SOIL
Soil is the top and thinnest layer of the earth’s crust. OR Is the upper surface layer of loose or unconsolidated materials which overlies the crustal rocks.
Soil composition
(components of soil)
1.
Inorganic components
2.
Organic matters
3.
Soil water
4.
Soil air
Inorganic matters
They include rock
particles or fragments. They are produced by weathering process. They include
all types of minerals such as silt, clay and sand, iron, calcium etc. they
account for 45% of the soil components.
Organic matter
This consists of
living things present in the soil such as bacteria, fungi and dead and decayed
plants and animals (humus). Humus is very important to soil fertility. Bacteria
plays a vital role in the decomposition of plant and animal remains. This constitute
5% of the soil components.
Soil water or
moisture (hygroscopic water)
Is the water
contained in the soil. It accounts for 25% of the soil. Water dissolves
various substances like salts,
potassium, calcium etc to form soil solution. This enables the plants to absorb
minerals from the soil.
SOIL AIR (soil
atmosphere)
Soil air comprises
gases which are contained in the pore spaces between soil particles. Soil air
is very important for the plant growth. A soil without air is not suitable for
plant survival. The micro organisms in the soil needs air. Air accounts for 25%
of the soil components.
Factors influencing
soil formation
1.
Parent materials
2.
Climate
3.
Living organisms
4.
Topography
5.
Time
They
are known as soil formers.
•
Climate and living
organisms are active soil formers
•
Parent materials
topography and time are passive soil formers.
Parent materials
These are the rocks
or debris from which soil develops. Parents materials determine the type and
quantity of soil produced. Example hard rocks undergo weathering so slowly
hence slowing down the soil formation process. Soft rocks undergo weathering
rapidly hence faster rate of soil formation. Mineral composition of the soil
and texture of the soil depends on the nature or parent materials.
Topography
Is the nature of the
surface of the landforms that make up the earth’s surface. Basing on aspects
the side that receives much sun, rainfall will develop thick vegetation cover.
This leads to the formation of a matured soil. On very flat landscape (swampy
condition), there will be no formation of matured soil.
Climate
Temperate and
rainfall are the dominant elements of climate that influence soil formation.
Direct effects
v It affects the rate of weathering
v It affects other soil forming processes i.e.
leading occur.
v It affects the work of erosional agents such as
water, wind and ice.
v Affects the rate of decomposition of organic
matter to form humus.
Living organisms
The presence of
micro-organisms such as bacteria causes the decay of plan and animal remains to
form humus. Humus is very essential component of a matured soil. It is black or
dark brown in colour.
v Plants facilitate weathering process
v Earthworms and other burrowing insects mixes the
organic remains with the mineral components to form a matured soil.
Time
Time is needed for
the factors to form matured soil. When the soil forming processes are left
undisturbed for a long time a matured soil develops some environments needs few
years while others needs many years for a matured soil to be formed.
Importance of soil
1.
Helps the plants to
grow. Soil contains water, air and nutrients for eh plants growth. It also
provides anchorage for the plants as well as protection of roots from damage.
2.
Support animal life.
Soils support plant life which is a source of food for animals especially the
herbivores.
3.
Habitat for
organisms. Soils function as a habitat for organisms such as burrowing rodents,
earthworms and termites.
4.
Building materials.
Soil is used to make bricks, tiles and whitewash. They are used in building of
houses, bridges and other structures. It is also used for roads constructions.
5.
Pottery and ceramics.
Soil is used in making pottery, ceramics and sculptures.
6.
Source of minerals.
Some soils contain minerals such as apatite from lateric soils which is used to
manufacture phosphate fertilizer; titanium; bauxite which is a source of
aluminium.
7.
Farming and
settlement. Fertile soil support agriculture both arable and pastoral farming.
Soil also influence settlement distribution. Fertile soil attracts many people
to settle in that area.
8.
Cultural and
medicinal value. Maasai uses red soil for body decoration. Some communities
mixes clay with herbs for medical purposes.
Soil properties
1.
Soil texture
2.
Soil structure
3.
Soil colour
4.
Soil permeability
5.
Soil porosity
6.
Chemical composition
Soil texture
Is the composition of
soil particles in terms of the size of those particles. Is the degree of
coarseness or fineness of the soil particles.
Gravel and sand soils
have few plant nutrients . It has large air space; water and air pass through
it so rapidly. Clay soil is the opposite.
|
Type
of soil |
Size
of particle in diameter |
|
Gravel
Sand
Silt Clay |
Larger
than 2mm Between
2mm – 0.02 mm 0.02mm
– 0.002 mm Less
than 0.002 mm |
· Gravel and sand soils have few plant nutrients . It
has large air space; water and air pass through it so rapidly. Clay soil is the
opposite
Importance of soil
texture
1.
It determines the
amount of water available in the soil
2.
It influences the
ease with which plant roots can penetrate the soil.
3.
It determine the
fertility retention capacity of the soil through its clay humus content.
Soil structure
Is the arrangement of particles in the soil to form large sized particles.
|
Type
of structure |
|
|
Granular
(crumb) |
Rounded
and porous particles groups with a diameter of up to 10 mm |
|
Platy
|
Plate
like flat particles that are arranged in a horizontal manner |
|
Prismatic
|
Vertical
prism like particles measuring between 0.5 and 10mm in diameter |
|
Columnar |
Similar
to prismatic but the particles are rounded at the top |
|
Blocky
|
Irregular
pieces of soil with sharp corners and edges. |
Soil colour
Different soils have
different colours. Soil colour is influenced by:
1.
Type of parent
material from which the soil has developed.
2.
Chemical composition
of the soil.
3.
The organic matter
content within the soil
4.
The drainage of the
area where the soil is found
Soil colour indicates
its component e.g. red and yellow colours = small iron contents. Red colour =
rich in ferric oxide. Yellow colour = hydrated ferric oxide. Dark colour = high
humus content.
Porosity
Refers tot eh amount of pore spaces in a soil.
It controls the amount of water that a soil can hold. Coarse sandy soils are
very porous, hence cannot hold water for long.
Chemical composition
Soil ph. Is the acidity or alkalinity of the
soil. It is the number of fine hydrogen ions in a soil solution. Soil PH is
measured by PH scale.
Soil acidity or alkalinity determines the chemical
environment is which plants grow and under which the soil micro organisms live.
Line is added to acidic soils to neutralize them.
Soil profile
Is the vertical arrangement of different layers
of the soil from the surface to the bed rock. These soil layers are also called
horizons.
There are
three main horizons:
1.
A horizon: Uppermost layer or top soil
2.
B horizon: intermediate layer or sub soil
3.
C horizon: lowest layer or weathered parent materials
·
Horizons A and B represent the true soil or the
solum
·
Horizons c is made up of disintegrating rock
material from which the soil forms.
·
Horizon D is the unweathered rock (bed rock).
Characteristics
of the soil profile
Horizon A
1.
It is rich in humus. Chemical and bacterial activities
are greatest.
2.
It is the zone of Eluviations. Eluviations is the
mechanical washing down of fine mineral particles from this layer into the
middle and lower layer. Materials are moved in suspension (Eluvial zone)
3.
It is the zone of leaching. Leaching is the process
where soluble minerals dissolve in water and the solution is moved from this
layer to the middle and lower layer, it is common in areas that experience high
rainfall and low evaporation.
Eluviations and
leaching causes the top soil to be deficient in minerals
Horizon B (characteristics)
1.
It is the zone of eluviations. Eluviations is the
deposition / accumulation of the materials that have been washed down by
eluviations from the upper layer of the soil.
2.
Precipitation takes place. This is the process in
which solid matter is formed from the solution that are washed down from the
upper layer through leaching.
Horizon C (characteristics)
(i)
It comprises weathered rock debris.
(ii)
It forms parent materials of the soil
(iii)
Mechanical weathering is common than chemical
weathering
(iv)
It is the beginning of true soil development
Horizon D
It is unweathered parent rock. It may sometime
be not part of parent materials.
Simple soil classification
Basing on texture; the following are the types
of soil
1.
Sand soil
2.
Silt soil
3.
Clay
Sand soil
v
It consists mainly of coarse and fine sand
v
It has large pore space
v
It is highly permeable
v
It has less amount of humus due to high leaching and
eluviation
v
It is easy to work upon
v
Plant roots can so easily penetrate through it.
v
The particles are unconsolidated (does not stick)
Silt soil
v
It consists of medium particles
v
It has sufficient amount of humus
v
It is well aerated soil
v
It is the most suitable soil for agriculture
Clay soil
v
It has fine particles
v
It is sticky when wet
v
It is less permeable (it cause water logging)
v
It contains high humus content
v
It is very difficult to work upon (heavy soil)
Loam soil
v
It is a mixture of various sizes of soil particles
v
It s good for agriculture
THE SOIL OF AFRICA
Latosol soils
v
They have a reddish colour due to high aluminium
andironoxides content
v
They lack mineral nutrients due to leaching of salts
and silica.
Dark grey and black soils
v
They occur in tropical areas which receive between
500mm – 800mm of rain a year.
v
They are grey to black in colour.
v
The upper layer contains lime and nitrogen compounds.
v
During dry seasons they dry and crack. During wet
season they become sticky and heavy.
Red loams
v
They occur in the wetter savanna areas
v
They are red or brown in colour being rich in silica
and iron oxides
v
They contain fair amounts of humus which makes them fertile
Desert soils
v
They occur in regions of very low rainfall where
temperatures are fairly high.
v
They have a
high lime content but no humus
v
They are yellow to red in colour
v
Most of African soils are from sedimentary, granitic
or volcanic rocks.
v
Some are formed from wind deposited and water
deposited sediment.
Major soil type of the world
A.
Tropical soils
Laterite:
formed in
humid tropical regions where heavy leaching takes place all the year.
Red soils:
Formed in
tropical regions with definite wet seasons during which heavy leaching takes
place.
Black soils:
Formed in
humid tropical regions where there are basalt rocks.
Desert soils:
Formed in hot dry regions. The soils are sandy
and salty.
B. Temperature soils
Podsols:
Formed in moist cool temperate regions. The
soils are leached.
Chernozems (black earth)
formed in
temperate grassland. They are rich in humus.
Brown earth:
Formed in temperate deciduous forest regions.
The soils are not leached.
Desert soils:
Formed in
temperate desert regions.
Other soils
Saline soils:
Formed in regions where evaporation is very
market.
Peat soils:
Formed in water logged conditions. The dead
vegetation in these soils is only partially decomposed because of the lack of
oxygen in the soil.
Limestone soils (terra Rossa).
These are determined by the limestone from which
they form. When they are red, they are called terra Rossa.
Recent soils:
Formed from materials recently deposited by
river, waves, glaciers and wind. They include loess and alluvia soils, outwash
plain.
Soil erosion
Is the removal of upper part of the soil by various
agents such as wind, waves and glacier(water).
Main agents of soil erosion
1.
Wind
2.
Running water
3.
Moving ice
4.
Waves
Causes of soil erosion
Poor farming practices. The following are the
poor farming practices that led to soil erosion
1.
The cultivation
of crops in regions which do not have a reliable rainfall.
2.
The plouging of land up and down the slope
3.
Shifting cultivation. This causes deforestation.
Deforestation exposes he soil to the agents of erosion.
4.
The cultivation of the same types of crop on a piece
of land year after year (monoculture)
5.
The cutting down of forests especially on higher
slopes.
6.
The overgrazing of grasslands
Effects of soil erosion
1.
Loss of productive soil. The top soil layer is the
most productive. When its eroded, it is lost forever.
2.
The plants are easily uprooted and blown away by wind.
3.
Destruction of structures such as roads and bridges.
4.
Lowering of water table. Its caused by the exposure of
the underground water reserves. Underground water will evaporate.
5.
Loss of vegetation. This is caused by the continued
loss of soil. The land remain bare and may turn into a desert.
6.
Silting water reservoirs. The eroded materials may
deposit in water reservoirs.
7.
Environmental pollution. Water may collect chemical
pollutants from forms and dumping grounds into rivers, lakes and oceans. These
pollutants may kills aquatic life such as fish etc.
8.
Clean sandy beach turns into muddy ones. This mud is
brought into the sea as sediments by rivers.
9.
Formation of fertile soil. The deposited areas form a
very fertile soil e.g. flood plain which are famous for agriculture.
10. Provide sand for
building and constructing.
Soil erosion control measures
1.
Planting of trees (afforestation and re-afforestation)
2.
Control of bush fire
3.
Controlled open grazing
4.
Practicing proper farming techniques
5.
Terracing to reduce the slope
6.
Contour ploughing to control surface runoff.
7.
Strip cropping. Planting crops in strips to break down
the speed of wind.
8.
Planting cover crop. To control erosion by rain water.
9.
Planting shelter belts. Large fields are divided into
small plots. Trees are planted around each plot.
10. Mulching: to cover
the surface with plant remains
11. Bush fallowing: to
allow land to regain fertility
12. Erecting brushwood
barriers. Brush or stone walls are constructed across the gullies to help trap
soil.
13. Construction of
cutoff drains. These are opening trenches which are dug across the slope and
the soil is heaped on their down slope sides to form a knit of ridge.
14. Construction of dams
and weirs. They are built across river valleys to control a river’s flow.
15. Use of artificial
waterways. This is a small channel constructed down a slope and in to which
surface runoff collects.
Soil conservation techniques
This includes all measures that are taken to
protect the soil from destruction
1.
Crop rotation. Growing crops with different nutrition
requirements in the same plot on rotational basis.
2.
Mixed farming. This helps toa improve soil fertility
3.
Proper application of fertilizers
4.
Use of organic manure. They add humus and improves the
moisture retention capacity
5.
Construction of drainage trenches. This prevents the
soil from becoming acidic.
6.
Proper ploughing techniques. This creates more air
spaces in the soil, thus increasing the rate of infiltration of water into the
soil.
7.
Intercropping.
Planting two or more crops concurrently on the same farm unit.
8.
Mulching
Advantages
Ø
Reduce evaporation of water from the soil
Ø
Protects the soil from erosion
Ø
Increases the humus content of the soil
Ø
Increase the rate of infiltration of water into the
soil
Ø
Provide a suitable habitat for burrowing animals
9.
Bush fallowing. It helps to improve the soil quality
in that the soil is able to regain its fertility during the period it remains
idle.
