SOIL FORMATION AND PROFILE DEVELOPMENT
Subject:
Agricultural Science
Class:
SS 1
Term:
First Term / 1st Term
Week:
Week 5
Topic:
SOIL FORMATION AND PROFILE DEVELOPMENT CONTENT
Previous Knowledge: The pupils have previous knowledge of
that was taught in their previous lesson.
Behavioural Objectives : At the end of the lesson, learners will be able to
- Explain the concept of soil formation
- List the factors of Soil Formation
- Describe the processes of Soil Formation
- Explain Soil Profile Development
- Say the importance of Soil Profile
Instructional Materials
- Wall charts
- Online Resource
- Textbooks
- Laptop
- Soil samples
- Flash cards
- www.edudelighttutors.com
Methods of Teaching
- Role modelling
- Questions and Answers
- Explanation
- Discussion
- Recitation
- Imitation
- Story Telling
- Dramatization
Content:
Introduction
The soil is a very important factor for agricultural activity and the survival of man. Soil is the loose weathered material covering the surface of the earth, which supports life. It can also be defined as the outermost part of the earth that is formed from the mixture of minerals and decaying organic matter which extends from the surface down to the limit of biological forces.
Factors of Soil Formation
The transformation of rocks into soil is referred to as soil formation. The first stage in the process of soil formation is the Weathering of rocks which involves the breaking of rocks into smaller particles and eventually into individual minerals that the rock is composed of. Weathering of rock can either be physical such as through water, wind, Pressure, ice and temperature, or chemical which involves changes in the chemical composition of the rock.There are a number of factors that influence the rate, type and quality of soil formed in a location.
There are five factors that determine or influence soil formation, namely: 1. Parent material
Climate Topography Living organisms Time Parent Material
The type of materials, that is structure, texture, and mineral composition present in the parent rock will also determine the type of soil formed. Parent materials are important in the following ways:
Determining the properties of the soil e.g coarse, grain soils are formed from granite or gneiss Coarse parent materials form coarse-textured and well-drained soils. Rocks containing calcite, feldspar will produce deep heavy soils like clayey soils. Quartz, Hornblende and mica will form poor soils like sandy soils.
Shale and limestone parent materials produce soils that are fine-textured, poorly drained and with high mineral content.
Climate: This the average weather condition of a place over a long period of time. Climatic factors include the following:
Rainfall Temperature Wind Relative humidity Pressure Cloud cover Sunlight
Rainfall: Rainfall helps in the dissolution of chemicals from the parent materials and also in transporting broken down materials.it determine the types of soil to be formed
Temperature: The effect of temperature leads to the expansion and contraction of rocks. Rock mineral with low expansively will break away to form soil. Also rock holes may retain water which freezes up at night and melts during the day, leading to disintegration of rocks and eventually soil formation.
Wind: Wind can carry materials which collide with rock surfaces leading to breaking of rocks into tiny particles.
Relative humidity: This can supply moisture thereby encouraging the breaking of rocks particles into soil.
Pressure: This can lead to the breaking of large rock particles into tiny soil particles.
High pressure on a rock may cause such rock to fall down and break into tiny pieces, resulting in the formation of soil.
Topography: Relief or topography describes the surface features of the earth’s crust such as hills, valleys or flat land. It may also mean the shape of the land in relation to the underlying rocks of the
earth surface. The effect may include the following: The shape of the land affects the movement and amount of water in the soil.
Hilly and sloppy surfaces support erosion and encourage soil formation. A flat land exposes the surface to equal environmental factors and therefore delays soil formation.However there maybe rapid accumulation of soil in the valley.
Living Organisms: This refers to the activities of plants, microbes, rodents, man. Their effects will help to encourage soil formation in the following ways;
Animals such as rodents, earthworms, termite, etc burrow into the rocks,thereby leaving the holes or cracks in the rocks; they exert pressures which break off fragments of rock.
The activities of man during farm operation and construction contribute to the breaking down of rocks into smaller particles especially when ploughing machines are employed.
The root of higher plants exert prying effect on rock to cause break down. Also plant’s root exude certain substances which may cause dissolution of rock minerals, hence resulting in soil formation.
Time: Time is an important factor to soil formation. The breaking down of rocks and decomposition of organic materials takes some time, the time may be short or long. Time determine the age of the soil, that is, a young or an old soil. Young soils have poor water-holding capacity, mineral content and life sustenance. Old soil are rich in everything because all the factors like climate, parent material, topography and living organisms take time to register their effects on the soil formed. Also it takes time for plants to decay and become part of the soil.
Processes of Soil Formation
This refers to all the activities that will lead to the weathering of rocks. Weathering of rocks is the disintegration/ breaking down of rocks into tiny particles to form soil. Weathering is a complex series of changes or processes which take place in transformation of rocks and their constituent minerals into soil. The changes or process of soil formation include the following:
Physical process Biological process Chemical process 1. Physical processes
Are mostly climatic factors like temperature, running water, ice, wind and pressure.
Temperature: Temperature changes bring s about different expansion and contractions of minerals. Day and night variation of the temperature in the desert can lead to alternate expansion and cooling of rocks, as rocks that cannot withstand these changes peels off or flake off.thus creating stress in the rocks, leading to the eventual break down of the rocks.
Running water/Rainfall: Rain water may form flood, rivers and streams which Carries small rock particles (stone) and pebbles which collide and get broken into smaller particles. Through theerosional forces by the running water, portions of rock could be cut off and crushed which later form soil minerals.
Ice: Ice is an important agent of weathering in temperate regions, extremely low temperatures brings about frozen of water which is collected in the crevices of rock. The increase volume of the ice exerts considerable pressure on its surrounding leading to the breakdown of the rock.
Wind: Wind is important in arid regions and in tropical areas, especially during the dry weather. Wind is air in motion. In the tropical areas especially during the dry weather, wind can be laden with sand particles and dunes which result in abrasive forces and expose massive rock thereby resulting in disintegration.
2. Biological Processes
This involves the activities of plants and animals in the breaking down of rocks to form soil. Plant roots penetrate mineral rock cervices which crack and cause physical splitting of the rocks and minerals Animals help in weathering by burrowing into the rock. Human beings also help through cultivation and ploughing
.Chemical weathering/process: Chemical weathering is the reaction of rock minerals with water. The processes include hydrolysis, hydration, solution, carbonation, oxidation and reduction.
Hydrolysis: Is the process by which the hydrogen ions from water molecule are used to replace the metallic elements in a rock mineral. This is otherwise known as the reaction of water with rock minerals catalyzed by hydrogen to form a strong alkaline, this disintegrates the rock minerals
KAlSi₃O₈ + H2O → HAlsi3O8 +KOH
Hydration: This the rigid attachment of water molecules to a rock mineral e.g the changing of haematite(red) to limonite(yellow). The newly formed rock will disintegrate due to instability.
Fe2O3+ 3H2O ___________ Red (haematite) Solution
This is the process by which a solvent (water) dissolves a solute (rock minerals or salts) to form solution. The substances formed will be carried away and deposited elsewhere to form soil.
Carbonation: This is the process by which certain atmospheric gases react with water molecules to form weak acids leading to the disintegration of rocks.
E.g. (i) Carbondioxide reacts with water to form weak trioxocarbonate iv acid. CO2 + H2O _____ H2CO3 (trioxocarbonate iv acid) (ii) Sulphur dioxide reacts water to form weak trioxosulphate iv acid. SO2 + H2O ______ H2SO3. (trioxosulphate iv acid)
Oxidation: This is the addition of oxygen to a rock mineral or removal of hydrogen from a rock mineral or the process of electron loss. During weathering, the most significant example of
Fe2O3. 3H2O yellow (limonite)oxidation is the conversion of iron ii to iron iii. rock minerals – Siderite is changed to haematite.
Reduction: This simply means the addition of hydrogen to a rock mineral or the removal of oxygen from a rock mineral or the process of electron gain.
Soil Profile Development Meaning of Soil Profile
This refers to the vertical section of the soil from the surface into the underlying unweathered materials. These soil layers can be differentiated by their color, organic matter content, porosity and depth. There are four(4) distinct layers or horizons or strata.
The top soil is referred as A-Horizon while the sub soil is B-Horizon. Parent material is C-Horizon and the bed rock is D-Horizon.
Soil Formation and Profile Development – Soil profile development
A-Horizon: This consists of two layers:
The organic matter layer and top soil. The organic matter layer contains dead plants and animals materials, decayed, decaying organic matter (Humus). This is usually dark in colour. The top soil is the upper layer of the soil immediately after organic matter layer. It contains bacteria, worms and fungi .It is rich in soil nutrients.
B-Horizon: The B-Horizon is the sub soil. It is greyish or brownish in colour. The sub soil can be divided into two layers based on its colour. The upper layer is dark in colour and contains coarse sandy loam, fibrous branches, taproots, bacteria, fungi, worms, termites. The second layer is yellowish in colour. It contains sandy loam, little of humus, taproots and is the richest part of the sub soil.
C-Horizon: The horizon-C or parent materials layer is reddish orange, gravelly and contains sandy clay soils. The clay content is very high and taproots can be seen here occasionally.
D-Horizon: This is the bed rock, and an area in which rocks had not been transformed. It consists of unweathered rocks.
Importance of Soil Profile It helps to determine the level of soil fertility. It determines the types of crop to be grown in a particular area.
The level of parent material determines the type of mineral salt available to plants. It determines the level of drainage and irrigation of the soil. It determines easy penetration of roots of crops.
A loosely packed subsoil reduces erosion and soil run-off
Presentation :
The topic is presented step by step
Step 1: The class teacher revises the old topic
Step 2: The class teacher introduces the new topic
Step 3: The class teacher allows the pupils to give their own contributions and gives room for pupils” participation
Class Teacher and Pupils Activities. Interaction or Participation
This involves class teacher and pupils’ interaction, conversation, imitation or modeling through discussion, play method or just by recitation or asking and answering questions that are related to the topic that has just been taught.
EVALUATION QUESTIONS
1. (a) What is rock weathering? (b) State two factors of agricultural importance of weathering.
2. Mention five factors of soil formation and any four.
3. Discuss briefly five factors which affect soil formation.
4. Discuss the role of climate in soil formation
5. Explain briefly four types of chemical weathering.
Conclusion :
The class teacher concludes the lesson by giving the pupils some notes on the topic that has just been taught. He goes round to mark and he does the necessary corrections.
