◄UNIT 3 ROCKSUNIT 5 WEATHER AND CLIMATE IN THE WORLD►

UNIT 4 SOILS

  • UNIT 4 SOILS

    Key Unit competence: The student-teacher should be able to explain the
    processes responsible for soil formation, causes and
    effects of soil erosion, and suggest measures for soil

    conservation.

    Soil is defined as the thin upper layer of earth in which plants grow, a black or
    dark brown material typically consisting of a mixture of organic remains, clay,
    and rock particles. Soil is one of humankind’s most important natural resources
    because all food production and many other human activities depend on it.
    Soil formation is influenced by both natural and human factors. They are parent
    rock, climate, living organisms, topography, human activities and time. They
    are described below.

    4.1.1 Parent rock
    The parent rock is the rock material that breaks down to form soil particles.
    Parent rocks differ in terms of structures; some are hard while others are soft.
    Therefore, they are weathered at different rates. The rate of weathering of a
    parent rock also differs according to whether it has or does not have joints.
    Joined rocks are more susceptible to weathering and readily break down to
    give rise to soil particles.

    The colour of the parent rock also determines the rate at which it absorbs heat,
    expands and contracts and therefore breaks down. Soil formation is likely to be
    faster on dark coloured rocks, compared to shiny ones which reflect heat. The
    composition of a rock directly determines the nature of the soil nutrients. For
    example, a quartz rock leads to the formation of sandy, porous soils, while a
    limestone rock leads to the formation of soils rich in lime. Weathering of laterite
    yields thin skeletal and poor laterite soils.

    4.1.2 Climate
    Through elements of climate mainly rainfall and temperature, climate
    influences the soil formation. Rain provides water which is the prerequisite
    for chemical weathering processes which leads to rock decomposition hence
    the formation of soils. Furthermore, high temperatures speed up the rate of
    chemical reactions. Because of this, soil formation in hot climates is faster than
    in cold regions.

    4.1.3 Relief or topography
    Topography also affects soil formation. It determines the rate of erosion of the
    surface rocks. Steep slopes are often more susceptible to erosion than gentle
    ones. This implies that there is a more rapid loss of nutrients on steep slopes
    compared to gentle ones. This explains why only shallow, skeletal soils can
    form on steep slopes. Although steep slopes are associated with thin soils,
    they are also associated with rapid rates of soil formation because the rate of
    exposure of the parent rock to agents of weathering is high. Gentle slopes tend
    to have deep, mature and well developed soils because the rate of removal of
    soils is low and the rate of deposition is high. Flat landscapes are susceptible to
    leaching therefore; poor soils tend to develop over such environment.

    4.1.4 Living organisms
    Vegetation and the activities of animals and bacteria determine the organic
    content of soil, along with all that is living in soil (algae, fungi, worms, and
    insects). The chemical composition of the vegetation contributes to the acidity
    or alkalinity of the soil solution. For example, broadleaf trees when decomposed
    tend to increase alkalinity whereas needle-leaf trees tend to produce higher
    acidity. Also decay of plants and animals supply the soil in humus and nutrients.
    Animals contribute to soil development through breaking down of vegetation
    and rocks into small particles that form the soil. The figure below represents

    the diversity of life in fertile soil that contributes to soil formation and recycling.

    4.1.5 Time

    All of the identified natural factors in soil development (parent rock, climate,
    biological activity, and topography) require time to operate. If the parent
    rock has been exposed to weathering for a long time soil formation will be
    complete compared to another rock that has been exposed to weathering for
    a comparatively shorter period. This implies that the longer the duration of
    interaction, the more developed and mature the soils will be.

    4.1.6 Human activities
    Human intervention has a major impact on soils: The use of fertilizer changes
    the natural properties of soils. Farming activities by use of heavy machinery
    causes soil compaction. This hinders water seepage and consequently soil
    development is hindered. Man cuts the vegetation (deforestation) hence
    reducing humus formation and consequently affecting the type of soil that
    develops. Mining activities allow water to percolate deeply hence weathering
    the bedrock. Reclamation of land leads to formation of waterlogged soils e.g.
    peat soil. Construction works and settlement require cementing and tarmacking
    of the foundations. This reduces soil permeability and water seepage thus

    limiting chemical weathering of the parent rock and premature soils.

    4.2.1 Causes of soil erosion
    4.2.1.1 Human made causes

    • Overgrazing
    This is the major cause of erosion in pastoral areas; it is caused due to
    overstocking of domesticated animals like cattle, sheep and goats.When these
    animals eat plants over a long time, they expose areas of soil which are often
    subsequently removed by wind or rain action, bush burning or over cultivation.

    • Deforestation
    Because of population increase, there is great pressure on forests in order to
    get cultivable land and land for settlement. The high demand for fire wood and
    charcoal both in rural and urban areas has posed a great threat on the natural
    forests. This has led to wide scale deforestation hence exposing the soil to rain
    and surface run off which erodes away the top soil.

    • Bush burning
    It is done in the dry season in nomadic areas, with the aim of improving the
    quality of pasture which will grow during the next rainy season. This instead
    destroys the soil cover, makes the soil exposed to all agents of erosion.

    • Over-cultivation of the land
    Ploughing disrupts the soil. Every year, the world population increases by
    93 million people and they need more food. Therefore, farmers plough more
    fields to produce more food. This increases pressure on our soil resources.
    Ploughing soil is the mechanical turning and loosening of soil to improve it
    for crops. Ploughing) soil removes the plant cover that holds soil particles in
    places, leaving soils open to wind and water erosion. This makes the land lose
    its fertility and becomes exhausted.

    • Rapid population increase
    Today there is population explosion in most areas of the world especially in less
    developed countries. There is massive human pressure on land, vegetation in
    search of food, this leads to the removal of vegetation cover which accelerate
    soil erosion.

    • Uphill and downhill cultivation
    This refers to cultivation of strips of land up and down a slope. In this kind of
    ploughing, fields are longer up and down a slope than they are across the slope.
    This provides an open ground, long enough for runoff to gather momentum
    enough to erode soil. These strips become man-made channels which can be
    enlarged into gullies by surface run-off.

    • Construction works
    Through the construction of roads, railways and housing complexes, reasonable
    forest areas are cleared. This produces gorges, quarry sites, and steep sided
    hills are dissected by roads and railways. This thus accelerates the speed of
    runoff and exposes the soils to agents of erosion.

    4.2.1.2 Natural causes
    • Heavy rainfall
    This is common in hilly areas where the speed of surface run off is high than
    infiltration . Rain drop stake away the top soils to the valleys.
    • Drought
    The current climatic change has resulted into limited vegetation ornovegetation
    cover. This makes the soil to be exposed to the agents of erosion (wind and
    moving water).

    • Winds
    Wind takes away the top soil in areas with limited vegetation cover and trees
    which would act as wind breakers. This is common in arid and semi- arid areas

    which experience high temperature and too much sunshine.

    River and wave action
    Shore lines of lakes and oceans as well as banks of rivers are eroded by the
    waves and running water respectively. This accelerates the rate of erosion

    • Steep slopes
    This has accelerated the rate of surface run off hence leading to soil erosion in
    highland areas in the world.

    4.2.2 Effects of soil erosion
    The following are the effects of soil erosion:

    • Limited mechanized agriculture
    The use of modern machines like tract or sis made difficult because of gullies
    which affect their movement in farms, this in turn affects crop production.

    • Destruction of crops


    Soil erosion destroys crops on farm yards. The eroded materials destroy the
    wind erosion destroys the growing crops especially in hilly areas and at times
    transport them to the lower valleys. This is common in the northern province

    of Rwanda.

    • Low soil productivity
    Soilerosionleadstothelossofsoilnutrientswhichinturnaffectstheyields.
    ThebaresoilinHilly slopes can no longer support crop production.
    • Famine
    Soilerosionleadstovegetationdestructionandthisaffectsrainfallformation.
    Thisphenomenon
    limits agricultural productivity hence famine.
    • Change of the Landscape
    It changes the landscape appearance and natural beauty of the areas affected

    because of presence of landslides, gullies and rills.

    • Siltation
    The eroded materials at times are deposited on people’s land or along riverbanks.
    Such silt displaces people and destroys their property. This is common in flat
    lands adjacent to hilly areas.
    • Flooding
    Floods cause great damages on communities and individuals. As most people
    are well aware,the immediate impacts of flooding include loss of human life, and
    damage to property, destruction of crops, loss of livestock, and deterioration of

    health conditions owing to waterborne diseases.

    • Destruction of transport system
    Roads are greatly destroyed because of soil erosion that results into gullies.
    These gullies are caused by surface run off in the affected areas. This limits
    movement of people, goods and services and requires urgent rehabilitation.
    This affects government ‘s budget.
    • Desertification
    Soil erosion depletes the vegetation cover which would act as a source of

    convection rainfall. Thus causes semi-arid conditions of climate to develop.

    The following are areas of severe soil erosion:
    • Mountains and highlands with very steep slopes
    Surface water run off occurs whenever there ise xcess water on a slope that can
    not be absorbed In to the soil or is trapped on the surface. Reduced infiltration
    due to soil compaction, crusting or Presence of steep slopes increases the run
    off. Run off fromagricultural land is greatest during rainy months when the

    soils are typically saturated.

    The steep relief accelerates the rate of surface runoff hence leading to soil
    erosion. This is common in volcanic and hilly areas. Severe erosion occurs in
    these areas because the speed of surface runoff is too high and takes away the
    top soil.

    • Glaciated highlands
    Moving ice and glacier on the major highlands carries away large quantities or
    eroded soil in form of moraine. This is deposited at the base of mountain and
    on out wash plains. Severer erosion by glaciers and melt water is common on
    most high and steep glaciated mountains.

    • Desert and semi-desert areas
    Soil erosion, is severe in desert areas because of limited vegetation cover. Very
    strong winds blow off the unconsolidated soil and attaches it from the ground.
    Soil is relocated elsewhere by strong winds where it is carried and deposited in

    other areas. This phenomenon is common in desert areas.

    • Soil conservation: it refers to the prevention of soils from erosion,
    degradation or loss of fertility while soil management refers to all the
    measures put in place to ensure proper use of soils in a sustainable
    manner.

    Soil erosion and depletion are the major threats to soil as a resource.
    Therefore, sound farming techniques must be employed to prevent soil
    erosion and impoverishments, and to ensure that agriculture is more
    profitable in the world, the following should be done:

    • Mulching: This method consists of covering the bare ground with
    a layer of organic matter such as straw. This helps to maintain soil

    moisture and limit rapid evaporation.

    Figure 4. 47: Mulching

    • Crop rotation: This is done to avoid the effects of monoculture and
    add mineral salts to soil, hence improving soil cohesion and reducing

    soil erosion.

    • Contour ploughing: This method prevents excessive soil loss, as
    gullies are less likely to develop and also reduce run-off so that plants
    receive more water. Row crops and small grains are often planted in
    contour pattern so that the plants can absorb much of the rain, and
    erosion is minimized

    • Terrace farming: This method is practically used on the steep slopes
    to avail enough flat land to grow. It is best way to reduce surface run off

    and soil erosion.

    Terracing - Agro-forestry
    • Intercropping method. It consists of growing different crops in
    alternative rows and sown at different periods to protect the soil from
    rain-wash.

    • Fallowing: Sometimes it is very important to allow a farmland to rest
    or lie fallow, so that the natural forces can act on the soil. Fallowing also
    increases the sub-soil moisture and improves the general structure of
    the soil.

    • Zero grazing: Reducing the number of animals grazing in an area
    according to the carrying capacity of land.

    • Afforestation: It is a process of planting trees in a virgin land without
    any trees to create a forest. Trees as wind breaks are planted and they
    reduce the speed of wind hence reducing soil erosion.

    • Growth of cover crops: This is used in gentle area to reduce splash
    erosion and surface run off e.g. the growth of Pumpkins and Yams in
    Buganda.

    • Education and mass mobilization: this can be done through education
    of farmers and pastoralists about the causes and effects of soil erosion,
    and how to avoid them. This should also be done through agricultural

    seminars, radio programs, and demonstration farms.

    ◄UNIT 3 ROCKSUNIT 5 WEATHER AND CLIMATE IN THE WORLD►