Topic outline

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  • UNIT 1:Introduction to Agriculture

    TOPIC AREA 1: SOIL SCIENCE

    Unit 1: Introduction to Agriculture

    Introduction

    The foods we eat every day are all deliberately produced at a given place. Examples include vegetables, meat, cereals, fruits and milk, among others. Some of them are produced in our country. Some are imported from other countries. You shall understand why foods are produced and why it is important to study Agriculture as a subject in this unit.

    Key unit competency

    After studying this unit, I should be able to:

    a) Define the term Agriculture.

    b) Explain correctly the importance of Agriculture.

    c) Explain the branches of Agriculture and their relevance to human life.

    d) Identify the different farming systems.

    Unit Outline

    1.1 Definition of Agriculture

    1.2 Socio-economic importance of Agriculture

    1.3 Branches of Agriculture

    1.4 Farming systems

    1.1 Definition of Agriculture

    Activity 1.1: Research Activity

    Using textbooks and the internet, find out the meaning of Agriculture. Interact with your classmates to know what they have found out. Prepare a report and present to the rest of your class.

    I have found out that…

    Agriculture is the Art and Science of cultivating land, growing crops and rearing livestock.

    The facts:

    Agriculture is regarded as an Art since it involves the application of human skills in processes such as milking, construction of farm structures, measuring land size and operating various farm machineries. As a Science, Agriculture requires application of various intellectual and practical skills. Examples are observation, experimentation and analysis. Below is a chart showing the various agricultural activities in categories of art and science.

    Self-evaluation Test 1.1

    1. Explain how Agriculture is being practised in your local environment.

    2. Do you think those Agricultural practices are important? Why?

    1.2 Socio-economic importance of Agriculture Generally, Rwanda’s economy relies heavily on Agriculture. About a third of Rwanda’s Gross Domestic Product (GDP) is accounted for by Agriculture. In Rwanda, the Agricultural sector meets about 90% of the national food needs and it generates more than 70% of the country’s export revenues. This subunit, seeks to appreciate the socio-economic importance of Agriculture to our households and country at large.

    Activity 1.2

    1. Go for a field trip to a nearby farm during the harvesting period. Ask as many questions as you can to the workers with the aim of understanding the various benefits of Agriculture. Note down the important points and prepare a report after the visit. 2. Try to think of other benefits that can come from practising Agriculture. 3. Do a class presentation to the rest of the class on your findings.

    I have discovered that…

    The socio-economic benefits of Agriculture can be presented in terms of:  

    • Food supply  

    • Source of employment  

    • Source of raw materials for industries 

     • Source of capital 

     • A recreational activity 

     • Foreign exchange earner  

    • Source of medicinal products

    The fact

    a) Food supply

    Agriculture provides food needed to give us energy to engage in productive activities. The food comes from crops such as cereals, vegetables and fruits, and from livestock products, such as eggs, meat and milk. Proper feeding promotes good health. A healthy nation leads to enhanced productivity in all sectors. This ensures that development takes place.

    Activity 1.3

    Visit a nearby agricultural market and list some of the most common agricultural produce being sold. Create a table and record the number of stalls in which certain foods are sold. Such foods can be vegetables, fruits or cereals (specify their names). From the results you will have obtained, come up with a bar graph.

    Discussion corner!

    From the graph in Activity 1.3 above, answer the following questions:

    a) Which food is the most common in the market?

    b) Which food is the least popular in the market?

    c) Find out possible reasons for the answers in (a) and (b) above.

    d) Give appropriate recommendations that can help to reduce the disparity.

    b) Source of employment

    As we have seen earlier, Agriculture accounts for a large part of Rwanda’s Gross Domestic Product (GDP). This means that many Rwandans, especially those living in rural areas, are engaging, either directly or indirectly, in the agricultural sector. Direct involvement in the agricultural sector is when one actually works on the farms. This can be as a farm manager or any kind of casual worker in the farm. Indirect involvement can be when one works in a crop processing company or any other industry that uses agricultural produce. Those working in industries that manufacture farm inputs and other agrochemicals are also under the indirect employment category. From these, we draw that indeed Agriculture is a crucial industry. We should therefore be keen on learning more about agriculture; not just to be employed, but to be the ones creating employment opportunities for others.

    Discussion corner!

    Think of other agricultural activities you can come up with to expand the scope of farming carried out in your locality. Note them down and present them in class.

    c) Source of raw materials for industries

    The various industries present in our country play a major role in developing our economy. In most industries, raw materials are usually processed into more useful commodities. Since Rwanda is an agricultural country, most industries in the country are agriculture-based. Examples of such industries are: • Leather tanning factory in Gikondo.

    • Inyange industries at Masaka that processes milk and fruits.

    • Food and fruit processing industry at Nyirangarama in Rulindo.

    • Tea factories such as Pfunda in Rubavu, Mulindi in Gicumbi and Rubaya in Nyabihu.


    • Breweries such as Bralirwa in Rubavu and Skol at Kigali.

    • Sugar factories such as Kabuye in Gasabo.

    • Rice processing units in Bugarama.
    As in any other industry, it is also important for the participants of the agricultural

    d) Source of capital


    There are two forms of farming; subsistence farming and farming for commercial purposes. Subsistence farming refers to the kind of farming whereby the farmer only focuses on producing food for his/her household. Farming for commercial purposes on the other hand is mainly for business. The farmer sells produce from his/ her farm in order to gain profits. From what they will have earned, the farmers will now be able to cater for their household needs and even have some extra cash to invest in other entrepreneurial ventures. The government also gets revenue through the taxes levied on farmers. Such income enables the government to finance its recurrent expenditure and the various national development projects; such as provision of education facilities, health facilities, road construction among others.

    e) Source of medicinal products

    Various agricultural products, from both animals and plants, have been used since time immemorial to treat various ailments and diseases. Despite the fact that today there is reduced use of herbs, some of these products are still being used as raw materials in the processing of the currently popular tablets and syrups.

    Health check! You should only take medicine that has been prescribed by a qualified doctor.

     f) Foreign exchange earner

    Activity 1.4: Research activity

    Find out the meaning of the term foreign exchange. You can use textbooks in the library or the internet. Note down the meaning and discuss with your friends in the classroom.

    I have discovered that…

    The money that the government earns from exporting produce to other countries is what we call foreign exchange.

    The facts

    Agriculture is a major source of foreign exchange for Rwanda. It accounts for about 70% of the country’s total export revenues. Some of the most valuable export crops in Rwanda include tea, coffee, fruits, vegetables, flowers (especially roses) and pyrethrum.

    g) A recreational activity

    Discussion corner!

    Read this story

    Uwase had a big compound in her home. She wondered what she was going to do to make her home more beautiful and welcoming. One day, she went to visit her friend Keza. Keza’s home had beautiful flower gardens at almost every corner of the compound. It looked amazing. Uwase was impressed. Study questions

    1. What would you advice Uwase to do in her big compound?

    2. What benefit is associated with Keza’s home?

    Apart from just providing food and all the other listed uses, agriculture can also be carried out solely for recreational reasons. An example would be when a farmer sets aside a small piece of land to plant flowers for beautification purposes. Flower gardens normally improve the general appearance of any given place. This can be done in private compounds, public places such as roads, schools, church compounds and hospitals. Also, fish ponds can be established to carry out fishing as a recreational activity.

    Activity 1.5

    With the help of your teacher, find places in your school compound that you can establish flower gardens. Look for appropriate plant seedlings and carefully plant them. You should be able to take care of those plant seedlings until when they are fully grown plants. 

    Self-evaluation Test 1.2

    Study the table given below about a coffee farmer called Habimana then answer the questions that follow. Habimana incurred the following expenses in his coffee farm in the year 2015.

    1.3 Branches of Agriculture

    Activity 1.6: Research activity

    Using reference books or the internet, find out the branches of Agriculture. Prepare a report and present it in class.

    I have discovered that…

    The branches of Agriculture are: 

    • Soil science

     • Crop husbandry 

    • Animal husbandry  

    • Agricultural economics

    • Agricultural engineering

     • Horticulture  


    The facts

    a) Soil science

    This is the study of soil as a natural resource that occurs on the surface of the earth. The specific aspects of soil studied include soil formation, classification and mapping, physical, chemical, biological and fertility properties of soil among others. These properties are studied in relation to the use and management of soils.

    b) Crop husbandry

    Crop husbandry refers to all agricultural activities done to crops from the time crops are planted to the harvesting time. It also includes agricultural produce processing and storage. All these practices aim at ensuring that a crop is provided with the best conditions for optimum growth in the field. This ensures optimum returns in terms of quantity and quality of produce.

    c) Animal husbandry

    This refers to management and care of farm animals for a profit. It can also be defined as the practice of selectively breeding and raising livestock to promote desirable traits in them for sale, sports, pleasure or research. In animal husbandry, genetic qualities and behaviors considered to be advantageous are further developed.

    d) Agricultural economics

    Discussion corner!

    Read this story

    Gatete grows plantains. His farm has grown bigger as compared to the one he had before. The produce is now becoming overwhelming. He needs better structures to help him get the best out of his farm. What should Gatete do?


    Agricultural economics is an applied field of economics. It is concerned with the application of economic theories in improving the production and distribution of food and other agriculture-related materials. Agricultural production involves the use of limited resources such as land, capital and labour. These resources are also known as factors of production. Each of these must be properly allocated in order to minimise costs while at the same time maximise on revenue. This will in turn result in high profits which is the sole objective of agricultural economics.

    Activity 1.7

    Visit to an agricultural farm

    Visit a nearby farm and find out practices that farmers carry out to ensure that the limited resources used in Agriculture are well-utilised. Note them down and present them in class.

    What I discovered...

    Some practices that help to ensure that Agricultural economics is achieved include:

    i. Application of principles of economics

    ii. Proper accounting iii. Proper record keeping

    iv. Good marketing strategies



    e) Agricultural engineering

    In this branch of Agriculture, engineering, science and technology are applied. Agricultural engineering mainly involves knowledge and usage of farm machines and equipment. It usually deals with the development of new systems and practices that aim at addressing problems of inefficiency facing the agricultural sector.

    f) Horticulture

    This is the branch of Agriculture that deals with the growing of highly perishable crops. Such crops require high level farm management skills, from planting time, to the time of harvest and also how the crop will be marketed.

    Activity 1.8

    1. Visit a horticultural farm and find out the types of crops grown there.

    2. Find out why the farmer chose to grow the crops. Are there any advantages of the crop over others?

    What I have discovered...

    Horticulture is divided into three main categories. These are:

    • Floriculture – Growing of flowers.

    • Olericulture – Growing of vegetables.

    • Pomoculture or pomology – Growing of fruits.

    Activity 1.9: Research Activity

    Find out about some of the problems that farmers face. Find out possible solutions and present them in class.

    The facts

    Some of the problems that face the agricultural sector and their solutions include:

    • Inadequate capital for farmers – Cooperative Societies have been established to help farmers obtain capital.

    • Unpredictable weather patterns – Farming methods that are independent of weather patterns have been established through use of irrigation systems and the green houses among others.


    • Crop pests and diseases – Pesticides and acaricides are used to deal with these respectively.

    • Animal parasites and diseases – Livestock can be sprayed using various chemicals to prevent them from succumbing to attacks by various external parasites and diseases. External parasites can be dealt with through use of dewormers and other drugs.

    • Inadequate knowledge and lack of proper farming skills – Farmers can be trained about the various farming techniques that will ensure that they have maximum output.

    • Inadequate awareness of proper farm inputs – Farmers can be encouraged to use certified seeds, effective farming machines and to keep records of all these to ensure accountability.

    Self-evaluation Test 1.3

    1. Among all the named branches of Agriculture, which ones do you think can be applied in your school farm and why?

    2. Why is floriculture popular in highlands?

    3. Why is studying soil important to farmers?

    4. Which three fields of study are combined with Agriculture in Agricultural Engineering?

    1.4 Farming systems

    Farming system refers to the way farm enterprises (resources) are organised and utilised. Different farm activities and enterprises are organised in various ways depending on the resources available to the farmer. These resources include land, capital, labour and human resource

    Activity 1.10
    Field trip

    1. Go for a field visit to a nearby farm and find out the various types of farming systems used.

    2. Compare and contrast the various farming systems you have learnt about from your trip.

    3. Come up with a report and do a presentation in the class.

    I have discovered that…

    Farming systems help farmers to organise and utilise their farm resources appropriately. The various types of farming systems are:

    • Monocropping 

    • Intercropping 
    • Pastoralism 

    • Stall-feeding

    • Intensive farming

    • Extensive farming 

    • Large scale farming 

    • Small scale farming

    The facts

    a) Monocropping

    This is the type of farming system whereby a single crop is grown on a large area. The farmer only grows one type of crop on the same piece of land throughout the farming season. In Rwanda, tea, coffee, potatoes, sugarcane and pyrethrum are the main crops planted using the monocropping system.

    Advantages of monocropping

    (i) Operations like weeding, disease and pest control and harvesting are easily carried out.

    (ii) It is easy to mechanise field operations.

    (iii)There is optimum utilisation of applied fetiliser and manure as a result of correct plant population establishment.

    Disadvantages of monocropping

    (i) Continuous growing of one crop may lead to depletion of a particular nutrient resulting in low crop yields. (ii) It encourages build-up of pests and diseases.

    (iii)It is difficult to control parasitic weeds on the crop, for example, the Striga spp in maize crops.

    (iv) In cases of crop failure, heavy losses are incurred.

    (v) There may be little profit realisation in cases of reduction in prices in the market.

    (vi) Lack of soil cover encourages erosion especially when crops that grow upwards are planted continuously.

    b) Intercropping

    This involves the growing of two or three crops in association. All the crops are planted on the same piece of land. Examples of combinations of crops that can be grown using this system are maize, beans and finger millets. Intercropping can also be referred to as interplanting.

    Activity 1.11: Research Activity

    Research on other crops that can be interplanted and list them down. What are the benefits of these?

    Advantages of intercropping

    (i) There are high crop yields per unit area.

    (ii) It ensures ample soil cover especially when cereals are interplanted with legumes. This minimises soil erosion.

    (iii)There is no total loss in cases of disease and pest outbreaks. It is hence an insurance against total loss.

    (iv) There is supplementation of nutrients in the soil especially when legumes are included.

    (v) Some plants can even act as nurse crops for other crops. An example is the maize crop which acts as a nurse crop for bean plants.

    (vi) Maximum utilisation of soil nutrients is ensured particularly when deep rooted plants are interplanted with shallow rooted plants.

    Disadvantages of intercropping

    (i) Carrying out of field practices, such as weeding, pest and disease control becomes difficult.

    (ii) It is not possible to mechanise the various field operations.

    (iii)There is wastage of fertiliser since some of the crops planted may not respond to some given types of fertilisers. It will therefore require the farmer to apply different forms of fertiliser in order to cater for all the types of crops planted.

    (iv) Use of herbicides becomes impossible since it may cause harm to some of the crops planted.

    c) Pastoralism

    Pastoralists are people who depend on livestock or the sale of livestock products for most of their income and for consumption. In this system, the livestock is mainly grazed on communally-managed or open-access pastures, and where there is at least some propensity of households or individuals to move seasonally with livestock. This is not common in the Rwandan culture.

    d) Stall-feeding

    This involves keeping and feeding an animal in a stall, especially with an aim of fattening it. It is also known as zero grazing. Rwanda is one of the most densely populated countries in Africa. With this condition every effort must be made both to increase agricultural output and to protect the soil from erosion. One of these efforts is to encourage stall feeding among farmers.

    The idea of stall-feeding also enables production of manure for composting. The compost manure is used in the farms to increase the organic content of the soil. This helps to increase the permeability of soil and also improve the soil’s water storage capacity, hence raising soil fertility, increasing yields and reducing erosion.

    1. Draw a model of stall-feeding and give examples of livestock that can be kept under stall feeding.

    2. How does stall feeding help small scale farmers?

    3. What are the advantages of intercropping over monocropping?

    e) Extensive farming

    This system of farming involves use of large portions of land, normally with low capital, labour and management investment. There is also very minimal mechanisation. Extensive farming is mostly carried out in marginal areas and wastelands.

    Advantages of extensive farming

    (i)   It is cheap due to low capital input.

    (ii)  It requires less labour input.

    (iii) It leads to proper utilisation of marginal areas and wastelands.

    (iv) It does not require high level management skills.

    Disadvantages of extensive farming (

    i)  It has low output due to the low investment in capital, labour and management skills.

    (ii)  The land is under-utilised; in terms of the available nutrients. (

    iii) It cannot be practised in densely populated areas since it requires large portions of land. (

    iv) It has low profit per unit area. This is because of the small amount of output obtained.

    f) Intensive farming

    The system is characterised by the use of a lot of labour, large sums of capital investment and high level management skills. In this system, agricultural mechanisation is practised and irrigation can also be applied where necessary. A good example of a farming method that applies intensive farming is horticultural farming; which usually results in high returns per unit area

    Advantages of intensive farming

    (i) There is maximum utilisation of land.

    (ii) It can be practised in areas that are densely populated.

    (iii)Due to the high level management skills involved, the intensive farming system often gives high yields and high quality produce.

    (iv) It results in high income and high net revenue (profit).

    Disadvantages of intensive farming

    (i) In the event of failure-due to poor or ineffective management, or disease/pest attack, heavy losses can be incurred.

    (ii) High initial capital is required.

    (iii)High labour costs are incurred.

    (iv) It requires high levels of skills and management.

    g) Large scale farming

    This entails the growing of crops and keeping of livestock in large hectares of land (usually over 20 ha). Large scale farming is mainly done for commercial purposes. Field operations are normally mechanised especially during land preparation, and in some cases, during planting and harvesting. Large scale farming can either be intensive or extensive.

    Advantages of large scale farming

    (i) It results in high yields.

    (ii) The farmer can take advantage of the economies of scale to increase profits.

    (iii)It promotes foreign exchange earnings for the country.

    (iv) It helps to create employment opportunities owing to the large labour force required where mechanisation is not possible.

    Disadvantages of large scale farming

    (i) A huge sum of capital investment is required for the purchase of farm inputs and machinery.

    (ii) A lot of labour force is required, especially where mechanisation is not possible. This is for instance in coffee and tea harvesting.

    (iii)High level management skills are required in order for good profits to be realised.

    (iv) Heavy losses can be incurred in the event of disease and pest attack.

    (v) It can only be practised where there are large tracts of land.

    h) Small scale faming

    Unlike in large scale farming, small scale farming is the growing of crops and keeping of livestock in a limited area of land. It is characterised by very minimal mechanisation. Most small scale farmers rely on their families for labour. During high seasons, casual labourers can be hired to supplement the family labour. Due to limited arable land, the majority of farmers in Rwanda are small scale farmers. Small scale farming can be practised both for commercial purposes and also for subsistence purposes. It can also either be extensive or intensive.

    Discussion corner!

    Discuss about some of the reasons that can lead farmers to being small scale farmers.

    The facts

    Conditions that lead to small scale farming include:

    (i) Lack of adequate land

    (ii) Limited capital for large scale farming

    (iii)Lack of market or incentives

    (iv) Government policies

    Advantages of small scale farming

    (i)   It requires low capital investment.

    (ii)  It has low labour requirement.

    (iii) The farmer can decide to sell surplus produce to generate income for the family.

    Disadvantages of small scale farming

    (i)     It has low output per unit area due to low investments in input and management.

    (ii)   It is less profitable compared to large scale farming.

    (iii)  The marketing of produce is difficult and unprofitable.

    Self-evaluation Test 1.5
    1. Which farming system is the most economical and why?

    2. Which farming system ensures maximum utilisation of land and how?

    3. Of the farming systems, which one ensures high output and why?

    4. Which farming system requires high level management and why?

    5. Which of the farming systems do you think is the most appropriate for upcoming farmers? Why is this the case?

    Remember the facts! • Agriculture is the art and science of cultivating land, growing crops and rearing livestock. • In Rwanda, the agricultural sector accounts for about 90% of the national food needs and it generates more than 70% of the country’s export revenues.

    • Agriculture has various branches. These include: 

    - Soil science  - Crop husbandry 

    - Animal husbandry 

    - Agricultural economics 

    - Agricultural engineering 

    - Horticulture

    • Farming systems are ways in which farm enterprises are organised and utilised. 

    • Types of farming systems include: 

    - Monocropping 

    - Intercropping 

    - Pastoralism 

    - Stall-feeding 

    - Extensive farming 

    - Large scale farming 

    - Small sale farming

    Test your competence 1

    1. Explain how agriculture affects human life.

    2. Why is Agriculture a popular practice in Rwanda?

    3. Discuss the various types of farming systems you have learnt about in terms of: 

    (a) The number of crops grown. 

    (b) The kind of product targeted.

    4. Why would you spread awareness to your community against encroaching into forests so as to acquire land for agriculture?

    5. What are some of the problems that farmers in your area face and what are the possible solutions you can give for these problems?

    6. How would you recommend the structure of your school farm to be improved so that it becomes more effective?

    7. (a) Which are the most commonly planted crops in your school farm and why? 

    (b) Which farming system is being used to plant those crops in your school farm?

    (c) Do you think the farming system being employed is appropriate? Give reasons for your answer.

    8. Choose the single word used to describe growing of vegetables.

    A. Floriculture

    B. Pomoculture

    C. Arboriculture

    D. Olericulture

    9. For horticultural farming to be successful there has to be a good transport system, electricity and high level farm management skills. Justify this statement.

    10. Why do you think intensive farming is the most appropriate for horticultural farmers?

    11. Referring to the branches of Agriculture, point out those that relate to the following subjects and also explain how they relate. 

    (a) Physics 

    (b) Chemistry 

    (c) Home science 

    (d) Biology 

    (e) Economics

    12. What do we call the type of farming where crops are grown and livestock are reared in large tracts of land?

    A. Extensive farming

    B. Intercropping 

    C. Large scale farming

    D. Pastoralism


    Files: 2
  • UNIT 2: Soil

    Agricultural practices are carried out on soil. This means we rely on land to do agriculture. It is therefore important for us to understand soil and its composition. Look at the pictures below. They show various types of soil. Which type of soil do you know? Which soil is suitable for farming?

    This unit is about soil and its importance in Agriculture.

    Key Unit Competency

    After studying this unit, I should be able to interpret soil formations, soil properties and the various types of soil.

    Unit Outline

    2.1 What is soil?

    2.2 Soil formation

    2.3 Types of soil

    2.4 Components of soil

    2.5 Soil profile

    2.6 Properties of soil

    2.7 Soil sampling and testing

    2.1 What is soil?

    Activity 2.1: Research Activity

    1. Find out the definition of soil and how it is formed. You can use the internet search engine and other reference books in the library.

    2. Present your findings in class.

    I have discovered that…

    Soil refers to the loose natural material which form the uppermost layer of the earth’s crust. Formation of soil from the parent material is referred to as soil genesis.

    The facts

    Soil is very important in our lives. It provides anchorage, nutrients and water to plants. The top soil, in particular, covers most of the earth’s surface. It forms the fertile soil which contains minerals, organic matter and living things. It is good for farming. For this reason, this layer forms the basis of agriculture.

    Self-evaluation Test 2.1

    1. How does soil support plant growth?

    2. What is top soil?

    3. What are some of the components of soil?

    2.2 Soil formation

    Activity 2.2

    1. Your teacher will show you a video on how soil is formed.

    2. From what you have seen in the video, note down the processes of soil formation. Describe how they happen and write a report. Present your findings to the rest of the class.


    I have discovered that…

    Soil is formed by weathering of rocks through various physical, biological and chemical processes. The rock from which soil is formed is referred to as the parent material or parent rock. Minerals particles form the main bulk of soil. The organic portion of the soil forms a small but very important part.

    The facts

    Weathering refers to disintegration of rock particles to form soil. It takes several hundreds of years for a centimeter of soil to be formed. The weathering process is brought about by physical, biological and chemical agents. It is in turn influenced by climate changes, parent rock material, living organisms, topography and time. All these are referred to as soil formation factors.

    The process of soil formation

    Soil formation, or soil genesis, takes place through a process called weathering. Weathering is the breaking down and alteration of the parent rock near the surface to form soil. The various forms of weathering include:

     • Physical weathering 

     • Biological weathering  

    • Chemical weathering 

     • Transport and deposition

    a) Physical weathering

    Physical weathering, also known as mechanical weathering, involves disintegration of rocks into smaller fragments by physical agents. The agents include climatic factors such as rainfall (running water), temperature changes, moving ice (glaciers) and wind.

    (i) Wind - When strong wind blows, it carries rock materials from the ground. These materials bounce on the ground and hit against each other hence breaking off into smaller fragments which form soil.

    (ii) Rainfall (running water) - When it rains, the raindrops hit the ground with some force making rock particles to be loosened and broken down. Running water also carries small stones which hit against each other along the river bed or on the ground surface resulting in further breaking off and wearing out of rock particles. This is how alluvial soils are formed and deposited on river banks; and in later stages on river plains.

    (iii) Moving ice (ice glaciers) - Moving ice also known as glacier, depending on its size, has the capacity to cause rocks to rub over each other as they are carried along the ground. This causes breakdown of rock particles into small pieces. In other words, glaciers have a grinding effect on rock surfaces.

    (b) Biological weathering

    Biological weathering involves the disintegration of rock and minerals due to the chemical and physical agents or organisms. Living organisms play a very important role in soil formation through various biological processes. Living organisms here act as agents of weathering. For example, when large animals such as cattle, buffaloes, camels, elephants and human beings move, they exert pressure on the rocks causing small fragments of rock to disintegrate. Also, animals moving in large herds are very effective in breaking rocks or stones to form soil.

    Physical activities of human beings such as mining, cultivation, quarrying and construction of buildings, railways and roads on the earth’s surface reduce the size of rocks into smaller particles.


    Fig. 2.6: Quarrying cause breaking of rocks to form soil

    Organisms living in the ground, including moles and earthworms, burrow the soil and break large soil particles into smaller pieces.  In the course of their living, organisms produce fluids or wastes which have chemicals that can cause corrosion.  Body fluids of most organisms contain ammonia, carbon dioxide and hydrogen.  When these come into contact with rock surfaces in the presence of water, they cause substantial corrosion. Roots of growing plants, on the other hand, penetrate small cracks in rocks and exert considerable pressure which eventually causes breakage of rocks. When these plants die, the roots decay leaving gaps in the rocks which are then occupied by water and air.  These form acids which dissolve minerals from rocks and corrode the rocks weakening them so that they are easily broken into fragments by other agents of weathering. Roots produce acids in the soil during respiration. These acids dissolve minerals from rocks.

    (c) Chemical weathering

    Discussion corner!

    What do you know about corrosive chemicals? Name them. Explain how they act.

    Rocks which form the parent material where soil comes from are made up of chemical substances which in the course of time undergo changes that alter the composition of rocks.  Chemical weathering is the actual decay or decomposition of rocks.  It involves various chemical reactions which take place between rock minerals, water and certain atmospheric gases like oxygen and carbon dioxide. Chemical weathering changes the chemical structure of the rock, making it unstable hence easy to disintegrate. Chemical weathering involves the following processes.

    (i) Carbonation - This term describes the action of carbon dioxide on rock minerals.  Carbon dioxide can dissolve in water to form a weak carbonic acid.  This acid can dissolve some of the rocks, especially marble and limestone.

    The resulting calcium bicarbonate is easily broken down since it is soluble in water.  Therefore, limestone parent rocks form soils by this chemical weathering process.

    (ii) Oxidation - This means the taking up of oxygen present in the air by an element or compound.  It is important to note that oxygen also oxidises many elements. It usually occurs in compounds which contain mainly iron and sulphur. The oxides which are formed take up more space and help in rock disintegration. For example, oxygen oxidises iron from olivine rocks into ferrous oxide, ferric oxide and red ferric oxide producing red soils.

    (iii) Hydration - This occurs when water combines with minerals present in the rocks. It causes softening of the original rocks, making them easy to break. The chemical composition of the rocks however, remains unchanged.  When dehydration occurs, on the other hand, the rocks often revert to their original forms.

    (iv) Dissolution - Water can dissolve any soluble minerals present in the rocks. When this occurs, the minerals that hold the rocks together are dissolved and the rocks easily disintegrate. For example, in areas where there is a lot of industrial smoke being produced, the gases produced dissolve in water to form corrosive substances which can weather rocks. Some of the industrial gases produced are sulphur dioxide, hydrogen among others. When these gases dissolve in water, they form weak acids. These acids cause rocks to be brittle hence ready to weather physically.

    (v) Hydrolysis - This occurs when the minerals in the rocks react with water. Chemical bonds in the minerals are broken by water, changing rocks from their original forms and making them easy to break. Hydrolysis best occurs where there is free movement of underground water.

    (d) Transport and deposition (accumulation)

    Wind acts as a transport agent and hence it can carry the weathered materials from one place to another.  Where a lot of weathering materials are deposited, there are deep soils. However, where the materials were carried from will be left bare or with very shallow soils. Bacteria and fungi initiate breakdown of plant materials on the surface and within the soil. Also, arthropods such as mites, springtails and termites are chiefly responsible for more severe breakdown of plant tissues.  Termites and earthworms mix organic matter with mineral fractions of the soil.

    Self-evaluation Test 2.2 
    1. Which human activities in your area do you think lead to soil formation?

    2. How can we encourage soil formation and still discourage soil erosion?

    2.3  Types of soils

    Activity 2.3

    Field trip

    1. Go for a field excursion in your neighborhood, or any appropriate place, and collect the various types of soil samples.
    2. Describe the various types of soils you will have gathered.

    3. Distinguish the various types of soils by touching (feel the various soil samples in between your fingers) and note down your inferences and conclusions. Use a table format.

    The facts

    The most common types of soils are clay soil, loam soil and sandy soil.

    (a) Clay soils

    Properties of clay soil

    (i) They have more than 50% clay particles  and between 0-45% silt and sand.

    (ii) They have a very high water holding capacity but their ability to release this water to plants is much less compared to that of loam soil. This hence makes it difficult to cultivate crops in clay soils.

    (iii) They are fine textured and smooth.

    (iv) Clay usually forms extremely hard clods or lumps when dry and is extremely sticky and plastic when wet.

    (v) They have a crystalline and platy structure and expose a relatively large surface area which is responsible for their physical and chemical properties.

    (vi) Clay soil has poor aeration and drainage but high capillarity. When wet, the clay particles expand and this impairs drainage.  It therefore, makes them become heavy causing tillage operations difficult and expensive.

    (vii) When containing the proper amount of moisture, it can be made into ribbons by squeezing between thumb and forefinger.

    (viii) They have high nutrient absorption ability. This increases the amount of nitrogen causing the soil to have pH that is between neutral and alkaline.

    (ix) Tubers and plant roots grown in such soils are greatly affected in their growth when the soil becomes dry.

    (x) Lack of moisture may lead to hastened maturity in plants, making them yield less than expected.

    (xi) The rate at which clay soils absorb water is low. Water therefore accumulates on the surface making them to be waterlogged.  They can be improved by drainage.

    (xii) This class of soil is particularly good for growing cotton and rice.

    (b) Loam soils

    Properties of loam soils

    (i) These are medium–textured soils which contain 30–50% sand particles, 40% silt and 20% clay, with about 4% of organic matter.

    (ii) They have good proportions of sand and clay in their composition.

    (iii) They are the most productive soils for crop production as they contain good amounts of plant nutrients and organic matter.

    (iv) They are high in soil water available for plant use and have a good water-holding capacity.

    (v) They are easy to till and do not erode as easily as sand soil, hence most crops do well in loamy soils.

    (vi) These soils can be improved by planting cover crops to maintain fertility and also by adding manures and fertilisers.

    (c) Sandy soils

    Properties of sandy soils
    (i) Sandy soils generally contain 80%
      sand particles, 10% silt and 10% clay
      and about 3% of organic matter.

    (ii) They are usually well drained, coarse
      textured and moderately fertile.

    (iii) They have a low water-holding capacity
      and capillarity, hence cannot retain
      enough water for plant use.
    (iv) These soils are more prone to erosion than either clay or loam soils. This is mainly because sandy soils have a less stable structure on the surface.

    (v) Deep rooted plants suited for arid regions can survive on sandy soils.

    (vi) They are easy to cultivate but are not fertile.  However, they can be improved by adding a lot of organic manure and fertilisers.

    Self-evaluation Test 2.3

    1. Describe the type of soil in your area.

    2. (a) Do you think the soil in your area is good for farming?

      (b) Which types of crops should be planted?  Why?

    2.4  Soil components

    Discussion corner!

    What do you think soil is made up of? Talk to your friend about this. Write a report and present it to class.

    Soil constituents refer to the components that make up soil. They include the following:  • Mineral particles/inorganic matter or rock particles 

     • Soil water  

    • Soil air  

    • Organic matter (humus) 

     • Soil living organisms

    The facts

    (a) Mineral particles/inorganic matter Mineral particles are also referred to as inorganic matter. 

    It forms the main framework of soil in which plants anchor their roots. The inorganic matter of the soil is made up of particles of rocks formed from parent rock by the weathering process. The mineral constituents of these depend on the mineral composition of the parent rock from which it was derived. There are spaces between the particles which are filled with air and water.

    Activity 2.4

    Finding out the sizes and shapes of particles of various types of soil Collect various soil samples from different places and observe them using a handlens or under a light microscope. Describe their shape. Draw the shapes in

    your notebook. Compare and contrast the sizes of the particles. Comment on their suitability for use in growing crops.

    The structures, texture and colour of the mineral particles are derived from the minerals found in the parent rock. Various soils are composed of particles of various sizes and shapes as shown below.

    (b) Soil water

    Activity 2.5

    Finding out if soil contain water

    Apparatus/ materials

    • A lump of soil

    • Transparent plastic containers with leads

    • Hand lens Procedures

    Procedures

    1. Dig out a lump of soil from a random  

    2. Put the lump of soil in a transparent plastic container with a lid.

    3. Place the container out in the sun and leave it for about 6 hours.

    4. Observe the sides and lid of the container.

    Discussion corner!

    1. What do you see?

    2. Why was the observation made? location in your school compound.

    The facts

    Soil contains water which comes from precipitation (rainfall) or through irrigation. The amount of water in the soil is determined by factors such as the rate of precipitation, evaporation rate, the amount and type of vegetation cover, the water storage capacity, temperature, gradient of the land, type of soil and altitude. Basically soil water exists in three forms, namely:

    • Superfluous water 

    • Capillary water 

    • Hygroscopic water

    (i)  Superfluous water  This is water that exists in the large air spaces (macro-pores) between the mineral particles. It is held by gravitational force and can be made available to plants for use through the roots.  This water is easily lost because it is loosely held by soil particles.  Its amount varies inversely with the amount of air available. It is important to note that this water is not very useful to plants. Too much of it in the soil limits aeration and it also drains away a lot of nutrients hence causing leaching.

    (ii)  Capillary water  This is underground water available to plants through the roots and occupies the micropores.  It is held with greater force by soil particles.  It dissolves plant nutrients.  It is also referred to as the available water since it leaves most of the macro-pores empty to allow aeration of the soil. 

    (iii) Hygroscopic water  This is water held strongly by the soil particles and exists as a thin film around the soil particles.  This water is subject to forces created by soil particles and therefore it is not available to the plant.  However not all soil particles have hygroscopic water. For instance sandy particles with weaker forces contain very little hygroscopic water whereas clay particles have a lot of hygroscopic water.

    Activity 2.6:

    Finding out the percentage of water in a soil sample

    1. Come up with a procedure to demonstrate the percentage of water in a given sample of soil. Your teacher may provide you with the following apparatus:  • Soil sample  

    • A porcelain dish  

    • Bunsen burner  

    • Stirring rod 

     • Desiccators  

    • Weighing balance  

    • Tripod stand  

    • Wire gauze  

    2. Record the steps you will need to follow, your results and conclusions.

    3. Write a report and do a presentation to the rest of the class on your findings.

    The facts

    Sample procedure for investigating the percentage of water in a soil sample

    1. Collect a sample of garden soil from a depth of about 20 cm.

    2. Weigh the empty porcelain dish and record the weight.

    3. Put some of the soil in the porcelain dish and weigh.

    4. Heat the dish with its content in an oven at a temperature of about 105°C for about 1 hour.

    5. While heating, the soil sample in the dish, it should be stirred to facilitate complete moisture evaporation.

    6. The soil sample should be heated until a constant weight is obtained.

    The results of the procedure can be recorded as shown below:

    • Weight of empty dish =  (a) gm

    •  Weight of fresh unheated soil =  (b) gm

    • Weight of dish + fresh unheated soil =  (a + b) gm

    • Weight of dish + heated soil =  c g

    • Weight of evaporated water = (b – c) gm Percentage of water in the soil can be calculated using this formula:

    I have discovered that…

    Soil contains a certain percentage of water; hence water forms a substantial proportion of soil.  However, the quantity of water in soil varies with different soils. For instance, sandy soil contains much less water compared to both clayey and loamy soils.

    The facts

    The following are importances of soil water: 

    (i) Water serves as a solvent for the plant nutrients (minerals) in the soil.

    (ii) It is an essential raw material used in the process of photosynthesis by plants.

    (iii) It is taken in by plants as a coolant in the process of transpiration.

    (iv) Most of the protoplasm in plant cells is made up of water.  It makes the plant cells turgid.  The movement of this water within the plant cells makes the plant to stand upright (erect).

    (c) Soil air

    The air content of soil consists of oxygen, carbon dioxide, nitrogen and other rare gases. Soil air is located in soil pores separated by soil particles. The content and composition of soil air is determined to a large degree, by soil–water relationships. Air simply moves into the soil pores that are not occupied by water.

    The amount of air in the soil is inversely proportional to the amount of water in the soil pore spaces. The pore size and distribution is influenced by soil texture and structure. The air in soil has remarkable influence on plant growth and soil organisms; especially for respiration of plant roots. The presence of air in the soil leads to oxidation, which converts part of organic matter into nitrates; a form readily available to the plants. When there is less oxygen in the soil, some plants may not do well. This is because their roots are not able to absorb water from the soil.  Excess carbon dioxide in the soil can cause harm to plant roots. A good soil for crop growing must contain an adequate amount of air. The air must circulate freely and continuously in order to keep oxygen at a level high enough for proper plant growth. For instance, there must be a balance between soil water and soil air for most crops to do well.  It is important to note that the nitrogen in the soil must be converted into nitrates by the nitrogen–fixing bacteria for it to be available for plant use.

    Activity 2.7

    Determining the presence and percentage of air in soil

    1. Find out how to establish the percentage of air in a soil sample. Your teacher may provide you with the following apparatus:

     • A small tin (of known mass)  

    • A large graduated glass  

    • Trough 

     • Stirring rod 

     • Ruler  

    • A knife  

    • 500 cm3 graduated cylinder  

    • Hammer 

     • A nail 

    2. Record the steps you will follow, your observations and conclusions.

    3. Write a report and make a presentation to the rest of the class.

    The facts

    Sample procedure for investigating the presence (percentage) of air in a soil sample

    1. Fill a tin of known mass, for example a 300 g jam tin, with water and transfer the water into the 500 cm3 cylinder.

    2. Then fill the tin with garden soil and use a ruler to cut clean the soil in the tin so that it fills just up to the brim.

    3. Place the tin with soil carefully into the water in the graduated cylinder. The tin should be placed upside down without pouring the soil out.

    4. Record the final volume of the soil and water in the cylinder.

    Note: It is important to note that the volume of the soil in the tin is equal to the volume of the tin.  This experiment could be repeated with different soil types such as clay and loam.

    5. Put some soil in glass of water as shown below. Note your observations

    I observed the following:

    • When the small tin with soil was placed in the water the level of the water rose. • Bubbles of air were also seen escaping from the small tin through the holes at the base of the tin. • While the bubbles were escaping the level of the water was dropping.

    The results of the procedure can be recorded as shown below:

    Volume of soil in cylinder = (a) cm3

    Original volume of water in the cylinder = (b) cm3

    Volume of soil and water in the tin = (a+c) cm3

    New volume of soil after air has escaped = (c) cm3 

    Percentage of air in the soil can be calculated as shown below:

    I have discovered that…

    Soil contains a certain percentage of air. Hence air is a component of soil. The percentage of air in any given soil depends on the type of soil.

    The facts

    The following are the importance of air in soil: 

    (i) Air is required for plant respiration.

    (ii) Oxygen in the soil combines with many elements in the soil so that they become available to plants.  For example oxygen combines with nitrogen to form nitrates which are used by plants.

    (iii) Plants and animals that occupy spaces in soil require oxygen for respiration.  These organisms are useful in the process of soil formation.

    (d) Soil organic matter

    Soil organic matter is derived from partially decayed and totally decomposed plant and animal remains. Organic matter that has totally undergone decomposition is called humus. Humus may be dark or brown in colour and is very rich in plant nutrients.  It is usually found at the top of the soil profile. Due to its dark colour, humus absorbs and retains a lot of heat. Therefore, soils rich in humus are relatively warm.

    The process of breaking down organic matter releases carbon dioxide into the atmosphere. Other substances such as sulphates (SO4)2-, phosphates (PO4)2-, nitrates (NO3)- and other nutrients are oxidised and released into the soil for plant use. Humus also cointains important minerals such as calcium (Ca2+), magnesium (Mg2+), potassium (K+) and ammonium (NH4)+ ions which are released to plants for their nutrition. It is important to note that a good supply of humus in the soil increases the amount of water absorbed and its availability in the soil.

    Activity 2.8

    Determining the percentage of organic matter in soil

    1. Come up with a procedure to find out the percentage of organic matter in various soil samples. Your teacher may provide you with the following materials and apparatus:

     • Silica dish/porcelain dish 

     • Fresh garden soil  

    • Weighing balance 

     • Tripod stand  

    • Bunsen burner  

    • Wire gauze  

    • Stirring rod  

    • Desiccator  

    2. Record the steps you will have followed and your results.

    3. Write a report and do a presentation to the rest of the class.

    The facts

    Sample procedure for investigating the presence (percentage) of organic matter in a soil sample

    1. Weigh the empty silica dish and record the mass.

    2. Collect fresh samples of garden soil from a depth of about 20 cm where there is a high likelihood of getting a good supply of humus.

    3. Put the collected soil sample in the dish and record the mass.

    4. Heat the soil sample in an oven at a temperature of about 105°C for two hours.

    5. Allow the moisture in the soil to evaporate.

    6. Cool the sample in desiccator-this is to prevent more moisture from being added into the soil.

    7. Weigh the cooled soil sample and record the new mass.

    8. Heat the cooled soil sample strongly over a Bunsen burner noting any change in appearance of the soil during the heating process.  The heating will remove the humus in the soil, converting it to gases. The gases then escape into the atmosphere.

    9. Cool the dish and soil sample in the desiccator. 10. Weigh it and record the new mass. (Note:This heating, cooling and weighing is repeated until a constant weight is obtained.)

    The results of the procedure can be recorded as shown below:  

     • Mass of silica dish = (a) g  

    • Mass of fresh soil  = (b) g  

    • Mass of burnt soil = (c) g  

    • Mass of strongly burnt soil = (d) g 

    • Mass of humus removed     = (c – d) g  

     • Percentage of humus in the soil can be calculated as shown below:

    I have discovered that…

    There is always a percentage of humus in a given soil sample. Hence soil contains organic matter or humus.

    The facts

    The following are the importance of organic matter in soil:

    (i) It is a major source of most plant nutrients such as nitrates, phosphorous, sulphur and calcium.

    (ii) Organic matter provides food for micro-organisms in the soil. These microorganisms promote the process of soil formation.

    (iii) Organic matter in the soil absorbs moisture and acts as a sponge, resulting in moisture retention.

    (iv) Organic matter binds soil particles together. It helps to maintain the structure, workability, aeration, water penetration and increases the water holding capacity of the soil.

    (v) Organic matter has a texture that helps increase the water holding capacity especially in sandy soils.

    (vi) The dark colour of humus makes it absorb and retain more heat in the soil thereby moderating soil temperature.

    (e) Soil living organisms

    Living organisms are a very important component of the soil. In fact, soil contains a variety of living organisms. They range from micro-organisms such as bacteria and fungi to insects, earthworms and rodents. These micro-organisms live in the micro-pores in the soil particles whereas the larger organisms burrow into the soil. When, larger organisms such as earthworms burrow into the soil, they make it well aerated and loose. On the other hand, micro-organisms such as bacteria, fungi and protozoa help in the decomposition of organic matter. Some bacteria, such as those of the rhizobium group, live in the roots of leguminous plants. They help in converting soil nitrogen into nitrates. These nitrates are later absorbed by plants. However, some of these micro-organisms may damage crops by causing diseases. Examples are bacterial and fungal diseases that attack crops.

    Activity 2.9

    Determining the presence of living things in a soil sample

    1. Come up with a procedure of an experiment to show presence of living organisms in soil. Your teacher may provide you with the following apparatus and materials:  

     • Fresh garden soil   

    • Porcelain dish   

    • 2 conical flasks   

    • Rubber corks   

    • Bunsen burner   

    • Strings   

    • 2 muslin bags   

    • Lime water   

    • A tripod stand

    2. Record the steps you will follow, your observations, reasons and inferences.

    3. Write a report and do a presentation to the rest of the class.

    The facts

    Sample procedure for investigating the presence of living organisms in a soil sample

    1. Collect a sample of fresh garden soil.

    2. Place half of the collected soil on a porcelain dish and heat it until you are sure that all the living organisms are dead. Then let it cool.

    3. Place the other half of the fresh garden soil in a muslin bag and suspend in the first conical flask containing lime water as shown in Fig. 2.14 A.

    4. Place the heated soil in another muslin bag and suspend it in another conical flask which has lime water as well. See Fig. 2.14 B.

    5. Leave the set up to stand for about 4 hours then make your observations


    I have observed that:

    Lime water in conical flask A turns milky while lime water in conical flask B remains clear.

    The lime water in conical flask A turns milky because living organisms present in fresh soil exhale carbon dioxide during respiration. This is what forms the white precipitate when it comes into contact with lime water.

    The lime water in conical flask B remains clear. This is because the soil living organisms in that soil sample were burnt to death during heating; hence no carbon dioxide was present to turn the lime water milky.

    What I have discovered!
    • Garden soil usually contains living organisms which respire actively. • These organisms take in oxygen and exhale carbon dioxide. 
    Importance of living organisms in the soil

    (i) Living organisms help in soil formation by physically breaking down the soil particles.

    (ii) The micro-organisms assist in the decomposition of organic matter in the soil.

    (iii) The larger organisms on the other hand burrow the soil and in the process they aerate it.

    (iv) Certain micro-organisms such as the rhizobium bacteria help fix free nitrogen in the atmosphere into nitrates. This makes it available to plants.

    Self-evaluation Test 2.4 

    1. Given the following information: 

    Weight of empty dish = 15 gm

    Mass of dish + fresh soil = 45 gm

    Mass of dish + heated soil = 40 gm Calculate: 

    (a) Mass of unheated soil only. 

    (b) Mass of heated soil only. 

    (c) Mass of evaporated moisture. 

    (d) Percentage of water in the soil.

    2. Given the following information: 

    Mass of silica dish + fresh soil = 36 g

    Mass of silica dish + burnt soil = 35 g

    Mass of silica dish + strongly burnt soil = 33 g 

    Calculate the percentage of humus that was in the original soil sample.

    3. What is the significance of soil living organisms in the soil?

    4. Why would it be important to establish the percentage of organic matter in a soil?

    2.5 Soil profile

    Activity 2.10(a)

    Field trip

    1. Go out into the field. With the help of your teacher, find a hole that has been recently dug. Observe, from the wall of the hole the various layers of soil. You can also visit a quarrying site to observe these layers.

    2. Make a drawing of what you have seen in your notebook.

    3. Back in your class, compare the layers you drew with the chart provided by the teacher. Did you get the layers right?

    I have discovered that…

    Soil profile is the vertical arrangement, or a cross-section of soil layers from the ground level (surface) to the parent rock.  These layers are known as horizons. The horizons differ in properties such as colour, texture, structure, porosity, organic matter content and chemical composition.

    The facts

    Soil profile can help to determine whether the soil is mature or recently formed. This depends on the number of horizons present. From the soil profile, we can also determine the origin of the parent material involved in soil formation. Every soil type has its own way of formation. The horizons in a soil profile are:

     • Top soil  

    • Subsoil  

    • Substratum (weathered rock)  

    Characteristics of the horizons in the soil profile

    (i)   Horizon A (Top soil)

    This is the uppermost soil layer which lies beneath the superficial layer (surface) and marks the beginning of the mineral soil. It is commonly known as the top soil.  It is characteristically dark in colour due to its high humus content. It is well aerated and contains active living organisms which break down and decompose organic matter into humus. Most plant roots and nutrients are found here. This zone is permeable to air and water and it is also well-drained.

    (ii)  Horizon B (Subsoil)

    This is the layer found immediately below the top soil (Horizon A) and is also referred to as subsoil.  Tap roots of large plants reach to this layer. The base of this layer is more compact and less aerated than their top soil. It also contains an impermeable layer called a hard pan. This hard pan impedes drainage and may prevent root penetration.  There are clay deposits in this zone because of the downward movement of clay colloids.  Sometimes minerals are leached from the subsoil and accumulate here, hence the subsoil layer is also referred to as layer of accumulation.

    (iii) Horizon C (Substratum or weathered rock)   

    This layer is also referred to as substratum or weathered rock.  This layer is found beneath the subsoil and is partly made of weathered rock with no humus.  Tap roots of large trees may reach this layer and draw water from it during the dry season.  The layer is hard, therefore impermeable to water.  During erosion, most parts of the horizon A and B are washed away to expose this layer.

    (iv) Horizon D (Bedrock)

    This layer is found below the weathered rock and is also referred to as the parent rock or bedrock.  This layer is completely impermeable to water and air.  Soil is formed from this rock.  The entire soil profile is from this horizon.  Water table is found in this layer.

    Activity 2.10 (b)

    Having learnt about the various layers of soil, refer to the diagram you drew in Activity 2.10(a) and label it (name the various layers you have learnt about correctly). Recognise the most important layer and explain why it is important.

    Influence of soil profile on crop production

    a) The suitability of a soil for agricultural production can be determined by the depth of the soil profile.  Farmers look at how deep the soil profile is to decide what crops to grow and how best to cultivate the land.

    b) Soils on steep slopes have their top fertile layers washed away. This type of soil has very thin or shallow top layers. Erosive agents especially water can easily wash it away. This makes such soils less fertile and they therefore cannot support growth of healthy plants.

    c) A deep soil having a well developed profile has great potential for agriculture. It is able to hold more moisture for plant use than a shallow one.

    d) A loosely packed subsoil layer allows easy penetration of roots, drainage and aeration.  It also ensures that erosion does not take place and reduces the degree of run-off. This layer must also be fairly deep. The maintenance of the top soil and subsoil ensures that fertile soils are available for plant growth.

    e) Most of the soil nutrients are contained at the top soil. This is vital to plants since most soil organisms, such as soil microbes and plant roots spread here.

    f) The top soil is usually better aerated. It has therefore more active microorganisms which decompose the vegetable matter into humus.

    g) The nature and composition of the mineral components of the bedrock have influence on the mineral components of the whole soil. Thus the mineral nutrients that a soil is able to supply to the plant largely depends on the mineral composition of the parent rock. If, for example, the parent rock lacked in some minerals, then the soil formed from it will also lack those same minerals.

    h) Crop production is influenced by root penetration into the subsoil and by the amount of moisture and nutrients held there. An impermeable subsoil will restrict root growth and penetration.

    i) The topography on which the soil develops greatly influences its properties. Soils that develop from slopes have shallow horizons A and B than soils developing from level topography.  Soils on level grounds are darker in colour than soils on steep slopes.

    Self-evaluation Test 2.5

    1. Explain how we can ensure that soil profile is well maintained.

    2. How important is the top soil to plant growth?

    2.6 Soil properties

    Soil properties can be explored from three perspectives. They include:  • Physical properties  

    • Chemical properties  

    • Biological properties 

    a) Physical properties of soil

    Activity 2.11

    Collect various soil samples from different areas in your locality. For each of the samples:

    1. Observe the colour and record.

    2. Pass the soil particles in between your fingers to feel how smooth or rough they are and record.

    3. Use a hand lens or light microscope to observe how the aggregate soil particles look like. Draw the shapes of the various soils in your notebook.

    What I have discovered The physical properties of soil include:  

    • Soil colour  

    • Soil texture 

     • Soil structure

    The facts

    (i) Soil colour

    Soils tend to have distinct variations in colour when looked at horizontally and vertically as well.  This property can help identify the nutrients in the soil. It also gives information on the present condition of the soil system. Soil colour is determined by the minerals present in the parent rock, the amount of organic matter and the amount of iron in the soil.  If a soil was formed from a rock containing a lot of iron compounds, it tends to be brownish or reddish in colour. Such soils are rich in oxidised iron.

    Soils rich in organic matter are usually black in colour. This is due to the presence of humus and other substances in the soil such as peat and more or less decomposed plant residues. The amount of water present may also determine the type of reactions that take place in the soil. It may also determine the colour of the soil. For example, soils which have a lot of water are poorly drained and they tend to develop a greyish colour. In arid areas, soils develop a high concentration of solute salts. They do not have organic matter and are generally whitish-yellow in colour. The combination of iron dioxide and organic content gives many soil types a brown colour. It is important to note that soil colours influence soil temperature. Dark soils absorb and retain more heat than light coloured soils.  High temperatures affect the activity of soil micro-organisms.  Soil micro-organisms will be more active in high temperatures. Under such conditions, the decay of organic matter is usually faster than it is in low temperatures.

    (ii) Soil texture

    The term soil texture refers to the relative proportions of the various sizes of mineral particles in soil. More appropriately, soil texture is a term commonly used to designate the proportionate distribution of the different sizes of mineral particles in a soil.Soil texture can also be defined as the coarseness or fineness of a soil sample when felt between fingers.  Some particles are large and therefore coarse in texture while others are small.  The small particles are fine to the feel between the thumb and the index finger hence giving a fine texture.

    Soil textural classification

    Most soils do not consist entirely of particles of the same size.  Most soils are a mixture of sand, silt and clay particles.  The texture of the soil determines its ability to absorb and retain water and soil nutrients. The following are the classes of soil according to their texture: • Clay soil

    • Sandy clay

    • Sandy loam

    • Clay loam

    • Loam soil

    • Silty loam

    • Silty clay These classes can be obtained using the textual triangle see fig 2.16. Follow any two component percentages to find the name of the soil type.For example, for a soil that is 75% silt and 50% clay. Find 50% along the bottom (clay) line. Follow the slanted (dotted) line until you reach the horizontal line for 75%. The soil type is clay loam.

    Activity 2.12

    Demonstrating that soil is made up of differently sized particles

    1. Come up with an experiment to demonstrate that soil is made up of differently sized particles. Your teacher may provide you with the following materials and apparatus:  

    • Garden soil  

    • Water  

    • Sodium carbonate  

    • A 250 cm3 measuring cylinder

    2. Record the steps you followed, the observations and results you obtained.

    3. Write a report and do a presentation to the rest of the class.

    The facts

    Sample procedure for investigating the sizes of various soil particles

    1. Collect fresh garden soil.

    2. Put about 50 g of the soil in the 250 cm3 measuring cylinder.

    3. Add sodium carbonate about four times the volume of water to help in dispersion of the soil particles.

    4. Cover the mouth of the cylinder with your hand and shake vigorously for about two minutes.

    5. Place the cylinder on the bench for about one hour or more to allow the contents to settle down.

    I observed that…

    The soil in the cylinder settles in various layers as shown in figure 2.17 below.

    • The heavy, coarse gravel settled first.

    • Followed by sand, silt and clay.

    • The humus and other organic matter remain floating on the water. 

    • The depth of each layer can be assessed by reading from the marks on the measuring cylinder.

    I have discovered that…

    Garden soil is a mixture of particles of different sizes.

    Activity 2.13

    Determining size of particles of different types of soil

    1. Perform an experiment to determine size of particles of different types of soil. Your teacher may provide you with the following apparatus and materials:  

    • Sieves of different mesh diameters 

     • Garden soil 

     • Containers  

    • Weighing balance 

    2. Record the steps you followed, the observations and the inferences made. 

    3. Write a report and do a presentation to the rest of the class.

    The facts

    Sample procedure of an experiment to determine sizes of different soil types

    1. Collect fresh garden soil.

    2. Place a known amount of soil into a container.

    3. Crush the soil lumps without breaking the particles.

    4. The crushed soil should be passed through the sieve with the largest mesh diameter (2.00 mm) and shaken vigorously.

    5. Observe the sizes of the soil particles that remain on the sieve and record.

    6. Repeat the process using the other sieves with mesh diameters of 0.2 mm and 0.02 mm (always using the soil that passes through the previously used sieve).

    I have observed that…

    Some larger soil particles are always left on the different sieves used. The soil particles left on the first sieve of mesh diameter 2.00 mm are called gravel; from the second sieve (0.2 mm), coarse sand particles; from the third sieve (0.02 mm), fine sand particles; from fourth sieve (0.002 mm), silt particles and whatever particles pass through the smallest sieve (0.0002 mm) are clay particles.

    Note: The proportions of the various soil particle sizes can be calculated based on the original mass of soil sample.

    I have found out that…

    • Soil is made up of differently sized particles. • Gravel particles are fairly large and heavy because they contain a lot of iron. • Sand particles are coarse textured and are very well aerated. When wetted and felt between the fingers, sand particles are coarse and gritty.

    • Silt particles are smooth and powdery. 

    They normally increase the water holding capacity of the soil. Therefore the higher the amount of silt in a soil, the greater the amount of water available for plant use in that soil. • Clay particles are fine and colloidal in nature hence their rate of water absorption is very good.  Clay particles are closely packed together and contain very small and few air spaces. They feel smooth, sticky and plastic when wet and can easily be molded. They form very hard lumps when dry. Such particles remain suspended in water for a very long time.

    Activity 2.14

    Determining the water-holding capacity of a given soil 

    1. Carry out an experiment to find out the water-holding capacity of various soil samples. Your teacher may provide you with the following apparatus and materials:  • Measuring cylinders   

    • Funnels    

    • Water 

    • Cotton wool 

     • Sandy soil 

    • Clay soil  

    • Loam soil  

    • Stop watch

    2. Record the steps you will have followed, your results and conclusions. 

    3. Write a report and make a presentation to the rest of the class.

    The facts

    Sample procedure for investigating the water holding capacity of various soil samples 

    1. Dry the soil samples in the sun.

    2. Crush all the soil samples except sandy soil.

    3. Plug the funnels with equal amounts of cotton wool.

    4. Place equal amounts of the three different types of soil into each of the funnels.


    5. Place each funnel onto a separate measuring cylinder as shown above, and then quickly pour 20 cm3 of water into each of the funnels. (The water should be poured into each of the funnels simultaneously as a fourth learner starts the stop watch.)

    6. Record the time taken for any known volume of water to drain through each of the soil types in each measuring cylinder. (Once the stop watch has been stopped, the funnels must be removed from each of the measuring cylinders so that no more water drains in. Label the respective measuring cylinders with the type of soil).

    7. Note the volume of water collected from each set up. In which soil was most water collected?

    I observed that…

    • After about half a minute, the first drop will come from the funnel containing sandy soil. • In the funnel containing loam soil, the first drop was seen after about one minute while the first drop from clay soil took about five minutes to drip into the measuring cylinder.

    Conclusion

    • Clay soil is least porous of the three types of soil while sandy soil is the most porous. • It can also be said that sandy soil has low water-holding capacity while clay has the highest water-holding capacity.

    • Loamy soil has an average porosity and water-holding capacity.

    • Soils found in low-lying areas and depressions which are characterised by dull colours and fine textures usually range between imperfectly drained and poorly drained.

    Activity 2.15

    Comparing capillarity in different soils

    1. Come up with an experiment to compare the capillary action of different soils. Your teacher may provide you with the following apparatus and materials:  

    • Long capillary tubes  

    • Trough 

     • Water  

    • Samples of sandy, clay and loamy soils 

     • Cotton wool  

    • Stop watch  

    • A ruler 

     • A clamp 

    2. Note down the steps you followed, and the observations and inferences you made.

    3. Write a report and do a presentation to the rest of the class.

    The facts

    Sample procedure for an experiment to compare the capillarity  action of different soils

    1. Collect the three types of soil, sandy, clay and loam and ensure they are adequately dried.

    2. Crush the loam and clay soil samples to fine particles except the sandy soil.

    3. Plug one end of each capillary tube with cotton wool.

    4. Put the samples of sand, loam and clay soils labelled A, B and C respectively in separate capillary tubes. 5. Using a clamp, hold the tubes upright in the water trough as shown in Fig 2.20.

    6. Put water into the trough to a depth of 5 cm.

    7. Remove the tubes from the trough after about 3–5 minutes and measure the height of water in every tube.

    8. At least six readings should be taken.

    9. Leave the experiment to stand overnight. Examine the height to which the water has risen in each kind of soil after 24 hours.

    10. Plot the results on a graph; mark the time in minutes along the horizontal axis and water height in centimeters along the vertical axis.

    11. Plot the graphs for each of the three samples on the same axes.

    Note: By comparing the three graphs, we can deduce the relationship of size of soil particles to capillarity action represented by the height of the water.

    I observed that

    • After some time, water rose up through the three tubes by capillarity action. The water rose fastest in clay soil followed by loamy soil within the first few minutes. After about 2 hours, the level of water in the clay soil was highest followed by loam soil and then sandy soil. • The water stopped rising first in sandy soil and it stopped rising last in clay soil. Conclusion

    • Clay soil has the highest capillarity of the three soil types. • Loam soil has average capillarity. It is therefore good for crop production. • Sandy soil has the lowest capillarity.

    (iii) Soil structure

    Soil structure refers to the physical appearance of soil in terms of how the individual soil particles are arranged, packed and aggregated. It is a term used to describe the overall arrangement or grouping of soil particles. Aggregated soil consists of many soil particles held or cemented together. They form natural units of compound
    particles/clusters or aggregates. The consistency of the soil changes with the amount of water present in the soil. Soil aggregates are often separated from adjoining surfaces by lines of weaknesses. When a soil sample is dry, its consistency is described as loose, soft, hard or very hard. When moist, its consistency is loose, friable or firm. Wet soils are sticky and plastic. This is especially true of clay soils. Soil organic matter is important in soil aggregation due to its binding effect.

    Types of soil structures

    Activity 2.16

    Go out into the field and collect soils from different places. (Be careful not to crush the soil samples). Label the soils depending on where they were obtained. Carry the soil samples carefully back to class. Observe the soil under a hand lens. Draw the various shapes of soil structures observed. Comment on the shapes of the structures and where the soil sample was obtained.

    The facts

    There are various types of soil structures. They are categorised according to the arrangement of the particles and the pore spaces in the soil. Soil structure depends on the kind and extent of aggregation. Aggregation is influenced by climate, living organisms, topography, parent material and time. Clay particles and humus influence soil structure by the way they cement or build the different soil particles into bigger and more stable aggregates. Secretions from plant roots may influence soil structure as well. The most common types of soil structures are:  

    • Crumb soil structure  

    • Granular soil structure  

     • Single–grained soil structure  

    • Prismatic and columnar soil structure  

    • Platy (plate-like) soil structure 

     • Block soil structure

     (a)  Crumb soil structure

    The soil particles here appear irregular in shape, small and rounded. They are not closely fitted together; that is the soil particles loosely adjoin with other aggregates. This soil is therefore soft, porous and permeable, yet it retains moisture. They are normally found on horizon A.

    (b)  Single–grained soil structure

    This is an elementary structure which forms no aggregates meaning that particles are not cemented together.  This structure is relatively non-porous with small and spherical particles mostly found at the top soil of sandy soils, arid climates and alkaline soils.

    (c) Granular soil structure

    The soil grains appear irregular in shape and aggregates are rounded with smoother edges.  The aggregates are loosely held together and when wet, the grains are highly porous. This is because the spaces in between are not readily closed. This structure can be found in the top soil horizon of cultivated soils and in the subsoil horizons of soils under grasses or bushes. The granular soil structure is the only arrangement influenced by practical methods of tillage. It contains organic material and a high water retention capacity.

    (d)   Platy soil structure

    The aggregates here appear in thin horizontal plate-like layers. It is actually a soil structure whose aggregates are arranged on top of one another in relatively thin horizontal plates, as in leaflets. The plates often overlap and impair permeability. This impedes drainage and root penetration. Soils with such structures are poorly drained and are not suitable for growing crops. The structure is mostly found at the top horizon of soils in forests and it is mainly found in clayey soils.

    (e)   Prismatic structure 

    The soil particles in this structure are cemented in vertically oriented pillars. The tops of the pillars could be shaped in such a way that they are level, plane and clean cut. This is what gives the structure the name prismatic.  Soils with such a structure are normally located in the subsoil horizons of arid and semi-arid lands.

    (f)   Columnar structure 

    When soil aggregates are arranged vertically but with flat rounded tops, they are said to have a columnar structure. The columns are similar to those in the prismatic soil structure; apart from the rounded tops. They are also found in the subsoils of arid and semi arid areas.

    (g)   Block soil structure

    The aggregates are arranged in rectangular blocks. The aggregates easily fit together along vertical edges. They make penetration of plant roots difficult because their angular edges fit closely. They are moderately permeable, poorly aerated and drained. The blocky structure is common in finely textured subsoils.

    Activity 2.17
    Having learnt about the various soil structures, refer to the ones you had drawn in Activity 2.16. Identify and name the various soil structures you had drawn.

    Influence of soil structure on crop production

    • A good soil structure ensures a good balance between soil water and air since soil structure influences the pore spaces in the soil. In fact the amount of air and water present in a soil sample depends on the pore spaces available.  This implies that soils with closely packed particles are poorly aerated and drained. • A good soil structure aids drainage thereby avoiding waterlogging.  Remember that most crops do well in well aerated and drained soils except for a few such as rice, which do well in waterlogged soils. • A good soil structure also ensures adequate water retention for the plants. 

    It also reduces accumulation of carbon dioxide in the soil through proper aeration. • Use of heavy machinery on wet soils destroys the structure thus decreasing permeability and aeration.  This results in high incidences of surface run-off and erosion.

    • Soil structure influences the water-holding capacity of a soil. A good soil should hold enough water for plant use.  A soil which cannot retain water, though fertile, may not be good for crop production as there will be no water available for plant use. • Soil living organisms respire and produce carbon dioxide which must be removed from the soil so that it does not build up to toxic levels.  This is facilitated by free circulation of air. The structure should allow free circulation of air by having enough pore space, which can be occupied by air as in granular or crumby soil structures. In such soils, the plant roots and microorganisms can get the oxygen they need while carbon dioxide is expelled easily. • Waterlogged soils may result from structures whose particles fit closely together. For example soils with platy structure have a higher capacity for holding water, hence such structures may be good for crops such as rice.

    b) Chemical properties of soil

    Chemical properties of soil are influenced by the following factors:  

    • The level of organic matter in the soil  

    • The amount of rainfall or precipitation  

    • The mineral rock from which the soil particles have been derived On decomposition, organic matter in the soil releases organic acids, like carbonic and nitric acids, which make the soil acidic. In high rainfall areas, soils tend to be acidic due to excessive leaching of bases like potassium hydroxide, calcium hydroxide and magnesium hydroxide. Soils derived from rock particles rich in minerals, such as aluminum, are usually acidic.

    The four major chemical properties of soil include:

     • Soil pH  

    • Salinity  

    • Cation Exchange Capacity (CEC)  

    • Carbon: Nitrogen ratio 

    (i)   Soil pH 

    Soils may generally be referred to as either acidic or basic. One of the most important chemical properties of soil is pH. Soil pH is a measure of the degree of acidity or alkalinity of a soil solution. It is expressed as the potential hydrogen, which is the hydrogen ion (H+) concentration in a soil solution. 

    Soil pH can be measured in two ways:  

    • Using a universal indicator solution: This solution results from the mixing of several acidic-base indicators put together. When this is added to a soil solution, the colour change is matched with the colours on the pH chart.

     • Using a pH meter: This is a device used to determine the pH of a soil solution. The equipment is expensive and may only be found in agricultural laboratories.

    The pH scale shown above runs from 0–14 corresponding to hydrogen ion concentration with pH 7 being the neutral point. The values in the pH scale that fall above 7 are alkaline while those less than 7 are acidic. This means that the lower the pH, the more acidic a soil solution is; that is solutions with low pH values are strongly acidic while those with high pH values are highly alkaline.

    Activity 2.18

    Determining soil pH using the Universal indicator

    1. Conduct a research in the library or by using the internet and find out how the universal indicator solution is used to determine soil pH. Note down your findings.

    2. Having known how the Universal Indicator solution is used, come up with an experiment to determine the pH of various soil samples. Your teacher may provide you with the following apparatus:   •     Test tubes  

     •     Universal indicator solution  

     •     A pH chart  

     •     Barium sulphate powder  

     •     Soil samples  

     •     Distilled water 

    3. Record the steps you will follow, the observations and readings, and note down the inferences you make. 4. Write a report and share with the rest of the class members.

    The facts
    A sample procedure for determining the soil pH of various soil samples:

    1. Place the soil samples in different test tubes to a height of about 1 cm.

    2. Add an equivalent amount of barium sulphate to the test tubes containing the soil samples-this helps to ensure flocculation and precipitation of colloidal clay.

    3. Fill the test tubes with distilled water to about 4 cm from the top.

    4. Shake the test tubes thoroughly.

    5. Allow the contents to settle; then add 8–10 drops of the universal indicator solution.

    6. Shake the test tubes again and allow the contents to settle.

    7. Hold each of the test tubes against the pH chart.

    8. Compare each colour on the pH chart with the colour of the suspension and note the pH of the colour which matches it most closely.


    I have discovered that…

    Various soil samples collected from different places have varying pH values. Various plants also have preferences for soils with specific pH values.

    (ii) Salinity  Soil

    salinity refers to the concentration of salts in a soil solution. This can be pronounced at the top soil surface. Salt solutions can move to the top soil surface by capillarity from the salt laden water table. They then accumulate due to evaporation of water. Salt can also accumulate due to human activities, such as use of potassium fertilisers, which accumulates phosphate salts. As soil salinity increases, it results in soil degradation.

    Activity 2.19

    Determining soil salinity using a salinity meter

    1. Perform the experiment below to determine the salinity of soil in an area of your choice. You may be provided with the following apparatus:   

    •     Soil sample    

    •     Distilled water  

     •     Weighing scale  

     •     Measuring cylinder  

     •     Jug or any container   

    •     Salinity meter 

    2. Follow the steps below: 

    (i) Take a sample of soil and leave it to dry in the sun.

    (ii) Crush the soil lumps after drying. Use a wide and heavy blunt object, such as a hammer.  

    (iii) Place 50 g of the dried soil in the jug and add 250 cm3 of distilled water. 

    (iv) Shake the content vigorously for about 3 minutes to enable salts in the soil to dissolve in the water. 

    (v) Allow the solution to settle for at least 1 minute. 

    (vi) Place the salinity meter in the solution and read the display.

    Note: Do not dip the salinity meter into the soil settled at the bottom of the container. Soil salinity can also be tested by use of a conductivity meter.

    3. Carry out research on the internet and find out how the conductivity meter is used.

    Note down the steps to follow and use it to measure soil salinity. (The conductivity meter will be provided by your teacher.)

    (iii) Cation exchange capacity of soil (CEC) 

    Cation exchange capacity of a given soil refers to the total capacity of a soil to hold exchangeable cations.  Factors influencing Cation Exchange Capacity of soil 

    • Nature of clay minerals in the soil-for example a CEC increases with the amount of clay and it also varies with the type of clay.

     • Texture of soil-that is finer textured soils have more mineral colloids than coarse textured ones. 

    • Organic matter – the higher the humus content the higher the CEC. The type of humus compound is also important. For example, humus which has been developed from monocotyledonous leaves are better.  • Soil pH – The higher the soil pH, the higher the CEC.

    Note:

    • The availability of a certain cation to plants will depend very much on the proportion of that cation in the cation exchange capacity of the soil. 

    • The replacement of cations by others is known as cation exchange.

     Importance of Cation Exchange Capacity of a soil (CEC) 

    • Cation Exchange Capacity of a soil is described as a measure of how much nutrients a soil can hold rather than how fertile a soil is.  However, it is very much correlated with natural soil fertility because it indicates the degree of weathering.

     • CEC guides a farmer on the level of fertilisers and liming to apply.  Nutrients should be applied to the soil in amounts which the soil can hold not in big surpluses which will leach away without being taken up by plants. 

    • On the other hand a soil with a high CEC requires high fertiliser application and or liming before nutrients can be available to plants.

    (iv) Carbon: Nitrogen ratio

    Carbon is an essential constituent of all living things. It occurs naturally in the atmosphere in form of carbon dioxide whereby it constitutes 0.03% of air by volume. Atmospheric carbon dioxide is the major source of carbon required by plants. The various processes contribute to the circulation of carbon in the atmosphere. These processes include those that use carbon from the atmosphere and those that replenish carbon into atmosphere.
     Nitrogen is one of the most important elements needed for plant growth.  It occurs naturally in the atmosphere in form of nitrogen gas (N2); it constitutes 78% of air by volume.  However, it is not available to plants in this free gaseous form. The various processes which contribute to the circulation of nitrogen in the atmosphere include those that use nitrogen from the atmosphere and those that replenish nitrogen into the atmosphere.
     Therefore, the Carbon: Nitrogen ratio through its selective influence on soil organisms, exerts a powerful control on nitrification and the presence of nitrogen in the soil. The nitrogen in the soil may be used by the soil microorganisms and higher plants, or it may as well be lost through leaching, or it may escape into the air in volatile form. For purposes of encouraging the useful microbial activity in the soil, it is important to maintain a good balance of the C:N ratio in the soil.

    (c) Biological properties of soil

    Living organisms are found almost everywhere on earth.  These living organisms have both positive and negative effects on their surroundings. Living organisms may include pests, parasites, decomposers, pathogens, predators, pollinators and nitrogen-fixing bacteria. The main soil micro-organisms you shall learn about in this section are bacteria.  The following are the two major categories of bacteria that are important in soil:

     • Symbiotic bacteria – Found in nodules of leguminous plants such as beans. They mainly include rhizobium bacteria.  

    • Non-symbiotic bacteria – Found in the soil which include azotobacter bacteria.

    Apart from decomposing organic matter, bacteria perform other useful functions like fixing free atmospheric nitrogen into the soil for plant use. This is done by the rhizobium bacteria. Nitrogen is converted into nitrates which are absorbed by plants. Some micro-organisms damage crops by causing bacterial and fungal diseases in plants. Nematodes live as parasites in plant roots and interfere with the nutrient and water uptake.  Clostridium and azotobacter are two genera of anaerobic soil bacteria that are dependent on plants for their activities. They can fix atmospheric nitrogen to nitrogenous matter. When these micro-organisms die, they decompose and release the nitrogen compounds into the soil for use by crops.  On the other hand, soil nitrogen may also be lost in form of ammonia, nitrogen gas or oxides of nitrogen. This may be due to the activity of certain denitrifying anaerobic bacteria which can oxidise ammonium on to ammonia gas; nitrates and nitrous acid are reduced to nitrogen and oxides of nitrogen such as nitrogen oxide.

    Self-evaluation Test 2.6

    1. How does the soil in your area behave when it rains? What does that say about its water-holding capacity?

    2. Why should a prospective farmer be keen on investigating the properties of soil in a land he/she is intending to cultivate first before embarking on any farming?

    2.7 Soil sampling and testing

    Soil sampling is the process of random collection of a small quantity of soil from a defined area of land. The soil then acts as a representative sample for laboratory testing and analysis.  The defined area of land where the sample is taken from is known as a sampling unit. Such a chosen area should be uniform in terms of slope, drainage, soil texture, soil colour, fertiliser usage and history of cropping. Soil is sampled in order to be tested for nutrients and soil pH. An analysis of the samples gives information about the fertility status of the soil. In fact, sampling should be done when crops show deficiency symptoms or whenever yields start to drop.

    (a) Soil sampling methods

    There are two methods of soil sampling which are usually used:

    • The traverse method

     • The zigzag method 

    (i)  The traverse method  This is where the sampling follows a line along diagonals of the field or a sampling unit. It is also referred to as the diagonal method.

    (ii) The zigzag method  This is where the sampling forms random zigzag patterns in the sampling unit.  Here locations are arranged in such a way that they are in a zigzag form as shown below.

    When sampling, the following areas should be avoided: 

    • Places where manure or organic matter have been heaped  • Fence lines or boundaries which may not give representative sample. 

    • Places near trees  

    • Swampy areas  

    • Dead furrows 

     • Footpaths in the field  

    • Ant-hills in the field  

    • Areas between slopes and the bottom line

    Soil sampling procedure

    When collecting a soil sample the following procedure should be followed carefully in order to get reliable results and a good representative sample.

    1. Clear sampling unit of any vegetation by scrapping it off.

    2. Make a vertical cut to a depth of 15-25 cm for crop land and 5 cm for top soil and 25-30 cm for subsoil.

    3. Take a slice from the vertical cut, preferably using a soil auger.

    4. Put the soil obtained from each site, for both topsoil and subsoil layers, in a clean container.

    Note: The above steps can be done on 10-20 sites depending on the sampling method being used.

    5. Remove any foreign materials from the collected soil samples.

    6. Dry the soil samples and crush them into smaller particles or colloids.

    7. Use the quartering technique to arrive at a small quantity of each representative sample.

    8. The samples should then be packed in sampling envelopes and dispatched to the laboratory with the following information given:  

    • Name and address of the farmer or locality of the farm  

    • History of fertilisers or manure use     

    • Crop to be grown 

     • History of land use 

     • Special features on the land  

    • Date of sampling

    Activity 2.20

    Perform a soil sampling procedure on your school farm. Record your results and present them in class.

    (c) Soil pH

    Soil pH, as we have seen previously, is a measure of the degree of acidity or alkalinity of a soil solution.

      Testing soil

    pH From our previous discussions, we saw that soil pH can be determined accurately. We only used the Universal indicator solution as one method of testing for soil pH. However, here we shall explore more ways of determining soil pH. It is important for farmers to understand the pH status of soil in their prospective and even present pieces of land to avoid disappointments. The following are some alternative methods of determining soil pH.

    (i) Using a pH meter

    The pH meter is an expensive equipment only found in research stations, colleges, universities and some teacher training colleges.  This method is also known as glass electrode method.  The pH meter consists of a thin-walled bulb of special glass which contains dilute hydrochloric acid, into which a platinum wire is dipped to make an electrical contact.

    This arrangement is sensitive to the hydrogen ion concentration of the solution to which it is immersed.  The pH meter method is accurate. However, the set up is expensive hence it is only limited to research work.

    Using colour indicators


    In soil pH testing, the colour indicators that can be used include the following:

    • Litmus paper

    • Universal indicator papers (pH papers)

    • Colour indicator dyes Some of the procedures of determining soil pH using colour indicators are outlined below.

    (i) Universal indicator method

    1. Place a little soil on clean porcelain plates.

    2. Pour 1–2 cm3 of the indicator on the soil.

    3. Wash the soil with the indicator and allow the liquid to drain away from the soil into a clean part of the porcelain.

    4. The colour of the solution is observed against the pH colour chat and the acidity or alkalinity of the solution estimated.

    (ii) Using universal indicator papers

    1. A small quantity of soil sample is taken and added to about 25 cm3 of distilled water which has been boiled to expel dissolved carbon dioxide.

    2. The mixture is shaken thoroughly and a piece of the pH paper is then dipped.

    3. The colour change of the pH paper is checked against the pH chart to give the correct pH value.

    (iii) Using a commercial soil testing kit

    • A small quantity of soil sample is taken and shaken with distilled water to make a soil solution. • A few drops of commercial indicator are added to the solution. • The colour of the soil solution is then compared with the colours on the colour chart.

    Note: The basis of this method is the fact that different indicators change colour at different pH values.

    (iv) Using litmus papers

    Although litmus papers can also be used, they only indicate whether a solution is acidic or alkaline and cannot give specific pH values. Remember that use of litmus papers is the simplest pH testing method. The litmus papers exist in two colours; blue to test for acidity and red to test for alkalinity. If the solution is acidic, blue litmus paper turns red, while the red litmus paper remains red. If the solution is alkaline, the blue litmus paper remains blue while the red litmus paper changes to blue. Litmus paper colour remains the same in neutral solutions. The limitation of this method is that the pH value is not known.

    Self-evaluation Test 2.7

    1. Select three areas that should be avoided during soil sampling and justify why they should be avoided.

    2. Which method or instrument of determining soil pH is the best according to you and why?

    Remember the facts!

    • Soil refers to the loose natural material on the uppermost layer of the earth crust.

    • Formation of soil from the parent material is known as soil genesis. • Soil is formed by weathering of rocks through various physical, biological and chemical weathering processes.

    • The common types of soils are clay soil, loamy soil and sandy soil. 

    • The quantity of water in any soil sample depends on the type of soil it is.

     • Soil contains a certain percentage of air. Air is a component of the soil. • Humus is normally made up of organic matter. • Soil usually contains living organisms which respire actively. • We should avoid burning soil and using harmful chemicals on soil. These might cause harm to the soil living organisms. • Soil profile is the vertical arrangement or a cross-section of soil particles in different layers from ground level. The layers of the soil profile are called horizons.

    • Physical properties of soil include: 

     − Soil colour 

     − Soil texture  

    − Soil structure 

    • Soil is made up of particles of different sizes. 

    They include:  

    − Gravel: fairly large and heavy.  

    − Sand: coarse-textured and very well aerated.  

     Silt: smooth and powdery.

     − Clay: fine and colloidal. • Soil structure refers to the physical appearance of soil in terms of how the individual soil particles are arranged, packed and aggregated. • Soil pH is a measure of the degree of acidity or alkalinity of a soil solution. 

     • Various plants have preferences for specific pH values. 

    • Cation exchange capacity (CEC) of a given soil refers to the total capacity of a soil to hold exchangeable ions.

    • Soil sampling is the process of random collection of a small quantity of soil from a defined area of land. The soil will then act as a sample for laboratory testing and analysis.

    Test your competence 2

    1. Explain what happens during soil genesis in your own words.

    2. Distinguish between the three types of soils described in this unit and give the appropriate uses of each type.

    3. (a) How does the water-holding capacity of a soil relate with texture? 

    (b) How does this affect crop farming?

    4. Explain the factors which determine the amount of soil water in any given type of soil.

    5. We must avoid burning soil and using dangerous chemicals on it. Explain why.

    6. Explain some of the activities which you think can lead to increased soil salinity.

    7. How does carbon:nitrogen ratio affect plant growth?

    8. Which activities do you think can destroy the physical soil structure and how?

    9. (a) Why is sandy soil more erodible than clay soil? 

    (b) What are some of the things you can do to prevent soil erosion?

    10. Mutesi has just acquired a piece of land which she plans to cultivate. However, the piece of land is largely dominated by sand soil. Explain what advice you would give Mutesi so that she can improve the soil fertility of the piece of land.

    11. Which soil structure is likely to encourage waterlogging?

    A. Single-grained soil structure

    B. Platy soil structure

    C. Crumb soil structure

    D. Granular soil structure