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3.0: SOIL

3.0: SOIL


3.0: SOIL

3.1: Soil formation

 concepts from this subtopic

  1. define soil
  2. Describe factors for soil formation
  3. Assess the importance of soil

Soil is the thin uppermost layer of the earth’s crust composed of inorganic and organic materials.

Components of soil

  1. Organic matter and living organism (5%)

¾    Decay of organic matter add nutrients in the soil such as humus

¾    Humus is the substance that is left over after plants and animals have undergone a long process of thorough decomposition done by earthworms, bacteria, fungi, and other microorganisms

Importance of humus to the soil

(a)    It improves the soil structure. 

(b)    It increases pore space, making it easier for air and water to penetrate the soil

(c)     It reduces exposure of soil to erosion.

(d)    It minimizes the leaching of nutrients;

(e)    It provides suitable medium for valuable soil organisms.

(ii)Inorganic matter (45%)

Inorganic matter constitutes 45% of the total volume of the soil. It is formed when the parent rock disintegrates into small particles. Mineral matter or inorganic particles in the soil range in size from stones (feldspar) to relatively small particles (clay), for example, feldspar to clays.


Soil water or moisture accounts for 25% of the soil. Water contained in the soil is called soil water. It is derived essentially from rainfall runoff and irrigation. Water is important for regulating soil temperature, dissolving and transferring nutrients, controlling chemical reactions in the soil as well as mechanical weathering. Water also plays a significant role of washing soluble minerals such as salts from the top soil to the sub-soil. This process is known as leaching


Soil contains gases which account for 25% of soil components. Air is contained in pores (spaces between soil particles) forming what is referred to as soil atmosphere. It is this air that provides oxygen for the metabolism of soil organisms. Air accelerates oxidation and biological activity. Well-aerated soil is productive while poorly-aerated soil is less productive. Soil air facilitates plant growth as plant roots can efficiently absorb water and mineral nutrients in the presence of oxygen. Soil air may include gases like oxygen, carbon dioxide and nitrogen.

Factors for soil formation

  1. Parent Rock Material

This is one of the Achief factors of soil formation. It determines soil type, color, depth, rate of soil formation, structure, texture, porosity and soil fertility. Parent rock influence soil maturity, therefore hard rocks take a long time to mature while soft rocks take a short time to mature. Shallow and poorly productive

  1. Climate

The most variable elements under climate are temperature, precipitation (rainfall) and wind Temperature affects decomposition of organic matter hence it influences the development of soil profile Rainfall and wind encourage the formation of soil due to their role in the erosion process On the other hand rainfall adds moisture which encourages chemical and physical weathering

  1. Living Organism

Some plants have nodules with bacteria which add nitrogen into the soil hence improve aeration of soil. Microorganisms are active in the decomposition of the organic matter to form humus on the other hand barrowing of animals and plant roots facilitate the state of both physical and chemical weathering hence lead to the formation of soil easily.

  1. Relief [Topography]

The role of relief in soil formation is mostly in indirect way. Relief influences climate and vegetation. The most important aspect of topography in soil formation, steep slopes areas soils are shallow due to erosion while on a gentle slopes and low land areas soils are deep due to deposition of materials.

  1. Time

This involves the duration that has been taken in the process of soil formation. Time determines the maturity of soil, when soil formation has taken a long time, soil tends to be mature i.e. they are deep and well developed.

Soil equation S= f(PCORT)

“S” stands for soil

“F” stands for factors

“P” stands for Parents rock

“C” stands for climate

“R” stands for Relief

“T” stands for time

Importance of soil

  1. It supports plants growth
  2. It determines the type of natural vegetation of a place
  3. It provides materials for construction and making pottery
  4. It is habitants for other organism such as ants and bacteria
  5. It is source of minerals
  6. It is cultural value. For example Masaai

3.2: Soil composition and properties

Soil properties

  1. physical properties
  2. Chemical properties
  3. biological properties. 

Physical properties of soil

  1. Porosity
  2. colour
  3. texture
  4. density
  5. structure

(i) Porosity is the total amount of pore spaces in a soil. For example, sand and clay

(ii)Soil colour

From soil colour it is easy to tell how a soil has been formed, its contents as well as its fertility. For example, a soil which is dark in colour is rich in humus, while red colour indicates the presence of ferrous minerals.

(iii) Soil texture This refers to the coarseness or fineness of a soil

These particles can be classified according to their size, from gravel, sand, silt to clay.

(iv) Soil density is dry mass per unit volume. As the density of any object is measured by mass divided by its volume, soil is measured in units expressed in grams per cubic centimetre (g/cm3) or megagrams per cubic meter (Mg/m3).

(v) Soil structure is the appearance of soil by arrangement of individual soil particles within the soil or the way soil grains are grouped together to form larger pieces of aggregates.

II. Chemical properties of the soil

The major chemical properties of soil are its acidity or alkalinity.

Acidity or alkalinity is determined by the amount of hydrogen ions in the soil and is commonly measured by a pH value.

Soil with a pH value 7 is said to be neutral, with a value less than 7 is acidic, and that with a value greater than 7 is alkaline.

Plants differ in their tolerance to acidity or alkalinity and this influences their distribution. For example, coffee does well in acidic soils while leguminous plants thrive well in alkaline soils because each of the crop is sensitive to a specific pH level. Human beings can treat the soil either to raise or lower its acidity. 

III. Biological properties of soil

The biological properties of soil refer to the variety and concentration of living organisms in the soil. Organisms that may be contained in the soil are earthworms, bacteria and fungi. In moist areas with a lot of plant remains, such as in the tropical forests, there is a high concentration of earthworms and bacteria, whereas in arid and semiarid areas living organisms are limited in numbers and diversity. Coniferous forests have a lot of moisture and plant remains but the concentration of living organisms is low because of the acidity of the soils. Biological soil properties include soil organisms and presence of organic matter. These organisms have various functions in the soil.

3.3: Soil profile and characteristics

Concepts from this sub-topic

a       Define soil profile

b       Soil profile and characteristics

Soil profile refers to the vertical sections of soil from the surface to the bed rock, characterized by distinct layers usually of different textures and colours.

Matured Soil profile consists layers which are

A- horizon. Is the topmost layer and can include organic matter to form humus.

 Horizon A varies in color from place to place for example dark, grey etc. this zone is also called the zone of Eluviation from which materials are washed down ward. It is in this place where leaching process takes place.

LEACHING is the washing down of nutrients in solution from the topmost layer to another layer

B- horizon. This zone is also known as the zone of accumulation. In this layer the materials washed from A horizon are deposited or accumulated.

C- horizon. Is the partially weathered parent rock from which the soil develops, it is underlined the D horizon which is the fresh [un weathered] parent rock.

 D-horizon (Bedrock). It is the un weathered parent rock. It is the parent in sense that it is the source of the in organic content of the soil.

3.4: Simple soil classification

Types of soil can be identified on the basis of its texture

(a)    Sandy soil consists mainly of sand, hence has sufficient air spaces. It is commonly referred to as light soil. This light soil allows water to pass through easily taking plant nutrients with it. Therefore, this soil type needs constant manure. It is suitable for horticulture and other root crops like carrots. In Tanzania, sandy soils are found in many places, but they are dominant in the coastal belt

(b)    ) Clay soil consists mainly of very fi ne particles, with tiny air spaces. It contains little air but can hold much water. Therefore, it is referred to as heavy soil and is diffi cult to cultivate. When it dries up it forms a very hard surface with many cracks. Clay soils are often rich in plant nutrients. Clay soils are found in Mwanza and Shinyanga regions in Tanzania, where they are known as mbuga soils.

(c)     Silt soil consists of particles of intermediate size between sand and clay. It has more air spaces than clay soils but less than sandy soils. Therefore, it is more suitable for agriculture than both sandy and clay soils. Silt soils are commonly found in river flood plain.

(d)    Loamy soil consists of particles of various sizes. The type of loamy soil depends on the proportion of sand, silt and clay in the soil.

It can be a sandy-loam soil if it contains more sand; silt-loam soil if it contains more silt; and clay- loam soil if it contains more clay. Loamy soils are well-aerated and drained. Most volcanic areas have clay-loam soils such as those found on the slopes of Mount Kilimanjaro in Northern Tanzania.

3.5: Soil erosion


a.       Define soil erosion

b.       Examine how agent of erosion works

c.       Describe types of soil erosion to social and economic activities

d.       Relate population growth and rate of soil erosion on the quality of life

e.       Demonstrate ways of controlling soil erosion

Soil erosion is the removal of the top soil by agents of erosion.

The agents of erosion are running water, waves, ice wind and human activities such as overstocking, removal of vegetation cover and cultivation on steep slopes without erosional control measures.

Types of soil erosion

         i.            wind erosion

       ii.             running water erosion

     iii.             wave erosion 

     iv.            ice erosion

       v.            geological

     vi.             accelerated erosion

Causes of soil erosion

(a) Deforestation This is a practice of cutting down trees. When trees are cut, the soil is exposed to rain droplets, sun and wind. The rain drops loosen the topsoil hence making it more vulnerable to erosion.

(b) Overgrazing This occurs when plants are subjected to intensive grazing over a long period of time or without sufficient recovery periods thus reducing vegetation and exposing the soil to agents of erosion. The practice reduces the compaction of soil and exposes the land to erosion which may result to desertification. Similarly, animal hooves make soil

(c) Shifting cultivation This is a common practice in areas with a sparse population. It is a system of cultivation whereby land is cleared by slash and burn methods.

(d) Monocropping This is a practice of planting the same crop on the same land for many years. This kind of farming exhausts the soil. The soil then deteriorates in quality, becomes loose and it can be easily eroded by using agents like water

(e) Building and excavation works Works such as construction of roads and buildings, mining and quarrying are also responsible for soil erosion. These works expose the inner soil to agents of erosion. These activities involve the removal of the top soil by machines which then accelerates the rate of soil erosion.

Relationship between human population growth and rate of soil erosion

(a)    As human population increases, the demand for land also rises. However, habitable and cultivable, land becomes inadequate to meet the increasing demand of human population. When the population increases, people start to cultivate on hills and valleys in order to produce enough food to feed the population.

(b)    With population increase, it becomes difficult to maintain soil conservation activities as the demand for grazing and farming becomes high, hence exposing land to agents of erosion

(c)     A large population needs more area for shelter, fuel wood and timber. Hence, people clear land to build houses, harvest building materials and firewood. All these activities expose the land to agents of erosion.

Effects of soil erosion

(a)    Destruction and loss of properties. Erosion can damage roads, houses and bridges. This is because soil erosion goes together with deattachment of rocks which act as the foundation of these buildings, roads and bridges

(b)    Source of materials for building and construction. When eroded material is deposited in form of sand it becomes a good resource for building and construction purpose

(c)     Environmental pollution. Eroded soil that is carried into rivers, lakes and oceans may contain chemical pollutants that may affect aquatic life and weaken some economic activities like fishing.

(d)    Siltation. Siltation reduces the water depth in the dams or river channels hence leads to water shortage that affects some investments like hydro-electric power thus accelerate the problem of power rationing.

(e)    Loss of productive soil. Soil erosion involves removal of the top fertile soil, leaving the subsoil which is less fertile. This in turn leads to low land productivity, increasing the likelihood of famine and hunger occurrences

Ways of controlling soil erosion

(a)    Use of proper farming techniques The use of proper farming techniques helps in conserving the land. These techniques are listed in the sections that follow: mulching, intercropping, terracing, strip cropping and crop rotation

(b)    Afforestation and reforestation; Afforestation involves planting of trees in areas where no forest existed before while reforestation is the planting of trees on land that had forests before

(c)     Education on environment and population growth should be provided to all people. This will help to create awareness about conservation of the environment, hence reduce soil erosion as there will be proper environmental management and optimum population growth that will conserve the environment.

(d)    Enacting restricted laws

(e)    Establishment of forest reserves



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