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Lesson Note

Subject: Agricultural Science

Lesson objective: by the end of of the lesson, should be able to:

1. Define the term soil;

2. Mention and explain the basic types of soil;

3. Identify and explain the components of soil.

Lesson Discussion


Soil can be defined as the topmost layer of the earth’s crust, where plants attach their roots to obtain nutrients and water for their metabolic activities. Soil is formed from the process of weathering. Organic matter or humus that has been obtained from dead and decomposed plant and animal materials is added later to this weathered rock known as mineral particles to make soil a complete medium for plant growth.

Types Of Soil

Soil is made up of basically four types. They are grouped based on the sizes of their particles and nature. They include sandy, clay, loamy and silty soil. These four are briefly explained below.

1. Sandy Soil

This type of soil has a high proportion of particles of 2.00mm – 0.02mm in diameter, it absorbs and loses water easily and is generally not good for agricultural production.

2. Loamy Soil

Agriculturally and generally speaking, loamy soil is fertile with high organic matter and less proportion of sand and clay mixture. It has a non-powdery and non sticky texture and so is good for agricultural and crop production.

3. Clay Soil

Clay soil has particles of less than 0.002 mm in diameter. It has small pores (spaces) among the particles. Clay encourages water logging, and so its difficult to work on.

4. Silt

Naturally, silt soil has fine particles and small pores (spaces), also daily sticks when wet. Its particles is between 0.002 mm to 0.002mm.

Composition Of Soil

Soil is made up of fine components and a number of other materials. These include organic matter, inorganic matter, soil air, soil water and organisms. These components are explained below.

1. Organic Matter

Organic matter comes from accumulated dead plants and animal remains. The decomposition of organic matter results in humus which is dark in colour. It forms about 45% of the total soil volume. It serves as a source of nutrients to plants. It prevents leaching, increases the water holding capacity of the soil and also improves soil aeration.

2. Inorganic Matter

Inorganic matter is the product of rock weathering. It is the fragments of rock which form 5 percent of soil volume, it also determines the type of mineral elements that are present in the soil, this provides an anchorage for the plant. It holds water and air for the activities of plants and animals.

3. Soil Air

It refers to the air or gases that are present in the soil. The gases occupy the soil pores which water did not occupy. Soil air forms about 25% of the total soil volume. The ability of air to circulate freely in the soil is called soil aeration.

4. Soil Water

This refers to the water that is present in the soil. The water content of soil is supplied by precipitation such as rain, dew, mist etc. Or irrigation (artificial application). It occupies the spaces which air did not fill. Soil water forms about 25% of the total soil volume, and dissolves minerals for plants to absorb easily with their roots.

4. Organisms

Organisms are plants and animals that lived in the soil. These are fungi, bacteria, earthworms and other higher animals. Animals in the soil feed on decayed plants and lower animals. The soil organisms improve the soil structure and granulation. By their burrowing activities, they leave holes for the crop roots to penetrate.

Done studying? See previous lessons in Agricultural Science

Take a quick test for this lesson

1. Define the term soil.

2. What are the basic types of soil? List and explain them.

3. What are the components of soil? Mention and explain them.

Questions answered correctly? Bravo!

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