Geography

Key Unit Competence:
 By the end of this unit, I should be able to investigate the different constituents and morphological properties of soil.

7.1. Soil constituents

Learning activity 7.1

Using previous knowledge learnt in Geography textbooks and other sources of

geographical information;

a. Define soil.

b. Name the constituents/components of soil.

c. Draw a pie chart showing the approximate composition of soil.

Soil is defined as a superficial covering on the earth’s crust having been derived from both inorganic and organic materials of the Earth’s crust. Soil is not merely a group of mineral particles. It also has a biological system of living organisms and some other components. It is a complex of five components:

– Inorganic materials: A matrix of mineral particles derived from varying degrees of breakdown of the parent-rocks through weathering. These particles vary in shape and size. They include the following:

a. Silica: In this context we can give an example of sand.

b. Silicate: This is predominant in clay and as an example we can talk

about aluminium silicate.

c. Oxides: These are product of metal rust and mineral oxidation, for

example iron oxides.

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.

– Organic matter or humus: It is made up of animal and plant wastes and decomposed animals and plants living in the soil. This forms the humus which is found on the surface of the earth. The humus is black or dark-brown in colour and it is formed from the breakdown of organic matter. Organic matter makes up 5% of the soil. It provides the soil with important benefits which include the following:

– It enhances the soil’s ability to hold and store water;

– It improves the soil structure;

– It reduces eluviation of soluble minerals from the top soil;

– It helps in soil aeration.

– Soil water and moisture: This refers to all the water contained in the soil together with its dissolved solids, liquids and gases. Soil water is held by capillary and absorptive forces both between and at the surface of soil particles. Soil water is a dilute solution of many organic and inorganic compounds, which is the source of plant mineral nutrients. The movement of water and dissolved minerals is called leaching, and the water is called gravity water. Water also moves upward in soil by capillary action. The water containing dissolved minerals is called capillary water. The soil water makes up 25% of the soil and it occupies the pore spaces in the soil. Soil water are useful in the following ways:

– The soil water dissolves various substances for example salts that

are derived from plant or animal remains forming solutions;

– The soil water helps plant to absorb minerals from the soils;

– The soil water washes away highly soluble minerals from the upper to the lower layers (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.

– The soil air: it 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.

– 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 then observe critically the nature of the soil constituents and then describe them.

7.2. Morphological properties, soil profile, soil catena and fertility of the soil

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:

Soil structure: This is the arrangement of the individual soil particles. Soil structure varies in size and shape. On the basis of 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 Ahorizon.

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 cling 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 has accumulated;

Soil texture: This is the proportion of sand, silt and clay within a soil. The soil particles can be grouped according to size. Particles that are larger than sand are grouped as stones. Soil texture is important due to the following reasons:

– Soil texture 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 easiness with which plant roots penetrate the soil.

– It determines the soil’s ability to retain humus from being washed away.

– It influences the aeration of the soil.

Soil colour: It is the property of the soil that is easily identified through seeing. The colour of a soil depends on the parent material making up the soil. The soil colour 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 black or dark brown has high organic matter content. Soils that are reddish brown are well drained whereas grey soils are infertile.

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 while clay is non-porous.

Soil pH (potential for Hydrogen): Soil pH is measured by a pH scale or chart numbered from 1 to 14. Soil pH is an indicator of the acidity or alkalinity of soil. It is also known as soil reaction. Numbers from 1 to 6.9 indicate acidity; number 7 indicates neutral state, while 8 to 14 indicate alkalinity. A soil pH of below 7 shows that the soil is acidic. Acidity of the soil increases from PH 6.9 to 0.A soil pH of 7 shows that the soil is neutral. A PH of above 7 shows that the soil is alkaline. Alkalinity of the soil increases from pH 7 to 14.

Soil consistency: This is the strength with which soil materials are held together or the resistance of soils to deformation and rupture. Soil consistency also refers to the easiness with which individual particles of soil can be rushed. This is done by the fingers or a cultivation tool. Soil consistency depends on the soil moisture content. Soil consistency is determined using wet, moist and dry soil samples.

Thickness (depth): In very dry regions, the soils are usually thin. In sub-humid regions soils are generally thick. Thin soils are not good for agriculture. Thick soils are good for agriculture (above 1 m of depth).

Soil moisture: This is the water in the soil. It is determined by soil texture and structure. Soil water helps in the movement of minerals up and down the layers of soil. Clay soils, for example, keep water for long. They become water-logged. Sandy soils allow water to pass easily. They are well drained.

Soil temperature: This is the degree of warmth or coldness in the soil. Soil temperature affects the germination of seeds and plant growth. It also influences soil moisture, air and availability of plant nutrients.

Soil air: This occupies the pore (or open) spaces that are not filled by water. It is controlled by drainage and soil texture. Poorly drained soils have little air. This is because the pores are filled with water.

Soil nutrients: These are chemical elements found in the soil. They help in plant growth and ensure the soil remains fertile. The three main nutrients are nitrogen (N), phosphorus (P) and potassium (K). Together they make up the trio known as NPK. Other important nutrients are calcium, magnesium and sulphur. Plants also need small quantities of iron, manganese, zinc, copper, boron and molybdenum, known as trace elements because only traces are needed by the plant.

Soil depth: The depth of soil profile from the top to parent material or bedrock or to the layer of obstacles for roots. It differs significantly for different soil types. It is one of basic criterions used in soil classification. Soils can be very shallow (less than 25 cm), shallow (25 cm - 50 cm), moderately deep (50 cm - 90 cm), deep (90cm - 150 cm) and very deep (more than 150 cm).

Soil density: It is expressed in two well accepted concepts as particle density and bulk density. In the metric system, particle density can be expressed in terms of mega grams per cubic meter (Mg/m3).

Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salinization can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean. It can also come about through artificial processes such as irrigation.

7.2.2. Soil profile and catena

a. 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 layers are known as soil horizons. They are marked using letters A, B, C and D.

i. 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 sub-layers:

O1: This is the uppermost layer consisting of freshly fallen dead organic matter such

as leaves, branches, flowers, fruits and dead parts of animals.

O2: This layer lies just below the O1 layer. Here, organic matter is found under

different stages of decomposition.

ii. Horizon A: This layer is also known as the top soil. It refers to the upper layer of soil, nearest the surface (Horizon O). This is where biological activities and humus content are at their maximum. This layer is rich in silica or other resistant minerals. It is the zone more affected by the leaching of soluble material and by the downward movement like minerals. Hence, this layer is known as the zone of ‘eluviation’. This horizon is divided into two sub-layers:

A1: This is dark and rich in organic matter, called ‘humus’. This layer has a mixture of

finely divided organic matter and the mineral elements.

A2: This layer is of light colour, with more sand particles and little organic matter. In

regions of heavy rainfall, the mineral elements are rapidly lost downwards in this

region. This is also known as podzolic or eluvial (E) or zone of leaching.

iii. Horizon B: This is called the sub soil. It is the layer below the top soil. Its colour is determined by the parent rock and presence of organic matter. It has fewer living organisms and is rich in clay deposits.

iv. Horizon C: This is the layer with recently weathered materials. It has low organic matter. It is the zone of deposition.

v. Horizon D: This is the parent rock. It contains the rock which is resistant to weathering.

b. Soil catena: 

Soil catena is a sequence of different soil profiles that occur down a slope. It shows the changes that take place in the soil from the top to the bottom of the slope. A long the slope, different soils develop. These are influenced by climate and angle of slope.

Upland areas have deep soils. This results from heavy weathering due to high rainfall and temperature. In those regions, the rain water reaches deep leading to a high degree of leaching. This makes the soils to become very mature. Thick layer of humus is developed in areas of thick vegetation. Along the slopes, the soils are thin. This is caused by movement of materials by gravitational forces. The soils are dry and less developed due to high runoff water on the slope. The run off is the rain water that moves freely down the slope. The soil profiles here have no top soil.

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. 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 washed by seepage out outflow.

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 soil to support plant growth. Fertile soil is the one that is rich in nutrients that plants use to grow. Examples of these nutrients are nitrogen, phosphorus and potassium.

Factors affecting soil fertility

The following are the major factors affecting soil fertility:

– Mineral matter: A matrix of mineral particles derived from varying degrees

of breakdown of the parent-rocks. The fertility of soil depends on the type of

mother rock to which the rock was derived.

– Organic matter or humus: The 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 more amount of water while the crops like sweet potatoes, cassava

required low quantity of water.

– Soil nutrients: these are chemical elements found in the soil. They help in plant

growth and ensure the soil remains fertile.

Application activity 7.2:

1. Basing on the knowledge and skills acquired from the above lesson,

differentiate soil porosity in Eastern Rwanda from those of Northern

Rwanda.

2. With the help of diagrams differentiate soil profile from soil catena.

3. Identify the factors influencing soil fertility

– Thickness (depth): Thin soils are not good for agriculture. Thick soils are good

for agriculture (above 1 m of depth).

– 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: This refers to the size of soil particles. Clay loam soil are much

fertile than other soil.

End unit assessment

1. Conduct a field work study around your school and collect soil samples

then study those samples to identify their constituents.

2. Describe the soil catena using diagram.

3. Distinguish:

a. Soil structure and soil texture

b. Soil colour and Soil PH

– Soil acidity and alkalinity: The basic soils are much more fertile than acidic soils.