◄UNIT 5 : ROCKS AND MINERALSUNIT 7: CLIMATE CHANGE ►

UNIT 6 : CLASSIFICATION OF SOILS AND SOIL FORMATIO

  • UNIT 6 : CLASSIFICATION OF SOILS AND SOIL FORMATIO

    Key Unit competence
    By the end of this unit, I should be able to explain the classification of soils 

    and factors responsible for the formation of the soil.

     Introductory activity
     Read the passage below and answer the questions that follow:
     Soil is defined as the thin layer of material covering the earth’s surface and 
    is formed from the weathering of rocks. It is composed of mineral particles, 
    organic materials, air, water and living organisms all of which interact slowly 
    but constantly.

    Most plants get their nutrients from the soil and they are the main source 
    of food for humans, animals and birds. Therefore, most living things on land 
    depend on soil for their existence.

    Soil is a valuable resource that needs to be carefully managed as it is easily 
    damaged, washed or blown away. If we understand soil and manage it 
    properly, we will avoid destroying one of the essential building blocks of our 

    environment and our food security.

     1. Identify major types of soil in the world
     2. Describe factors responsible for soil formation
     3. Assess the importance of soil to man
     4. Discuss the major causes of soil erosion and suggest what should 

    be done to prevent it

     6.1. Definition of the soil
    Soil is a dynamic natural body capable of supporting a vegetative cover. It 
    contains chemical solutions, gases, organic refuse, flora, and fauna. The physical, 
    chemical, and biological processes that take place among the components of a 

    soil are integral parts of its dynamic character.

     6.2. Classification of the major types of soil in the world, 
    factors and processes of soil formation
    This section presents briefly the classification of the major types of soil in the 

    world, factors and processes of soil formation are briefly described.

     Learning activity  6.1
     1. Make research on the major types of soils in the world.
     2. Identify factors influencing soil formation

     3. Discuss on processes leading to the formation of the soil

     6.2.1. Classification of the major types of soil in the world
     The classification of soils is either based on geographic regions, where 
    the soils are well-developed from the parent material by the normal soil
    forming action of climate and living organisms.  Another way of classifying 
    the soil is based on the level of weathering, which is related to geographic 
    environments, but also under the same geographical region you can find 

    different types of soils which reflect the level of weathering.   

    A) Soil classification based on geographical regions
     The soil classification based on geographical regions, include three soil 
    classes: zonal soil, intrazonal soil and azonal soil.

     i) Zonal soils 
    These are soils that cover a wide geographic region in the world. They depend 
    on the major climatic zones, vegetation and living organisms in areas where the 
    landscape and climate have been stable for a long time.  They are common on 
    gentle slopes. They are found both in tropical and temperate regions. 
    This kind of soil has the following types: Tundra Soils, Podzols, Brown forest 
    Soils, Lateritic Soils / Latosols / Ferralsols, Chernozem / Prairie / Steppe, 
    Grumusol / Reddish Brown Soils, Desert (Seirozems and Red Desert) Soils.

    ii) Intrazonal soils
     These are soils that mainly develop due to relief of the area and the nature of 
    parent rock. These soils reflect the dominance of a single local factor, such as 
    parent rock or extremes of drainage that prevail over the normal soil-forming 
    factors of climate and living organisms. They are divided into three types:
     • Calcimorphic or calcareous soils which develop on limestone 
    parent rock (rendzina and terra rossa);
     • Halomorphic soils which contain high levels of soluble salts (e.g. 
    sodium ions) which render them saline. 
    • Hydromorphic soils that have constantly high water content which 
    tends to suppress aerobic factors in soil-formation.
     
    iii) Azonal soils 
    Azonal soils have a more recent origin and occur where soil-forming processes 
    have had insufficient time to operate fully. They lack well-developed horizons 
    because of immaturity or other factors that have prevented their development 
    such as excessive soil erosion. They are skeletal soils resulting from erosion and 
    deposition. They lack clear soil horizons. They are common in volcanic regions, 
    glaciated regions and areas blown by winds. They include dry sand, loess, 
    moraine soils, and marine soils, alluvial and volcanic soils. 

    The map below shows the major soil types of the world

    B) Soil classification based on level of weathering 
    Basing on the level of weathering, the American soil taxonomy has classified 
    soils into 12 soil orders which reflect the level of weathering (slight, intermediate 

    and strong) plotted on the chart and briefly described below:

    Table: Major soil orders according to American classification

    6.2.2. Soil formation factors
     Soil formation is a function of five factors which include parent material, 
    climate, biology (living organisms), relief (topography), and time. They are 
    classified passive (parent material, relief “topography” and time) and dynamic 
    (climate and biology “living organisms)”. Recent studies have shown that human 
    activities can have an impact on soil development. These factors interact as a 

    system to form soils. The roles of these factors are briefly hereafter described:  

    Parent rock
     Physical and chemical weathering of rocks in the upper lithosphere provides 
    the raw mineral ingredients for soil formation. This helps to determine the type 
    of soil, mineral composition and texture. For instance, granite and sandstone 
    disintegrate to form sandy soils rich in quartz, volcanic lavas form clay soils with 

    low quartz content and plants decompose to form loam rich in humus.

    Climate 
    The moisture (rainfall), evaporation and temperature changes determine the 
    chemical reactions and physical breakdown of rocks. Climate also affects rate 
    and type of weathering. For example, heavy rainfall results into deep soils due 
    to heavy weathering and leaching, wind in deserts is responsible for formation 

    of loess soils. 

    Living organisms 
    Plants, animals and microbes are living organisms that affects soil development. 
    Dense vegetative cover protects a soil from being eroded away by running water 
    or wind.  .  Burrowing animals and worms mix organic remains with mineral soil 
    component.  - Roots penetrate and add more porosity, improve soil depth and 
    aeration. Micro-organisms such as bacteria cause plant and animal remains to 

    decay into humus

     Topography 
    The topography represents the slope of the relief. The slope of the land and its 
    aspect (the direction it faces) all influence soil development. Steep slopes are 
    generally subject to rapid surface runoff of rainfall and less infiltration of water, 
    whereas on gentler slopes runoff decreases with an increasing infiltration.  As a 
    consequence, rapid runoff on steep slopes can erode soils as fast, or faster 
    than soil can develop on them. Steep slopes result in shallow immature soils 
    due to severe erosion and prevent the formation of a soil that would support 
    abundant vegetation, 

    On     gentler slopes there is higher infiltration and less runoff. More water is 
    available for soil development and to support vegetation growth, so erosion is 
    not as intense. Well-developed soils typically form on land that is flat or has a 

    gentle slope.

     Time
    All of the mentioned above natural factors in soil development require time to 
    operate. This determines the depth of weathering and the period of operation 
    of soil formation processes. Briefly, the longer the time taken by soil forming 

    processes the deeper and well developed soil is.

     6.2.3. Processes of the soil formation
    The formation of soil requires numerous processes.  Soil is said to be formed 
    when organic matter has accumulated and colloids are washed downward, 
    leaving behind deposits of clay, humus, iron oxide, carbonate, and gypsum, 

    producing a distinct layer called the ‘B’ horizon.     

    Weathering: Weathering is the process by which the rocks break down into 
    small particles to form soil. It is the combined action of physical weathering, in 
    which rocks are fractured and broken, and chemical weathering, in which rock 
    minerals are transformed to softer or more soluble forms.

    Mineralization:     This is the process through which organic matter is further 
    decomposed into mineral compounds. Mineral content in humus may be further 
    converted to inorganic matter e.g. silica.
     Humification: Humification is the process by which organic matter is 
    decomposed to form humus, a task performed by soil organisms. 

    Eluviation:      Eluviation is the downwards movement of fines particles such as 
    clay and the leached soluble materials from upper layers of the soil (‘A’ horizon) 
    to another lower layer within the soil.

    Illuviation    : This is the process of accumulation of clay, aluminum and iron 
    usually from A and E horizons to B horizons.

    Leaching:     Leaching is the removal of soluble material in solution.  It is the 
    process by which water removes leached materials (organic and inorganic) in 
    solution from the upper horizon to the underlying horizon. It operates vertically 
    but not sideways. 

    Laterization:     Laterization is leaching of soils in warm and humid climates. It is 
    a process that occurs after the soluble mineral substances have been leached. 
    After leaching, the insoluble mineral compounds derived from the parent rock 
    remain on top, hence forming lateritic soils that are stony.

    Calcification:     This is the process in which calcium carbonates accumulates in 
    the ‘B’ horizon; particularly characteristic of low rainfall areas such as arid and 

    semi-arid climates.

     Application activity  6.1
     1. Soil forms continuously, but slowly, from the gradual breakdown of 
    rocks through weathering:
     a. Explain how organisms contribute to the formation of soil 
    b. Describe any three other processes leading to the soil formation 
    2. With reference to the knowledge and skills you have acquired in this 
    unit, discuss the difference between zonal soils, azonal soils and 
    intrazonal soils.
     3. Based on the level of weathering, describe the soils orders according 

    to American soil classification  

    6.3. Soil erosion: causes, effects, appropriate soil 
    management and the conservation measures and importance of soil 
    Learning activity  6.3
     1. What does soil erosion means?
     2. Identify major causes of soil erosion 

    3. Discuss on the effects of the soil erosion

     6.3.1. Cause of soil erosion 
    The predominant causes of soil erosion are either related to naturally occurring 
    events or influenced by the presence of human activity. If we want to prevent 
    soil from going away, we need to understand different factors contributing to 
    the soil erosion.  Some of the major causes of soil erosion include:
    – Overgrazing also causes excessive loss of water from the soil causing 
    it to become loose and fine grained and easily eroded. 
    – Rainfall: In a particular heavy rain result to excessive soil erosion and 
    thus poorly aerated
    – Drought: A long dry weather deprives the soil of moisture which holds 
    the soil together causing particles to loosen making it to be easily 
    brown by wind. 
    – Some human works in relation with excavation activities such as 
    quarrying, open-cast mining, building of estates and road construction 
    which loosen and expose the soil to erosion agents.
    – Slope of the landscape: The physical characteristics of the land can 
    contribute to the soil erosion. For example, steep slopes accelerate 
    soil erosion while gentle slopes experience less erosion, places with 
    rugged terrain experience gulley erosion while hilly and steep areas 
    experience rill and gulley erosion.
    – Poor cultivation techniques such as pulling hoe along the surface 
    when removing weeds which loosens the soil and when it rains it’s 
    washed away,  ploughing of land down slope which accelerates 
    soil erosion, cultivation of steep slopes and along river banks which 
    encourages soil erosion, burning which destroys vegetation covering 

    the soil exposing it to erosion agents etc.  

    6.3.2. Effects of soil erosion 
    Some of the greatest effects of soil erosion include:
    – Loss of topsoil: Soil erosion lowers the agricultural productivity of 
    land when fertile top soil is eroded.
    – Desertification: Soil erosion contributes to desertification when top 
    soil is eroded leaving bare ground destroying vegetation. 
    – Water pollution: Serious soil erosion is responsible to water pollution 
    when agro-chemicals and other chemicals are carried to rivers, lakes 
    or oceans.
    – Flooding: Another effect of soil erosion is that it contributes to flooding 
    by blocking river channels causing them to burst their banks during the 
    rainy season flooding the adjacent areas.
    – Alteration of the landscape: Soil erosion can cause significant 
    alteration to the natural shape of the land. For example, it can make 
    huge valleys to occur on plain lands.
    – Reduced organic and fertile matter: Removing topsoil that is heavy 
    with organic matter will reduce the ability for the land to regenerate new 

    flora or crops.

    – Eye and respiratory problems: Soil erosion especially one caused 
    by wind can cause eye and respiratory problems. The latter can happen 
    when people inhale the dust and soil particles being carried away by 
    the wind into their lungs. Eye problems can also occur when the dust 
    particles from wind erosion enter into the eyes.
    – Water siltation:     Persistent soil erosion causes siltation of water 
    reservoirs reducing their utility. For example,  H.E.P. generation, 
    navigation and fishing 
    – Destruction of properties:     It may cause collapsing of structures such 
    as buildings and bridges when soil around them is eroded weakening 

    their foundation.

    6.3.3. Appropriate soil management and the conservation 
    measures
    Generally, when it comes to finding solutions for soil erosion, the most useful 
    techniques found tend to be those that highlight reinforcing the structure of the 
    soil, and reducing processes that affect it.
    – Careful tilling: Due to the activity of preparing land for growing that 
    involves break up the structure of the soil, doing less tilling with fewer 
    passes will preserve more of the crucial topsoil 
    – Crop Rotation: If farmers want to keep their land happy and healthy, 
    they are strongly advised to apply crop rotation.  Growing crops which 
    require different nutrients on the same piece of land on rotational basis 
    to prevent exhaustion of particular mineral nutrients from the soil.   
    – Mixed farming: This involves growing crops and keeping animals on 
    the same farm. Consequently, manure from animals is used to enrich 
    the soil with minerals and improve its structure. 
    – Increased knowledge: another  major factor for preventing soil 
    erosion is education more and more people who work with the land on 
    why it is a concern, and what they can do to help reduce it.
    – Contour Ploughing: Ploughing across the slope rather than down 
    the slope. This practice helps to trap water on horizontal furrows thus 
    preventing excessive soil removal. 
    – Terracing: Through dividing the slope into a series of wide steps, crops 
    can be grown on them. This helps to trap the soil from being carried 
    away by running water and also traps water allowing it to gradually 
    infiltrate into the soil.  
    – Afforestation and reafforestation: Vegetation play a big role in 
    preventing soil erosion: 
    • Leaves reduce the force of rain drops preventing soil particles from 
    being removed.
     • Plants  protect the soil, more dense plant cover yields less damage 
    from erosion.
     • Vegetation increases the rate of infiltration of rain water into the soil 
    thus reducing runoff. 
    • Roots bind the soil particles together. 
    • Decayed vegetation provides humus which binds the soil particles 
    together. 
    – Planting wind breakers: Planting hedges or trees around plots in 
    large fields acts as wind breakers and also trap soil being carried by 
    water.  
    – Regulating livestock numbers: Matching the number of animals 
    kept to the carrying capacity of land.  
    – Paddocking: Overgrazing can also be prevented by paddocking which 
    ensures there is always pasture for animals and no area is overgrazed.
    – Constructing Gabions:  Construction of wire mesh boxes which are 
    filled with soil.  This allows water to pass through but trap the soil then 
    vegetation gradually grows on the trapped soil. 
    – Planting Cover Crops: Planting crops which cover the soil properly 
    and holds the soil in place e.g. sweet potato vines.  
    – Mulching: This practice consist of covering the soil with crop residues.  
    • It helps reducing the impact of rain drops on the soil.  
    • Decays enriching soil with nutrients. 
    • Reduce the rate of moisture evaporation from the soil.
     
    6.3.4. Economic importance of the soil
    – Soil provides physical support for the rooting system of plants and 
    protects root system from damage.  
    – It is a conducive habitat for burrowing animals and bacteria necessary 
    for breakdown of organic matter into humus. 
    – Soil acts as a medium through which nutrients and air are made available 
    to plants.  
    – It provides mineral elements to plants e.g. nitrogen, calcium, phosphates, etc.  
    – Serve as a construction material for building and other infrastructure. 
    Example, clay is used for making bricks and tiles. 
    – Clay soil is used in ceramics such as making pots.  
    – Source of minerals especially to expectant mothers. 
    – Soil contains valuable mineral elements such as alluvial gold.  
    – Soil supports plant life which is a source of food for people and animals 
    especially herbivores.  Soils are used for medicinal purposes e.g. clay 

    is mixed with some herbs for medical purpose in some communities.

    Application activity 6.3  

    Study carefully this photograph and answer the questions that follow

     1. Suggest what could be the cause of  the colored  river
     2. Examine the effects of soil erosion
     3. If you had a chance to become a chairperson in charge of environmental 

    conservation, what would you suggest to handle the above cases?

    Skills Lab
    Identify any area affected by soil erosion and explain to the local people 

    what should be   done to slow down the washing away of soil. 

    End unit assessment
     1. With reference to your knowledge and skills, show difference 
    between three categories of the soil in the world.
     2. Explain how soil erosion is one of the major problem challenging 
    agriculture.
     3. Soil is one of the amazing products of nature and without which 
    there would be no life. Justify 
    4. Most farmers in the northern province of Rwanda use terracing as a 
    measure of soil conservation. 
    a. Explain why terracing is mostly used in this area.
     b. Describe other soil conservation techniques used in your   area.
     c. Show how these techniques are helpful to environmental 

    sustainability.

    ◄UNIT 5 : ROCKS AND MINERALSUNIT 7: CLIMATE CHANGE ►