1. CONCEPTS OF SOIL

What is soil?

SOIL IS SOUL OF INFINITE LIFE

The word soil come from Latin word ‘solum’ means floor or ground. It is one of the most

important natural resources which are the heart of terrestrial ecosystem, and

understanding of soil system is key success of any human use of land.

Agronomist, Engineers, Pedologist, Ecologist, Geologist, Ecologist, Microbiologist,

Farmers, and Laymans think of soil in different ways and for different purposes. The

concept of soil depend on what use we have for it.

The farmers makes a living by growing crops in soil; an engineers uses soil as a

foundation for roads and houses; the soil scientist studies soil to understand and manage

it properly, and a parent sees soil as something to be removed from a child.

‘Soil’ as a general term usually denotes the unconsolidated, thin, variable layer of mineral

and organic matter, usually biologically active, that covers most of the earth’s land

surface.

‘Soils’ are complex biogeochemical materials on which plants may grow. They have

structural and biological properties that distinguish them from the materials from which

they normally originate.

They also are dynamic ecological systems, providing plants with support; water nutrient

and air for growth and also supporting a large population of microorganisms that recycle

the materials of life.

Function of soil in our ecosystem

1. Medium of plant growth

The soil mass provides physical support, anchoring the root system so that the

plant does not fall over

Plant roots depend on the prices of respiration to obtain energy. Allowing c02 to

escape and fresh o2 to enter the root zone. This ventilation is accomplished via

the net work of soil pores

The soil pores also absorb rain water and hold it can be used by roots.

Soils also moderates temperatures fluctuations occurred in the root environment

Good soils protect plants from toxic concentration by ventilating gasses,

decomposing/absorbing organic toxins or by suppressing toxins producing

organism.

Soil supply plants with inorganic mineral nutrients in the form of dissolved ions

in amounts and relative proportions appropriate for plant growth.

It is true that plants can grown in nutrient in nutrient solutions with out soil

(hydroponics), but then the plant support functions of soils must be engineered

into the system and maintained at a high cost of time, effort and management.

2. Regulatory of water supplies

Soil properties are principal factor controlling the fate of water in the hydrologic

system. Water loss, utilization, contamination and purification are all affected by

the soil.

If the soil (permeable and deep) allows the rain to soak in some of the water may

be stored in the soil and used by the tress and other plants, while some may seep

slowly down through the soil layers the ground water, eventually entering the

river over a period of month or year as base flow.

If the water is contaminated, as it soaks through the upper layer of soil it is

purified and cleared by soil process that remove many impurities and kill potential

disease organism.

3. Recycler of raw materials

With in the soil, waste products and dead bodies of plants, animals and peoples are

assimilated and their basic elements are made available for reuse by the next

generation of life.

The soil system plays a pivotal role in the major geo chemical cycles

4. Habitat for soil organisms

Soils harbor much of the earth’s genetic diversity. It provides habitat for a marad

of living organism, from small mammals and reptiles to tiny insects to

microscopic cells of unimaginable numbers and diversity.

5. Engineering medium

In human built ecosystem soil plays an important role in as an engineering

medium.

Soil is not only an important building materials in the form of earth fill and bricks

(baked soil materials), but also provides the foundation for virtually every road,

air port and house we build.

1.2 HISTORY OF SOILS

Historians place the beginning of agriculture in Mesopotamia about 7000 years

ago. In ancient Egypt, civilization flourished along the Nile. Here silts were a

blessing at the river replenished the land with a new addition of soil with each

flood farming the river delta was a sustainable practice until the construction of

dams on the river.

The earliest investigator into the nature of soils could be called Edaphologists

(those who study soil as a habitat for organism and, particularly as the medium in

which plants grow).

Those who study soil as geologic entity its origin, morphology geology and

taxonomy are called Penologists.

Xenophon, a Greek historian(430-355B.C) is credited with recording the merits of

soil enriching crops when he wrote, “ whatever weeds are upon the ground, being

turned into the earth, enrich the soil as much as dung’’

Cato (234-149 B.C) recommended that intensive cultivation. Crop rotation the use

of legume and manure for soil improvement.

Early 17th century an experiment performed by Van Helmont (1577-1644) he

reasoned that water was the “ principal of vegetation’’

Von Liebig stated that carbon was derived from soluble soil forms rather than

from CO2. He also postulates the law of minimum, which stases that the growth of

plants is limited by the essential elements presents in the least relative amount.

J. B .Lawes and J.H. Gilbert (1843) were established the first modern agricultural

experiment station (UK). On Rothmasterd station the gave many theories on soil

plant nutrients.

In the history of Pedology, V.V .Dokuchaiev and N.M Siberstev were developing

new concepts of soil. They stated that soils as independent, natural bodes, each

with unique combination of climate, living matter parent materials, relief, and

time. They hypothesized that properties of each soil is reflected on the basis of

soil forming factors.

In the 1920s C.F.Marbut adapt the Russian approach to the American

system.1938 the U.S department of agriculture yearbook contained the first

extensive soil classification system for the United States.

In 1975 U.S National co-operative soil survey under the leadership of Guy Smith,

developed system of soil classification.

1.3 COMPONENTS OF SOIL

Mineral particles, organic matter, air and water are the components of soil. The

relative proportion of these four components greatly influence the behavior and

productivity of soil.

Half of soil volume consists of solid materials (minerals and organic); and the

other half consists of pore spaces field with air and water.

a, The Solid Components

Soil has two kinds of solid components; mineral derived from weathering rocks,

and organic materials from plants and microorganisms.

Mineral(inorganic)

Except in the case of organic soils, most of a soil’s solid frame work

consists of mineral particles.

The mineral particles of soil and rocks are nearly always distinct,

crystalline, substances; appreciable amount of iron, Calcium, Potassium,

appreciable or magnesium.

The mineral particles present in soils an extremely variable in size. The

larger soil particles which include stones, gravel and coarse sands are

generally rock fragments and they are aggregates of several different rock

fragments and they are aggregates of several different minerals smaller

particles tend to be made of a single mineral.

The three major inorganic soil particles are sand (2.0-0.05mm), silt (0.05-

0.002mm) and clay (< 0.002mm).

The smaller particles of clay (< 0.001mm) which have large surface area

per unit of mass have colloidal properties.

Soil organic matter

It consists of a wide range of organic (Calcareous) substances, including

living organism (the soil biomass).

Organic matter comprises only a small fraction of the mass of a typical

soil by weight typical well-drained mineral surface soil contain from 1-

6% of organic matter.

Organic matter binds mineral particles into a granular soil structure that is

largely responsible for the loose, easily managed condition of productive

soils.

Organic matter also increases the amount of water a soil can hold and the

proportion of water available for plant growth.

During the decay of plant tissue, large complex molecules form that

remain in the soil as a resistant by products. These organic colloids are

hummus, which is responsible for the brow and black color of the top

layer of some soils.

b, Soil pores

The solid mineral particles and organic coatings are the soils skeleton. The pores

between the particles control the soil’s ventilation, water intake, water storage and

drainage.

The size and shape of pores and the total pore space are important. Smaller pores,

for instance, hold water well, but large interconnected pores are needed for water

and air to move frelly into and out of the soil.

The soil air

It is the channel for the movement (diffusion) of oxygen and other gasses it is

the soil’s connections with earth’s atmosphere.

When water enters the soil, it displaces air from some of the pores; the air

content of a soil is therefore inversely related to its water content.

When the smaller pores are field with water the ventilation systems becomes

clogged.

The soils drains from heavy rain or irrigation, large pores are the first to be

filled with air, followed by medium sized pores, and finally the small pores, as

water is removed by evaporation and plant use.

The soil water

Water is of vital importance in the ecological functioning of soils. The

presence of water in soils is essential for the survival and growth of plants and

other soil organisms.

Water is held with in soil pores with varying degree of tenacity depending on

the amount of water present and the size of the pores. The attraction between

water and the surface of soil particles of soil particles greatly restricts the

ability of water to flow.

Soil water is never pure water, but contains hundreds of dissolved organic and

inorganic substances; it may be more accurately called the soil solution. An

important function of the soil solution is to serve as a constantly replenished

dilute nutrients solution bringing dissolved nutrient elements to plant roots.

1.4 SOIL MINERALS

Minerals are natural occurring inorganic material, distinct physical

characteristics, internal structure and definite composition.

The soil mineralogy is primarily determined by soil parent material and

weathering process that have occurred.

- Based on the occurrence/abundance they are broadly classified into Essential and

Accessory mineral

Essential minerals

Are those mineral which are very important, they dictate the

charctestics and proprieties of rocks. They are chief constituents of

rock.

Quartz, Feldspar, Mica, Amphiboles, and Divines are good examples

of essential minerals.

Accessory minerals

Are those minerals which are presenting low quantities and they don’t

dictate the characteristics of rock. They are not chief constituent of

rock.

Apatite, Pyrites, Zircon are examples

- Based on origin/ formation, the minerals are broadly classified as primary and

Secondary minerals.

Primary minerals.

Are those minerals formed at high temperature and pressure conditions and

inherited from the parent rock (Igneous rock and Metamorphic rock) with

out being chemically changed.

Examples of primary minerals including Quartz, Olivine, Feldspar

(Orthoclase and Plagioclase), Mica (Biotite and Muscovite), Pyroxenes,

Amphiboles and others.

Secondary minerals.

Are those minerals formed from primary minerals due to metamorphism. It

is formed as a result of chemical alteration and dissolution of primary

minerals and subsequent precipitation of secondary minerals. Formations

of secondary minerals are a complex process.

Common secondary minerals including Gypsum, Carbonate, phosphorus

minerals, Rock phosphate, oxides, aluminum hydroxides oxides and oxy

hydroxides, iron, manganese, zinc, copper, boron, molybdenum, sodium

minerals, Clay minerals (layer silicates) including Kaolinite, Smectite,

Vermiculite, Chlorite, Mica etc.

1.5 SOIL PROFILE

Soil profile: is a vertical cross section of the soil with depth exposing through all its

horizons and extending into the parent material.

Soil horizon: is a layer of a soil, approximately parallel to the soil surface, differing in

soil properties and characteristics from adjacent layers below or above it.

Each horizon is different from other horizons in the profile. usually but not

always, the difference between adjacent horizons above or below, are quite

obvious to the eye (color, structure differences) or the difference can be felt by the

fingers(variation in clay content).

Horizons with in the soil may vary in thickness and have some what irregular

boundaries, but generally they are parallel to the land surface. The boundaries

between horizons in profile ranges from indistinct to abrupt and clear. Some

boundaries are relatively smooth (the horizon is the same thickness in that volume

of soil) other have tonguing patterns (large vertical variation). Usually a horizon

is separated from others horizons:

The horizon has accumulated more humus and is dark colored

The horizon has had some of its clay and humus moved to greater depth and

is a leached horizon.

The horizon has accumulated or produced more clay than is in other horizon

and

The horizon has accumulated some secondary minerals (Calcite, silca, or

iron oxides), forming horizons such as lime zones or hard pans.

There are four types of soil horizons

1. Master/Major horizon

2. Subordinate horizon

3. Transition horizon

4. Diagnostic horizon

1. Master/Major horizon

Five master soil horizon are recognized and are designated using the capital letters O, A,

E, B, C & R

O- HORIZONS

Organic horizon in which most leaves, stem, litter are subjected to humification

and mineralization.

It is found above the mineral soil. Common in forested areas and are referred to as

forest floor.

Which have three sub horizons:

Oi horizon: plants and animal parts are slightly decomposed (Fibric material)

Oe horizon: partially decomposed (Hemic materials).

Oa horizon: highly decomposed (Sapric material)

A-HORIZONS

It the top most mineral horizon consisting of:

- Organic matter which give the soil a color darker than that of the lower

horizons.

- Elluviation of clay, iron or alumunium with resultant concentration of

quartz or other resistant minerals of sand or silt size.

- Maximum biological activities.

Which have sub horizons

Ah horizon: contains high amounts of humus (>1% OM)

Ap horizon: ploughed (cultivated) layers

Ag horizon: gleyed (spot of different color/mottles exists)

- Due to fluctuation of water (water coming and going)

- Anaerobic situation which cause Fe & Mn reduction.

E –HORIZONS

It a zone characterized by maximum elluviation (leaching) of silicates clays and

iron and alumunium oxides

It leaves concentration of resistant minerals such as quartz, sand and silt.

Commonly occurs above the B- horizon and below A-horizon. Generally lighter

in color (bleached) than either the A- horizon above it or horizon below.

It is common in soils developed under forests (high rain fall areas) but the rarely

occur in soils developed under grassland.

The sub division

Eg horizon: bleached layer with spot of different color

B –HORIZONS

It zones of illevation (accumulation). Small particles that have washed from O, A

or E horizons (iron, aluminum oxides, silicate clays) have accumulated because

of filteratin (lodging) or lacking of enough water to move them deeper

Early B- horizon development stages of soil may have only redder (orange,

yellow brown) colors of weathering caused by the color iron and aluminum

hydrous oxide.

In this horizon the structural formation is good (blocky or prismatic).

Sub horizons are;

Bg horizon: gleyed

Bh horizon: humus accumulation

Bs horizon: sesquoxide (iron and aluminum hydrous oxide) accumulation

Bt horizon: translocated clay

Bw horizon: alteration by weathering (distinctive color or structure)

By horizon: gypsum accumulation

Bz horizon: salt accumulation

C –HORIZONS

It is a mineral horizon or layer excluding bed rock, which is relatively affected by

pedogenic processes. It is unconsolidated material underlying the solum (A and

B) horizons.

It may or may not be the same as the parent material from which the solum

formed.

Few root, massive gravel and rocks are found.

The sub horizons are

Cg horizon: glying

Ck horizon: accumulation of CaCo3

Cm horizon: cemented layer of silca and calcium.

Cx horizon: fragipan ( high bulk density, brittle hard pan, silty texture)

Cy horizon: accumulation of gypsum

Czhorizon: cementing material of salt

R-Layers

These are underlying consolidated rock, with little evidence of weathering. It is

hared bed rock like basalt, granite or sand stone and digging is impractical. In

these layer when the rock cracks it filled with soil.

Sub division with in master horizons

Arabic numerals following horizon letter indicate vertical subdivision within a

horizon. Often distinctive layers exists with in a given master horizon and these

arte indicted by numeral following the letter designation. For example, if three

different combinations of structures and colors can be seen in the B horizon, then

profile may include B1-B2-B3 sequence.

Arabic numerals infront of master horizons indicate discontinuities.

For example losses of A horizon over limestone B horizon would have the

B written as 2B to show parent material change (discontinuities).

2Cg indicates apparent materials with strong gleying and of differing

parent material than the horizon above.

For organic layer and and volcanic soil we don’t use lithological discontinuities. It

is only used for soil forms discontinuities by geological process.

2. Transition horizon

Transition layers between the master horizons (O, A, E, B, and C) may be

dominated by properties of one horizon but also have prominent characteristics of

another

e.g. AE, EB, BE, BC

Letters combinations such as E/B are used to distinct the transition horizons.

where distinct parts of the horizon have properties of E while other parts have

properties of B.

3. Subordinate horizon

Specific horizon characteristics may be indicated by a lower case letter following

of master horizon designation.

The distinctions of these horizons include special physical properties and the

accumulation of particular material, such as clays and salts.

a…..organic matter highly decomposed sm….. Cementing materials iron oxide

b…..buried soil horizon zm…. Cementing materials salt

c…..concretions or nodules ym……Cementing materials gypsum

d….dense unconsolidadated materials n……accumulation of sodium

e….organic matter partial decomposed. o…... accumulation sesquoxide

f…. frozen p……plow/cultivated

ff….permafrost (no ice) q…… accumulation of silca

g….gleying r……weathering of soft bed rock

h….accumulation of humus s…… accumul. of OM, Fe, Al oxide

i…. organic matter slightly decomposed ss…..sillickensides

j…..jarosite t……accumulation of silicate clays

jj …Crytoturbation v……plinthite

k…accumulation of carbonate w….. Weatherable

m…cementation or induration x….. fragipan

km...Cementing materials CaCo3 y…….. accumulation of gypsum

qm....cementing material silca z…… accumulation salts

4. Diagnostic horizon

Are used in the in the new soil taxonomy system to differentiate among soils

orders sub orders, great groups and sub groups.

Epipedons: the diagnostic horizon that occur at the soil surface

Mollic: soft, a thick, dark friable not strongly acidic

Antropic: people made mollic horizon

Histic: organic surface horizon underlain by mineral

Melanic: thick, black, friable horizon, formed in volcanic areas

Albic: strongly leached E horizon

Ochric: thin or light colored surface

Plaggen: people cause, high humus horizon

Endopedons: the diagnostic horizon that occur below the surface

Agric: tillage caused clay and humus accumulation

Argillic: clay accumulation

Cambic: color or weakly developed B horizon

Kandic: argilic horizon of kaolinite like clays

Natric: like an argillic but with high exchangeable sodium content

Oxic: thoroughly weathered B horizon

Somic: an acidic, humus accumulation, tropical B horizon

Spodic: an acidic, cool area B horizon with an accumulation oh humus

iron aluminum oxides(sesquoxides)

Glossic: degrading argillic kandic or nitric

Endopedon(accumulation of solublized substances)

Calcic: calcium carbonate accumulation

Gypsic: gypsum accumulation

Salic: soluble salt accumulation

Sulfuric: horizon high in sulfides

Hardpan horizons

Duripan: silca cemented hardpan usually having carbonates

Fragipan: dense, brittle, minimal cemented hard pan

Petrocalcic: hard carbonate cemented

Petrogypsic: hard gypsum cemented

Placic: very hard, iron cemented

2. SOIL FORMATION AND DEVELOPMENT

It describes mixtures of minerals or hard rock as it changes into loosened material

in which plants and other organisms will be able to live and into which air and

water move.

The parent material from which soils developed varies widely around the world

and from one location to another only a few meters apart. Knowledge on these

materials their source of origins, mechanisms for their weathering, and means of

transport and deposition are essential to understand soil genesis.

2.1 SOIL FORMING ROCKS

Rocks are natural occurring inorganic consolidated material consists of more than

one mineral.

They are classified into three major divisions

1. Igneous rock

2. Sedimentary rock

3. Metamorphic rock

1. Igneous rock

When molten magma from under the earth’s crust is exposed on the surface or at

different depth in the earth.

During volcanic eruption molten heated liquid materials erupt from the ground.

Based on the eruption classified into

Extrusive igneous rock

Intrusive igneous rock

Extrusive igneous rock

The expelled igneous rock (from volcanic eruption) cools on the surface rapidly. It is

formed out side the earth surface and cause natural disasters.

They glassy and amorphous (low crystallization) and formed fine textured soils.

(Clay and clay loam)

e.g. Basalt, Andesite, Rhyolite

Intrusive igneous rock

Magma cools near the surface but not expelled into exposure to air. The rock

formed inside the earth surface. Those minerals in material less rapidly cooled

from small crystals in the rock mass.

They are high crystallizations and formed coarse textured sized soils (sandy

soil)

e.g. Granite, Diorite, Gabbro

Igneous rock IS composed of primary mineral such as

Light colored: Quartzite, Muscovite, and Feldspar

Dark colored: Biotite, Augite, and Hornblend

In general, dark colored minerals contain iron and magnesium and are eaisly

weathered. There fore dark colored igneous rock such as Gabbro and Basalt are

more easily broken down than are granite and other light colored igneous rock.

Decomposition of igneous rock depends on

The chemical composition (silica content)

Formation of rock (cooled magma)

2. Sedimentary rock

Those rocks formed by weathering and erosion process from pre existing rocks

other process like transformation.

Rock minerals and soil particles or soluble substances that become consolidated

or cemented into hard masses.

They are the most common type of rock encountered covering about 75% of the

earth’s land surface.

Based on decomposition and re cementation classified as

Mechanical sedimentary rock

Chemical sedimentary rock

Organic sedimentary rock

Mechanical sedimentary rock

Materials formed by loose sediment at suspension. Moving water (river) carry

stones, gravel, sand, silt, clay

e.g. Conglomerates, Sandstones, Shale

Chemical sedimentary rock

Materials formed due to precipitation of solutions/sediment

e.g. Calcite, Dolomite, Limestones

Organic sedimentary rock

Formed from either plant or animals origin. Due to decomposition of organic

matters.

e.g. Coal, Lignite, Anthrasite

Decomposition of sedimentary rocks depends on

Chemical composition

Cementing agent

3. Metamorphic rock

It is formed by metamorphism of pre existing igneous and sedimentary rocks.

Those rocks masses are subjected to tremendous heat and pressure.

It may be as hard as or harder than, the igneous, sedimentary rock from which

they formed, but they weathered to produce similar soils.

Based on the profound modification /layer or foliation classified as

Foliated/Layered Metamorphic rock

Non foliated/ non layered Metamorphic rock

Foliated/Layered metamorphic rock

The layer formaton are non massive. It is soft and easily exposed to weathered.

e.g. Schists, Slate, GnesisNon foliated/ non layered metamorphic rock

The layer formations are massive. It is very hard and takes long time to

weathered.

e.g. Marble, Qurtizite Decomposition of metamorphic rock depends on chemical composition & texture. 2.2 SOIL FORMING FACTORS

Five soil forming factors primarily responsible for the character of the developed

soil.

1. Parent material (unconsolidated material or rock)

2. Climate (primarily temperature and precipitation)

3. Biota (living organisms, organic residues)

4. Topography (slope, aspect, elevation)

5. Time

1. Parent material

Geological processes have brought to the earth’s surface numerous parent

materials in which soils form.

They include exposed bed rock, but the largest soil areas are formed from

unconsolidated materials.

The nature of parent materials profoundly influences soil characteristic.

e.g. Sandy texture soil from coarsed grain, quartz rich materials such as

Granite or Sand stone

The chemical and mineralogical composition of parent material also influences

both chemical weathering and natural vegetation

e.g. Presence of limestone in parent materials will delay the development of acidity.

Parent material also influences the quality and type of clay minerals present in the

soil profile.

Parent material classified with regarding to the mode of placement in their current

location.

Residual ( formed in place from rock)

Transported (Colluvial, Alluvial, Marine, Lucustrine, Glacial, Folian)

Accumulated plant debris (organic matter)

2. Climate

It is the most influencing factors acting on parent material because it determines

the nature and intensity of the weathering that occurs over large geographic areas.

The principal climatic variables influencing soil formation are effective

precipitation and temperature.

Precipitation

To be effective in soil formation, water must penetrate into the regolith. The

greater the depth of water penetration, greater in weathering and soil

development.

Percolating water stimulates weathering reaction and helps differentiate soil

horizon.

In the process of chemical weathering (hydration, hydrolysis reduction and

dissolution) water has major roles.

Rainfall comes togther with ions which increases weathering process. Leaching

of clay, carbonates soluble salts and accumulation in the horizon.

Temperature

It influences reactions in soil forming process. For every 10oc rise in temperature,

the rate of biochemical reaction more than doubled.

Fluctuation of hot and cold temperature causes expansion and contraction on the

parent material (rock and minerals).

Temperatures controls evapotranspiration indirectly affects the effective rainfall

(exist in the soil).

If warm temperature and abundant water are present in the profile at the same

time, the process of weathering leaching and plant growth will be maximized.

When the temperature increases, the soil become more gray color, amount of

organic matter reduced due to decomposition and absorption.

Direct effects of climate on soil formation

Lime: (in alkaline soils) can accumulate at shallow depth. In low rain fall areas

calcium carbonate not leached when little water present.

Acidic soil: humid areas due to intensive weathering and leaching out of basic

cations.

Erosion of soils on sloping land constantly removes developing soil layers.

Deposition of soil materials down slope covers developing soil layers.

Indirectly affects action on vegetation.

3. Biota

Living plants and animals and their organic wastes, residues have marked

influences on soil development.

Organic matter accumulation, biochemical weathering, profile mixing nutrient

cycling, and aggregates stability are all enhanced by the activities of organisms in

the soil.

Vegetation

Forest land

Tree leaves falling on the forest are the principal source of organic matter.

In humid forest vegetation may develop many horizon

Organic matter in the forest floor and a thin A horizon.

Grass land

Much of organic matter added to the soil is from deep fibrous grass rot

system

Soils generally have thicker A horizon and a deeper distribution of organic

matter.

Burrowing animals

Moles, gophers, prairie dogs, earth worms, ants and termites are important in soil

formation when they exists in large numbers

Bring the lower horizon to the surface

Encouraging movement of water and air into the subsurface layers.

Mixing the lower and upper layers horizons (pedoturbation).

Ingest soil particles and release organic residues

Increase stability of soil aggregates.

Micro organisms

Slowly decomposing organic matter and forming weak acid that dissolves mineral

faster than that does pure water.

Human interferences’

Destruction of natural vegetation, subsequent tillage, mining and urbanizing areas.

4. Topography

Influences soil formation primarily through its modification of water and

temperature.

It relates to the configuration of the land surface and is described in terms of

differences in elevation, slope and land scape position

Steep slopes

Encourage erosion of surface layers

Allow less rainfall to enter the soil before running off

There is no possibility for formation of deep soil

Less effective moisture exists

Less diverse plant cover, low organic matter.

Gentle slopes

More water passing vertically through them

The profile is deeper

The vegetation are more luxuriant high organic matter

Topography affects the absorbance of solar energy in a given land scape

A higher temperature on south and west slopes results in greater loss of water

by evaporation; the net result in regions where water is limiting is often soils

with thinner horizons and less vegetation cover than soils on north and east

slopes.

It can also interact with parent material. In many land scapes, topography reflects

the distribution of residual, colluvial and alluvial parent materials.

Residual : upper slopes

Colluvial : covering the lower slopes

Alluvial : filling the valley bottom

5. Time

The degree of which material with in soil to change depends on the amount of

time.

For soil development, the length of time required to developed layer, called

genetic horizons depending on many interrelated factors of climate, nature of

parent material, the organism and topography.

Horizon develop most rapidly in warm, humid, forested climates where there are

adequate water to move.

In rates of weathering, where the other factor of soil formation are favorable.

Organic matter may accumulate to form darkened A horizon… Decade

Incipient B horizon has become discernible on humid region….40years

The formation of B horizons with altered colors and structures…Centuries

The accumulation of silicates clays usually become noticeable…Thousands years

A mature, deeply weathered soil ………………hundreds of thousands year

When we speak of ‘young’ or ‘mature’ soil, we are not so much referring to the

age of the soil in layers as to be degree of weathering and profile development.

Residual parent material has generally been subjected to soil forming process for

longer periods of to time than material transported.

The age of soil detected by half life carbon.

2.3 Weathering of rocks

Weathering is biochemical process that involves destruction and synthesis. The

original rocks and minerals are destroyed by both physical distigration and

chemical decomposition.

It breaks up rocks and a mineral modify or destroy their physical and chemical

characteristics and carries away the soluble products.

It is a continuous reaction in the soil development until there is even not reactant

is present (with out noticing the change).

Weathering proceeds below and with in the solum

Geochemical weathering

- Occurred below the solum, in C horizon.

Pedochemical weathering

- Decomposition, distigration, modification

- All associated with biological and other soil forming factors

- Common in A and B horizons

The two major types of weathering are

1. Physical weathering (disitegration)

2. Chemical weathering (decomposition)

1. Physical weathering (disitegration)

Weathering includes breaking or grinding particles to smaller size with out

change in the chemical composition.

Effect of climate on physical weathering.

Temperature

In hot region

Rocks heat up during the day and cool down at nigh, causing alternating

expansion (heating) and contraction (cooling) of their constituents’ minerals

which cause the rock to crack apart.

Because the outer surface of a rock is often warmer or colder than the inner, more

protected portion, some rocks may weathered by exfoliation (peeling away of

outer layers).

e.g. low land areas, desert areas

In temperate regions

Rocks exposed for lower temperature (freezing and thawing/wetting and drying).

The process accelerated if ice forms in the surface cracks.

e.g. high land areas

Water (Rainfall)

When loaded with sediment, with sediment, water has tremendous cutting power.

Erosion, removal/rolling of material at the bottom of stream (collision-breaks the

rock/mineral)

e.g Gorges, Ravins, Valleys, rounded river bed rock, beach sand gravel