Post on 19-Apr-2017
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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