AGRICULTURAL SCIENCE

THEME: AGRICULTURE ECOLOGY 

CLASS – SENIOR SECONDARY SCHOOLS (SS 1) 

TOPIC – SOIL FORMATION AND PROFILE DEVELOPMENT

LEARNING AREA

1. Meaning of Soil

2. Soil Formation and Profile Development

3. Principles of Soil Classification

PERFORMANCE OBJECTIVES 

By the end of the lesson, most of the students should have attained the following objectives –

1. list factors of soil formation.

2. make a sketch of a soil profile.

3. discuss the basic principles of soil classification.

INSTRUCTIONAL MATERIALS

The teacher will teach the lesson with the aid of:

1. Diagram of soil profile.

2. Dug or existing soil profile.

CONTENT OF THE LESSON

MEANING OF SOIL

Soil is the top layer of the earth’s surface which made up of living and non living things such as water, leaf litters, rocks, humus, dead animals, living animals, air, soil particles – sand, loam and clay.

FACTORS OF SOIL FORMATION

1. PARENT ROCK – Soil minerals form the basis of soil. They are produced from rocks (parent material) through the processes of weathering and natural erosion.

2. ORGANISMS – Soil formation is influenced by organisms (such as plants), micro-organisms (such as bacteria or fungi), burrowing insects, animals and humans.

3. CLIMATE – Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow but, with heat and moisture, they are relatively rapid.

4. TOPOGRAPHY – The shape, length and grade of a slope affects drainage. The aspect of a slope determines the type of vegetation and indicates the amount of rainfall received. These factors change the way soils form.

5. TIME – Soil properties may vary depending on how long the soil has been weathered.

Source materials – How soil is formed?

SOIL FORMATION PROCESS

1. WEATHERING – This is the process of breaking down of rock (parent material) into smaller particles to form soil. Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material.

2. ACCUMULATION OF MATERIALS – Materials such as organic matter and decomposing materials or new mineral materials are added to the soil by the forces of ice, water, or wind and they accumulate over time.

3. LEACHING – Leaching is the removal of soluble components of the soil column by water.

4. TRANSFORMATION – Transformation is the chemical weathering of soil particles, including silt, sand, and clay minerals as well as the change of organic materials into degradation-resistant organic matter.

5. CLASSIFICATION – Calcification occurs when the removal of water via evapotranspiration exceeds precipitation causing the upward movement of dissolved alkaline salts from the groundwater.

Source materials – Process of Soil Formation – Mcrobenotes.com

SOIL PROFILE DEVELOPMENT

Soil profile is defined as the vertical section of the soil from the ground surface downwards to where the soil meets the underlying rock.

The soil is arranged in layers or horizons during its formation. These layers or horizons are known as the soil profile. It is the vertical section of the soil that is exposed by a soil pit. The layers of soil can easily be identified by the soil colour and size of soil particles. The different layers of soil are:

1. Topsoil

2. Subsoil

3. Parent rock

Each layer of soil has distinct characteristics.

Soil profile helps in determining the role of the soil as well. It helps one to differentiate the given sample of soil from other soil samples based on factors like its colour, texture, structure, and thickness, as well as its chemical composition. In mineral horizons (A, B, and C-horizons) and parent rock or R-horizon.

1. O – Horizon (Organic) – The O horizon is the upper layer of the topsoil which is mainly composed of organic materials such as dried leaves, grasses, dead leaves, small rocks, twigs, surface organisms, fallen trees, and other decomposed organic matter.

2. A – Horizon (Topsoil) – This layer is rich in organic material and is known as the humus layer. This layer consists of both organic matter and other decomposed materials. The topsoil is soft and porous to hold enough air and water.

3. B – Horizon (Subsoil) – It is the subsurface horizon, present just below the topsoil and above the bedrock. This layer holds enough water than the topsoil and is lighter brown due to the presence of clay soil. The soil of horizon-A and horizon-B is often mixed while ploughing the fields.

4. C – Horizon (Bedrock) – It is a compacted and cemented layer. Different types of rocks such as granite, basalt and limestone are found here. Apart from the rocks, minerals, and layers, soil profile also consists of a water content, which is referred to as soil moisture.

THE BASIC PRINCIPLES OF SOIL CLASSIFICATION

Soil classification is the process of categorizing soil based on its properties, composition, and origin. The classification of soil is important for various purposes such as agriculture, engineering, construction, and environmental studies. The basic principles of soil classification include:

1. Particle Size – Soil particles are classified based on their size, including gravel, sand, silt, and clay. The relative proportions of these particle sizes determine the soil texture, which influences properties like drainage, fertility, and compaction.

2. Soil Structure – Soil structure refers to the arrangement of soil particles into aggregates or clumps. The structure of soil affects its ability to retain water, nutrients, and air, as well as its resistance to erosion and compaction.

3. Soil Color – Soil color is influenced by factors like organic matter content, mineral composition, and drainage conditions. Soil color can provide information about soil fertility, drainage properties, and weathering processes.

4. Soil pH – Soil pH is a measure of the acidity or alkalinity of soil, which affects nutrient availability, plant growth, and microbial activity. Soils can be classified based on their pH ranges, such as acidic, neutral, or alkaline soils.

5. Organic Matter Content – Soil organic matter includes decaying plant and animal material, microbial biomass, and humus. Organic matter content influences soil structure, nutrient cycling, water-holding capacity, and soil fertility.

6. Soil Horizon – Soil horizons are distinct layers within the soil profile, each with different physical, chemical, and biological properties. The presence and arrangement of soil horizons can be used to classify soil types and determine soil properties.

7. Soil Texture – Soil texture is determined by the relative proportions of sand, silt, and clay particles in soil. Soil textures are classified into categories like sandy soils, loamy soils, and clayey soils based on their particle size distribution.

8. Soil Classification System – Several soil classification systems are used worldwide, including the USDA Soil Taxonomy, the World Reference Base for Soil Resources (WRB), and the Unified Soil Classification System (USCS). These systems provide a framework for categorizing soils based on their physical, chemical, and biological properties.

PRESENTATION

TEACHER’S ACTIVITIES

Teacher,

1. displays soil profile diagram, conduct students to existing soil profile and discuss.

2. summarizes the lesson on the board.

STUDENT’S ACTIVITIES

Students,

1. examine soil and Soil profile.

2. copy as the teacher writes.

CONCLUSION

To conclude the lesson for the week, the teacher revises the entire lesson and links it to the following week’s lesson.

LESSON EVALUATION 

Teacher asks students to:

1. list factors of soil formation.

2. make a sketch of a soil profile.

3. discuss the basic principles of soil classification.