Agricultural Science
Topic: Soil Formation And Profile Development
Contents
- Definition Of Soil
- Factors Of Soil Formation
- Processes Of Soil Formation
Objectives
At the end of this article, readers should be able to:
1. Define soil.
2. List and discusss factors of soil formation.
3. Discuss the process of soil formation.
4. Discuss rock weathering.
5. Eunmerate processes involved in rock weathering.
6. Mention four major elements of climate.
7. List the chemical processes in the weathering of rocks.
Definition Of Soil
Soil can be defined as unconsolidated, weathered materials found in the uppermost layer of the earth surface on which plants grow.
Factors Of Soil Formation
Soil formation is greatly controlled by five factors which are:
1. Climate
2. Parent materials
3. Topography
4. Biotic (living organisms)
5. Time.
1. Climate: Elements of climate such as rainfall, temperature, wind and pressure are all very important of soil formation.
1. Temperature: The alternating heating and cooling of rocks result in the continual expansion and contraction which eventually result in cracks in the rocks and its consequent breakdown into small pieces to form the soil. Temperature affects the rate of chemical weathering of rocks.
2. Rainfall: The action of running water from rainfall cause the gradual wearing away of rocks during erosion to form soil.
Rainfall enhances vegetative growth of plants whose roots cause further breakdown of rocks, while the rain water transports rock particles after disintegration.
3. Wind: High wind velocity in deserts carry with it other tiny rocks which collide with one another or other rocks, resulting in the breaking down of rocks into tiny pieces to form soil.
4. Pressure: High pressure on a hanging rock may cause such rock to fall down and break into tiny pieces, resulting in the formation of soil.
2. Parent Material: Parent materials constitute the major materials from which soil is formed. They are igneous, sedimentary and metamorphic rocks. Parent materials determine the chemical composition of the soil that is formed. It also contains different minerals which account for differences in the fertility of the soil formed from each of the different types. Parent materials determine the physical characteristics of the soil. Hardnedd of parent materials affects the rate of soil formation.
3. Topography: The shape of the ground in relation to the underlying rock of the earth’s surface is known as topography. That is, topography is:
1. The shape of the land that influences the movement and amount of water in the soil.
2. Hilly land slopy surfaces which support erosion and encourage soil formation.
3. A flat land exposes the whole surface to equal environmental factors and therefore delays soil formation.
4. More soil formation in the valleys than at the top of the hills.
4. Biotic Factors (Living Organisms) The activities of living organisms help to speed up the process of soil formation.
1. Termite, earthworm, rodent mix the mineral and organic matter together, and this results in the formation of soil.
2. They also allow water and air into the soil which eventually react with rocks to cause their breakdown into soil.
3. The activities of man during tillage and other farm operations indirectly help to break rocks into tiny pieces to form soil.
4. Microbes also improve soil aeration and water percolation. This enhances chemical and physical weathering.
5. Microbes help on the decomposition of organic matter in the soil.
6. They influences the organic matter content of the soil.
5. Time: Time also plays an important role in soil formation. It takes a long time for mature soil to be formed.
1. It takes a long time for small pieces of rock to disintegrate into grains of soil.
2. It also takes a long time for plants to decay and become part of the soil.
3. It takes time for rainfall to leach chlorides, sulphates and carbonates from the soil.
4. It takes a short time in the formation of immature soil.
5. Time also determines whether or not the soil is well developed.
Processes Of Soil Formation
The process of soil formation is called Weathering. Weathering is defined as the disintegration or break down of rocks into tiny pieces to form soil in other words, weathering can also be defined as the breaking down of rock masses (rock minerals) into simpler forms through the agents of physical, chemical and biological processes.
The Processes of Soil Formation Include:
1. Physical process
2. Chemical process
3. Biological process
1. Physical process: Agents of physical weathering are temperature, ice, wind, water and pressure.
1. Temperature: The alternating heating and cooling of the rocks produce pressure within the rocks and cause them to break down into pieces.
2. Wind: As a result of the grinding of rock surfaces by solid materials carried by wind, water and moving ice (glacier), rocks break down to form soil.
3. Ice: The conversion of water inside cracks in rocks into ice results in increase volumes results in more pressure being exerted on the rock walls which eventually break into smaller pieces.
4. Water: Running water carries some fragments of rocks along its course and these rub against the surface of rocks in the river bed, thus breaking off small pieces of rocks.
2. Chemical Process: Agents of chemical weathering include solution, carbonation, hydration, hydrolysis and oxidation.
a. Explanation of chemical weathering of rocks:
i. Chemical weathering is the decomposition of rocks by chemical agents formed through the reaction of water with atmospheric gases such as air (oxygen and carbon dioxide).
ii. As some minerals in the rocks are dissolved and others change into new chemical products, therefore disintegration of rocks occurs.
b. Discussion of chemical weathering of rocks;
a. Hydration:
i. Occurs when water combines with or binds to some minerals.
ii. Intact water may bind to silicates, or oxides of iron.
iii. Hydrated compounds are soft and easily fragmented.
iv. It does not usually affect the chemical composition of hydrated product.
b. Carbonation:
i. This is brought about by the combination of carbonate or bicarbonate ions with rock minerals.
ii. The carbon dioxide released to the atmosphere from various sources combines with water/rain water to form carbonic acid.
iii. The carbonic acid attacks the minerals and CO2 in the rock.
iv. In limestone or marbles, the calcite present is dissolved.
c. Oxidation/Reduction:
Atmospheric oxygen and free oxygen in rain water carry out this reaction.
i. Minerals containing iron (Ferrous form) manganese and sulphur are the ones most frequently affected by this reaction.
ii. When they are exposed to air and water, the ferrous ion is oxidized to ferric Fe2+ state.
iii. The change in valency from Fe++ to Fe+ destabilizes the molecule and leads to disintegration.
d. Hydrolysis:
i. A decomposition reaction in which silicate minerals are broken down.
ii. Water molecule is split into H and OH-ions H20 >> H+ + OH-
iii. The H+ replaces the cation from the mineral structure while the mineral is released for plant uptake.
iv. Hydrolysis is the breaking of the chemical bond in the mineral by water.
Solution
Water can dissolve any soluble mineral present in rocks and carry them from the place of reaction it the land is sloppy.
v. If the land is not sloppy, the products of the soil solution accumulate in that place and form particular types of soil.
Examples: Stalactite and stalagmite.
3. Biological Process
This involves the activities of plants and animals in the breaking down of rocks to form soil.
i. It is caused by the activities of plants and animals in the breaking down of rocks to form soil.
i. It is caused by the action of animals like earthworms, termites and other soil organisms.
ii. Movement of these organisms cause smal fragments of rocks to disintegrate.
iii. Earthworms and termites burrow into the rocks and break off fragments of rocks.
iv. The roots of growing plants penetrate rocks through crevices, exerting pressures which split some rocks.
v. The activities of man during farm operations such as ploughing and harrowing also break down rocks into tiny pieces.
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