The soil

1. Unit 6: The Soil
Learner Notes

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Unit 6: The Soil
Introduction
One of the first things you have probably learned about plants in previous grades was that they
needed certain things to live and grow like water, sunlight, and soil. Plants use these to live and
grow and each has its own job in the larger system. Man is dependent on soils as they are key to
sustaining life- affecting air and water quality, the growth of plants and crops ant the health of the
entire planet and soils are in turn dependent on man and the use he makes on them (Puri, 2010).
1. What is soil?
Soil can be defined as the organic and inorganic material on the surface of the earth that provides
the medium of plant growth. It is the extremely thin but precious skin covering our planet that
sustains all terrestrial life forms and contributes nutrients and filters contaminants to aquatic
environments.
2. Soil Formation
All soils initially come from rocks, this is termed the ‘parent material’. Geological factors such as
hard rock, lakes, river and glacier deposits, windblown silt or sand are the starting point from
which soil formation begins and they are the parent material for soil. Soils are key part of the
geological cycle, which includes rock formation, land scape evolution and weathering to produce
soil followed by erosion, deposition, and additional soil formation (Singer & Munns, 1992).
2.1. Factors in Soil Formation
The factors influencing soil formation are:
 Parent material
Parent material is rock or material from which soil is formed. The minerals found in the parent
material contribute to the soil’s fertility, the kind and amounts of minerals that will be found in
the soil.
 Climate
Climate is one of the most important factors affecting the formation of soil .Temperature and
precipitation play a major role in soil formation. Water is the main agent in weathering. The
amount of water entering the soil depends on how much falls and runs off the surface. The amount
of precipitation that falls influences the kind of soil formed and the rate at which it is formed at.
Precipitation and temperature also determine the kind and amount of vegetation that grows in an

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area and temperature determines how much precipitation falls as rain or snow. Warmer
temperatures and an abundance of water speed up the formation of soil, in some cases rather
dramatically. Whereas cooler temperatures and less precipitation slow down soil formation (Puri,
2010).
 Biota (Organisms)
The biotic factors include both plants and animals which is organic matter. Plant fix atmospheric
pressure and add it to the soil. Organic matter is added to the soil surface by plant tops and to the
surface of the plant roots. These factors influence the rate of soil formation and the kinds of
products formed (Singer & Munns, 1992).
 Topography
Topography is a characteristic of landscape. Slope angle and length are features of topography that
govern the amount of water that runs off or enters soil. Topography basically changes the
development of soil on a local or regional scale depending on the slope. Soil that develops on
moderate to gentle slope is often better drained than soil found at the bottom of valleys while
steep topographic gradients inhibit the development of soils because of erosion (Singer & Munns,
1992).
 Time
It is difficult to answer how old soil is but all soils are not the same age because the factors of soil
formation act continuously over time (Singer & Munns, 1992).
These factors are combined to produce an endless range of soils. Physical, biological and chemical
processes transform the parent material into soils, reducing the parent material size and convert
the individual minerals into soluble products that can be carried by the soil (Singer & Munns, 1992).
Figure 1: The development of a soil is influenced by five interrelated factors.

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2.2. The Soil Profile
If you look in a soil pit, you will see various layers in the soil. These layers are called soil horizons.
The arrangement of these horizons in a soil is known as a soil profile. Soil scientists, who are also
called pedologists, observe and describe soil profiles and soil horizons to classify and interpret the
soil for various uses.
Figure 2: The Soil Profile
Soil horizons differ in a number of easily seen soil properties such as color, texture, structure, and
thickness. Other properties are less visible.
O HORIZON- also known as humus, this is the utmost top layer of soil that is made up of living and
decomposed materials like leaves, plants, and bugs (organic materials). This layer is thin and
usually dark in color (Puri, 2010).
A HORIZON- This is the layer that we call “topsoil” and it is located just below the O Horizon. This
layer is made up of minerals and decomposed organic matter and is also very dark in color. This is
the layer that many plants roots grow in and where biological activity occurs. Soil organisms such
as earthworms, arthropods, nematodes fundi and many other bacteria are found in this layer (Puri,
2010).
B HORIZON- We call this layer “subsoil” and it is located just below the A Horizon. This layer has
clay and mineral deposits such as iron and aluminum and less organic materials than the layers
above it. Plant roots penetrate through this layer but it has very little humus. This layer appears to
be brownish or reddish in color due to the clay and iron oxides(rust) washed down from the A
horizon (Puri, 2010) .
C HORIZON- This is the layer is called “regolith” and it is located just below the B Horizon. This
layer is made up of lumps or more likely unbroken and un-weathered rock and only a little bit of
organic material is found here. Plant roots are not found in this layer. Parent material may also be
found here (Puri, 2010).

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R HORIZON- This layer is called the “bedrock” or “parent rock” and is located below the C Horizon,
at the base of the soil profile. Unlike other horizons, the R horizon is comprised of large continuous
masses of rock which cannot be excavated by hand (Singer & Munns, 1992).
3. Components of the Soil
Soils are made up of four basic components: air, water, minerals and organic material. In most
soils, minerals represent 45% of the total volume, water and air about 25% each and 5% of organic
material. The minerals portion consists of three distinct particle sizes that can be classified as
sand, silt and clay.
Figure 3: A pie chart representing the Components of the Soil
4. Types of Soil
4.1. Sand
This type of soil has the biggest particles and the size of these particles does determine the degree
of exposure to air and drainage that the soil allows. Sand is granular and consists of rock and
mineral particles that are very small. Sandy soil is formed by the breakdown and weathering of
rocks such as limestone, granite, quartz and shale. It is easier to cultivate if it is rich in organic
material but then it allows drainage more than is needed, thus resulting in over-drainage and
dehydration of the plants in summer. Sandy soil retains moisture and nutrients.
The sand soil properties
 The fertility of the sand soil is low.
 It is well ventilated soil that has low absorption of the water.
 The size of its particles is large.
 The color of the sand soil is yellow.
 It is loosely packed and has the fastest and the greatest drainage of the water.

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4.2. Silt
Silt soil is made up of fine particles. Like clay the soil holds water but doesn’t have good
ventilation around the roots. Some people think that adding sand to clay soil will open it up but
adding sand to clay creates a low grade concrete and most plants don't do well with this hardened
soil.
The silt soil properties
 The color of the silt soil is grey.
 The size of its particles is medium (between the sand soil particles and the clay soil
particles).
 It is moderately ventilated soil that has medium absorption of the water.
 It is moderately compacted and has the medium drainage of the water
 It is also highly fertile.
4.3. Clay
Clay soil particles are very small and compact. Gardens with these types of soil particles don’t
work well because the air has a hard time getting to the roots. The soil absorbs and holds water
and creates a drainage problem. This badly affects healthy root and plant growth. Clay is very fine,
small inorganic particles, worn down from rock.
The clay soil properties
 The color of the clay soil is dark (black).
 The size of its particles is small.
 It is fertile.
 It has highly compacted (hard).
 It is poorly ventilated soil that has high absorption of the water.
 It has the lowest drainage of the water.
4.4. Loam
Loam soil is a mixture of sand, clay and silt. This type of soil normally has very fine particles
making it one of the most ventilated. It also contains humus which is ideal for supporting plant
growth. Loam soil contains plant nutrients and allows animals like earthworm to thrive in it. This is
the most commonly used soil for agriculture throughout the world.
The loam soil properties
 It has well drainage and good ventilation levels.
 Loam soils have the ability to maintain nutrient levels.

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5. Soil Properties
5.1. Soil Texture
Textures range from clay, sand, and silt at the extremes, to a loam which consists of the other
types of soil. The main influence of texture is on the permeability which generally decreases with
decreasing particle size.
5.2. Soil Air
According to (Buckman & Brady, 1960), soil air is found in those spaces between the soil particles
that are not filled with soil water. The amount of air in a soil depends on how firmly the soil is
compacted. It differs from that of the atmosphere as it is not continuous because it is located in a
maze of soil pores separated by soil solids. Soil air also generally has a higher moisture content
than the atmosphere with a relative humidity approaching 100% when soil moisture is optimum.
The content of carbon dioxide found in the soil air is higher and that of oxygen is lower than that
found on the atmosphere. In well-aerated soil at least 20% of its volume is made up of air.
5.3. Soil Water
Soil water can be classified into three types, namely hygroscopic, capillary and gravitational water.
Hygroscopic water occurs as a thin film of water around each soil particle. Capillary water is that
water held in the small spaces between the soil particles and gravitational water is the water
which drains downwards through the soil. There are two major factors concerning soil water that
are important to obtain an idea of the significance of water as a component in water. These are:
 Water is held within soil pores with different degrees of tenacity depending on the amount
of water present
 Together with the soil’s dissolved Salts, soil water makes up a solution (soil solution) which
is important as a medium for supplying nutrients to growing plants (Buckman & Brady, 1960).
5.4. Soil Temperature
Chemical and biological activities in the soil are influenced by temperature for example
nitrification does not begin until soil temperatures reach about 4°C, the most favorable limits
being 8°C to 9°C. Soil temperatures for seed germination may vary thus making soil temperature
an important ecological factor. At low temperature there is little decay by decay causing micro-
organisms (Buckman & Brady, 1960).
5.5. Soil Acidity vs. Alkalinity
Acidity or alkalinity of soil (the pH of the soil) influences the biological activity in soil and the
availability of certain minerals. Thus the pH of soil has a greater influence on the growth and
development of plants. Soil p.H is an important consideration for several reasons like: many plants
and life forms prefer either alkaline or acidic conditions, some diseases thrive when soil is acidic or
alkaline and the p.H level can affect the availability of nutrients in the soil (Puri, 2010).

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Figure 4: An image showing the different p.H levels in the soil when planting different vegetables
6. Human Impact on the Soil
6.1. Erosion
Erosion removes mineral particles, organic material and nutrients form the soil reducing its
thickness and water holding capacity. Eroded soil then becomes a pollutant in streams and
reservoirs. The time required to form new soil is so long that from a person’s point of view soillost
during erosion is lost forever. Soil erosion is a natural geologic process however, humans can
accelerate the process by removing cover. Accelerated erosion occurs at 10-1000 times the natural
rate. Erosion can happen in all of the biomes on earth, and can be caused by removing trees or
grasses. This can decrease local water quality and contribute to poor health in populations in the
area. Removing the soils generally leads to other types of degradation and reduced food production
(Singer & Munns, 1992).
Human Causes (Seshoka, Keats, Earle, Dilley, & Rampou, 2002)
 Monoculture
 Ploughing on marginal land
 Lack of crop rotation
 Planting crops against the gradient of slopes
 Overgrazing
 Use of chemicals e.g. pesticides
 Deforestation
Animal Causes (Seshoka, Keats, Earle, Dilley, & Rampou, 2002)
 Mass migrations e.g. wildebeest
 Imbalance and /or overpopulation of one type of species

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Physical Causes
 Erosion by water
 Erosion by wind
Effects of Soil Erosion on People
 Reduction in the availability of farmland
 Drop in food production
 Rise in food prices
 Increase in unemployment
 Increase in poverty
 People migrate to urban areas – rural depopulation
Effects of Soil Erosion on the Environment
 Loss of vegetation
 Increase in run-off
 Silting of dams
 Desertification
Management Strategies to Prevent and Control Soil Erosion (Seshoka, Keats, Earle, Dilley, &
Rampou, 2002)
Political and Economic Changes
 Land distribution and land reform
 Education and training
 More scientific approaches to farming Technical Changes
 Contour ploughing
 Strip farming methods
 Avoiding overgrazing and over cropping
 Biological diversity
 Conserving Wetlands
Figure 5: An example of a piece of land that has undergone soil erosion

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6.2. Desertification
Desertification is the extreme degradation of productive land in arid and semi-arid areas. This is
most common in the tropical savannah and grasslands. This can create poor quality vegetation, and
the spreading of deserts to areas that weren't deserts before (Singer & Munns, 1992).
Figure 6: An image showing desertification
6.3. Acidification
Acidification occurs when the basic cations (like Calcium and Magnesium) leach from the soil,
leaving the acidic cations in the soil (Hydrogen, Aluminum, iron and manganese). The pH decreases
and soil becomes more acidic. This is a natural process in weathering. However, the use of certain
fertilizers to provide food, like anhydrous ammonia, causes soil to become more acidic much
faster. This can occur in all biomes (Singer & Munns, 1992).
Figure 7: An image showing soils contaminated by Iron and Sulphur

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6.4. Deforestation
Deforestation is the permanent destruction of forests in order to make the land available for other
uses like housing and urbanization, to harvest timber to create commercial items such as paper,
furniture and homes, to create ingredients that are highly prized consumer items, such as the oil
from palm trees, to create room for cattle farming. Deforestation is considered one of the most
common causes of global climate change as it has an impact on the global carbon cycle. Gas
molecules that absorb thermal infrared radiation are called greenhouse gases. If greenhouse gases
are in large quantity, they can force climate change, according to studies done. While oxygen (O2)
is the second most abundant gas in our atmosphere, it does not absorb thermal infrared radiation,
as greenhouse gases do. Carbon dioxide (CO2) is the most prevalent greenhouse gas. The
deforestation of trees not only lessens the amount of carbon stored, it also releases carbon dioxide
into the air. This is because when trees die, they release the stored carbon. (Seshoka, Keats, Earle,
Dilley, & Rampou, 2002)
Common methods of deforestation are burning trees and clear cutting. It is a very severe problem.
Figure 8: An image showing Deforestation
6.5. Salinization
This build-up of salt on the soil surface is called salinization. This is a very big problem in the
desert and savannah biomes. In arid or semi-arid regions, too much or too little irrigation water can
lead to an increase of soluble salts, reducing plant growth. Salts such as chlorides, sulfates and
bicarbonates of Na, Ca, and Mg accumulate as water evaporates form the soil. This can cause
physical soil damage, and the ability to grow plants thus only the most salt tolerant species can be
grown (Singer & Munns, 1992).

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Figure 9: An image showing salt surface accumulation

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References
Buckman, H. O., & Brady, N. C. (1960). The Nature and Properties of Soil (Sixth ed.). New York:
The Macmillan Company.
Puri, S. (2010). Soil Chemistry. New Delhi: Mehra Offset Press.
Seshoka, A., Keats, G., Earle, J., Dilley, L., & Rampou, S. (2002). Focus on Geography Grade 11.
Cape Town: Maskew Miller Longman.
Singer, M. J., & Munns, D. N. (1992). Soils, An Introduction (Second ed.). (P. Corey, Ed.) New York:
Macmillan Publishing Company.