Soil – SS2 Geography Lesson Note (Second Term)

Soil is the thin uppermost layer of the earth’s crust on which plants grow. It is made up of five components: mineral matter, organic matter, water, air and living organisms. Soil forms from weathered parent materials, is arranged in horizons (A, B and C), and supports agriculture, building, construction and vegetation.

Behavioural objectives

By the end of the lesson, students should be able to:

• define soil and list its five components;
• state the importance of each component of soil to agriculture;
• describe the properties of sandy, clay and loamy soils;
• list the processes of soil formation;
• describe the A, B and C horizons of a soil profile;
• state the importance of soil.

What are the components of soil?

Soil is made up of five components:

1. Inorganic or mineral matter
2. Organic matter
3. Soil water
4. Soil air
5. Living organisms

Mineral matter, organic matter, water and air are collectively referred to as the physical components of the soil. Living organisms are referred to as the biological component of the soil.

Mineral or inorganic matter

Mineral matter is made up of small rock fragments in the soil. It forms the bulk of the soil, about 45% of the total volume. It consists of gravel, stones, sand, silt and clay.

Importance of mineral matter to agriculture

1. It forms the solid part of the soil, which provides support for plants.
2. It is the main source of plant nutrients such as nitrogen, calcium, magnesium and iron.
3. It is the home or habitat of all soil-living organisms.
4. It holds water and air for the activities of plants and animals.
5. It has a moderating effect on soil temperature.
6. It affects soil porosity.

Organic matter

Organic matter is the remains of decomposed plants and animals. It is about 5% of the total volume of the soil. It includes leaves, roots of plants, crop residues and animal dung.

When these materials are deposited on the soil, they decay and form a dark colour on the upper part of the soil. This dark material is organic matter, which is also called humus.

Importance of organic matter to agriculture

1. It is very rich in plant nutrients.
2. It is the habitat of many soil microorganisms.
3. It prevents leaching where it is present in adequate amounts.
4. It prevents soil erosion and the evaporation of soil water.
5. It allows good drainage and holds water in the soil for plant use.
6. It improves the structure of the soil by binding particles together.
7. It increases the water-holding capacity of the soil.
8. It moderates the soil temperature.
9. It has a moderating effect on the pH value.

Soil water

Soil water is the water in the soil, usually obtained from rain or irrigation. It makes up 25% of the total volume of the soil and is found in the pore spaces. When there is too much water in the soil (covering the soil surface), the soil is said to be waterlogged.

A waterlogged soil can be improved by drainage to make it more productive. When there is a complete lack of water in the soil for a very long time, plants cannot absorb water even when it is supplied again. This condition is called the permanent wilting point, and the plant can die at this stage.

Importance of soil water to agriculture

1. Water is an important agent of weathering of rocks in the soil.
2. It helps to dissolve plant nutrients into solution form.
3. It is an essential raw material for photosynthesis.
4. It is needed for the hydrolysis of many food nutrients and for enzymatic activities.
5. Loss of water from the plant through transpiration helps to cool the plant.
6. The presence of water makes tillage easy and improves the soil structure.
7. It promotes the activities of soil organisms.
8. It is needed for the germination of seeds.

Soil air

Soil air is the gases present in the pore spaces between the soil particles. The amount of soil air varies with the amount of soil water, the size of the pore spaces, the type of soil and the amount of living organisms in the soil. Air is about 25% of the total volume of the soil. The ability of air to circulate freely in the soil is called aeration.

Importance of soil air to agriculture

1. Oxygen is necessary for the growth and development of plants.
2. Oxygen in the soil promotes easy germination of seeds.
3. Soil organisms require oxygen for respiration.
4. Excess carbon dioxide in the soil, when combined with water, can cause acidity and aid the weathering of rocks.
5. Air is needed in soil reactions, particularly the carbon and nitrogen cycles.

Living organisms

Living organisms are the plants and animals that inhabit the soil. They range from microscopic organisms to bigger organisms. Some are beneficial while others are harmful to crops and livestock.

The most common groups of soil organisms include bacteria, fungi, viruses, nematodes and insects such as termites and soldier ants. Others are millipedes, centipedes, earthworms, snails, reptiles and mammals (for example rats and other rodents).

Importance of living organisms to agriculture

1. They help to improve the aeration of the soil.
2. They help to decompose organic materials in the soil to form humus.
3. They aid the percolation of water.
4. They improve the structure of the soil.
5. Some organisms, such as bacteria, help to fix nutrients in the soil.
6. Some are pests and parasites, and some cause diseases of crops.
7. Some soil microorganisms produce acidic materials which help to break down rocks.

What are the types of soil and their properties?

There are three main types of soil: sandy soil, clay soil and loamy soil.

Sandy soil

• A soil is said to be sandy if the proportion of sand particles in a sample of soil is very high.
• The particles are mainly quartz (SiO₂).
• The particles are 2.0 mm in diameter.
• Sandy soil is not very good for farming because it is poor in plant nutrients.

Properties of sandy soil

• It is coarse, grainy and gritty.
• It is loose with large pore spaces.
• It is well aerated and cannot hold water.
• Percolation is high but capillarity is low.
• It is not sticky when wet.

Clay soil

• A soil is said to be clayey if the proportion of clay in a sample of soil is very high.
• The particles are less than 0.002 mm in diameter.
• It is a heavy soil because it is very difficult to work on or cultivate.

Properties of clay soil

• It is fine-grained and smooth.
• The particles are tightly packed with little pore space.
• It is poorly aerated and can hold water.
• Percolation is low but capillarity is high.
• It is sticky when wet and hard when dry.

Loamy soil

• Loamy soil is a mixture of sand and clay with a high proportion of organic matter.
• It is the most fertile soil and the best soil for farming.

Properties of loamy soil

• It is moist and loose.
• It contains a lot of organic matter (humus).
• It does not support erosion and waterlogging.
• It is well aerated and can hold water.

What are the characteristics of soil?

Certain features or characteristics are used to identify or describe a soil sample. These are:

1. Soil colour
2. Soil texture
3. Soil structure
4. Permeability
5. Porosity
6. Soil pH

What are the processes of soil formation?

The following are the processes of soil formation:

1. Weathering of parent materials
2. Action of chemicals
3. Further disintegration
4. Appearance of biological activities
5. Formation of humus
6. Final soil formation

What is a soil profile?

A soil profile is a vertical section through the soil to the underlying rocks, showing a series of horizontal layers. These layers are called horizons. A typical soil profile has three horizons: the A, B and C horizons.

A-horizon

• It is also called the topsoil.
• It is the surface layer of the soil profile.
• It contains more humus or organic matter.
• It is the zone of eluviation, where minerals are leached down the profile.
• It has the maximum biological activity of plants and soil microorganisms.

B-horizon

• It is also called the subsoil.
• It is the zone of illuviation, where soil minerals accumulate, especially those leached from the A-horizon.
• It contains a lot of mineral matter.
• Organic matter from the A-horizon is converted into inorganic matter here.
• This zone contains leached materials.

C-horizon

• It is also called the zone of parent materials.
• It contains weathered parent rocks.
• It is the material from which the topsoil and subsoil are formed.
• Beneath the C-horizon is the bedrock.

Importance of the soil profile

1. It determines the level of fertility of the soil. For example, a thick topsoil shows a high level of soil fertility.
2. It helps the farmer to know the type of crop to grow.
3. It determines the level of root penetration, drainage and aeration.
4. It determines the rate of percolation of water and the degree of soil erosion.

What is the importance of soil?

1. Agriculture: It provides the only medium through which agriculture is practised to provide food for man.
2. Building: All buildings are sited and erected on soil.
3. Construction: Various forms of construction, such as roads, railways and industries, are built on soil.
4. Vegetation: All vegetation types have their basis in soil. Without soil, plants cannot get support and nutrients for growth.
5. Storage of nutrients: Soil provides plants and crops with all the nutrients for their continuous existence.
6. Habitat of organisms: Soil provides a habitat for most organisms, especially bacteria, earthworms, termites and rodents.
7. Reservoir for water and air: Soil acts as a reservoir for the air and water that living organisms need to survive.
8. Provision of minerals: Soil is the source of all minerals used by man.

Evaluation

1. Define soil.
2. List the five components of soil and state the approximate percentage of the total volume that each of mineral matter, organic matter, water and air makes up.
3. State four properties of clay soil.
4. Name the three horizons of a soil profile and state which one is called the topsoil.
5. Why is loamy soil the best soil for farming?

Assignment

1. State five importance of organic matter to agriculture.
2. Differentiate between sandy soil and clay soil under these headings: particle size, aeration, percolation and capillarity.