Soil: Meaning, Types, Composition and Properties of Soil II



Welcome to Class !!

We are eager to have you join us !!

In today’s Agricultural Science class, We will continue learning about Soil. We hope you enjoy the class!






  • Process of Soil Formation
  • Soil Structure, Texture and PH
  • Experiments on Soil


soil types of soil agric science classnotesng



Weathering is defined as the disintegration or breakdown of rocks into tiny particles called soil.

The processes of soil formation include:

  1. Physical process
  2. Chemical process
  3. Biological process


A) PHYSICAL PROCESS: The agents of physical weathering are temperature, ice, wind, water and pressure

      1. Temperature: The alternating heating and cooling of rocks produce pressure within the rock which makes them break into smaller pieces.
      2. Wind: Solid materials carried by soil surfaces.
      3. Ice: The conversion of water inside the cracks of rocks into ice results in an increase in volume. This results in more pressure on the rocks which eventually break into smaller pieces.
      4. Water: Running water carries some fragments of rocks in the river bed, thus breaking off small pieces of rocks.


B) CHEMICAL PROCESS: Agents of chemical weathering include: solution, carbonation, hydration, hydrolysis and oxidation.

      1. Hydration: It occurs when water combines with or binds to some minerals e.g.
        • i) Calcium Sulphate changes to gypsum CaSO4+2H20 → CaSO4.H2O
        • ii) Red Haematite changes to hydrated yellow Haematite or Limonite Fe203 +3H20 = Fe203.H20
        • iii) CuS04 + 5H20 = CuS04.5H 20
      2. Carbonation: The carbon dioxide released to the atmosphere combines with acid. This weak acid reacts with rocks to form soil.
      3. Oxidation/Reduction: When minerals containing iron, manganese and sulphur are exposed to air and water, the famous iron is oxidized to the ferric state.

4F0C03 + O2         =          2FeO3 + 4CO2

(Iron II Carbonate)              (Iron Oxide)

4) Solution: reaction of water with soluble particles/minerals present in the rock and eventually displacing them given the sloe of the soil.

5) Hydrolysis: involves the breaking down of chemical bonds in rock minerals by water.

CaSiO3                      + 2H2O                       → H2SiO3                   +          Ca(OH)2

Calciumsilicate            water                              silica                                calcium

(Wollastonite)                                                                                                          silicate hydroxide


C) BIOLOGICAL PROCESS: This involves the activities of plants and animals in the breaking down of rocks to form soil. It can happen in the following ways;

        • The root of plants penetrates the rocks to form soil.
        • Earthworm and termites burrow into the rocks and break off fragments.
        • Activities of man during soil tillage causes the rock to breakdown to form soil.



  1. What is weathering?
  2. List and explain the three processes of weathering.




  1. Soil Structure: This refers to the ways in which the different particles of the soil are packed or arranged. Good Soil structure will promote a good yield of crops. Soil Structure can be preserved in the following ways:
          • Planting of cover crops
          • Application of manure
          • Avoidance of overgrazing
          • Mulching
          • Avoidance of clean clearing


The types of soil structures are:

      • Single grained structure
      • Crumb Structure
      • Plate-like structure
      • Spheroidal structure
      • Prismatic structure
      • Block-like structure



      • It determines how fertile the soil is.
      • A good soil structure supports aeration.
      • It also prevents erosion and waterlogging.
      • A good soil structure promotes the activities of soil microbes.
      • A good soil structure supports the growth of crops.


Soil Structure



Soil structure 2


  1. Soil Texture: This is the relative proportion of various particles of the soil. It also refers to the degree of fineness or coarseness of the various soil particles. The particles which make up a soil sample include; gravel, sand, silt and clay are usually referred to as primary particles of the soil. The name and sizes of the various soil particles are shown in the table below;
Name of Particles Range of Particle diameter
Clay Below 0.002mm
Silt 0.002 – 0.02mm
Fine Sand 0.02 – 0.2mm
Coarse Sand 0.2 – 2.0mm
Gravel Above 2mm



  • By feeling
  • By mechanical analysis through sieving
  • By sedimentation
  • By moulding



  • It is useful in the evaluation of the ability of soil to supply mineral nutrients.
  • It supports soil microorganism essential for crop growth.
  • It determines the type of crop to grow on the land.
  • It enables the farmer to know the type of soil on his farm.
  • It determines the movement of air and water in the soil.


  1. SOIL TEMPERATURE: This refers to the hotness or coldness within the soil.



  • It determines the rate of formation and decomposition of organic matter.
  • High temperature will impede the activities of micro-organisms.
  • Optimum temperature promoted seed germination.
  • It determines the population of soil microbes.
  • It determines the maturity or ripening of fruits.


  1. SOIL PH: This is the measure of the degree of acidity or alkalinity of the soil. It can also be defined as the measure of the concentration of hydrogen ions (H+) in the soil. A PH scale is use to determine soil acidity as shown below.

Note that;

  • PH2 is strongly acidic
  • PH6 is slightly acidic
  • PH7 is neutral
  • PH8 is slightly alkaline
  • PH13 is strongly alkaline



  1. Leaching
  2. Use of acid fertilizers
  3. Presence of acid parent materials
  4. Nutrient uptake by plants
  5. Presence of sulphur in the soil




Soil acidity can be removed by the application of liming materials which are rich in calcium. Examples of liming materials are:

  1. Slaked lime Ca(OH)2
  2. Quick lime Cao
  3. Calcium bicarbonate Ca(HCO3)2
  4. Wood ash
  5. Limestone CACO3
  6. Dolomite or Calcite
  7. Basic slay
  8. Gypsum



A soil profile is defined as the vertical section of the soil, showing a series of horizontal layers of different types of soil. The horizontal layers are called HORIZONS




  1. The A – Horizon: It is also called the topsoil, it is rich in organic materials; most soil organisms reside here.
  2. The B – Horizon: It is rich in minerals which are carried or leached down by percolating water.
  3. Horizon C: It represents the type of material from which topsoil and subsoil are derived.
  4. The D – Horizon: It is called the bedrock.



  1. Sedimentation experiment.
  2. Experiment to determine the moisture content of the soil.
  3. Experiment to determine the porosity/water holding capacity of the soil.
  4. Experiment to determine the capillarity of the soil.
  5. Experiment to demonstrate the preserve of microorganism in the soil.
  6. Experiment to determine the percentage of organic matter in a soil sample.



  1. What is soil?
  2. List 5 factors of soil formation.
  3. What is weathering?
  4. Explain the processes of physical weathering.
  5. Define (i) Soil acidity (ii) Soil structure (iii) Soil texture.



Essential Agricultural Science for Senior Secondary Schools by O.A. Iwena, Chapter 11, pages 65-87



  1. Describe sedimentation experiment with the aid of an appropriate diagram.
  2. (a) List five ways of removing soil acidity   (b) Define soil PH.




We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will be learning about Farm Machinery and Implements. We are very much eager to meet you there.


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