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Principles of Soil
Science
(ASS 106)for
Under Graduate Students
(International Agriculture Prog.)
Fac. of Agric., Cairo Univ.
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Principles of SoilScience
By
Prof. Dr. Mohamed Desouky
Prof. of Soil Chemistry
Soil Sciences Dept.,
Fac., of agric., Cairo Univ.
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Course Objectives
Identify the nature, origin and functions of soils. Describe the soil texture, structure, Porosity and color.
Define the soil CEC and pH.
Explain the concept of soil fertility. Define soil water content, classification, potential and
variability.
Describe elementary aspects of soil formation.
Name different horizons in soil profile.
Identify the soil orders according to USA soilclassification system.
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Course Contents
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Module1 : Soil Genesis
- Introduction:What is soil- Ecological functions of soil
- Major components of soil- Rocks and Minerals
- Soil forming processes
- Soil forming factors
- Soil Development and profile
Module2: Soil Physics:
- Physical Properties of Soil:
- Soil texture- Soil structure
- Soil porosity
- Soil color
- Soil water- Soil air
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Module3: Soil Mineralogy, Chemistry and Fertility
- Structure of clay minerals
- Cation exchange capacity (CEC)- Soil pH
- Nutrient availability and fertilizers
Module4: Soil Survey and classification
- Soil classification
- Soli Survey
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Soil Genesis
Module 1
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INTRODUCTION
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What is Soil?
-Soil is one of the three major natural resources,alongside air and water.
-It is one of the marvelous products of nature andwithout it there would be no life.
-Soil is a resource on which we depend in many
ways.
-Soil does not look the same from place to place.
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Soil Definition
Defining soil is a difficult task due to :
1- There are so many people who work with
soil in its many functions.
2 - Soil is a medium that we use for various
functions.
In agriculture, for growing plants.
In engineering, for supporting structures.
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Soil Definition
Soil is the decomposed thin surface layer of
lithosphere (Earth crust) consists of mineraland organic constituents formed as the result
of reaction between lithosphere and each of
hydrosphere, atmosphere (troposphere) andbiosphere, and suitable for plant growth.
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Bio-sphere
Hydro-sphere
Tropo-sphere
Soil
Lithosphere
Soil
Interactions Between Different Spheres
To Form Soil
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- Explanation:- Soil, therefore, is a loose mass of broken and
chemically weathered rock mixed with organic
matter (Humus).
- Humus is partially decomposed organic matterand is most abundant in topsoil.
- Soil is a mixture of mineral and organicconstituents that are in solid, gaseous and aqueous
states.
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Darkened topsoil and reddish subsoil layers are
typical in some regions.
Soil Profile
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Soil Components
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Soil consists of different Phases
Solid Phase:Inorganic andOrganic-Particles
Liquid Phase (soil solution)
Gaseous Phase (soil air)
Biological Phase: Soil
flora and fauna
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Biological Phase:Each soil has a distinctive floraas well as fauna of bacteria, fungi, algae, protozoa,nematodes etc.
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Soil Components
For explanation: the soil solid particles packed
loosely forming a soil structure filled with pore
spaces. These pores contain soil solution (liquid)and air (gas). Accordingly, soil is often treated as
a three state system.
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Soil is made up of 4 main
components:
AMineral materials: rocks and minerals. B - Organic matter: residues of plants and
animals live in soil.
C- Soil solution.
DSoil air.
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The main components of soil
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Soil Components
AMineral Materials
(Rocks and Minerals)
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I - Chemical Composition of Lithosphere
Earth Crust Consists of 92 Elements. They called(Lithophile elements)
These elements combined together to from minerals
(e.g., Silicate minerals) Only eight elements out of the lithophile group
represent 99% of the mass of the earth crust (O, Si,AL, Fe, Ca, Na, K, Mg). All other elements representonly 1% of the mass of earth crust.
Only three out of the eight elements (O, Si and Al)represent about 83% of the mass of earth crust. Theseelements called the skeleton elements.
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46.6%
27.7%
8.1%
5.0% 3.6% 2.8% 2.6% 2.1%
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
O Si Al Fe Ca Na K Mg
Percentages of the Eight Most Common
Elements in the Earth's Crust by Weight
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Soil Lithosphere Ele
49 47 O
33 27 Si
7 9 Al
4 5 Fe
1 4 Ca
1 3 K
1 3 Na
1 2 Mg
2 0.01 C
0.10 0.02 N
Lower in soil due to
leaching
Higher in soil due to
organic matter
Chemical composition
of lithosphere vs. Soil
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II - Mineral Composition of
Lithosphere
(Rocks and Minerals)
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Rocks
Definition: rock could be defined as anaturally occurring solid substance or
aggregate of minerals and/or mineraloids.The Earth's outer solid layer (lithosphere) ismade of rocks. In general rocks are of threetypes, namely, igneous, sedimentary, and
metamorphic. These three types of rocksexist as the effects of three basic geologicalprocesses.
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1 - Igneous Rocks
Igneous rocks are crystalline solids which form directly from thecooling of magma. The earth is made of igneous rock - at least at thesurface where our planet is exposed to the coldness of space.
Igneous rocks are given names based upon two things:
1- composition (what they are made of) and,
2- texture (how big the crystals are). Igneous rocks are divided into two main categories:
1- plutonic rock(intrusive rocks) result when magma cools andcrystallizes slowly within the Earth's crust (example granite),
2 - volcanic rock (extrusive rocks) result from magma reaching the
surface either as lava or fragmental ejects (examples pumice andbasalt).
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MINERALS, MAGMAS, AND VOLCANIC ROCKS
Granite
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2 - Sedimentary Rocks
Sedimentary rocks are called secondary, because
they are often the result of the accumulation of
small pieces broken off of pre-existing rocks.In most places on the surface, the igneous rocks
which make up the majority of the crust are
covered by a thin veneer of loose sediment. These
sediments get compacted and cemented togetherforming a sedimentary rock. The rock is made as
layers of this debris.
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There are three main types of sedimentary rocks
Mechanical sedimentary rocks (Clastic ): It is an accumulations of little pieces
of broken up rock which have piled up and been "lithified" by compaction and
cementation. (Example: sandstone, conglomerate, mudstones and shale)
Chemical sedimentary rocks(evaporites): These rocks form when standing
water evaporates, leaving dissolved minerals behind. These are very common inarid lands, where seasonal "playa lakes" occur in closed depressions (Example:
limestone and dolomite)
Organic deposition rocks: any accumulation of sedimentary debris caused by
organic processes. Many animals use calcium for shells, bones, and teeth. Thesebits of calcium can pile up on the seafloor and accumulate into a thick enough
layer to form an "org. deposition rock. (Example: organic lime stone, Phosphatic
rocks and coal)
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Sedimentary rock
sediments
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3 -Metamorphic Rocks
The metamorphic gets their name from "meta" (change)and "morph" (form). Any rock can become a metamorphicrock.
Metmorphic rocks are formed by subjecting of any rock todifferent temperature and pressure conditions than those inwhich the original rock was formed. These temperaturesand pressures are always higher than those at the Earth'ssurface and must be sufficiently high so as to change the
original minerals into other mineral types or else into otherforms of the same minerals (e.g. by recrystallisation).Common metamorphic rocks include schist, gneiss, andmarble.
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Metamorphic rocks
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Minerals
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Minerals
Definition: Mineral is a naturally occurring solidsubstance that has a definte chemical composition, aspecific crystalline structure and specific physicalproperties.
Traditional definitions excluded organically derived
material. By comparison, a rockis an aggregate of minerals
and/ormineraloids and does not have a specificchemical composition.
Minerals range in composition from pure element andsimple salt to very complex silicates with thousands ofknown forms. The study of minerals is calledmineralogy.
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Classification of minerals
Many methods are used to classify minerals including:I - Chemical Classification:Based on chemical
composition of mineral, in which the mineral are
categorized according to anion group.
II - Classification according to origin (primary andsecondary)
Chemical Classification of Principal Minerals
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Chemical Classification of Principal Minerals
based on chemical composition
Type Examples
Elemental Gold (Au) , Copper (Cu), Iron (Fe), Sulfur (S)
Sulfides Galena (PbS), Pyrite (FeS2)
Oxides & Hydroxides Hematite (Fe2O3), Brucite {Mg(OH)2}
Halides Halite (NaCl), Flurite (CaF2)
Sulphate Anhydrite (CaSO4), Gypsum (CaSO4.2H2O)
Phosphate Apatite {Ca5F(PO4)3}
Silicates Quartz (SiO2), Orthoclase (KAlSi3O8)
Carbonate Calcite {CaCO3}, Dolomite (Ca, Mg) (CO3)2
Classification of Principal Minerals Based on Origin
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Classification of Principal Minerals Based on Origin
(Primary and secondary Minerals)
Primary Minerals
Quartz SiO2
Feldspars Orthoclase (KAlSi3O8) ,
Mica Muscovite KAl3Si3O10(OH)2
Secondary Minerals
Secondary Clay
Minerals
Kaolinite {Al2Si2O5(OH)4}
Montmorillonite
{X(Al,Mg)4Si8O20(OH)4}
Calcite CaCO3
Dolomite (Ca,Mg) (CO3)2
G sum CaSO4. 2H2O
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Differences between minerals and
rocks A mineral is a naturally occurring solid with a definite chemical
composition and a specific crystalline structure.
A rockis an aggregate of one or more minerals. (A rock mayalso include organic remains and mineraloids.)
Some rocks are predominantly composed of just one mineral.For example, limestone is a sedimentary rockcomposed almost
entirely of the mineral calcite.
Other rocks contain many minerals, and the specific minerals ina rock can vary widely.
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The vast majority of the rocks of the Earths crust consist of
quartz, feldspar, mica, chlorite, kaolon, calcite, epidote,
olivin, augite, hornblend, magnetite, hematite, limonite, and afew other minerals.
Quartz micaolivine
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B - Organic matter
B O i
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B - Organic matter
Organic matter in soil implies exudates and residues of allorganisms live in and/or on soil. After death of living organismsand plants, their residues begin to decay.
Soils often have varying degrees of organic compounds in different
states of decomposition. The freshly organic residues added to the soil attacked by micro-
organisms.Micro-organisms attack organic residues to gain foodand energy required to their biological activities.As a result ofmicro-organisms attack, organic residues transform to intermediatecomponents, then to final simple products.
Humus is the final stable product of the decomposition of organicresidues. Among all living organisms exist either on the surface orsubsurface the soil, green plants are the most dominant component
of soil organic colloid.
H f t i tt th t h d d t
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Humus refers to organic matter that has decomposed toa point where it is resistant to further breakdown oralteration.
In soil, humus usually exists combined with mineralpart of soil forming organo-mineral complex.
humus, as a reserve of macro and micro-nutrients, is avital component affecting soil fertility.
Humus also absorbs water acting as a moisture reservethat plants can utilize.
It also increases formation of stable soil aggregatesproviding pore spaces.
Many soils, including desert and rocky-gravel soils,
have no or little organic matter. Others, have highcontents of organic matter.
Soils that contain contents of organic matter higher than18% called organic soil such as peat (histosols).
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C- Soil solution
S il s l ti
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Soil solution
All water exists in soil called liquid phase. Also, the terms of soil
moisture and soil solution named as liquid phase. Water dissolves a range of molecules and ions. Soil solution contains
soluble materials which exist in forms of free hydrated ions, organicand inorganic substances, plant nutrients such as nitrate, ammonium,potassium, phosphate, sulfate, calcium, and micronutrients such as
zinc, iron and copper, as well as gases in the dissolved state. The most common cations exist in soil solution are Ca2+, Mg2+, Na+,K+, NH4+, whereas the most common anions are HCO3-, Cl-, SO4=and NO3-.
respect concentration of sodium exist in solutions of arid soils thatgreatly impact plant growth. Soil pH can affect the type and amountof anions and cations that soil solutions contain.
Very important chemical reactions take place in the soil solution.
Plants and microorganisms mainly draw their necessary nutrientsfrom soil solution.
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D - Soil air
D Soil air
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D - Soil air
Air exists in the spaces (pores) found between soil
aggregates and solid particles. All void volume of the soilcan be occupied by either air or water: The amount of air insoil is thus inversely related to the amount of water present.
Composition of soil air is similar to that in the atmospherewith the exception of oxygen, carbon dioxide and water
vapor. However, in the soil air, oxygen usually is replaced by
carbon dioxide. Oxygen is used by plant roots and soilmicrobes during respiration, and carbon dioxide is released.Thus, in the soil, oxygen levels are generally less than
atmospheric levels and the carbon dioxide levels aregenerally greater than atmospheric levels.
The soil air also contains water vapor and, in many cases, isat 100% relative humidity.
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Formation of Soil
(Rock to Soil)
i i
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- Rocks are transformed into soil by physical and chemical changes that
occur at the Earth's surface. Naturally occurring physical and chemical
forces cause rock weathering.
- The first step in soil formation happens when mineral material from rocks
and organic matter from plants and animals are combined together.
Combination of mineral and organic matter gives soil its unique properties.
- Over time both the mineral material and organic matter are transformedinto new materials; these are then moved through the soil by percolating
water, so that the more soluble compounds are finally lost completely.
- Nature of these inputs, transformations, movements and losses
determine what type of soil will form.
The altered rock material may accumulate over the parent rock, or it may be
washed or blown to other sites. Soil formation begins at once on the loose
rock material (regolith),doesnt matter whether it formed in place or was
transported.
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Weathering
Definition:
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Definition:
Weathering is scientific term for the break-up of rocks.This term is used because climate, and the prevailing weather,
is the main factor that eventually transforms rock to soil.
Weathering defines the set of physical, chemical and biological
processes which decay and break rock down into smaller
pieces.
Weathering is disintegration and decomposition of rock at ornear the surface of the earth. It affects the rocks in place and
no transport is involved. This distinguishes weathering from
erosion.
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Weathering Processes
- Physical Weathering
- Chemical Weathering
- Biological weathering (Biochemical
weatheringWeathering processes can act independently and in
concert as well as at different scales of observation
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1Physical Weathering
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Physical Weathering
Physical weathering is the breakdown of rock material into smaller and smaller
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y gpieces with no change in the chemical composition of the weathered material.
Why do rock break up?
Because it formed under conditions of temperature higher than that of earthssurface. Therefore, rocks materials are no longer stable when they are exposedto surface conditions.
Factors of physical weathering:Expansion and Contraction - the thermal heating and cooling of rocks causing
expansion and contraction. When rocks composed of several different mineralsare exposed to heat they experience different rates of expansion.
Frost Action - Water can penetrate rocks along small cracks. When waterfreezes, its volume expands by 10%. The force exerted by expanding ice isenough to break open cracks. This form of weathering is called 'frost
shattering'. In addition, when rocks exposed to water, different minerals oftenhave different rates of swelling and shrinkage; this can initiate stresses withinthe rock that eventually cause it to fracture.
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By widening existing cracks, more of the rock surface is
exposed to the elements. This accelerates the weathering
process and hastens the disintegration of the rock.
Exfoliation - process in which curved plates of rock are
stripped from a larger rock mass.
Abrasion - physical grinding of rock fragments.
Other types - Cracking of rocks by plant roots and burrowing
animals.
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2 - Chemical Weathering
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2- Chemical Weathering
Chemical Weathering Reactions lead to an alterationin mineral and chemical composition of parent
material, disappear of low resistance mineral, andformation of new minerals (secondary clayminerals)
Water molecule plays an important role in chemicalweathering through different reactions.
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Chemical Weathering
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Chemistr of Weathering
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Chemistry of Weathering
Processes of chemical weathering
1- Dissolution
2- Hydration
3- Hydrolysis
4- Carbonation
5- Oxidation-Reduction
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1 -DissolutionIn this process minerals simply
dissolve in water. A few minerals such as
Halite (table salt) and fluorite are completelysoluble in water. Such these minerals dissolve
and washed away in solution.
1 Di l ti
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1- Dissolution
Water is a good solvent for minerals
containing ionic bonds such as Halite
mineral :
NaCl(s) ------------> Na(aq) + Cl(aq)
Solutes moves to depressions to form
Oceans and Seas
In this process minerals of ionic bonds such as Halite and
Fluorite simply dissolve in water and washed away in solution.
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2
Hydration: during hydration minerals absorbwater, but unlike hydrolysis there is no ion formation,
the water molecule remains intact. When a mineral
undergoes hydration its physical and chemical
properties can be altered. think about how physicalcharacteristics are altered as it absorbs water. When
some minerals become hydrated they can also become
weakened physically.
2 H d ti
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2- Hydration
It is a coordination between molecules ofmineral and water:
CaSO4 + 2 H2O ---------> CaSO4. 2 H2O
Anhydrite Gypsum
Sparingly soluble Relatively soluble
High Density low density
Hard Soft
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3- Hydrolysis: This is the most common form of chemical
weathering. It occurs when water molecules desperate into
two charged particles, H+ (a hydrogen ion) and OH-.
Hydrogen and hydroxyl ions attack the bonds that holdminerals together. Hydrolysis not only cause rock
disintegration but also changes the chemical nature of the
mineral. Hydrolysis is very important process in soil.
3 H d l i
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3- Hydrolysis
Small portion of water molecules dissociate to H+ and
OH- ions (10^-7 mole for each) :
H2O H+ + OH-
Hydrolysis is the reaction of a mineral with dissociatedH+ and OH- ions:
K Al Si3O8 + HOH --------> H Al Si3O8+ KOH
Orthoclase H-Clay
High Density Low Density
Hard Soft
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4 - CarbonationThis is an accelerated form of
hydrolysis, which caused by biological activity within thesoil.
When CO2 comes into contact with water, a proportion
of it dissolves to form carbonic acid. CO2 is derived from
root respiration and organic decomposition.
All acids are rich sources of hydrogen ions; carbonation
therefore enhances hydrolysis.
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4- Carbonation
Carbon dioxide gas dissolves in water forming
carbonic acid:
H2O + CO2 H+ + HCO3- H2CO3
Carbonic acid increase the solubility of
minerals such as calcite:
CaCO3 + H2CO3 -------> Ca(HCO3)2
Calcite Ca-bicarbonate
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5 - Oxidation and reduction When exposed to the
atmosphere some minerals undergo chemical changes;some are 'oxidized' and others are 'reduced'. In its simplest
form oxidation can be regarded as a mineral's tendency to
take up oxygen, while reduction is its ability to lose
oxygen. However, this narrow definition has beenexpanded so that it also refers loss or gain of electrons.
Although chemical process can become quite
complicated, the main point is that mineral's oxidation
state can weaken it.
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5 - Oxidation- Reduction Reactions
Oxid- Red Reactions is transfer of electron
between reagents
OX + e- Red Red OX + e-
Fe2+ Fe3+ + e-
Example:
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a p e:
Weathering of Pyrite Mineral and its transformation
to hematite mineral:
First Step: Transformation from sulfide to sulfate
2 FeS2 + 7O2 + 2H2O = Fe2+ + 4SO42- + 4 H+
Second Step: Oxidation of Ferrous to Ferric4 Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O
..............................................................................
4FeS2 + 15O2 + 14H2O = 4Fe(OH)3 + 8 H2SO4
Factors which affect the rate of chemical
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weathering are:
1 - Particle sizethe smaller the particle size, the
greater the surface area and hence the more rapid the
weathering
2Temperature and precipitation - chemical
weathering occurs more rapidly under conditions of
high temperature and precipitation (rainfall). In
general, the rate of chemical weathering reactions
doubles with every 10 C increase in temperature.
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3 - Biological Weathering
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The word bio means life. Thus biotic weathering is anytype of weathering that is caused by living organisms.
Most often the responsible of biotic weathering is plant
roots. These roots can extend downward, deep into rock
cracks in search of water and nutrients. In the process theyact as a wedge, widening and extending the cracks.
Bi l i l th i
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Biological weathering
Vegetative and micro-organisms play an importantrole in biological weathering and in soilformation.
The most important exudates formed as a result of
biological activity are:-CO2
-Organic acids
-Chelating Agents-EnzymesThese reagents play an important role inbiological weathering.
Effect of CO2 and H+ released from
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organic acids
1 - CO2 derived from root respiration and decomposition oforganic residues, dissolves in water to form carbonic acid.H2O + CO2 H+ + HCO3- H2CO3
Its a source of H+ ions. Carbonic acid react with carbonate
minerals and rocks (refer to carbonation)
2- H+ released from carbonic acid and other organic acids(oxalic-, citric-, tartaric- acids) attacks silicate minerals. Thesame action of hydrolysis:
2NaAlSi3O8(Albite) + 2H2CO3 + 9CO2 -> 2Na+ +2HCO3- +4H4SiO4 + Al2Si2O5(OH)4
(Clay mineral Kaolinite)
- These reactions are important in weathering and decomposition
of rocks and minerals.
Effect of chelating agents produced
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through biotic activity
Formation of complexes and chelates with metal cations Large molecules (e.g., organic molecules) have the ability to make
complexes and chelates with metal cations.
Organic molecules have functional groups (e.g., R-OH, R-NH2, R-SH). These groups have the ability to donate lone pair of electrons tometal cation (e.g., Zn, Cu,..) form a complex.
If the molecule has more than one functional group (3 or more), eachone donates lone pair of electron to metal cation and form a productcalled chelate compound
This reaction remove the metal cation from minerals and rocksleaving voids. These voids lead to decomposition of rocks andminerals.
Chelation and Complexation
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Fe chelated with EDTA
Chelation and ComplexationChelating agents like EDTA can chelate metals (Fe, Mn, Zn,)
exist in structure of primary minerals and rocks
Fe
WEATHERING PRODUCTS
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WEATHERING PRODUCTS
Sediments are the by-product of weathering.Sediments includes, altered materials in smaller size, and some new
minerals.
The major products of weathering are a blanket of loose, decayed rock
debris, known as regolith, and rock bodies modified into sphericalshapes.
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WEATHERING PRODUCTS
Soluble Insoluble
Salts of Na, K
Ca and Mg
OCEANS
SOILPrimary
MineralsSecondary
Minerals
ResistantQUARTZ
Clay Minerals
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Rock weathering
Lithosphere
Soil
Atmosphere
Hydrosphere
Rock weathering
Physical
Chemical
Biochemical
Biosphere
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Mineral Stability
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yMinerals crystallize from a melt at different temperatures during the migration and
emplacement of the magma.
Those crystallize at higher temperatures will be the least stable at the surface. It is
obvious that quartz is the most stable mineral in the weathering environment, and
will be a dominant constituent of sediments and sedimentary rocks.
Mineral stability series
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Factors of Soil Formation
Factors of Soil Formation
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- Soil-forming factors are:
climate, parent material, topography, living organisms and time.
- So, these factors are expressed in a similar way to a mathematical
equation;
Soil formation = climate, parent material, topography, living
organisms (plants and animals) and time.
- These five factors determine the nature of the initial inputs, how are
transformed and how quickly they are moved and lost from the soil.
- Because soils form under a range of conditions, it is not surprising
so many different soil types are found throughout the world.
Weathering produces soils Soils may or may not remain in place:
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Weathering produces soils. Soils may or may not remain in place:
- Soil could be divided to two types:
1-Residual soil (sedentary soil): Forms in place by the weathering
of bedrock. Has not been transported.
2 -Transported soil: material breaks down in one area and isdeposited as soil in another area; there's movement, via some type
of geological process (e.g. river, wind, etc.) NOTE: The Rio
Grande Valley and Soils of Nile river and its delta are transported
soil.
- Soil may be a combination of residual and transported material.
1 - Parent Material
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Common types of soil parent materials are:1. Residuum- soils formed in residuum are soils that form in mineral
strata in place.
2. Glacial till- soils formed in residual minerals left behind by
glaciers.
3. Alluvium- (Alluvial soil): soils from sediments that were deposited by
rivers and streams in valleys throughout the world. Soils of Nile valley
and Delta.
4. Colluvium- soils formed in deposits that resulted from mass
movement due to gravity, not water. (Aeolian soil)
5. Marine- soils formed in the residues of ancient sea beds (now dry
land)
6. Lacustrine- soils formed in residues of ancient lakes (now dry
land).
- Soils formed from gabbro and basalt (Black and dark graycrystalline rocks) are clayey and fertile
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crystalline rocks) are clayey, and fertile.
- Soils formed on sandstone are sandy.- Those on shale are clayey.
- Soils formed on limestone are clayey.- Volcanic ash, most of it develops into high-quality soil for crop
production.
-Organic soil: According to the definition used in Soil Taxonomy,
organic soils must have organic carbon >18%.Organic Soil contains high amounts of OM could be >80%, versuslow mineral content.Organic soil are derived in environment allow good growth ofplants and slowly decomposition of dead residues.
Usually formed in depressions of stagnant water (swamps, marshes,moos, bogs) in which anaerobic is favored.
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2 - Climate
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- Every place on earth has climate that can be described on the basisof its many components. The two components that most stronglyinfluence soil formation are precipitation (rainfall) andtemperature.
- It is no wonder that each climatic zone has a characteristic patternof soils. climate is the most important factor influencingweathering.
- Precipitation influences the conversion of parent material into soilin many ways. The greater the rainfall amount, the more rapid therate of weathering and leaching.
-Temperature has a marked influence on the rate of weathering
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-Temperature has a marked influence on the rate of weathering.
As a rule: Every 10-degree rise in temperature, the rate of
chemical reactions increases by a factor of two to three.
- The combined influence of precipitation and temperature is
probably as important as either one of them individually.
- Intense chemical weathering occurs in hot humid regions anddevelops thick regolith.
- Chemical weathering is minimal in deserts and polar regions.
- Laterites form in humid climates where only Al2O3 (Bauxite)
and Fe(OH)3 remain (the most strength weather) .
3 - Living organisms
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The soil and the organisms living on and in it, comprise an ecosystem. Theactive components of the soil ecosystem are the vegetation, fauna, including
microorganisms (bacteria, fungi), ants, man and many other livingorganisms shape each small part of the soil.
Plants play important rules in both physical and chemical weathering. Plantsstabilize soil profiles. Animals (including man) tend to destabilize the soilprofile, increasing weathering.
Living organisms produces humic acids that are powerful weatheringagents and help speed up weathering of rock particles.
The amount of humus in the soil is a direct result of how much plant residuehas been incorporated into it.
Most of the trees in the world's forests can be divided into two groups, thehardwoods with broad leaves and the softwoods (conifers) with needles.
Soils formed under conifer forests tend to be the most acidic with lowerbuffering capacity.
Grassland regions have the most fertile soil for agriculture.
4 - Topography
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p g p y
Topography is shape and features of soil surface.
Major topographical features are easily recognized in the field (e.g.mountains, valleys, ridges, crests, sinks , plateaux, floodplains).
Topography has a significant impact on soil formation as it determinesrunoff of water, and its orientation affects microclimate which in turnaffects vegetation.
For soil to form: The parent material needs to lie relativelyundisturbed so soil horizon processes can proceed.
Water moving across the surface strips parent material away impedingsoil development.
Water erosion is more effective on steeper, un-vegetated slopes.
The importance of relief is recognized by recurring sequence of soilforming on slopes in a generally undulating landscape. The termcatena used to describe a sequence of contiguous soils extending fromhill to top of a hill slope.
Soil catena
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Si th ti t l th l k it h b d th
5 - Time
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-Since the reaction rates are slow, the longer a rock unit has been exposed, the
more likely to be weathered.
- Time acts on soil formation in two ways:
The value of a soil forming factor may change with time (e.g.
climatic change, new parent material).
The extent of a pedogenetic reaction depends on the time for
which it has operated.
- Very old soils are formed on weathered consolidated rocks (e.g., granite,basalt).
- The most recent, large changes were associated with alternating glacial and
interglacial periods of the Pleistocene.
- One of our youngest soils are formed on alluvial or lacustrine, materials
generally have not had as much time to develop as the surrounding uplandsoils.
- Young in age are also colluvial soils, where sediment transport occurred
recently.
A hypothetical soil development across time is shown
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A hypothetical soil development across time is shown.
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Soil Formation Processes
Four processes integral to soil formation
1Weathering2Transformation: [chemical weathering, turnover]
3 - Transportation - Transporting usually is water. More rarely wind
or glacial ice.
- Translocations [secondary clay minerals, base cations]
- Additions and Removal [surface or subsurface]
4 - Deposition - Occurs when energy necessary to transport particles
is no longer available. Deposition due to the gentle settling of mineral
grains. Can also be result of chemical precipitation due to changing
conditions.
Soil Profile
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Definition: the soil profile not only refers to the top layers of soil,but also includes all the underlying layers down to the unalteredparent material on which the soil has formed. In case ofsedentary soil, the profile extended to the rock of parentmaterial. However, in case oftransported soil, the profileextended to water table.
Soil profile reveals the historical development of the soil byrecording the main inputs, translocations and losses.
Soil profile can be divided to few layers or horizons based ondifferences in color, texture, structure, etc, .
Horizons
is the distinctive layers in the soil profile. Soil profile usually consists of at least three horizons calledmaster horizons.
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These horizons are not present in all soil profiles. In areas of rapid
erosion, horizons of B & C may be present or C only. In some areas no
soil profile will develop at all.
Mineral Composition of Soil Solid Particles (Solidphase)
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phase)Soil solid particles could be divided into two types according to their
origin. These two types are; mineral particles which originated fromrocks and minerals exist in the parent materials, and organic particleswhich originated from residues of living organisms.
Soil particles varied widely in diameter and ranging from very smalldiameter less than 2m like clay particles, to coarse particles whichhave diameters as high as 2mm (e.g., coarse sand) or higher (e.g.,gravel).
Large particles like coarse and fine sand mainly consist of primaryminerals those exist in parent material and earth crust. On the otherhand, the smaller particles like clay fraction consist of secondary clayminerals. The secondary clay minerals were not exist in the parent.They are formed during the weathering of rocks and minerals of earthcrust.
General Questions:
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1- What are the products of weathering?
2- What is the difference between primary and secondary minerals?
3- The major components of soil are:
A - ..
B - ..
C - ..
D - ..
4 - Lithosphere skeleton elements are: .., ,
5- Contents of elements of C and N in soil are much higher than theirs in
lithosphere due to.
6 - The main types of rocks are:
A - ..
B - ..
C - ..7 - Many methods are used to classify minerals including:
A - ..
B - ..
7 - Based on Chemical composition, mineral are classified into 8 groups
including;
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including; .., ., .,
., , ..
8 - The most common cations exist in soil solution are , ., ..,
and ., whereas the most common anions are ., .,., and ..
9 - Soil solution represents
10 - Soil air consists of gasses including; , , , .
11- Weathering processes are;A - ..
B - ..
C - ..
12 - Compare between physical and chemical weathering?
13 - The factors of physical weathering are:A - ..
B - ..
C - ..
14 - The processes of chemical weathering are:A - .. B - ..
C - .. D- ..
E
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E - .
15 - The following equation represents ,
And supply the missed words
K Al Si3O8 + HOH --------> H Al Si3O8+ KOH
.................. ......................
16 - Factors which affect the rate of chemical weathering are:
A - ..
B - ..
17 - The most important exudates formed as a result of biological activity are:A - ..
B - ..
C - ..
D - ..
18 - The factors of soil formation are;A - ..
B - ..
C - ..
D - ..
E - ..................................
20 What are the difference between residual and sedentary soil?
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20What are the difference between residual and sedentary soil?
21"Soil may or may not remain in place". Based on this statement, soil
could be divided into two types; 1)-
2)- ..
22- Intense chemical weathering occurs in ,
whereas, weathering is minima in
23Hydration process means
whereas, hydrolysis process means
22 - The common types of parent materials are;A - ..
B - ..
C - ..
D -
- The main horizons of profile are;A - ..
B - ..
C - ..
D -
24 - The two important characteristics of colloidal particles are:
A
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A - ..
B - ..
25 - The types of soil colloids are:
A - ..B - ..
25 - The types of secondary clay minerals are;
A - ..
B - ..
- The sources of electrical charge on clay minerals
- The sources of electrical charge on humus (organic matter)
- Compare between kaolinite and montmorillonite menrals
The main types of soil structure are You can modify the soil structure by adding
The main methods of determination of soil structure are
.. and .
-
What do you know about:
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- The quantity of organic matter in the soils of Egypt
- Humus formation
- Mineralization and immobilization process- The chemical composition of humus
- List the functional groups of humus
- Define: Ion exchange reaction, Exchangeable ions, CEC
- Define: Soil-pH, Base saturation, soil buffering capacity
- The factors controlling soil-pH
- The factors affecting soil buffering capacity
- Reclamation of alkali soil
- List the essential elements and the form available to plants
- Classify essential elements
- Explain the effect of soil pH on nutrients availability to plants- The types of chemical fertilizers
- Mention an example for each type of chemical fertilizers
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50 - Write scientific definition for the following:
1Soil
2Rock3Mineral
4Physical weathering
5 - Chemical weathering
6Bio-chemical weathering7Parent material
8Soil Texture
9Soil structure
10USDA textural triangle
11Field Capacity
12Organic matter