can vary depending on factors such as soil type, moisture
content, and environmental conditions. However, the following gases are typically found
in soil air:
1) Nitrogen (N2): Nitrogen makes up the largest portion of soil air, typically around 78%
to 80%. It is an essential component of the atmosphere and is relatively inert in soil.
2) Oxygen (O2): Oxygen levels in soil air can vary widely, depending on factors like soil
moisture and biological activity. In well-aerated soils, oxygen levels can be similar to
atmospheric levels, around 20.9%. In waterlogged or poorly aerated soils, oxygen levels
may drop significantly.
3) Carbon Dioxide (CO2): Soil air contains carbon dioxide, typically ranging from 0.3%
to 1% or even higher, depending on soil conditions and microbial activity. Plants respire
and release carbon dioxide into the soil, while soil microorganisms produce it during the
decomposition of organic matter.
4) Water Vapor (H2O):The moisture content of the soil determines the amount of water
vapor in soil air. Soil air can hold varying levels of water vapor, depending on soil porosity
and temperature.
5) Trace Gases: Soil air may also contain trace amounts of other gases, including argon
(Ar), methane (CH4), and various trace gases such as nitrous oxide (N2O) and hydrogen
(H2). The concentrations of these gases are usually very low.
RENEWAL OF SOIL AIR
Renewal of soil air is essential for the health and productivity of soil ecosystems. some
factors and processes that contribute to the renewal of soil air:
1. Diffusion: Gases, such as oxygen (O2) and carbon dioxide (CO2), can diffuse into
and out of the soil through the pore spaces. This process is driven by differences in
gas concentrations between the soil and the atmosphere. Plant roots and soil
organisms also facilitate the exchange of gases during respiration.
2. Aeration: Aeration refers to the process of mechanically introducing air into the
soil. This can be done through practices like tilling, ploughing, or soil aeration
machines. Aeration helps to improve gas exchange in compacted soils and can be
especially important in agricultural and horticultural contexts.
3. Root Respiration: Plant roots consume oxygen for respiration, and they release
carbon dioxide as a by product. This process creates gradients in gas concentrations
within the soil, driving the exchange of gases between the soil and the atmosphere.

Basic Introduction of Soil Science | 7

 4. Microbial Activity: Soil microorganisms also respire, consuming oxygen and
releasing carbon dioxide. Their activity contributes to the renewal of soil air and
the cycling of nutrients within the soil.
5. Temperature Changes: Temperature fluctuations can influence the movement of
gases within the soil. Warmer temperatures generally promote the diffusion of
gases out of the soil, while cooler temperatures may slow down this process.
6. Rainfall and Water Movement: Rainfall can help to flush out gases from the soil,
promoting the renewal of soil air. Water movement in the soil can create channels
for gas exchange as well.
7. Plant Growth and Decomposition: The growth of plants and the decomposition
of organic matter in the soil contribute to changes in gas concentrations. As organic
matter breaks down, it releases gases, including carbon dioxide. Plant roots and the
mycorrhizal fungi associated with them can also influence soil gas dynamics.
8. Soil Texture: Soil texture affects the pore size distribution within the soil. Soils
with a greater proportion of larger pores typically have better aeration and gas
exchange compared to soils with predominantly smaller pores.
SURFACE SOIL AND SUB- SURFACE SOIL
Surface soil and sub-surface soil are terms used to describe different layers of soil found
beneath the Earth's surface. These layers differ in their characteristics and properties, and
they play important roles in various ecological and agricultural processes.
Surface Soil:-
▪ Surface soil, also known as topsoil, is the uppermost layer of soil found near the
Earth’s surface.
▪ It is the layer where most of the biological and chemical soil activity occurs, and it
contains organic matter such as decomposed plant and animal materials, microbes,
and minerals.
▪ Surface soil is typically rich in nutrients and is crucial for plant growth and
agriculture.
▪ It is often darker in color due to the presence of organic matter, and it is relatively
loose and easy to work with.
Sub-Surface Soil:-
▪ Sub-surface soil, also referred to as subsoil, is the layer of soil located immediately
below the surface soil/topsoil.
▪ It typically contains less organic matter than surface soil and may have a lighter
color.
▪ Sub-surface soil is important for water retention and drainage as it influences the
movement of water through the soil profile.

8 | Guru Dayal, Dr. Ram Bharose & Raghvendra Kumar Kushwaha

 ▪ Roots of some plants may extend into the sub-surface soil layer to access nutrients
and moisture not present in the surface soil.
▪ Sub-surface soil can vary in composition and characteristics depending on factors
like geological history, climate, and local conditions.
Soil Profile
A soil profile is a vertical cross-section of the soil layers or horizons that exist beneath the
Earth's surface in a particular location. It provides a visual representation of the various
soil horizons or layers, each with distinct characteristics, composition, and properties.
These profiles help in soil classification, land-use planning, agriculture, construction, and
environmental assessments.
A typical soil profile consists of several horizons, which are generally categorized into the
following layers (from top to bottom):
O-Horizon:- The top layer, also known as the organic horizon, consists of decomposed
organic matter like leaves, plant debris, and humus. This layer is typically found in forested
areas.
• OL1- un-decomposed leaves, branches, animal excreta, etc. which can be easily
distinguished by naked eyes.
• OL2- partially decomposed plant and animal residues which can not be decomposed
by naked eyes.
A-Horizon:- Often referred to as the topsoil, this layer is rich in minerals and organic
material. It's where most plant roots grow and is crucial for agriculture and natural
vegetation.
• A1- Upper most mineral soil horizon, dark in colour due to presence of more organic
matter.
• A2- it is the eluviation horizon also called as washing out horizon. Here eluviation
of oxide, clays take place.
• A3- it is the transition horizon between A and B horizon which have similar
properties of A.
E-Horizon:- The eluviation horizon is characterized by the leaching or removal of
minerals and nutrients. It may not always be present in every soil profile.
B-Horizon:- The subsoil or accumulation horizon is where minerals and nutrients leached
from the A and E horizons accumulate. It often contains minerals like iron, clay, or organic
matter, giving it distinct coloration and composition.
B1-it is the transition horizon between A and B horizon which have similar properties ofB.
B2- It is illuvial horizon or washing in horizon. Where the above washed-out oxides and
clays settle or accumulate.

Basic Introduction of Soil Science | 9

B3-it is the transition horizon between B and C horizon which have similar properties of B.
C-Horizon:- Also known as the regolith, this horizon contains weathered parent material
but lacks the distinct properties of the horizons above it.
R-Horizon:- The bedrock horizon is the bottom layer of the soil profile, consisting of un-
weathered rock material or solid bedrock. Biological activities reduce in this horizon.

Soil profiles vary greatly from place to place based on climatic conditions, plant cover,
parent soil, topography and human activities. Understanding and analyzing soil profiles is
important for agriculture, land management, environmental management and many other
areas.
Important points-
• Diverse Soil Types: India is home to a wide variety of soil types, but they can be
broadly categorized into major groups, including Alluvial Soils, Black Soils
(Regur), Red Soils, Laterite Soils, Arid and Desert Soils, Forest and Mountain
Soils, and Saline and Alkaline Soils.

10 | Guru Dayal, Dr. Ram Bharose & Raghvendra Kumar Kushwaha

• Agricultural Significance: India's agriculture largely depends on its soils. The
country's diverse soil types allow for the cultivation of a wide variety of crops, from
rice and wheat in the Indo-Gangetic plains to coffee and tea in the southern hills.
• Soil Erosion: Soil erosion is a significant issue in India, particularly in hilly and
forested areas. It results from deforestation, improper land use, and excessive
rainfall, leading to loss of fertile topsoil.
• Soil Health and Fertility: Maintaining soil health and fertility is crucial for
sustainable agriculture. Farmers use various practices, including crop rotation,
organic farming, and balanced fertilization, to enhance soil productivity.
• Conservation Efforts: India has implemented various soil conservation and
watershed management programs to combat soil erosion and degradation.
Terracing, afforestation, and check dams are some of the techniques used.
• Soil Testing and Research: Soil testing labs across the country help farmers
analyze their soil's nutrient content and pH levels. This information assists in
making informed decisions regarding fertilizer and nutrient management.
• Challenges: India faces challenges related to soil degradation, urbanization
encroaching on agricultural land, and pollution affecting soil quality. Sustainable
soil management practices are essential to address these issues and ensure food
security.
• E Horizon (Eluviation Horizon): Also known as the leaching horizon, the E
horizon is characterized by the removal (eluviation) of minerals and nutrients that
have leached down from above. It is typically lighter in color.
• Soil Formation: Soil profiles develop over long periods through weathering,
erosion, deposition, and the accumulation of organic matter. The unique
characteristics of a soil profile depend on factors such as climate, parent material,
vegetation, and time.
• Pedogenesis: The process of soil formation, known as pedogenesis, involves the
transformation of geological material into a mature soil profile, with distinctive
horizons and properties.
• Soil Texture: Soil texture refers to the relative proportions of sand, silt, and clay
particles within the soil profile. It significantly influences soil characteristics such
as water-holding capacity, drainage, and aeration.
• Soil Horizons as Indicators: The properties and arrangement of soil horizons in a
profile can provide valuable information about the soil's fertility, nutrient content,
drainage, and suitability for various land uses, including agriculture, construction,
and environmental conservation.
• Soil Conservation: Understanding soil profiles is essential for sustainable land
management and conservation practices. Knowledge of soil horizons helps in
implementing erosion control measures and optimizing agricultural practices.

Basic Introduction of Soil Science | 11

 • Soil Surveys: Soil profiles are examined and documented in soil surveys conducted
by soil scientists and geologists. These surveys provide crucial data for land use
planning, agriculture, and environmental management.
• Soil Amendments: Understanding soil composition helps in determining the need
for soil amendments such as lime to adjust pH, organic matter additions to improve
fertility, and irrigation management for moisture control.
• Parent Material: The composition of soil is influenced by the parent material,
which is the underlying geological material from which soil forms. Different types
of parent materials contribute to variations in soil composition.
• Living Organisms:Soil is teeming with life, including microorganisms (bacteria,
fungi, protozoa), earthworms, insects, and plant roots. These organisms play vital
roles in nutrient cycling, decomposition, and soil structure improvement.
• Nutrients: Soils contain essential nutrients for plant growth, including
macronutrients (nitrogen, phosphorus, potassium) and micronutrients (iron, zinc,
manganese). The availability of nutrients depends on soil pH and organic matter
content.
• pH Level:Soil pH measures the acidity or alkalinity of the soil. It influences
nutrient availability and microbial activity. Soils can be acidic, neutral, or alkaline,
with a pH scale ranging from 0 to 14.
REFERENCE-
1. Dilip Kumar Das-Introductory Soil Science, chapter 1 “Concept of soil” pp 1-8.
2. T D Biswas and S K Mukherjee– Textbook of Soil Science, chapter 1“Soil
Development- The Earth” pp 3-14.
3. R K Mehra – Textbook of Soil Science, chapter 1 “Soil introduction” pp 1-15.
4. ISSS– Soil Science an introduction, chapter 1 “Soil and human society” pp 1-38.

*****

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