Fundamentals of Soil Science (Soil formation, Structuexture)

SAC 101 Fundamentals of Soil Science (2 + 1)
Lecture notes
Dr.M.Sheik Dawood
Dr.A.Bhaskaran
Anbil Dharmalingam Agricultural College and Research Institute
Tamil Nadu Agricultural University
Navalur Kuttapattu

Tiruchirapalli - 620 009
SAC 101 Fundamentals of Soil Science (2 + 1)
SOIL SCIENCE IS “The science dealing with soil as a natural resource on the
surface of the earth, including Pedology (soil genesis, classification and mapping),
physical, chemical, biological and fertility properties of soil and these properties in
relation to their management for crop production”
Soil Science has six well defined and developed disciplines.
1. Soil fertility : Nutrient supplying properties of soil
2. Soil chemistry : Chemical constituents, chemical properties and the
chemical reactions
3. Soil physics : Involves the study of physical properties
4. Soil microbiology : Deals with micro organisms, its population,
classification, its role in transformations
5. Soil conservation : Dealing with protection of soil against physical loss by
erosion or against chemical deterioration i.e excessive
loss of nutrients either natural or artificial means.
6. Soil Pedology : Dealing with the genesis, survey and classification
Views on Soil (Science)
The term SOIL was derived from the Latin Word “SOLUM” Means FLOOR
 For a Layman soil is dirt or debris
 For an Agriculturist soil is a habitat for plant growth (to grow crops)
 For a Mining Engineer soil is a debris covering the Rocks
 For a Civil Engineer soil is a material on which road bed or house bed is
formed
 For a Home Owner soil is a mellow or loamy or hard material
Definitions
Whitney (1892): Soil is a nutrient bin which supplies all the nutrients required for
plant growth
Hilgard (1892): Soil is more or less a loose and friable material in which plants, by
means of their roots, find a foothold for nourishment as well as for other conditions of
growth”
Dokuchaiev (1900): Russian scientist - Father of soil science - Soil is a natural body
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composed of mineral and organic constituents, having a definite genesis and a
distinct nature of its own.
 Joffe (1936): “Soil is a natural body of mineral and organic constituents
differentiated into horizons - usually unconsolidated - of variable depth which differs
among themselves as well as from the underlying parent material in morphology,
physical makeup, chemical properties and composition and biological
characteristics”.
Jenny (1941): Soil is a naturally occurring body that has been formed due to
combined influence of climate and living organisms acting on parent material as
conditioned by relief over a period of time.
Ruffin and Simonson (1968): Soil is a mixture of Earth’s uppermost mantle of
weathered rock and organic matter
Soil Science Society of America (1970):
(i) Soil is the unconsolidated mineral matter on the surface of the earth that has been
subjected to and influenced by genetic and environmental factors of parent material,
climate (including moisture and temperature effects), macro and microorganisms and
topography, all affecting over a period of time and producing a product, that is “SOIL”
that differs from the material from which it is derived in many, physical, chemical,
biological and morphological properties and characteristics.
(ii) The unconsolidated mineral material on the immediate surface of the earth that
serves as a natural medium for the growth of land plants.
As soil provides nutrients, water, air and anchorage and supports life on Earth,
it can be called as Sole Of Infinite Life (SOIL)
Development of Soil Science
History of development of Soil Science: Refer to books for details
List of International Soil Scientists
1. Van Helmont (1577 – 1644)
2. Theoder De Saussure
3. John Woodward
4. Boussingault (1802 – 1882)
5. J.V. Liebig (1803 – 1873)
6. J.B.Laws & J.H. Gilbert (1855)
7. J.T.Way (1856)
8. R.Warrington (1876)
9. E.W. Hilgard (1860)
10.V.V. Dokuchaiev (1846-1903)
11.K.D.Glinga (1914)
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12.C.F.Marbut (1927)
13.Hens Jenny (1941)
Indian Scientists
1. J.W.Leather (1906)
2. Madam Scholasky (1932)
3. Wadia et al. (1935)
4. Viswanath & Ukil (1943)
Soil as a three dimensional body
Soil is a three dimensional body having length, breadth and depth. They form
a continuation over the land surface and differ in properties from place to place. Its
upper boundary is air or water and lower boundary is the rock lithosphere.
Composition of soil on volume basis
Mineral matter : 45%
Organic matter : 5%
Soil water : 25%
Soil air : 25%
Composition of Earth crust (% by weight)
Non - metallic Oxygen O2-
46.60
%
74.32% ( ¾th
)
Silica Si4+
27.72
%
Metallic Aluminium Al3+
8.13% ¼th
of the total
Iron Fe2+
5.00%
Calcium Ca2+
3.63%
Sodium Na+
2.83%
Potassium K+
2.59%
Magnesiu
m
Mg2+
2.09%
Others - 1.41%
Eight elements are abundant – 98.6%
Soil can be compared to various systems of animals
Digestive system : Organic matter decomposition
Respiratory system : Air circulation & exchange of gases
Circulatory system (blood) : Water movement within the soil
Excretory system : Leaching out of excess salts
Brain : Soil clay
Colour : Soil colour
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Approaches of Soil Study
Two Concepts: One treats soil as a natural body, weathered and synthesized
product in nature (Pedology) while other treats soil as a medium for plant growth
(Edaphology).
Pedological Approach: The origin of the soil, its classification and its description are
examined in Pedology. (from Greek word pedon, means soil or earth).
Pedology is the study of soil as a natural body and does not focus on
the soil’s immediate practical use. A pedologist studies, examines and
classifies soil as they occur in their natural environment.
Edaphological Approach: Edophology (from Greek word edaphos, means soil or
ground) is the study of soil from the stand point of higher plants.
Edaphologists consider the various properties of soil in relation to plant
production. They are practical and have the production of food and fibre
as their ultimate goal. They must determine the reasons for variation in
the productivity of soils and find means for improvement.
Origin of earth
Earth is one of the 9 planets (8 excluding Pluto) orbiting the SUN in the Solar
System. The Universe is composed of several Galaxies. Our solar system is part of
Milky way galaxy which is disk shaped with about 1,00,000 million stars of varying
sizes. Our solar system consists of 9 planets and 31 satellites, a belt of asteroids.
Theories about origin of Earth
Nebular hypothesis
This hypothesis suggests that the solar system formed through the
condensation of a nebula which once encircled the Sun (Immanuel Kant, 1775) and
LaPlace, 1796). The outer planets formed first, followed by Mars, the Earth, Venus,
and Mercury. This hypothesis suggests a sequential origin from outermost planet to
innermost. As per this hypothesis, Mars evolved earlier than the Earth. This
hypothesis is widely accepted to explain origin of different planets
Chamberlin-Moulton planetesimal hypothesis
A catastrophic hypothesis, proposed by Thomas Chamberlin and Forest
Moulton (1905), in which the planets of the Solar System are seen to arise from an
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encounter between the Sun and another star. In this scenario, the gravity of the
passing star tears a succession of bolts from the solar surface. Bolts coming from
the side nearer the star are thrown out to distances comparable with those of the
giant plants, while those from the far side of the Sun are ejected less violently to the
distances of the terrestrial planets.
From the inner remains of these bolts formed the initial cores of the planets.
The outer parts expanded and cooled into a huge swarm of solid particles spread out
in a disk rotating about the Sun in a plane determined by the motion of the passing
star. The cores gradually grew into planets by gathering in the planetesimals, most of
the growth taking place in the outer parts of the Solar System where material was
more plentiful. Eventually it became clear that, like other variants of the encounter
idea, not enough angular momentum could be conveyed to the ejected material to
explain the state of the Solar System as it exists today.
Tectonic plate theory
Tectonic plates are layers of rock inside the earth. This crust and upper
mantle form lithosphere. Under the lithosphere lies a fluid rock layer called
asthenosphere. The rocks in Tectonic plates are able to float upon the
asthenosphere. There are seven large and several small plates. The largest plates
include the pacific, North American, Eurasian, Antarctic and African plates.
Continental drift theory
The huge land masses of the earth are in constant movement. The continents
and the oceans rest upon rock plates. They move relative to each other at rates of
few centimeters a year. Until 200 million years ago, the continents were connected
into one single land mass called Pangaea. This Pangaea splits and pieces drifted
apart creating 2 new continents160 million years ago. The land mass that was to
become India separated and moved north ward (140 million years ago) and hit
another landmass leading to the eruption of the Himalayas. Australia separated from
Antarctica (100 million years ago). The two super continents then pulled apart east
and west opening the Atlantic oceans.
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The continents are still moving even today. The continental drift makes the
Pacific Ocean smaller, the Atlantic Ocean larger and the Himalayan Mountains taller.
The Atlantic oceans become wider, Mediterranean Sea collapses and Australia
reaches equator in 60 million years.
Age of the earth
So far scientists have not found a way to determine the exact age of the Earth
directly from Earth rocks because Earth's oldest rocks have been recycled and
destroyed. Nevertheless, scientists have been able to determine the probable age of
the Solar System and to calculate an age for the Earth by assuming that the Earth
and the rest of the solid bodies in the Solar System formed at the same time and are,
therefore, of the same age.
The ages of Earth and Moon rocks and of meteorites are measured by the
decay of long-lived radioactive isotopes of elements that occur naturally in rocks and
minerals and that decay with half lives of 700 million to more than 100 billion years to
stable isotopes of other elements. These dating techniques, which are firmly
grounded in physics and are known collectively as radiometric dating, are used to
measure the last time that the rock being dated was either melted or disturbed
sufficiently to rehomogenize its radioactive elements.
These ancient rocks have been dated by a number of radiometric dating
methods and the consistency of the results give scientists confidence that the ages
are correct to within a few percent. An interesting feature of these ancient rocks is
that they are not from any sort of ‘primordial crust’ but are lava flows and sediments
deposited in shallow water, an indication that Earth history began well before these
rocks were deposited.
In Western Australia, single zircon crystals found in younger sedimentary
rocks have radiometric ages of as much as 4.3 billion years, making these tiny
crystals the oldest materials to be found on Earth so far. The source rocks for these
zircon crystals have not yet been found. The ages measured for Earth's oldest rocks
and oldest crystals show that the Earth is at least 4.3 billion years in age but do not
reveal the exact age of Earth's formation.
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The best age for the Earth (4.54 Ga) is based on old mineral grains (zircon)
with U-Pb ages of 4.4 Ga have recently been reported from sedimentary rocks in
west-central Australia. The oldest dated moon rocks, however, have ages between
4.4 and 4.5 billion years and provide a minimum age for the formation of our nearest
planetary neighbor. The age of 4.54 billion years found for the Solar System and
Earth is consistent with current calculations of 11 to 13 billion years for the age of the
Milky Way Galaxy (based on the stage of evolution of globular cluster stars) and the
age of 10 to 15 billion years for the age of the Universe.
Geological Time Scale of Earth – Development of Life
Era Period Age
(m.Yr)
Organism
Archean Archean > 3500 Lifeless
Precambria
n
Precambrian -do- Soft bodied plant & animals
Palaeozoic Cambrian 500 Algae and shell bearing molluscus
Ordovician 400 Molluscus & sea weeds
Silurian 360 Land plants & breathing land animals
Devonian 320 True fishes and ampibians
Carboniferous 280 Coal forming materials , non
flowering plants
Permian 246 Reptiles
Mesozoic Triassic 235 Marine life & reptiles
Jurassic 185 Dinosours
Cretaceous 139 Many plants & fishes
Cenozoic Tertiary
Eocine 20 Mammals
Myocine 29 Mammals
Pliocine 10 Mammals
Quartanary
Plistocine 01 Mammals
Recent time Mammals
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Radius, distance from sun and revolution period
S.No Planet Radius
(km)
Distance from
sun(m.k)
Revolution period (day)
1 Mercury 2439 57.9 88
2 Venus 6052 108.2 245
3 Earth 6378 149.4 365.25
4 Mars 3397 227.9 687
5 Jupiter 71398 778.3 4333
6 Saturn 60000 1427 10743
7 Uranus 23620 2870 30700
8 Neptune 24300 4496.5 60280
9 Pluto 1150 5970 90130
Interior of Earth
The Earth Ball consists of 3 concentric rings namely Crust, Mantle and Core.
Crust: 5 to 56 km on the surface of Earth. Density of rocks is 2.6 to 3.0 g cc-1
.
Distance: 5 to 11 km in oceans and 35 to 56 km in the continents.
Mantle: A massive solid to semi solid layer below the crust; 2900 km in thickness;
comprises mixed metals and silicate and basic rocks with density of 3.0 to 4.5 g cc-1
.
Core: Innermost portion of Earth, 3500 km in thickness, contains molten metals like
Nickel, iron; average density: 9.0 to 12.0 g cc-1
.
Exterior of Earth: Solid lithosphere, Liquid hydrosphere, a d gaseous atmosphere.
The atmosphere is of 320 km above the lithosphere / hydrosphere. It contains
78.08% N; 20.9% O; 0.033% CO2 and 0.93 % other gases (H, NH3, H2S, SO2, O3,
He, Argon, Neon, Krypton, Xenon)
70% of Earths surface is covered by water (Hydrosphere). Sea water contains 3.5%
salts (minerals)
Rocks
Rocks are the materials that form the essential part of the Earth’s solid crust. “Rocks
are hard mass of mineral matter comprising one or more rock forming minerals”.
Rocks are formed from the molten material known as magma. The study of rocks is
called Petrology (in Greek, petra means rock, logos means science). Petrology deals
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with the description of rocks; petrogenesis is the study of the origin of rocks.
Formation of rocks:
1. Cooling and consolidation of molten magma within or on the surface of earth
= Igneous or Primary rocks
2. Transportation and cementation of primary rocks = Sedimentary or
Secondary rocks
3. Alteration of the existing primary and secondary rocks = Metamorphic rocks
1. Igneous rocks (primary or massive rocks)
These are first formed in the earth crust due to the solidification of molten
magma. Based on the mode of formation, they are further classified as extrusive and
intrusive rocks
Extrusive rocks (or volcanic rocks)
These rocks are formed due to the consolidation of magma on the surface of
the earth. The magma, when it flows on the Earth surface is called LAVA. E.g. Basalt
Intrusive rocks (or plutonic rocks)
These rocks are produced due to solidification of magma below the surface of the
earth. Plutonic – intrusive rocks solidifies at greater depth and Hypabassal rocks
solidifies at shallow depth from the surface. E.g. Granite, syenite, diorite, Gabbro
etc. Rocks formed in vertical cracks are called dykes and in horizontal cracks are
called sills.
Vesicular rocks: Molten magma cools on the surface. Steam of water is entrapped
into rocks and forms vesicles.
Based on the silica content, rocks are also classified as
1. Acid rocks : >66% SiO2 (Granite, Rhyolite)
2. Intermediate : 56 to 65% SiO2
(Sub acid rocks 60 to 66% SiO2 (Syenite and Trachyte))
(Sub basic rocks 56 to 60 % SiO2 (Diorite and Andesite))
3. Basic rocks : 40 to 55% (Gabbro, basalt)
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Igneous rocks
S.No Rocks Origin Essential
minerals
Common
minerals
Average
specific
gravity
Remarks
i. Granite Plutonic
holocrystalline
Quartz (20
to 30%)
Hornblende,
magnetite,
mica
2.64 Light
coloured
white or
reddish
ii. Syenite Plutonic
Holocrystalline
Quartz,
orthoclase
Hornblende,
magnetite,
biotite
2.80 Light
coloured
white or
reddish
iii. Diorite Plutonic
Holocrystalline
Quartz Hornblende,
magnetite,
biotite
2.85 Darker
iv Gabbro Plutonic
Holocrystalline
Labradorite,
augite,
olivine
Hornblende,
ilmenite
3.0 Blakish
v. Dolerit
e
Hypabasal Labradorite,
augite,
olivine
Hornblende,
ilmenite
3.0 Blakish
vi. Basalt Volcanic
crystalline with
glassy mass
Labradorite,
augite,
olivine
Hornblende,
ilmenite
3.0
2. Sedimentary rocks
These rocks are formed from the consolidation of sediments accumulated
through wind or water action at the surface of the earth. Many are deposited in layer
or formed through chemical reactions as precipitates from aqueous solutions.
Sediments may contain various size particles cemented together by substances like
SiO2, Fe2O3 or lime. These rocks are also called as clastic rocks.
Based on the origin, the sedimentary rocks are classified as
1. Residual : Laterite
2. Transported
a. Deposited as solids in suspension : Sandstone, shale
b. Deposited by chemical precipitation : Limestone, ironstone
c. Deposited through agency of organic matter : Peat, Phosphatic deposits
Based on the grain size, sedimentary rocks are classified as
1. Rocks with boulder pebbles sized minerals (Rudaceous) : Conglomerate
2. Rocks with sand size particles (Arenaceous) : Sandstone
3. Rocks with silt size particles (silt rocks) : Siltstone
4. Rocks with clay size particles (Argillaceous) : Shale
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Sedimentary rocks
S.No Rock Mineral composition Colour and structure
1. Sandstone Mainly quartz with some CaCO2, iron
oxides and clay
Light to red, granular
2. Shale Clay minerals, quartz and some
organic matter
Light to dark thinly
laminated
3. Limestone Mainly calcite with some dolomite, iron
oxides, clay, phosphate and organic
matter
Light grey to yellow,
fine grained and
compact
3. Metamorphic rocks
These are formed from igneous and sedimentary rocks under the influence of
heat, pressure, chemically active liquids and gases. Change may occur in mineral
composition or texture or both. The changes due to water is called
hydrometamorphosis and due to pressure is called dynamometamorphosis
Sand stone : Quartizite
Shale : Slate/mica, schist
Lime stone : Marble
Granite : granite gneiss
Dolerite : Hornblende gneiss
Metamorphic rocks
S.No. Rock Mineral
composition
Colour and structure
1. Gneiss Formed from granite Alternating light and dark colours, banded
and foliated
2. Schist Formed from basalt
or shale
As original rock, foliated
3. Quartzite Formed from
sandstone
Light ot brown, compact and uniform
texture, foliated structure
4. Slate Formed from shale Grey to black, compact and uniform
texture, foliated structure
5. Marble Formed from lime
stone
Light red, green, black, compact fine to
coarse texture, foliated structure
Minerals
Minerals are naturally occurring solids with a definite chemical composition and
crystal structure. “Solid substances composed of atoms having an orderly and
regular arrangement”
When molten magma solidifies, different elements present in them freely
arrange in accordance with the attractive forces and geometric form. Silica
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tetrahedron is the fundamental building blocks for the formation of different minerals.
(SiO2). Different silicate minerals are ortho silicates, ino-silicates, phyllosilicates and
tecosilicates. There are non-silicate minerals also. These are different oxides,
carbonates, sulphates, phosphates etc.
Minerals that are original components of rocks are called primary minerals.
(feldspar, mica, etc.). Minerals that are formed from changes in primary minerals and
rocks are called secondary minerals (clay minerals). Those minerals that are chief
constituents of rocks are called as essential minerals (Feldspars, pyroxenes micas
etc) and those which are present in small quantities, whose presence or absence will
not alter the properties of rocks are called accessory minerals (tourmaline,
magnetite etc).
Of the >2000 known minerals, only few occur in abundance in the Earth crust.
Minerals (arranged in the order of
their crystallization)
Important constituents Percent
distribution
Primary minerals
Ferro magnesium minerals
Ortho-ino silicates 16.8
Olivine Fe, Mg
Pyroxenes Ca, Na, Fe, Mg
Amphiboles Ca, Na, Fe, Mg, Al, OH
Phyllo Silicates 3.6
Biotite K, Fe, Mg, Al, OH
Muscovite K, Al, OH
Non-Ferro Magnesium
Tecto Silicates
Feldspars 61.0
Anorthite Ca, Al
Albite Na, Al
Orthoclase K, Al
Quartz
Secondary clay minerals
Minerals Na, K, Ca 11.6
Others Mg, Fe, Al, OH 6.0
Secondary clay minerals
Clay minerals in soils are formed from primary minerals due to weathering
processes. These clay minerals are of size <0.002 mm and are considered to be the
most reactive part of soil. Important soil properties like nutrient and water holding
capacity are controlled by clay minerals. These minerals are layered silicates
consisting of silica tetrahedron and aluminium octahedron.
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1) 1 silicon tetrahedron + 1 aluminium octahedron = 1:1 clay mineral (Kaolinite)
2) 2 : 1 non-expanding clay mineral i. Black mica (Biotite)
ii. White mica (Muscovite)
iii. Weathered mica (illite)
3) 2 : 1 expanding clay mineral i. partially expanding (Vermiculite)
ii. Fully expanding (Montmorillonite)
4) 2 : 2 clay mineral (chlorite)
Non-silicate minerals
Oxides: Haematite (Fe2O3)
Limonite (Fe2O3, 3H2O)
Goethite (FeO (OH) H2O)
Gibbsite (Al2O3H2O)
The red, yellow or brown colours in soils are due to the presence of goethite and
haematite, which occur as coatings on the surface of soil particles.
Carbonates: Calcite (CaCO3)
Dolamite (CaMgCO3)
Sulphates: Gypsum (CaSO4.2H2O)
Phosphates: Apatite (Rock phosphate Ca3(PO4)2 - primary source of phosphorus
Physical properties of minerals
1. Color
2. Streak
3. Fracture/ cleavage
4. Hardness
5. Luster
6. Crystal form
7. Taste
8. Specific gravity
9. Magnetism
10.Effervescence (fizz)
11.Birefringence
12.Fluorescence
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Additional reading
Color
 Denotes the natural colour of the mineral
 The most obvious, but least reliable.
 Calcite has more colours
 Sulfur and Pyrite have same colour
Streak
• Refers to the colour of the powder form of the mineral When an unknown mineral is rubbed against a piece of
unglazed porcelain (streak plate) it produces a colored line.
• Hematite - red
• Magnetite - Black
• Talc - white
Fracture and Cleavage
These terms describe the way a mineral breaks Fracture is the nature of the surface produced as a result of its
breakage
Conchoidal - curved surface
Uneven - Uneven surface
Hackly - Jagged surface
Earthy - Like chalk
Even - Smooth
Cleavage
Some minerals break along certain well defined planes called cleavage planes.
Gypsum - 1 set
Calcite - 2 sets
Flourite - 3 sets
Hardness
This is how resistant a mineral is to being scratched. We use the Mohs scale to classify a given minerals hardness.
Try to scratch the unknown mineral with various items, such as a fingernail (hardness of about 2.5), a coin (3), a steel nail
(5.5) and a steel file (7)
MOHS SCALE OF HARDNESS
Mineral Hardness Mineral Hardness
Talc 1 Feldspar 6
Gypsum 2 Quartz 7
Calcite 3 Topaz 8
Fluorite 4 Corundum 9
Apatite 5 Diamond 10
Luster
The way a mineral reflects light
Metallic (Magnetite); sub-metallic, Vitreous (Opal), Resinous (Pyrite), Pearly, Adamentine (Diamond), silky (Asbestos) and
greasy
Crystal form
Crystal structure is the result of regular grouping of atoms that are homogeneous. A crystal is a polyhedral form, which
means it is a geometric solid. It has a specific set of faces, corners and edges, which is consistent with the geometric packing
of the atoms
There are 6 basic crystal forms
1. Isometric
2. Tetragonal
3. Hexagonal
4. Orthorhombic
5. Monoclinic
6. Triclinic
Taste
This property is used to identify the mineral halite (salt)
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Specific Gravity
This characteristic relates to the minerals density. If the mineral is heavy for its size, then it has a high specific gravity
Magnetism
Is the mineral magnetic (try using a compass), or is it attracted by a magnet? This property is characteristic of Magnetite.
Effervescence
When some minerals are exposed to acids, they begin to fizz (calcite)
Birefringence
This is also known as double refraction. Birefringent minerals split the light into two different rays which gives the illusion
of double vision in this Iceland Spar Calcite
Fluorescence
Some minerals display the phenomenon of photoluminescence.
They "glow" when exposed to UV light.
Opal and Fluorite
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Weathering of Rocks and Minerals
Rocks and minerals are formed under a very high temperature and pressure,
exposed to atmospheric conditions of low pressure and low temperature and they
become unstable and weather. Soils are formed from rocks through the intermediate
stage of formation of Regolith which is the resultant of weathering.
The sequence of processes in the formation of soils is :
Weathering of rocks and minerals → formation of regolith or parent material →
formation of true soil from regolith.
Rock → Weathring → Regolith → soil forming factors and processes → True soil
(otherwise)
Two processes involved in the formation of soil are
1i) Formation of regolith by breaking down (weathering) of the bed rock.
2ii) The addition of organic matter through the decomposition of plant and animal
tissues, and reorganization of these components by soil forming processes
to form soil.
Weathering: A process of disintegration and decomposition of rocks and
minerals which are brought about by physical agents and chemical processes,
leading to the formation of Regolith (unconsolidated residues of the weathering rock
on the earth’s surface or above the solid rocks). (OR)
Parent material: It is the regolith or at least it’s upper portion. May be defined as the
unconsolidated and more or less chemically weathered mineral material from which
soil are developed.
Weathering involves three process
Physical (or) mechanical - disintegration
Chemical - decomposition
Biological
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Different agents of weathering
Physical/ Mechanical
(disintegration)
Chemical
(decomposition)
Biological
(disintegration +
decomposition)
1.Physical condition of
rock
1.Hydration 1.Man & animals
2.Change in temperature 2.Hydrolysis 2. higher plants &
their roots
3.Action of H2O 3.Solution 3.Micro organisms
- fragment & transport 4.Carbonation
- action of freezing 5.Oxidation
- alternate wetting & drying 6.Reduction
- action of glaciers
4. Action of wind
5. Atmospheric electric phenomenon
I. Physical weathering: the rocks are disintegrated and are broken down to
comparatively smaller pieces, with out producing any new substances
1. Physical condition of rocks:
 The permeability of rocks is the most important single factor. Coarse textured
(porous) sand stone weather more readily than a fine textured (almost solid)
basalt.
 Unconsolidated volcanic ash weather quickly as compared to unconsolidated
coarse deposits such as gravels.
2. Action of Temperature:
 The variations in temperature exert great influence on the disintegration of
rocks. During day time, the rocks get heated up by the sun and expand.
 At night, the temperature falls and the rocks get cooled and contract. This
alternate expansion and contraction weakens the surface of the rock and
crumbles it because the rocks do not conduct heat easily.
 The minerals with in the rock also vary in their rate of expansion and
contraction. The cubical expansion of quartz is twice as feldspar.
 Dark coloured rocks are subjected to fast changes in temperature as
compared to light coloured rocks.
 The differential expansion of minerals in a rock surface generates stress
between the heated surface and cooled un-expanded parts resulting in
fragmentation of rocks.
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 This process causes the surface layer to peel off from the parent mass and
the rock ultimately disintegrates. This process is called Exfoliation
3. Action of Water: Water acts as a disintegrating, transporting and depositing agent.
i) Fragmentation and transport: Water beats over the surface of the rock when the
rain occurs and starts flowing towards the ocean
 Moving water has the great cutting and carrying force.
 It forms gullies and ravines and carries with the suspended soil material
of variable sizes.
 Transporting power of water varies. It is estimated that the transporting
power of stream varies as the sixth power of its velocity i.e the greater
the speed of water, more is the transporting power and carrying capacity
Speed/Sec Carrying capacity
15 cm fine sand
30 cm gravel
1.2 m stones (1kg)
9.0 m boulders (several tons)
The disintegration is greater near the source of river than its mouth
ii) Action of freezing:
Frost is much more effective than heat in producing physical weathering
 In cold regions, the water in the cracks and crevices freezes into ice and the
volume increases to one tenth
 As the freezing starts from the top there is no possibility of its upward
expansion. Hence, the increase in volume creates enormous out ward
pressure which breaks apart the rocks
iii) Alternate wetting and Drying:
Some natural substances increase considerably in volume on wetting and
shrink on drying. (e.g.) smectite, montmorillonite
 During dry summer/ dry weather - these clays shrink considerably forming
deep cracks or wide cracks.
 On subsequent wetting, it swells.
 This alternate swelling and shrinking / wetting or drying of clay enriched rocks
make them loose and eventually breaks
iv) Action of glaciers:
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 In cold regions, when snow falls, it accumulates and change into a ice sheet.
 These big glaciers start moving owing to the change in temperature and/or
gradient.
 On moving, these exert tremendous pressure over the rock on which they
pass and carry the loose materials
 These materials get deposited on reaching the warmer regions, where its
movement stops with the melting of ice
4. Action of wind:
 Wind has an erosive and transporting effect. Often when the wind is laden
with fine material viz., fine sand, silt or clay particles, it has a serious abrasive
effect and the sand laden winds itch the rocks and ultimately breaks down
under its force
 The dust storm may transport tons of material from one place to another. The
shifting of soil causes serious wind erosion problem and may render cultivated
land as degraded
(e.g) Rajasthan deserts
5. Atmospheric electrical phenomenon
It is an important factor causing break down during rainy season and lightning breaks
up rocks and or widens cracks
II. Chemical Weathering
Decomposition of rocks and minerals by various chemical processes is called
chemical weathering. It is the most important process for soil formation.
Chemical weathering takes place mainly at the surface of rocks and minerals
with disappearance of certain minerals and the formation of secondary products
(new materials). This is called chemical transformation.
Feldspar + water → clay mineral + soluble cations and anions
Chemical weathering becomes more effective as the surface area of the rock
increases. Since the chemical reactions occur largely on the surface of the rocks,
therefore the smaller the fragments, the greater the surface area per unit volume
available for reaction. The effectiveness of chemical weathering is closely related to
the mineral composition of rocks.
(e.g) quartz responds far slowly to the chemical attack than olivine or pyroxene.
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Average mineralogical composition (%)
Composition Granite Basalt Shale Sand Stone Lime Stone
Feldspar 52.4 46.2 30.0 11.5 -
Quartz 31.3 - 2.3 66.8 -
Pyroxines-
amphiboles
- 44.5 - - -
FeO minerals 2.0 9.3 10.5 2.0 -
Clay minerals 14.3 - 25.0 6.6 24.0
Carbonates - - 5.7 11.1 76.0
Chemical Processes of weathering:
1.Hydration: Chemical combination of water molecules with a particular substance
or mineral leading to a change in structure.
Soil forming minerals in rocks do not contain any water and under go hydration when
exposed to humid conditions. Upon hydration there is swelling and increase in
volume of minerals. The minerals loose their luster and become soft. It is one of the
most common processes in nature and works with secondary minerals, such as
aluminium oxide and iron oxide minerals and gypsum. (e.g)
2. Hydrolysis: Most important process in chemical weathering. It is due to the
dissociation of H2O into H+
and OH-
ions which chemically combine
with minerals and bring about changes, such as exchange,
decomposition of crystalline structure and formation of new
compounds. Water acts as a weak acid on silicate minerals.
This reaction is important because of two reasons.
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i) clay, bases and silicic acid - the substances formed in these reactions - are
available to plants
ii) water often containing CO2 (absorbed from atmosphere), reacts with the
minerals directly to produce insoluble clay minerals, positively charged metal
ions (Ca++
, Mg++
, Na+
, K+
) and negatively charged ions (OH- HCO3
-) and some
soluble silica – all these ions are made available for plant growth.
3. Solution: Some substances present in the rocks are directly soluble in water. The
soluble substances are removed by the continuous action of water and
the rock no longer remains solid and form holes, rills or rough surface
and ultimately falls into pieces or decomposes. The action is
considerably increased when the water is acidified by the dissolution of
organic and inorganic acids. (e.g) halides, NaCl
NaCl + H2O Na+
, Cl-
H2O ( dissolved ions with water)
4. Carbonation: Carbon di oxide when dissolved in water it forms carbonic acid.
2H2O + CO2 → H2CO3
This carbonic acid attacks many rocks and minerals and brings them into
solution. The carbonated water has an etching effect up on some rocks, especially
lime stone. The removal of cement that holds sand particles together leads to their
disintegration.
CaCO3 + H2CO3 → Ca(HCO3)2
(calcite) slightly soluble (Ca bi carbonate) readily soluble
5. Oxidation: The process of addition and combination of oxygen to minerals. The
absorption is usually from O2 dissolved in soil water and that present in atmosphere.
The oxidation is more active in the presence of moisture and results in hydrated
oxides.(e.g) minerals containing Fe and Mg.
4FeO (Ferrous oxide) + O2 → 2Fe2O3 ( Ferric oxide)
4Fe3O4 (Magnetite) + O2 → 6Fe2O3 (Haematite)
2Fe2O3 (Haematite) + 3H2O → 2Fe2O3 .3H2O(Limonite)
6. Reduction : The process of removal of oxygen and is the reverse of oxidation and
is equally important in changing soil colour to grey, blue or green as
ferric iron is converted to ferrous iron compounds. Under the
conditions of excess water or water logged condition (less or no
23

oxygen), reduction takes place.
2Fe2O3 (Haematite) - O2 → 4FeO( Ferrous oxide) -reduced form
In conclusion, during chemical weathering igneous and metamorphic rocks can be
regarded as involving destruction of primary minerals and the production of
secondary minerals.
In sedimentary rocks, which is made up of primary and secondary minerals,
weathering acts initially to destroy any relatively weak bonding agents (FeO) and the
particles are freed and can be individually subjected to weathering.
III Biological Weathering
Unlike physical and chemical weathering, the biological or living agents are
responsible for both decomposition and disintegration of rocks and minerals. The
biological life is mainly controlled largely by the prevailing environment.
1. Man and Animals
 The action of man in disintegration of rocks is well known as he cuts rocks to
build dams, channels and construct roads and buildings. All these activities
result in increasing the surface area of the rocks for attack of chemical agents
and accelerate the process of rock decomposition.
 A large number of animals, birds, insects and worms, by their activities they
make holes in them and thus aids for weathering.
 In tropical and sub tropical regions, ants and termites build galleries and
passages and carry materials from lower to upper surface and excrete acids.
The oxygen and water with many dissolved substances, reach every part of
the rock through the cracks, holes and galleries, and thus brings about
speedy disintegration.
 Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and
bodies of the dead animals, provides substances which react with minerals
and aid in decaying process.
 The earthworms pass the soil through the alimentary canal and thus brings
about physical and chemical changes in soil material.
2. Higher Plants and Roots
The roots of trees and other plants penetrate into the joints and crevices of
the rocks. As they grew, they exert a great disruptive force and the hard rock may
break apart. (e.g.) papal tree growing on walls/ rocks.
24

The grass roots form a sponge like mass, prevent erosion and conserve
moisture and thus allowing moisture and air to enter in to the rock for further action.
Some roots penetrate deep into the soil and may open some sort of drainage
channel. The roots running in crevices in lime stone and marble produces acids.
These acids have a solvent action on carbonates. The dead roots and plant residues
decompose and produce carbon dioxide which is of great importance in weathering.
3. Micro- organisms
In early stages of mineral decomposition and soil formation, the lower forms of
plants and animals like, mosses, bacteria and fungi and actinomycetes play an
important role. They extract nutrients from the rock and N from air and live with a
small quantity of water. In due course of time, the soil develops under the cluster of
these micro-organisms. These organisms are closely associated with the decay of
plant and animal remains and thus liberate nutrients for the next generation plants
and also produce CO2 and organic compounds which aid in mineral decomposition.
Weathering of Minerals
There are many factors which influence the weathering of minerals.
1Climatic conditions
2Physical characteristics
3Chemical and structural characteristics
1. Climatic Conditions
The climatic condition, more than any other factor- tends to control the kind
and rate of weathering. Under conditions of low rainfall, there is a dominance of
physical weathering which reduces the size and increases the surface area with little
change in volume.
The increase in moisture content encourages chemical as well as mechanical
changes and new minerals and soluble products are formed.
The weathering rates are generally fastest in humid tropical regions as there
is sufficient moisture and warmth to encourage chemical decomposition.
The easily weatherable minerals disappear on account of intense chemical
weathering and more resistant products (hydrous oxides of Fe and Al) tend to
accumulate
Climate controls the dominant type of vegetation which in turn controls the bio
chemical reactions in soils and mineral weathering.
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2. Physical Characteristics
i) Differential composition
ii) Particle size
iii) Hardness and degree of cementation
3. Chemical and structural characteristics
 Chemical: For minerals of given particle size, chemical and crystalline
characteristics determine the ease of decomposition. (e.g.) gypsum –
sparingly soluble in water, is dissolved and removed in solution form under
high rainfall.
 Ferro magnesium minerals are more susceptible to chemical weathering than
feldspar and quartz
 Tightness of packing of ions in crystals: Less tightly packed minerals like
olivine and biotite are easily weathered as compared to tightly packed zircon
and muscovite (resistant)
 Chemical weathering of Silicates: The most important silicates are quartz,
feldspar and certain Ferro magnesium minerals
Weathering products of common silicate minerals
Minerals Composition Decomposition products
Minerals Others
Olivine (Fe,Mg)2SiO2 serpentine,limonitehaemati
te, quartz
Some Si in solution,
carbonates of Fe and Mg
Pyroxenes Fe, Mg Clay, calcite, limonite Some Si in solution,
carbonates of Ca and Mg
Amphibole Ca- silicates Haematite,quartz -do -
Biotite Al -do -
Plagioclase Calcic Clay, quartz,calcite Some Si in solution, Na
and Ca carbonates
Sodic -do-
Orthoclase Potassic Clay, quartz Some Si in solution,
potassium carbonate
Quartz Quartz grains Some Si in solution
Weathering sequence:
The minerals remain in the soil and regolith have led to the following generalized
order of weathering
Quartz (most resistant) > muscovite > K feldspar > Na and Ca feldspar > biotite,
hornblende, augite > olivine > dolomite and calcite > gypsum (least resistant)
26

Soil Forming Factors
The soil formation is the process of two consecutive stages.
11. The weathering of rock (R) into Regolith
22. The formation of true soil from Regolith
The evolution of true soil from regolith takes place by the combined action of soil
forming factors and processes.
1. The first step is accomplished by weathering (disintegration & decomposition)
2. The second step is associated with the action of Soil Forming Factors
Factors of soil formation
Dokuchaiev (1889) established that the soils develop as a result of the action of soil
forming factors: S = f ( p, cl, o )
Further, Jenny (1941) formulated the following equation
S = f (cl, o, r, p, t, …)
Where,
cl – environmental climate
o – Organisms and vegetation (biosphere)
r – Relief or topography
p – Parent material
t- Time
… - additional unspecified factors
The five soil forming factors, acting simultaneously at any point on the surface of the
earth, to produce soil
Active soil forming factors: i) Climate, ii) organism (vegetation or biosphere)
These factors supply energy and act upon other soil forming factors
Passive soil forming factors i) Parent material, ii) Relief, iii) Time
These are the factors on which active soil forming factors act upon
The passive soil forming factors are those which represent the source of soil forming
mass and conditions affecting it. These provide a base on which the active soil
forming factors work or act for the development of soil.
Parent Material: It is that mass (consolidated material) from which the soil has
formed.
i) Sedentary: Formed in original place. It is the residual parent material. The parent
material differ as widely as the rocks
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ii) Transported: The parent material transported from their place of origin. They are
named according to the main force responsible for the transport and redeposition.
1a) by gravity - Colluvial
2b) by water - Alluvial , Marine , Locustrine
c) by ice - Glacial
d) by wind - Aeolian
Colluvium: It is the poorly sorted materials near the base of strong slopes
transported by the action of gravity.
Alluvium: The material transported and deposited by water is, found along major
stream courses at the bottom of slopes of mountains and along small streams
flowing out of drainage basins.
Lacustrine: Consists of materials that have settled out of the quiet water of lakes.
Moraine: Consists of all the materials picked up, mixed, disintegrated, transported
and deposited through the action of glacial ice or of water resulting primarily
from melting of glaciers.
Loess or Aeolian: These are the wind blown materials.
When the texture is silty - loess; when it is sand - Aeolian.
The soils developed on such transported parent materials bear the name of
the parent material; viz. Alluvial soils from alluvium, colluvial soils from colluvium etc.
In the initial stages, however, the soil properties are mainly determined by the kind of
parent material.
Endodynamomorphic soils: With advanced development and excessive leaching,
the influence of parent material on soil characteristics gradually diminishes. There
are soils wherein the composition of parent material subdues the effects of climate
and vegetation. These soils are temporary and persist only until the chemical
decomposition becomes active under the influence of climate and vegetation.
Ectodynamomorphic soils: Development of normal profile under the influence of
climate and vegetation.
Soil properties as influenced by parent material: Different parent materials affect
profile development and produce different soils, especially in the initial stages.
 Acid igneous rocks (like granite, rhyolite) produce light-textured soils (Alfisols).
 Basic igneous rocks (basalt), alluvium or colluvium derived from limestone or
basalt, produce fine-textured cracking-clay soils (Vertisols).
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 Basic alluvium or aeolian materials produce fine to coarse-textured soils
(Entisols or Inceptisols).
 The nature of the elements released during the decaying of rocks has a
specific role in soil formation. (e.g.) Si and Al forms the skeleton for the
production of secondary clay minerals.
 Iron and manganese are important for imparting red colour to soils and for
oxidation and reduction phenomena.
 Sodium and potassium are important dispersing agents for day and humus
colloids.
 Calcium and magnesium have a flocculating effect and result in favorable and
stable soil structure for plant growth.
2. Relief or Topography: The relief and topography sometimes are used as
synonymous terms. They denote the configuration of the land surface. The
topography refers to the differences in elevation of the land surface on a broad scale.
The prominent types of topography designations, as given in FAO Guidelines
(1990) are:
Land surface slope
1.Flat to Almost flat 0 – 2 %
2.Gently undulating 2 - 5 %
3.Undulating 5 – 10 %
4.Rolling 10 – 15 %
5.Hilly 15 –3 0 %
6.Steeply dissect > 30 % with moderate range of elevation( <300 m)
7.Mountainous > 30% with great range of elevation (>300 m)
Soil formation on flat to almost flat position: On level topographic positions,
almost the entire water received through rain percolates through the soil. Under such
conditions, the soils formed may be considered as representative of the regional
climate. They have normal solum with distinct horizons. But vast and monotonous
level land with little gradient often has impaired drainage conditions.
Soil formation on undulating topography: The soils on steep slopes are generally
shallow, stony and have weakly- developed profiles with less distinct horizonation. It
is due to accelerated erosion, which removes surface material before it has the time
to develop. Reduced percolation of water through soil is because of surface runoff,
29

and lack of water for the growth of plants, which are responsible for checking of
erosion and promote soil formation.
Soil formation in depression: The depression areas in semi-arid and sub humid
regions reflect more moist conditions than actually observed on level topographic
positions due to the additional water received as runoff. Such conditions (as in the
Tarai region of the Uttar Pradesh) favour more vegetative growth and slower rate of
decay of organic remains. This results in the formation of comparatively dark-
coloured soils rich in organic matter (Mollisols).
Soil formation and Exposure / Aspect: Topography affects soil formation by
affecting temperature and vegetative growth through slope exposures (aspect}. The
southern exposures (facing the sun) are warmer and subject to marked fluctuations
in temperature and moisture. The northern exposures, on the other hand are cooler
and more humid. The eastern and western exposures occupy intermediate position
in this respect.
3. Time: Soil formation is a very slow process requiring thousands of years to
develop a mature pedon. The period taken by a given soil from the stage of
weathered rock (i.e. regolith) up to the stage of maturity is considered as time. The
matured soils mean the soils with fully developed horizons (A, B, C). It takes
hundreds of years to develop an inch of soil. The time that nature devotes to the
formation of soils is termed as Pedologic Time.
It has been observed that rocks and minerals disintegrate and/or decompose
at different rates; the coarse particles of limestone are more resistant to
disintegration than those of sandstone. However, in general, limestone decomposes
more readily than sandstone (by chemical weathering).
Weathering stages in soil formation
Stages Characteristic
1 Initial Un weathered parent material
2 Juvenile Weathering started but much of the original material still un weathered
3 Virile Easily weatherable minerals fairly decomposed; clay content
increased, slowly weatherable minerals still appreciable
4 Senile Decomposition reaches at a final stage; only most resistant minerals
survive
5 Final Soil development completed under prevailing environments
 The soil properties also change with time, for instance nitrogen and organic
matter contents increase with time provided the soil temperature is not high.
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 CaCO3 content may decrease or even lost with time provided the climatic
conditions are not arid
 In humid regions, the H+
concentration increases with time because of
chemical weathering.
B. Active Soil Forming Factors
The active soil forming factors are those which supply energy that acts on the mass
for the purpose of soil formation. These factors are climate and vegetation
(biosphere).
1. Climate
Climate is the most significant factor controlling the type and rate of soil formation.
The dominant climates recognized are:
 Arid climate: The precipitation here is far less than the water-need. Hence the
soils remain dry for most of the time in a year.
 Humid climate: The precipitation here is much more than the water need. The
excess water results in leaching of salt and bases followed by translocation of
clay colloids.
 Oceanic climate: Moderate seasonal variation of rainfall and temperature.
 Mediterranean climate: The moderate precipitation. Winters and summers are
dry and hot.
 Continental climate: Warm summers and extremely cool or cold winters.
 Temperate climate: Cold humid conditions with warm summers.
 Tropical and subtropical climate: Warm to hot humid with isothermal
conditions in the tropical zone.
Climate affects the soil formation directly and indirectly.
Directly, climate affects the soil formation by supplying water and heat to react with
parent material. Indirectly, it determines the fauna and flora activities which furnish
a source of energy in the form of organic matter. This energy acts on the rocks and
minerals in the form of acids, and salts are released. The indirect effects of climate
on soil formation are most clearly seen in the relationship of soils to vegetation.
Precipitation and temperature are the two major climatic elements which contribute
most to soil formation.
Precipitation: Precipitation is the most important among the climatic factors. As it
percolates and moves from one part of the parent material to another. It carries with
31

it substances in solution as well as in suspension. The substances so carried are re
deposited in another part or completely removed from the material through
percolation when the soil moisture at the surface evaporates causing an upward
movement of water. The soluble substances move with it and are translocated to the
upper layer. Thus rainfall brings about a redistribution of substances both soluble as
well as in suspension in soil body.
Temperature
 Temperature is another climatic agent influencing the process of soil
formation.
 High temperature hinders the process of leaching and causes an upward
movement of soluble salts.
 High temperature favors rapid decomposition of organic matter and increase
microbial activities in soil while low temperatures induce leaching by reducing
evaporation and there by favour the accumulation of organic matter by
slowing down the process of decomposition. Temperature thus controls the
rate of chemical and biological reactions taking place in the parent material.
Jenney (1941) computed that in the tropical regions the rate of weathering proceeds
three times faster than in temperate regions and nine times faster than in arctic.
2. Organism & Vegetation
Organism:
 The active components of soil ecosystem are plants, animals, microorganisms
and man.
 The role of microorganisms in soil formation is related to the humification and
mineralization of vegetation
 The action of animals especially burrowing animals to dig and mix-up the soil
mass and thus disturb the parent material
 Man influences the soil formation through his manipulation of natural
vegetation, agricultural practices etc.
 Compaction by traffic of man and animals decrease the rate of water
infiltration into the soil and thereby increase the rate of runoff and erosion.
Vegetation
 The roots of the plants penetrate into the parent material and act both
mechanically and chemically.
32

 They facilitate percolation and drainage and bring about greater dissolution of
minerals through the action of CO2 and acidic substances secreted by them.
 The decomposition and humification of the materials further adds to the
solubilization of minerals.
 Forests – reduces temperature, increases humidity, reduce evaporation and
increases precipitation.
 Grasses reduce runoff and result greater penetration of water in to the parent
material.
Soil Forming Processes
The pedogenic processes, although slow in terms of human life, yet work
faster than the geological processes in changing lifeless parent material into true soil
full of life.
 The pedogenic processes are extremely complex and dynamic involving
many chemical and biological reactions, and usually operate simultaneously
in a given area.
 One process may counteract another, or two different processes may work
simultaneously to achieve the same result.
 Different processes or combination of processes operate under varying
natural environment.
 The collective interaction of various soil forming factors under different
environmental conditions set a course to certain recognized soil forming
processes.
The basic process involved in soil formation (Simonson, 1959) includes the following.
 Gains or Additions of water, mostly as rainfall, organic and mineral matter to
the soil.
 Losses of the above materials from the soil.
 Transformation of mineral and organic substances within the soil.
 Translocation or the movement of soil materials from one point to another
within the soil. It is usually divided into two aspects. 1) movement of solution
(leaching) and 2) movement in suspension (eluviation) of clay, organic matter
and hydrous oxides
A. Fundamental Soil forming Processes
Humification: Humification is the process of transformation of raw organic matter
33

into humus. It is extremely a complex process involving various organisms.
First, simple compounds such as sugars and starches are attacked followed by
proteins and cellulose and finally very resistant compounds, such as tannins, are
decomposed and the dark coloured substance, known as humus, is formed.
Eluviation: It is the mobilization and translocation of certain constituent’s viz. Clay,
Fe2O3, Al2O3, SiO2, humus, CaCO3, other salts etc. from one point of soil body to
another. Eluviation means washing out. It is the process of removal of constituents in
suspension or solution by the percolating water from the upper to lower layers. The
eluviation encompasses mobilization and translocation of mobile constituents
resulting in textural differences. The horizon formed by the process of eluviation is
termed as eluvial horizon (A2 or E horizon).
Translocation depends upon relative mobility of elements and depth of percolation.
Illuviation: The process of deposition of soil materials (removed from the eluvial
horizon) in the lower layer (or horizon of gains having the property of stabilizing
translocated clay materials) is termed as Illuviation. The horizons formed by this
process are termed as illuvial horizons (B-horizons, especially Bt) The process leads
to textural contrast between E and Bt horizons, and higher fine: total clay ratio in the
Bt horizon.
Horizonation: It is the process of differentiation of soil in different horizons along the
depth of the soil body. The differentiation is due to the fundamental processes,
humification, eluviation and illuviation.
B. Specific Soil Forming Processes
The basic pedologic processes provide a framework for later operation of more
specific processes
Calcification: It is the process of precipitation and accumulation of calcium carbonate
(CaCO3) in some part of the profile. The accumulation of CaCO3 may result in the
development of a calcic horizon.
Calcium is readily soluble in acid soil water and/or when CO2 concentration is high in
root zone as:
CO2 + H2O → H2CO3
H2CO3 + Ca → Ca (HCO3)2 (soluble)
Ca (HCO3)2 → CaCO3 + H2O + CO2 (precipitates)
The process of precipitation after mobilization under these conditions is called
34

calcification and the resulting illuviated horizon of carbonates is designated as Bk
horizon (Bca).
Decalcification: It is the reverse of calcification that is the process of removal of
CaCO3 or calcium ions from the soil by leaching
CaCO3 + CO2 + H2O → Ca(HCO3)2 (soluble)
(insoluble)
Podzolization: It is a process of soil formation resulting in the formation of Podzols
and Podzolic soils.
In many respects, podzolization is the negative of calcification. The calcification
process tends to concentrate calcium in the lower part of the B horizon, whereas
podzolization leaches the entire solum of calcium carbonates.
Apart from calcium, the other bases are also removed and the whole soil becomes
distinctly acidic. In fact, the process is essentially one of acid leaching.
The process operates under favorable combination of the following environments.
i) Climate: A cold and humid climate is most favorable for podzolization
ii) Parent material: Siliceous (Sandy) material, having poor reserves of weatherable
minerals, favor the operation of podzolization as it helps in easy percolation of
water.
iii) Vegetation: Acid producing vegetation such as coniferous pines is essential
iv) Leaching and Translocation of Sesquioxide: In the process of decomposition of
organic matter various organic acids are produced. The organic acids thus formed
act with Sesquioxide and the remaining clay minerals, forming organic- Sesquioxide
and organic clay complexes, which are soluble and move with the percolating water
to the lower horizons (Bh, Bs).
Aluminium ions in a water solution hydrolyze and make the soil solution very acidic.
2Al +6H2O → 2 Al(OH)3 + 6H+
As iron and aluminium move about, the A horizon gives a bleached grey or
ashy appearance. The Russians used the term Podzols (pod means under, the zola
means ash like i.e. ash-like horizon appearing beneath the surface horizon) for such
soils.
To conclude, the Podzolization is a soil forming process which prevails in a
cold and humid climate where coniferous and acid forming vegetations dominate.
The humus and Sesquioxide become mobile and leached out from the upper
35

horizon s and deposited in the lower horizon.
4. Laterization: The term laterite is derived from the word later meaning brick or tile
and was originally applied to a group of high clay Indian soils found in Malabar hills
of Kerala, Tamil Nadu, Karnataka and Maharashtra. It refers specifically to a
particular cemented horizon in certain soils which when dried, become very hard,
like a brick. Such soils (in tropics) when massively impregnated with sesquioxides
(iron and aluminium oxides) to extent of 70 to 80 per cent of the total mass, are
called laterites or latosols (Oxisols). The soil forming process is called Laterization
or Latozation. Laterization is the process that removes silica, instead of
sesquioxides from the upper layers and thereby leaving sesquioxides to concentrate
in the solum. The process operates under the following conditions.
i) Climate: Unlike podzolization, the process of laterization operates most favorable
in warm and humid (tropical) climate with 2000 to 2500 mm rainfall and continuous
high temperature ( 25°C) throughout the year.
ii) Natural vegetation: The rain forests of tropical areas are favorable for the
process.
iii) Parent Material: Basic parent materials, having sufficient iron bearing
ferromagnesian minerals (Pyroxene, amphiboles, biotite and chlorite), which on
weathering release iron, are congenial for the development of laterites.
5. Gleization: The term glei is of Russian origin means blue, grey or green clay. The
Gleization is a process of soil formation resulting in the development of a glei (or gley
horizon) in the lower part of the soil profile above the parent material due to poor
drainage condition (lack of oxygen) and where waterlogged conditions prevail. Such
soils are called hydro orphic soils.
The process is not particularly dependent on climate (high rainfall as in humid
regions) but often on drainage conditions.
The poor drainage conditions result from:
 Lower topographic position, such as depression land, where water stands
continuously at or close to the surface.
 Impervious soil parent material, and.
 Lack of aeration.
Under such conditions, iron compounds are reduced to soluble ferrous forms.
The reduction of iron is primarily biological and requires both organic matter and
36

microorganisms capable of respiring anaerobically. The solubility of Ca, Mg, Fe, and
Mn is increased and most of the iron exists as Fe++
organo - complexes in solution or
as mixed precipitate of ferric and ferrous hydroxides.
This is responsible for the production of typical bluish to grayish horizon with mottling
of yellow and or reddish brown colors.
6. Salinization: It is the process of accumulation of salts, such as sulphates and
chlorides of calcium, magnesium, sodium and potassium, in soils in the form of a
salty (salic) horizon. It is quite common in arid and semi arid regions. It may also
take place through capillary rise of saline ground water and by inundation with
seawater in marine and coastal soils. Salt accumulation may also result from
irrigation or seepage in areas of impeded drainage.
7. Desalinization: It is the removal by leaching of excess soluble salts from horizons
or soil profile (that contained enough soluble salts to impair the plant growth) by
ponding water and improving the drainage conditions by installing artificial drainage
network.
8. Solonization or Alkalization: The process involves the accumulation of sodium
ions on the exchange complex of the clay, resulting in the formation of sodic soils
(Solonetz).
All cations in solution are engaged in a reversible reaction with the exchange sites
on the clay and organic matter particles.
The reaction can be represented as:
Ca.Mg.2NaX → Ca++
+ Mg++
+2Na+
+ x-6
(+3CO3
2-
→ Na2CO3 + MgCO3 + CaCO3)
(Where X represents clay or organic matter exchange sites)
9. Solodization or dealkalization: The process refers to the removal of Na+
from
the exchange sites. This process involves dispersion of clay. Dispersion occurs
when Na+
ions become hydrated.
Much of the dispersion can be eliminated if Ca++
and or Mg++
ions are concentrated in
the water, which is used to leach the soonest. These Ca and Mg ion can replace the
Na on exchange complex, and the salts of sodium are leached out as:
2NaX + CaSO4 Na2SO4 + CaX (leachable)
10. Pedoturbation: Another process that may be operative in soils is pedoturbation.
It is the process of mixing of the soil. Mixing to a certain extent takes place in all
soils. The most common types of pedoturbation are:
37

 Faunal pedoturbation: It is the mixing of soil by animals such as ants,
earthworms, moles, rodents, and man himself
 Floral pedoturbation : It is the mixing of soil by plants as in tree tipping that
forms pits and mounds
 Argillic pedoturbation: It is the mixing of materials in the solum by the
churning process caused by swell shrink clays as observed in deep Black
Cotton Soils.
Development of Soil Profile
The development of soil profile is a constructive process where in
disintegrated material resulted from weathering of rocks and minerals gets converted
into a soil body.
Definition of soil profile: The vertical section of the soil showing the various layers
from the surface to the unaffected parent material is known as a soil profile.
The various layers are known as horizons. A soil profile contains three main
horizons.
They are named as horizon A, horizon B and horizon C.
 The surface soil or that layer of soil at the top which is liable to leaching and
from which some soil constituents have been removed is known as horizon A
or the horizon of eluviation.
 The intermediate layer in which the materials leached from horizon A have
been re-deposited is known as horizon B or the horizon of illuviation.
 The parent material from which the soil is formed is known as horizon C.
A Study of soil profile is important as it is historic record of all the soil forming
processes and it forms the basis for the study in pedagogical investigations. Soil
profile is the key for the soil classification and also forms the basis for the practical
utility of soils. A hypothetical mineral soil profile will include O, A, B, C and R master
horizons and all the possible sub-horizons.
Master horizons and sub horizons
O horizon - It is called as organic horizon. It is formed in the upper part of the mineral
soil, dominated by fresh or partly decomposed organic materials.
 This horizon contains more than 30% organic matter if mineral fraction has
more than 50 % clay (or) more than 20 % organic matter if mineral fraction
has less clay.
 The organic horizons are commonly seen in forest areas and generally absent
38

in grassland, cultivated soils.
 O1 - Organic horizon in which the original forms of the plant and animal
residues can be recognized through naked eye.
 O2 - Organic horizon in which the original plant or animal matter can not be
recognized through naked eye.
 A horizon - Horizon of organic matter accumulation adjacent to surface and
that has lost clay, iron and aluminium.
 A1 - Top most mineral horizon formed adjacent to the surface. There will be
accumulation of humified organic matter associated with mineral fraction and
darker in Colour than that of lower horizons due to organic matter.
 A2 - Horizon of maximum eluviation of clay, iron and aluminium oxides and
organic matter. Loss of these constituents generally results in accumulation of
quartz and other sand and silt size resistant minerals. Generally lighter in
Colour than horizons above and below.
 A3 - A transitional layer between A and B horizons with more dominated
properties of A1 or A2 above than the underlying B horizon. This horizon is
sometimes absent. Solum.
 B horizon - Horizon in which the dominant features are accumulation of clay,
iron, aluminium or humus alone or in combination. Coating of sesquioxides
will impart darker, stronger of red Colour than overlying or underlying
horizons.
 B1 - A transitional layer between A and B. More like A than B.
 B2 - Zone of maximum accumulation of clay, iron and aluminium oxide that
may have moved down from upper horizons or may have formed in situ. The
organic matter content is generally higher and Colour darker than that of A2
horizon above.
 B3 - Transitional horizon between B and C and with properties more similar to
that of overlying B2 than underlying C.
 C horizon - It is the horizon below the solum (A + B), relatively less affected by
soil forming processes. It is outside the zone of major biological activity. It
may contain accumulation of carbonates or sulphates, calcium and
magnesium
 R - Underlying consolidated bed rock and it may or may not be like the parent
39

rock from which the solum is formed.
Besides, lower case letters are used to indicate the special features of master
horizons. This case letters follow the subdivisions of master horizons. eg. Ap -
ploughed layer eg. B2t - illuvial clay
When two or more genetically unrelated (contrasting) materials are present in a
profile as in the case of alluvial or colluvial soils then the phenomenon is known as
lithological discontinuity. This is indicated by the use of Roman letters as prefixes to
the master horizons. eg. Ap, B2, II B22, IIIC
Special Features
Soil Individual or Polypedon: The Soil Survey Staff (1960) defined the soil
individual or polypedon (Pedon, Ground) as a natural unit of soil that differs from its
adjoining unit on the landscape in one or more properties.
The term pedon has been proposed for small basic soil entities that are part of the
continuum mantling the land.
A pedon is the smallest volume that can be called "a soil". The set of pedons must fit
within the range of one series and occur in a contiguous group to form a polypedon.
A polypedon is therefore, defined as a contiguous similar pedons bounded on all
sides by "not-soil or by pedons of unlike characters. It is a real physical soils body
which has a minimum area of more than 1 sq. km and an unspecified maximum
area.
The unconsolidated materials above the parent rock including A, B and C horizons
are called Regolith
The weathered soil material including O, A and B horizons are called Solum
40

Physical Properties
Physical properties (mechanical behaviour) of a soil greatly influence its use
and behaviour towards plant growth. The plant support, root penetration, drainage,
aeration, retention of moisture, and plant nutrients are linked with the physical
condition of the soil. Physical properties also influence the chemical and biological
behaviour of soil. The physical properties of a soil depend on the amount, size,
shape, arrangement and mineral composition of its particles. These properties also
depend on organic matter content and pore spaces.
Important physical properties of soils.
1. Soil texture
2. Soil structure
3. Surface area
4. Soil density
5. Soil porosity
6. Soil colour
7. Soil consistence
Soil Texture
Definition: Soil texture refers to the relative proportion of particles or it is the relative
percentage by weight of the three soil separates viz., sand, silt and clay or simply
refers to the size of soil particles.
The proportion of each size group in a given soil (the texture) cannot be easily
altered and it is considered as a basic property of a soil. The soil separates are
defined in terms of diameter in millimeters of the particles
Soil particles >2 mm in diameter are excluded from soil textural determinations.
Stones and gravels may influence the use and management of land because
of tillage difficulties but these larger particles make little or no contribution to soil
properties such as water holding capacity and capacity to store plant nutrients and
their supply.
Gravels : 2 – 4 mm
Pebbles : 4 – 64 mm
Cobbles : 64 – 256 mm
Boulders : > 256 mm
Particles less than 2 mm is called fine earth, normally considered in chemical and
41

mechanical analysis.
The components of fine earth: Sand, Silt and Clay (Soil separates. The size limits
of these fractions have been established by various organizations. There are a
number of systems of naming soil separates.
1(a) The American system developed by USDA
2(b) The English system or British system ( BSI )
3(c) The International system (ISSS)
(d) European system
i) USDA
Soil Separates Diameter (mm)
Clay < 0.002
Silt 0.002 – 0.05
Very Fine Sand 0.05 – 0.10
Fine Sand 0.10 – 0.25
Medium Sand 0.25 - 0.50
Coarse Sand 0.50 - 1.00
Very Coarse Sand 1.00 – 2.00
ii) BSI
Soil separates Diameter (mm)
Clay < 0.002
Fine Silt 0.002 – 0.01
Medium Silt 0.01 – 0.04
Coarse Silt 0.04 – 0.06
Fine Sand 0.06 - 0.20
Medium Sand 0.20 - 1.00
Coarse Sand 1.00 – 2.00
iii) ISSS
Soil separates Diameter (mm)
Clay < 0.002
Silt 0.002 – 0.02
Fine sand 0.02 – 0.2
Coarse sand 0.2 – 2.0
42

iv) European System
Soil
separates
Diameter (mm)
Fine clay < 0.0002
Medium clay 0.0002 –
0.0006
Coarse clay 0.0006 – 0.002
Fine silt 0.002 - 0.006
Medium silt 0.006 - 0.02
Coarse silt 0.02 - 0.06
Fine sand 0.06 - 0.20
Medium sand 0.20 - 0.60
Coarse sand 0. 60 - 2.00
Sand:
 Usually consists of quartz but may also contain fragments of feldspar, mica
and occasionally heavy minerals viz., zircon, Tourmaline and hornblende.
 Has uniform dimensions
 Can be represented as spherical
 Not necessarily smooth and has jagged surface
Silt:
 Particle size intermediate between sand and clay
 Since the size is smaller, the surface area is more
 Coated with clay
 Has the physico- chemical properties as that of clay to a limited extent
 Sand and Silt forms the SKELETON
Clay:
 Particle size less than 0.002 mm
 Plate like or needle like in shape
 Belong to alumino silicate group of minerals
 Some times considerable concentration of fine particles which does not
belong to alumino silicates. (eg). iron oxide and CaCO3
 These are secondary minerals derived from primary minerals in the rock
 Flesh of the soil
 Knowledge on Texture is important
43

 It is a guide to the value of the land
 Land use capability and methods of soil management depends on Texture
Particle size distribution/ determination
The determination of relative distribution of the ultimate or individual soil
particles below 2 mm diameter is called as Particle size analysis or Mechanical
analysis.
Two steps are involved
1i) Separation of all the particles from each other ie. Complete dispersion into
ultimate particles
2ii) Measuring the amount of each group
Separation
S.No Aggregating agents Dispersion method
1 Lime and Oxides of Fe & Al Dissolving in HCl
2 Organic matter Oxidizes with H2O2
3 High concentration of electrolytes
(soluble salts)
Precipitate and decant or filter with
suction
4 Surface tension Elimination of air by stirring with
water or boiling
After removing the cementing agents, disperse by adding NaOH
Measurement
Once the soil particles are dispersed into ultimate particles, measurement can be
done
i) Coarser fractions – sieving – sieves used in the mechanical analysis
corresponds to the desired particle size separation
 For 2 mm, 1 mm and 0.5 mm – sieves with circular holes
 For smaller sizes, wire mesh screens are used ( screening)
ii) Finer fractions – by settling in a medium
The settling or the velocity of the fall of particles is influenced by
 Viscosity of the medium
 Difference in density between the medium and falling particles
 Size and shape of object
Stokes' Law:
Particle size analysis is based on a simple principle i.e. "when soil particles
44

are suspended in water they tend to sink. Because there is little variation in the
density of most soil particles, their velocity (V) of settling is proportional to the square
of the radius 'r' of each particles.
Thus V = kr2
, where k is a constant. This equation is referred to as Stokes' law.
Stokes (1851) was the first to suggest the relationship between the radius of
the particles and its rate of fall in a liquid. He stated that "the velocity of a falling
particle is proportional to the square of the radius and not to its surface. The relation
between the diameter of a particle and its settling velocity is governed by Stokes'
Law:
dw)-(ds
gr
9
2
V 2
η
=
where
V - velocity of settling particle (cm/sec.)
g - acceleration due to gravity cm/ sec2
(981 )
ds - density of soil particle (2.65)
dw - density of water (1 )
η - coefficient of viscosity of water (0.0015 at 4°C)
r - radius of spherical particles (cm).
Assumptions and Limitations of Stokes' Law
1. Particles are rigid and spherical / smooth. This requirement is very difficult to
fulfill, because the particles are not completely smooth over the surface and
spherical. It is established that the particles are not spherical and irregularly
shaped such as plate and other shapes.
2. The particles are large in comparison with the molecules of the liquid so that
in comparison with the particle the medium can be considered as
homogenous. i.e. the particles must be big enough to avoid Brownian
movement. The particles less than 0.0002 mm exhibit this movement so that
the rate of falling is varied.
3. The fall of the particles is not hindered or affected by the proximity (very near)
of the wall of the vessel or of the adjacent particles. Many fast falling particles
may drag finer particles down along with them.
4. The density of the particles and water and as well as the viscosity of the
medium remain constant. But this is usually not so because of their different
chemical and mineralogical composition.
45

5. The suspension must be still. Any movement in the suspension will alter the
velocity of fall and such movement is brought by the sedimentation of larger
particles (> 0.08 mm). They settle so fast and create turbulence in the
medium.
6. The temperature should be kept constant so that convection currents are not
set up.
Methods of Textural determination
Numerous methods for lab and field use have been developed
i) Elutriation method – Water & Air
ii) Pipette method
iii) Decantation/ beaker method
iv) Test tube shaking method
v) Feel method – Applicable to the field – quick method – by feeling
the soil between thumb and fingers
Feel Method
Evaluated by attempting to squeeze the moistened soil into a thin ribbon as it
is pressed with rolling motion between thumb and pre finger or alternately to roll the
soil into a thin wire.
Four aspects to be seen
i) Feel by fingers
ii) Ball formation
iii) Stickiness
iv) Ribbon formation
Soil Textural Classes
To convey an idea of the textural make up of soils and to give an indication of
their physical properties, soil textural class names are used. These are grouped into
three main fractions viz., Sand, Silt and Clay.
According to the proportion of these three fractions a soil is given a name to indicate
its textural composition. Such a name gives an idea not only of the textural
composition of a soil but also of its various properties in general.
On this basis soils are classified into various textural classes like sands clays, silts,
loams etc.
Sands: The sand group includes all soils in which the sand separates make up at
least 70% and the clay separate 15% or less of the material by weight. The
46

properties of such soils are therefore characteristically those of sand in contrast to
the stickier nature of clays. Two specific textural classes are recognized in this group
sandy and loamy sand although in practice two subclasses are also used Loamy fine
sand and loamy very fine sand.
Silt: The silt group includes soils with at least 80% silt and 12% or less clay.
Naturally the properties of this group are dominated by those of silt. Only one textural
class - Silt is included in this group.
Clays: To be designated a clay a soil must contain at least 35% of the clay separate
and in most cases not less than 40%. In such soils the characteristics of the clay
separates are distinctly dominant, and the class names are clay, sandy clay and silty
clay. Sandy clays may contain more sand than clay. Likewise, the silt content of silty
clays usually exceeds clay fraction
Loams: The loam group, which contains many subdivisions, is a more complicated
soil textural class. An ideal loam may be defined as a mixture of sand, silt and day
particles that exhibits the properties of those separates in about equal proportions.
Loam soils do not exhibit dominant physical properties of sand, silt or clay. Loam
does not contain equal percentage of sand, silt and clay. However, exhibit
approximately equal properties of sand, silt and clay.
Determination of Textural Class: In the American system as developed by the
United State Department of Agriculture twelve textural classes are proposed.
The textural triangle: It is used to determine the soil textural name after the
percentages of sand, silt, and clay are determined from a laboratory analysis. Since
the soil's textural classification includes only mineral particles and those of less than
2mm diameter, the sand plus silt plus clay percentages equal 100 percent. (note that
organic matter is not included.) Knowing the amount of any two fractions
automatically fixes the percentage of the third one.
To use the diagram, locate the percentage of clay first and project inward
parallel to sand line. Do likewise for the per cent silt and project inward parallel to
clay line and for sand, project inward parallel to silt. The point at which the
projections cross or intersect will identify the class name. Some times, the
intersecting point exactly fall on the line between the textural classes. Then it is
customary to use the name of the finer fraction when it happens. (eg). Soil containing
40% clay, 30% sand and 30% silt - called as clay rather than clay loam.
47

Importance of Soil Texture
 Presence of each type of soil particles makes its contribution to the nature
and properties of soil as a whole
 Texture has good effect on management and productivity of soil. Sandy soils
are of open character usually loose and friable.
 Such type of the texture is easy to handle in tillage operations.
 Sand facilitates drainage and aeration. It allows rapid evaporation and
percolation.
 Sandy soils have very little water holding capacity. Such soils can not stand
drought and unsuitable for dry farming.
 Sandy soils are poor store house of plant nutrients
 Contain low organic matter
 Leaching of applied nutrients is very high.
 In sandy soil, few crops can be grown such as potato, groundnut and
cucumbers.
 Clay particles play a very important role in soil fertility.
 Clayey soils are difficult to till and require much skill in handling. When moist
clayey soils are exceedingly sticky and when dry, become very hard and
difficult to break.
 They have fine pores, and are poor in drainage and aeration.
 They have a high water holding capacity and poor percolation, which usually
results in water logging.
 They are generally very fertile soils, in respect of plant nutrient content. Rice,
jute, sugarcane can be grown very successfully in these soils.
 Loam and Silt loam soils are highly desirable for cultivation
 Generally, the best agriculture soils are those contain 10 – 20 per cent clay, 5
– 10 per cent organic matter and the rest equally shared by silt and sand
48

Fundamentals of Soil Science (Soil formation, Structuexture)

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