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UNIT 7 :SOILS
Key Unit Competence: The student-teachers should be able to investigate
the different constituents and morphologicalproperties of the soil.
Introductory activity
i. Identify the major components of the soil.
ii. Using the experiences gathered from previous classes on soils, explain the
contents of each components of the soil.
iii. Describe the morphological properties of soil.iv. Explain what makes soil to be more productive.
7.1. Soil constituents
Activity 7.1
Using previous knowledge learnt in Geography textbooks and other
sources of geographical information;
a) Define soil.b) Differentiate the constituents/components of soil.
7.1.1. Definition of the soil
Soil is a dynamic natural material composed of fine particles in which plants
grow. It is defined as a natural body consisting of layers (soil horizons) that
are composed of mineral materials, organic material, air and water. Mineral
materials originate from weathered rocks whereas organic materials are
products of decayed plants and animals.
7.1.2. Soil constituents
The figure below shows the five major constituents of the soil with theirproportions, as described in the following paragraphs:
Soil is not merely a group of mineral particles. It also has a biological system of
living organisms and some other components.
A brief description of each of the five constituents of the soil is given in the
following paragraphs:
i) Inorganic materials, also called mineral matter is matrix of mineral
particles derived from varying degrees of breakdown of the parent-rocks
through weathering. These particles vary in shape and size. Sand, silt
and clay are the three major contents of mineral matter. They include the
following. Oxides which are products of metal rust and mineral oxidation
(e.g. iron oxides), are also important constituents of the soil.
Inorganic matter provides important plant nutrients; determine soil
aeration, soil texture and drainage. It also gives support to the plants.
Inorganic matter makes up 45% of the soil.
ii) Organic matter is made up of decayed plants and decomposed animal
products. Some living animals and plants are also important part of the soil.
The decayed remains of plant and animal materials, partially transformed
by bacterial action, are collectively called humus, which provides the soil
with brown or dark color. Organic matter makes up 5% of the soil.
Organic matter has among other the following benefits:
-Humus helps in soil aeration, enhances the soil’s ability to hold and
store water and improves the soil structure- It helps plants to extract soil nutrients.
─ Humus reduces eluviation of soluble minerals from the top soil and
supplies nutrients and minerals to the soil.
─ Soils that contain humus are quite workable and have a good
capacity to retain water. Humus is also an important source of food
for microscopic soil organisms.
iii) Soil water and moisture refers to all the water contained in the soil
together with its dissolved solids, liquids and gases. Soil water is a dilute
solution of many organic and inorganic compounds, which is the source
of plant nutrients. The water in the soil moves by gravity (downward)
or by capillarity (upward) movement. Gravitational water is the water
that passes through the soil under the influence of gravitation, whereas
capillary water is soil water that clings to soil particles as a result of
surface tension. Capillary water moves in all directions through the soil
from areas of surplus water to areas of water deficit. The movement of
water and dissolved minerals is called leaching, and the water is called
gravity water. The soil water makes up 25% of the soil and it occupies the
pore spaces in the soil.
Soil water has among others the following benefit:
• It dissolves various minerals and organic substances that derive from
plant or animal remains forming solutions;
• The soil water helps plant to absorb minerals from the soils;
• The soil water removes highly soluble minerals from the upper layers
to the lower layers, and this process is known as leaching;
• The soil water brings soluble minerals from the lower to the upper
horizons of soil through capillarity;
• The soil water is very important in the sense that it provides a medium
within which most of the chemical processes of the soil formation take
place;
• The soil water provides a medium through which living organisms and
soil bacteria operate during the decomposition of organic matter.
iv) The soil air occupies the pore space between soil particles which is
not filled with water. The soil is normally lower in oxygen and higher in
carbon dioxide content. The soil air includes gases from biological activity
and chemical reactions. The air or gases make up 25% of the soil. The air
in the soil is important in the following ways:
• It facilitates plant growth by supplying oxygen to the root hairs;
• It supports micro-organisms which are found in the soil;
• It helps in the process of weathering known as oxidation which is
responsible for breaking down rocks to form soils.
v) Biological system or living organisms and bacteria: The living
organisms and bacteria help to decompose the organic matter into humus.This is sometimes classified together with organic matter/humus.
Application Activity 7.1
Carry out a tour around your school. Observe critically the nature of thesoil constituents and make a their description
7.2. Morphological properties, profile and catena and fertility of the soil
Activity 7.2
Observe the illustration provided below and answer the questions that follow:
1. From your observation of the above illustrations, which picture
represents: (i) Soil structure, (ii) Soil texture (iii) Soil colour (iv) Soil
profile (v) Soil catena (vi) Soil porosity.
2. Differentiate soil structure from soil texture.
Soil properties refer to the chemical and physical characteristics of the soil and
these include mainly: structure, texture, colour, porosity, pH and consistency.
7.2.1. Soil properties
The main properties of the soil include the following:
i. Soil structure
The soil structure is the arrangement of the individual soil particles. Soil
structure varies in size and shape. Based on the shape, the following types of
soil structure exist: granular, prismatic, platy, columnar and blocky.
• Granular and crumb: They are individual particles of sand, silt
and clay grouped together in small, nearly spherical grains. Water
circulates very easily through such soils. They are commonly found in
the A-horizon of the soil profile.
• Platy: It is made up of soil particles aggregated in thin plates or sheets
piled horizontally on one another. It is commonly found in forest soils,
in part of the A- horizon.
• Prismatic: The soil particles are formed into vertical prism-like
particles. Water circulates with greater difficulty and drainage is poor.
They are commonly found in the B-horizon where clay has accumulated.
• Columnar: It is similar to prismatic, but the particles are rounded at
the top.
• Blocky: These are soil particles that bound together in irregular
square or angular blocks having more or less sharp edges. Relatively
large blocks indicate that the soil resists penetration and movement
of water. They are commonly found in the B-horizon where clay hasaccumulated.
Structure is important because it controls drainage, rooting of plants, and how
well the soil delivers nutrients to plants. The shape of individual peds, shown
here, controls a soil’s structure.
ii. Soil texture
Soil texture refers to the mixture of sizes of its particles and the proportion of
different sizes, namely sand, silt and clay within a soil. Particles that are larger
than sand are grouped as stones.
Soil texture is important due to the following reasons:
• It influences the amount of air and water available within the soil. Silt
and clay soils have a high water holding capacity. Sandy soils have a
low water holding capacity.
• It also influences the amount of water and the ability of the roots to
pass through it.
• It determines the plant roots penetration and growth in the soil.
• It determines the soil’s ability to retain humus from being washed away.• It influences the aeration of the soil.
Measures of the ratio of clay, silt, and sand determine soil texture. As an
example, points 1 (horizon A), 2 (horizon B), and 3 (horizon C) (see figure
above) designate samples taken at three different horizons in the Miami silt
loam in Indiana. Note the ratio of sand to silt to clay shown in the three pie
diagrams and table.
iii. Soil colour
Soil color is important, because it sometimes suggests its composition and
chemical makeup. It is influenced by the parent rock, organic matter, moisture
content and minerals. Soils have a variety of colours. They include red, yellow,
black, grey, white and brown. A soil that is dark brown has high organic matter
content. Soils that are reddish brown are well drained whereas grey soils are
infertile.
iv. Soil porosity/permeability
This refers to the number of pore spaces in the soil. The pore spaces relate to
the portion of the soil space occupied by air and water. This is determined by
the arrangement of the soil particles.
Soils vary in porosity. Soils with large pores, for example sand are porous whileclay is non–porous.
v. Soil pH (potential for Hydrogen)
Soil pH is an indicator of the acidity or alkalinity of soil. It is also known as soil
reaction. The pH scale measures acidity (lower pH) and alkalinity (higher pH).
The complete pH scale ranges between 0 and 14. Numbers from 1 to 6.9 indicateacidity; number 7 indicates neutral state, while 8 to 14 indicate alkalinity.
The pH scale measures acidity (lower pH) and alkalinity (higher pH). The
complete pH scale ranges between 0 and 14.
vi. Soil consistency
Soil consistency is the strength with which soil materials are held together or
the resistance of soils to deformation and rupture. It also refers to the ability
with which individual soil particles of soil can be eroded. The consistency of
the soil can be tested on the field using cultivation tool or fingers, or throughlaboratory tests.
7.2.2. Soil profile and catena
i. Soil profile
Soil profile refers to the vertical arrangement of the soil in layers from the
ground/surface to the parent rock/bedrock or mother rock. It can be as little
as 10 cm thick in some places or as deep as several meters in others. The layersare known as soil horizons. They are marked using letters A, B, C and D.
i) Surface horizon O: This layer is also known as the superficial layer. This
layer includes organic litter, such as fallen leaves and twigs. These are
fresh or partially decomposed organic matter. This zone has two sublayers:
OL: This is the uppermost layer consisting of freshly fallen dead
organic matter such as leaves, branches, flowers, fruits and dead
parts of animals.
OF : Called fermentation layer, it lies just below the OL layer. Here,
organic matter is found under different stages of decomposition.OH : Lies beneath OF layer and is made mostly by humus
ii) Horizon A: This layer is also known as the top soil horizon. It refers to the
upper layer of soil, nearest the surface (Horizon O). In this layer biological
activities and humus content are at their maximum. Called eluvial or
outwash horizon, the zone is the the most affected by the leaching of
soluble mineral and removal of finest solid material downwards. This
horizon is divided into two sub-layers:
AA : Is dark and rich in organic matter ‘humus’. It is characterized
by abundant proportion of organic matter compared to other subhorizons,
mixed with mineral matter.
AE : Layer is of light colour, with more sand particles and little organic
matter. In regions of heavy rainfall, the mineral elements are rapidly
washed downwards in this region. This is also known as podzolic or
eluvial (E); referred to as zone of leaching.
iii) Horizon B, also called the sub-soil horizon, or zone of illuviation is the
layer below the top soil, characterized by clay and oxides iron deposits,
which is why it is also called zone of inwashing, or illuvial horizon.
iv) Horizon C: This is the mineral layer made of recently weathered parent
materials, with little organic matter content.
v) Horizon D is the half-weathered or unaltered parent rock.
ii. Soil catena
A soil catena is a series of distinct but co-evolving soils arrayed down a slope. It
shows the changes that take place in the soil from the top to the bottom of the
slope, but the term is also used to describe the lateral variation in soils over a
hillslope.
The development of soil catena is influenced by climate and angle of slope.
Along the slopes, the soil horizons are thin as the slope angle increases, butthey become thicker in gentle sloping topographies.
A soil catena consists of three main divisions called complexes namely: Elluvial
complex, the colluvial complex and the illuvial complex.
• The elluvial complex: This is the upper convex slope where weathered
materials are washed out downwards. It includes the summit and free
face of the hill. Erosion predominates the free face hence; the soils are
skeletal and with shallow soil profile.
• The colluvial complex: This is the lower concave slope where there
is gradual deposition of eroded material at footslope areas or in their
nearby. The soil of the colluvial complex is thus moderately drained
and retains certain moisture hence, supports agriculture and grazing.
• The illuvial complex: This occupies the valley bottoms where fine
materials are inwashed through seepage.
Soil catena is important in the following ways:
• The soil catena (elluvial complex) is used for settlement;
• The soil catena is used for rock quarrying for construction especially
on free face which has no vegetation;
• The valley bottoms are used for brick lying (e.g. Ruliba Clay Works
along River Nyabarongo in Rwanda);
• The illuvial complex is used for agriculture due to the deep soil profile
derived from deposition;
• The valley bottoms are used for rice growing and growth of vegetables
and yams, etc.
7.2.3. Soil fertility
Soil fertility is the ability of the soil to support plant growth. Fertile soil is the
soil that is rich in nutrients needed for the growth of plants: the most important
nutrients include: Nitrogen, Phosphorus and Potassium and Magnesium.
Factors affecting soil fertility
The following are the major factors affecting soil fertility:
• Mineral matter: A matrix of mineral particles derived from varying
levels of breakdown of the parent-rocks.
• Organic matter or humus: a fertility of soil depends on the amount
of organic matter or humus available in soil. The higher amount of
organic matter reflects the higher level of soil fertility.
• Soil water or soil solution: The required optimum amount of water
depends on the type of crops to be grown in specific area. Some crops
such as rice require much water while the crops like sweet potatoes,
cassava required low quantity of water.
• Soil nutrients are chemical elements found in the soil. They help in
plant growth and ensure the soil remains fertile.
• Thickness (depth): Thick soils (above 1 m of depth) are good for
agriculture whereas thin soils are not good, because they don’t allow
the development of root system.
• Soil permeability: This is the ability of the soil to allow water to pass
through it. Permeable soils are much more fertile than non-permeable soils.
• Soil texture refers to the size of soil particles. Clay loam soil are much
fertile than other soil.
• Soil acidity and alkalinity: The basic soils are much more fertile thanacidic soils.
Application Activity 7.2
1. Basing on the knowledge and skills acquired from the above lesson,
differentiate soil porosity in Eastern Rwanda from soil porosity of
volcanic soils in the Northern Rwanda.
2. With the help of diagrams differentiate soil profile from soil catena.3. Identify the factors influencing soil fertility.
Skills lab
Make a field study on your local area, and select the best type of soil suitable for
increasing production in agriculture.
End unit Assessment
1. Conduct a field trip around your school, collect soil samples and
study them to identify their constituents.
2. Explain the importance of soil catena
3. Establish the difference between:The soil structure and the soiltexture, the soil colour and the soil pH