UNIT 7 CLASSIFICATION OF SOILS AND SOIL FORMATIO
UNIT 7: CLASSIFICATION OF SOILS AND SOIL
FORMATION
Key unit competence:
By the end of this unit, I should be able to explain the factors and processes of soilformation.
Introductory activity 7:
Use internet and other geographical resources to research on factors and
processes responsible for soil differentiation and formation.
7.1 Classification of major soil types of the world
Learning activity 7.1
Make research on:
a. The major types of soils in the world.
b. The difference between zonal soils, azonal soils and intrazonal soils.
Soil is the uppermost layer of the land surface that plants use and depend on for
nutrients, water, and physical support. Soils consist of weathered rock mixed with
organic material that is derived from decaying plants and animals. There are three
major types of soils in the world, namely zonal soil, intrazonal soil and azonal soil.
7.1.1 Zonal soils
These are soils that cover a wide geographic region in the world. They depend on the
major climatic zones, vegetation and living organisms in areas where the landscape
and climate have been stable for a long time. They are common on gentle slopes.
They are found both in tropical and temperate regions.
This kind of soil has the following types: Tundra Soils, Podzols, Brown forest Soils,
Lateritic Soils / Latosols / Ferralsols, Chernozem / Prairie / Steppe, Grumusol / ReddishBrown Soils, Desert (Seirozems and Red Desert) Soils.
7.1.2 Intrazonal soils
These are soils that mainly develop due to relief of the area and the nature of parent
rock. These soils reflect the dominance of a single local factor, such as parent rock
or extremes of drainage that prevail over the normal soil-forming factors of climate
and living organisms. They are divided into three types:
• Calcimorphic or calcareous soils which develop on limestone parent rock
(rendzina and terra rossa);
• Halomorphic soils which contain high levels of soluble salts (e.g. sodium ions)
which render them saline.
• Hydromorphic soils that have constantly high water content which tends to
suppress aerobic factors in soil-formation.
7.1.3 Azonal soils
Azonal soils have a more recent origin and occur where soil-forming processes have
had insufficient time to operate fully. They lack well-developed horizons because
of immaturity or other factors that have prevented their development such as
excessive soil erosion. They are skeletal soils resulting from erosion and deposition.
They lack clear soil horizons. They are common in volcanic regions, glaciated regions
and areas blown by winds. They include dry sand, loess, moraine soils, and marinesoils, alluvial and volcanic soils.
The map below shows the major soil types of the world
Application activity 7.1
Make research on:
a. The major types of soils in the world.b. The difference between zonal soils, azonal soils and intrazonal soils.
7.2. Distribution of the major types of soil in the world
Learning activity 7.2.
Use the world map or Atlas to locate the distribution of major soil types.
Climate is an essential factor in soil formation. Therefore, the world’s major soil types
are distributed following the world’s climatic distribution.
7.2.1. Tropical soils
These include lateritic soils, red soils, black soils and deserts soils
• Lateritic soils: These are reddish brown soils that are developed under humid
tropical forest vegetation. These soils have granular dark reddish brown
surfaces underlain by reddish friable clay B-horizons. This type of soil is found
in regions with heavy rainfall where leaching is dominant throughout the year.
• Red soils: These are soils that develop in a warm temperate moist climate under
deciduous or mixed forests. They have thin organic mineral layers overlying
a yellowish-brown leached layer resting on a red horizon. They are found in
tropical areas where leaching takes place due to heavy rainfall.
• Black soils: These are found in humid tropical regions where the basalt rocks
are common.
• Desert soils: These are soils that develop under sparse shrub vegetation
in warm to cool arid climates. They have a light-colored surface soil, usually
underlain by calcareous material and a hard pan layer. They are found in both
arid and semi-arid regions. They are usually sandy and salty.
7.2.2. Temperate soils
i. Podzols: These are leached soils usually found in cool temperate regions.
ii. Chernozems: Chernozems also called black earth are found in extensive
temperate grasslands and contain a lot of humus.iii. Brown earth: These soils are found in temperate deciduous forested areas.
They are not leached.
7.2.3. Other soils
There are various types of soil whose characteristics are not determined by climate.
These occur in many regions. They include:
i. Mountains soils: These are stony and unstable soils in highlands. They are
usually eroded and transported to the lower valleys.
ii. Saline soils: They are found in areas where evaporation is dominant.
iii. Peat soils: They are found in water-logged areas. They contain dead
vegetation which is partially decomposed because of the lack of oxygen in
them.
iv. Limestone soils: They are common in limestone regions; when they are red
they are called “terra rossa”.
v. Alluvial soils: They are formed from deposited materials (sand, clay and silt)
along the river banks and lakeshores.
The following map shows the major types of soils of the world according to the
major climatic zones.
Application activity 7.2
1. Use the above information from the map to describe the major types of
soils around your school
2. Draw a sketch map of the world and mark the major soil types
7. 3. Processes and factors of soil formation
Learning Activity 7.3.
Use internet and other geographical resources to research on the processes of
soil formation
7.3.1 Processes of soil formation
Numerous processes are involved in the formation of soil or pedogenesis and
the creation of the profiles, structures and other features described below by the
combined effect of physical, chemical, biological and other processes working on
soil parent material. Soil is said to be formed when organic matter has accumulated
and colloids are washed downward, leaving behind deposits of clay, humus, iron
oxide, carbonate, and gypsum, producing a distinct layer called the ‘B’ horizon.
i. Calcification: This is the process in which calcium carbonates accumulates
in the ‘B’ horizon; particularly characteristic of low rainfall areas such as arid
and semi-arid climates.
ii. Eluviation: Eluviation is the downwards movement of fines particles such
as clay and the leached soluble materials from upper layers of the soil (‘A’
horizon) to another lower layer within the soil.
iii. Illuviation: This is the process of accumulation of clay, aluminum and iron
usually from A and E horizons to B horizons.
iv. Mineralization: This is the process through which organic matter is further
decomposed into mineral compounds. Mineral content in humus may be
further converted to inorganic matter e.g. silica.
v. Humification: Humification is the process by which organic matter is
decomposed to form humus, a task performed by soil organisms.
vi. Weathering: Weathering is the process by which the rocks break down into
small particles to form soil. It is the combined action of physical weathering,
in which rocks are fractured and broken, and chemical weathering, in which
rock minerals are transformed to softer or more soluble forms.
vii. Leaching: Leaching is the removal of soluble material in solution. It is the
process by which water removes leached materials (organic and inorganic)
in solution from the upper horizon to the underlying horizon. It operates
vertically but not sideways.
viii. Laterization: Laterization is leaching of soils in warm and humid climates.
It is a process that occurs after the soluble mineral substances have been
leached. After leaching, the insoluble mineral compounds derived from theparent rock remain on top, hence forming lateritic soils that are stony.
7.3.2 Factors of the soil formation
Soil-forming factors are both passive (parent material, topography and time) and
dynamic (climate, living organisms and man’s activities). These factors work together
as a system to form soils.
The major factors that influence soil formation are shown below:
i. Climate
The moisture (rainfall), evaporation and temperature changes determine the
chemical reactions and physical breakdown of rocks. This results in soil profile
development.
ii. Relief or topography
Topography also affects soil formation. Slopes that are too steep cannot have full soil
development because gravity and erosional processes remove water and materials.
Flat areas encourage water percolation/infiltration which favors the development
of deep soil profile, whereas steep relief accelerates water surface runoff, hence a
shallow soil profile.
iii. The nature of parent rock
Physical and chemical weathering of rocks in the upper lithosphere provides theraw mineral ingredients for soil formation. These rocks supply the parent materials,
and their composition, texture, and chemical nature help determine the type of soil
that forms. A weathering of fine grained rock texture tends to form a shallow soil
profile, while dark colored rocks are easily weathered and form deep soil profile. Clay
minerals are the principal weathered by-products in soil.
Vegetation and the activities of animals and bacteria determine the organic content
of soil, along with all that is living in soil (algae, fungi, worms, and insects). The
chemical composition of the vegetation contributes to the acidity or alkalinity of
the soil solution. For example, broadleaf trees when decomposed tend to increase
alkalinity whereas needle-leaf trees tend to produce higher acidity. Also decay of
plants and animals supply the soil in humus and nutrients. Animals contribute to soil
development through breaking down of vegetation and rocks into small particles
that form the soil. The figure below represents the diversity of life in fertile soil thatcontributes to soil formation and recycling.
iv. Time
All of the identified natural factors in soil development (parent rock, climate,
biological activity, and topography) require time to operate. This determines the
depth of weathering and the period of operation of soil formation processes.
v. Human factor
Human activities have a major impact on soils. The use of fertilizers changes the
natural properties of soils. Soil erosion has greatly increased due to agriculture and
construction. Approximately 1.2 billion hectares (3.0 billion acres) of Earth’s soils
suffer degradation through erosion caused by human misuse. An example of soil
loss through sheet and gully erosion on a northwest Iowa farm One millimeter of soil
lost from an acre weighs about 5 tons. Planting on the contour prevents water fromflowing straight down the slope and thus reduces soil erosion.
Application activity 7.3:
1. Visit your local area and describe the soil formation processes in that area
2. Use the acquired knowledge from the above lesson and explain how
different factors, namely climate, living organisms, parent rock, relief, time
and man influence soil formation.
3. Which of the above processes (in 1 above) are the most predominant.Justify your answer.
7.4. Soil erosion
7.4.1. Causes of soil erosion
When vegetation is removed from soil, the soil is exposed to the direct action of
rain and wind. Rain and wind can erode the topsoil and carry it away, destroying the
soil’s structure. Also, without plants, soil development slows and sometimes stops
because humus is no longer being produced.
Soil erosion is the physical removal or washing away of soil by several agents such as
human activities (cultivation and building) and natural processes including running
water, strong winds, moving glaciers, animals. Soil erosion occurs when overlandflow moves soil particles downslope.
The causes of soil erosion are both man-made (human activities) and natural.
i. Man-made causes
a. Over-cultivation of the land
Ploughing disrupts the soil. Every year, the world population increases by 93 million
people and they need more food. Therefore, farmers plough more fields to produce
more food. This increases pressure on our soil resources. Ploughing soil is the
mechanical turning and loosening of soil to improve it for crops.
Soils have not always been managed effectively. Ploughing) soil removes the plant
cover that holds soil particles in places, leaving soils open to wind and water erosion.
This makes the land lose its fertility and becomes exhausted. Over cultivation is
usually caused by increasing population and scarcity of land for farming and food
production, and over-cultivation of available croplands.
b. Poor methods of farming
Growing of the same crops and constant ploughing of the land on steep slopes using
poor methods of farming also encourages soil erosion especially in highland areas.
c. Deforestation
Because of population increase, there is great pressure on forests in order to get
cultivable land and land for settlement. The high demand for fire wood and charcoal
both in rural and urban areas has posed a great threat on the natural forests. This has
led to wide scale deforestation hence severe soil erosion.
d. Bush burning
It is done in the dry season in nomadic areas, with the aim of improving the quality
of pasture which will grow during the next rainy season. This instead destroys the
soil cover, makes the soil exposed to all agents of erosion.
e. Rapid population increase
Today there is population explosion in most areas of the world especially in less
developed countries. There is massive human pressure on land, vegetation in search
of food, this leads to the removal of vegetation cover which accelerate soil erosion.
f. Overgrazing
It is a major cause of erosion in nomadic areas. It occurs as a result of overstocking
of domesticated animals like cattle, sheep and goats. The animals are too many for
the available land. They destroy all the grasses hence exposing the soil and thisaccelerates soil erosion especially water runoff and wind erosion.
ii. Natural causes
a. Heavy rainfall
This is common in hilly areas where the speed of surface run off is high than infiltration.
Rain drops take away the top soils to the valleys; e.g. North Western Rwanda.
b. Drought
The current climatic change has resulted into limited vegetation or no vegetation
cover. This make the soil to be exposed to the agents of erosion (wind and moving
water).
c. Winds
Wind takes away the top soil in areas with limited vegetation cover and trees which
would act as wind breakers. This is common in arid and semi-arid areas which
experience high temperature and too much sunshine.
d. River and wave action
Shorelines of lakes and oceans as well as banks of rivers are eroded by the waves
and running water respectively. This accelerates the rate of erosion.
e. Steep slopes
Steep slopes increase on the speed of surface run-off leading to high rate of soilerosion.
7.4.2. Areas of severe soil erosion
The following are areas of severe soil erosion:
i. Mountains and highlands with very steep slopes
Surface water runoff occurs whenever there is excess water on a slope that cannot
be absorbed into the soil or is trapped on the surface. Reduced infiltration due to
soil compaction, crusting or presence of steep slopes increases the runoff. Runoff
from agricultural land is greatest during rainy months when the soils are typicallysaturated.
The steep relief accelerates the rate of surface run off hence leading to soil erosion.
This is common in mountainous and hilly areas. Severe erosion occurs in these areas
because the speed of surface runoff is too high and takes away the top soil.
ii. Glaciated highlands
Moving ice and glacier on the major highlands carries away large quantities or
eroded soil in form of moraine. This is deposited at the base of mountain and on
outwash plains. Severe erosion by glaciers and melt water is common on most high
and steep glaciated mountains.
iii. Desert and semi-desert areas
Soil erosion, is severe in desert areas because of limited vegetation cover. Very
strong winds blow off the unconsolidated soil and detaches it from the ground. Soil
is relocated elsewhere by strong winds where it is carried and deposited in otherareas.
7.4.3. Effects of soil erosion
The following are the effects of soil erosion:
i. Limited mechanized agriculture
The use of modern machines like tractors is made difficult because of gullies which
affect their movement in farms, this in turn affect crop production.
ii. Destruction of crops
Soil erosion destroys crops on farmyards. The Wind erosion destroys the growing
crops especially in hilly areas and at times transport them to the lower valleys. This iscommon in the northern province of Rwanda.
iii. Low soil productivity
Soil erosion leads to the loss of soil nutrients which in turn affects the yields. The
bare soil in hilly slopes can no longer support crop production.
iv. Famine
Soil erosion leads to vegetation destruction and this affects rainfall formation. This
phenomenon limits agricultural productivity.
v. Change of the Landscape
It changes the landscape appearance and natural beauty of the areas affectedbecause of presence of landslides, gullies and rills.
vi. Siltation
The eroded materials at times are deposited on people’s land or along river banks.
Such silt displaces people and destroys their property. This is common in flat lands
adjacent to hilly areas.
vii. Flooding
Floods cause great damages on communities and individuals. As most people are
well aware, the immediate impacts of flooding include loss of human life, damage
to property, destruction of crops, loss of livestock, and deterioration of healthconditions owing to waterborne diseases.
viii. Destruction of transport system
Roads are greatly destroyed because of soil erosion that results into gullies. These
gullies are caused by surface run off in the affected areas. This limits movement
of people, goods and services and requires urgent rehabilitation. This affectsgovernment’s budget.
Application activity 7.4.
1. Make a fieldwork study around your school and research on causes of soil
erosion and analyze their impact on the environment.
2. Basing on your experience on soils and topography, compare the effectsof severe soil erosion in the provinces of Rwanda.
7.5. Appropriate soil management and the conservation measures
The impact of soil loss on society is potentially disastrous as population and food
demands increase. The techniques described in the following paragraphs present
however some of management and soil conservation measures which enable
humans to use efficiently and sustainably the soil. They are presented and briefly
described as follows:
i. Agroforestry
This is a system of soil conservation where trees are planted around crops. Some
trees add nutrients to the soil while others protect crops from wind. This practice is
common in the northern province of Rwanda.
ii. Afforestation
It is a process of planting trees in a virgin land without any trees to create a forest.
Trees as windbreaks are planted and they reduce the speed of wind hence reducing
soil erosion. Therefore, afforestation is the creation of a new forest.
iii. Re-afforestation
It is the replanting of trees where they have been cut. This is usually done in steep
slopes where erosion is rampant. In several areas in Rwanda, this act has been done
under the system of countrywide tree planting. This has been done in order to
protect the environment and conserve soils.
iv. Contour ploughing
It is the farming practice of planting across a slope following its elevation contour
lines. These contour lines create a water break which reduces the formation of rills
and gullies during times of heavy water run-off; which is a major cause of soil erosion.
The water break also allows more time for the water to settle into the soil.
v. Crop rotation
It is a system of farming that involves growing of different types of crops in the same
area in sequenced seasons. It is done so that the soil of farms is not used for only one
set of nutrients. It helps in reducing soil erosion and increases soil fertility and crop
yields.
vi. Terracing
It is a piece of sloped plane that has been cut into a series of platforms, which
resemble steps, for the purposes of reducing erosion. This type of landscaping,
therefore, is called terracing. Terraces are commonly used on hilly or mountainous
terrain. Terraced fields decrease both erosion and surface runoff, and may be used
to support growing crops that require irrigation, such as rice.
vii. Use of fertilizers
It is important for farmers to use both artificial and organic fertilizers where necessary.
This improves the quality of the soils. Fertilizers help the soils to support vegetation
which plays a great role in protecting soils against erosion.
viii. Mulching
Mulching is the process of covering the top soil with plant material such as leaves,
grass, crop residues, straws etc. A mulch cover enhances the activity of soil organisms
such as earthworms and reduces the movement of soil. As the mulch material
decomposes, it increases the content of organic matter in the soil.
ix. Strip cropping
It is a method of farming which involves cultivating a field partitioned into long,
narrow strips which are alternated in a crop rotation system. It is used when a slope
is too steep or when there is no alternative method of preventing soil erosion.
x. Dry farming
This is also called Dry land Farming, the cultivation of crops without irrigation
in regions of limited moisture, typically less than 20 inches (50 centimeters) ofprecipitation annually.
Application activity 7.5.
Make a field study in your local area and analyze the appropriate measures taken
by the local community to conserve soil.
1. Discuss the role of community work (Umuganda) in conserving the soils
for sustainable development in your area.2. Study the illustration given below and answer the following questions:
7.6. Economic importance of soil
Soils are important to humans in various ways:
i. Agriculture
Soil has vital nutrients for plant growth. As a result, it is used in agriculture to nourish
plants. The roots of a plant receive nutrients from the soil to help plants grow.
ii. Building
Some soils provide important materials for building purposes. Soil can be used as
building materials such as sand and clay for, bricks, Tiles, block and concrete making.
Soil compaction increases the density of the soil and improves the load support
which is done as part of the building process.
iii. Pottery
Clay soil is used in making ceramics, or pottery. When water is added to clay soil, it
can be used to create the ceramics. Any type of ceramic can be created with the claysoil, such as vases, bowls, cups or other sculptures.
iv. Medicine
Some soil types are commonly used in the production of anti-biotics. Microbes
created in the soil are harmful to bacteria that is why soil is used in medicine.
Medicines created by soil include skin ointments, tuberculosis drugs and anti-tumor
drugs.
v. Mining industry
Some soils contain valuable minerals e.g. gold, diamond, etc. which helps in
industrial development.
vi. Habitat for animals
Some soils act as homes for various organisms like insects, worms, termites androdents and many others.
Application activity7.6
Using the above information on importance of soils, conduct a field work studyaround your school and investigate the impact of soils on the community.
End unit assessment
1. Deforestation in many parts of the world has resulted into severe soil
erosion and its associated effects.
a. Make a field study in your home area and evaluate the effects of soil erosion.
b. Identify other major causes of soil erosion in your area.
2. Most farmers in the northern province of Rwanda use terracing as a
measure of soil conservation.
a. Explain why terracing is mostly used in this area.
b. Describe other soil conservation techniques used in your area.
c. Show how these techniques are helpful to environmental sustainability.
3. Soil is a source of livelihood in the world because it is used in many ways.
Apart from agriculture, how can you use soil for your own survival in yourarea?