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Principles of Soil

Science

(ASS 106) for

Under Graduate Students

(International Agriculture Prog.)

Fac. of Agric., Cairo Univ.



Principles of Soil Science

By

• Prof. Dr. Mohamed Desouky

Prof. of Soil Chemistry

Soil Sciences Dept.,

Fac., of agric., Cairo Univ.



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.



Course Contents



Module – 1 : 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

• Module – 2: Soil Physics:

- Physical Properties of Soil:

- Soil texture

- Soil structure

- Soil porosity

- Soil color

- Soil water- Soil air



Module – 3: Soil Mineralogy, Chemistry and Fertility

- Structure of clay minerals

- Cation exchange capacity (CEC)- Soil pH

- Nutrient availability and fertilizers

Module – 4: Soil Survey and classification

- Soil classification

- Soli Survey



Soil Genesis

Module 1



INTRODUCTION



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.



http://www.scienceimage.csiro.au/mediarelease/images/cracked_soil.jpg

http://www.atmos.albany.edu/daes/atmclasses/atm301/soil_xsec.jpg



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.



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.



Bio-sphere

Hydro-sphere

Tropo-sphere

Soil

Lithosphere

Soil

Interactions Between Different Spheres

To Form Soil



- 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.



Darkened topsoil and reddish subsoil layers are

typical in some regions.

Soil Profile



Soil Components



Soil consists of different Phases

Solid Phase: Inorganic and

Organic-Particles

Liquid Phase (soil solution)

Gaseous Phase (soil air)

Biological Phase: Soil

flora and fauna



Biological Phase: Each soil has a distinctive floraas well as fauna of bacteria, fungi, algae, protozoa,

nematodes etc.



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.



Soil is made up of 4 main

components:

• A – Mineral materials: rocks and minerals.• B - Organic matter: residues of plants and

animals live in soil.

• C- Soil solution.

• D – Soil air.



The main components of soil



Soil Components

A – Mineral Materials

(Rocks and Minerals)



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.



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



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



II - Mineral Composition of

Lithosphere

(Rocks and Minerals)



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.



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).



MINERALS, MAGMAS, AND VOLCANIC ROCKS

Granite



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.



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)



Sedimentary rock

sediments



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 slate, schist, gneiss,and marble.

http://en.wikipedia.org/wiki/Metamorphic_rock

http://images.nationalgeographic.com/wpf/media-live/photos/000/011/custom/1192_1280x1024-wallpaper-cb1267711128.jpg



Metamorphic rocks

http://homepage.mac.com/yuee/H/AC11.jpg

http://images.nationalgeographic.com/wpf/media-live/photos/000/011/custom/1192_1280x1024-wallpaper-cb1267711128.jpg



Minerals



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 rock is an aggregate of minerals

and/or mineraloids and does not have a specificchemical composition.

• Minerals range in composition from pure element andsimple salt to very complex silicates with thousands of known forms. The study of minerals is calledmineralogy.



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



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



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

Kolinite {Al2Si2O5(OH)4}

Montmorillonite

{ X(Al,Mg)4Si8O20(OH)4}

Calcite CaCO3

Dolomite (Ca,Mg) (CO3)2

G sum CaSO4. 2H2O



Differences between minerals and

rocks• A mineral is a naturally occurring solid with a definite chemical

composition and a specific crystalline structure.

• A rock is 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 rock composed almost

entirely of the mineral calcite.

• Other rocks contain many minerals, and the specific minerals ina rock can vary widely.



The vast majority of the rocks of the Earth’s crust consist of

quartz, feldspar, mica, chlorite, kaolon, calcite, epidote,

olivin, augite, hornblend, magnetite, hematite, limonite, and afew other minerals.

Quartz micaolivine

http://un2sg4.unige.ch/athena/mineral/images/quartz2_orpierre.jpg

http://www.answersincreation.org/curriculum/geology/MicaSheetUSGOV.jpg



B - Organic matter

B O i



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 food and energy required to their biological activities. As a result of micro-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



• 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).



C- Soil solution

S il s l ti



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.



D - Soil air

D Soil air



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.



Formation of Soil

(Rock to Soil)



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.

Rocks are transformed into soil by physical and chemical changes that occur

at the Earth's surface.

Over time both the mineral material and organic matter are transformed into

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.

Naturally occurring physical and chemical forces cause rock weathering.

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), doesn’t matter whether it formed in place or was

transported.



Weathering

Definition:



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.



1 – Physical Weathering



Physical Weathering

• Physical weathering is the breakdown of rock material into smaller and smaller



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 earth’ssurface. Therefore, rocks materials are no longer stable when they are exposed to 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.



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.



2 - Chemical Weathering



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.



Chemistr of Weathering



Chemistry of Weathering

Processes of chemical weathering

1- Dissolution

2- Hydration

3- Hydrolysis

4- Carbonation

5- Oxidation-Reduction



1 - Dissolution In 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



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.



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



2- Hydration

It is coordination between molecules of mineral and water:

CaSO4 + 2 H2O ---------> CaSO4. 2 H2O

Anhydrite Gypsum

Sparingly soluble Relatively soluble

High Density low density

Hard Soft



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



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



4 - Carbonation This 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.



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



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 chemial process can become quite complicated,

the main point is that mineral's oxidation state can weaken

it.



5 - Oxidation- Reduction Reactions

• Oxid- Red Reactions is transfer of electron

between reagents

• OX + e- → Red • Red → OX + e-

• Fe2+ → Fe3+ + e-

Example:



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 Ferric 4 Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O

..............................................................................

4FeS2 + 15O2 + 14H2O = 4Fe(OH)3 + 8 H2SO4

Factors which affect the rate of chemical



weathering are:

1 - Particle size – the smaller the particle size, the

greater the surface area and hence the more rapid the

weathering

2 – Temperature 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.



3 - Biological Weathering



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



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



organic acids

1 - CO2 derived from root respiration and decompostion of

organic residues, dissolves in water to form carbonic acid.

H2O + CO2 < – > H+ + HCO3- < – > H2CO3

2- H+ released from carbonic acid and other organic acids

(oxalic-, citric-, tartaric- acids) attacks silicate minerals:

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



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 f unctional groups (e.g., R-OH, R-NH2, R-SH). These groups have the ability 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 donate lone pair of electron to metal cation and form a product“called” chelate compound

• This reaction remove the metal cation from minerals and rocksleaving voids. These voids lead to decomposition of rocks andminerals.

Chelation and Complexation



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



WEATHERING PRODUCTS

Sediments are the by-product of weathering.Sediments includes, altered materials in smaller size, and some new

minerals.

Products of Weathering: the major products of weathering are a

blanket of loose, decayed rock debris, known as regolith, and rock bodies modified into spherical shapes.



WEATHERING PRODUCTS

Soluble Insoluble

Salts of Na, K

Ca and Mg

OCEANS

SOILPrimary

MineralsSecondary

Minerals

ResistantQUARTZ

Clay Minerals



Rock weathering

Lithosphere

Soil

Atmosphere

Hydrosphere

Rock weathering

Physical

Chemical

Biochemical

Biosphere



Mineral Stability



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



Factors of Soil Formation

Factors of Soil Formation



- 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), 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 somany different soil types are found throughout the world.

Weathering produces soils Soils may or may not remain in place:



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



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



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 of plants and slowly decomposition of dead residues.

Usually formed in depressions of stagnant water (swamps, marshes,moos, bogs) in which anaerobic is favored.



2 - Climate



- 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



-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 regoliths.

- 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



• 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



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



Si th ti t l th l k it h b d th

5 - Time



-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



A hypothetical soil development across time is shown.



Soil Formation Processes

Four processes integral to soil formation

1 – Weathering2 – Transformation: [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



• 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 of sedentary soil, the profile extended to the rock of parentmaterial. However, in case of transported 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 called“master horizons”.



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)



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 2µm 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.

mod-1. princ. soil science-last version

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