SOIL

SOIL
Soil refers to the top most material on the earth’s surface and constitutes the outer most layer of the earth’s crust.

Table of Contents

• Soil
• Soil Formation 
• Soil Profile 
• Factors Affecting soil Formation 
• Process Of Soil Formation
• Soil PH
• Soil Erosion 

SOIL

Introduction:-

Soil formation is a long term process. It takes several million years to form a thin layer of soil. As soil is a complex mixture of various components, its formation is also more complex. The Formation of a particular type of soil depends upon the physico-chemical properties of the parent rock, intensity and duration of weathering, climatic and other parameters.

Soil Formation

Soil is the thin surface layer on the earth, comprising mineral particles formed by the breakdown of rocks, decayed organic matter, living organisms, soil water and soil atmosphere. Soil formation starts with weathering of rock masses and accumulation of weathered particles on the surface of the rock masses. The components of soil are found in all three states of matter.

Soil Profile

A vertical section of the soil from surface down to the bedrock, showing the different horizons is called the soil profile. The horizons or layers have different colours. Generally a soil profile has four horizons:

• Top soil: It contains organic matter and humus.
• Subsoil: It contains weathered rock material, sand, silt and clay. This layer contains salts and minerals and less humus,
• Substratum: It consists of large fragments of weathered parent rock material.
• Bedrock: It provides a base to the three layers above. The layer is unweathered.

Factors affecting Soil formation

1. Nature of the parent material/parent rock.

The parent rock material is the rock material that breaks down into rock particles and may influence the nature of the soil in terms of fertility, mineral composition, depth, colour and the final soil profile. Parent rock may be hard or it may be soft. Hard parent rock is normally resistant to weathering and as a result skeletal soils are formed. On the other hand relatively soft rocks are easily broken down into soil particles and the results into a higher rate of soil formation.

Stages Of Soil Formation

2. Climate

Climate influences soil formation through its role in weathering that leads to the formation of soil. In areas of heavy rainfall adequate moisture is provided for the process of chemical weathering. In addition, in the desert areas soil formation through physical weathering processes like exfoliation are common. High temperatures accelerate chemical weathering leading to high rate of soil formation unlike in areas of lower temperatures where soil formation through chemical weathering is limited. In very cold regions like mountain tops, the nature of soil formation is through physical weathering processes like frost action or freeze and thaw. Climate also determines the nature of vegetation and animal life that consequently contribute to the soil formation through the addition of humus.

3. Living Organisms/Biota

These include bacteria, insects, mammals (animals), human beings and plants. Bacteria play an important role in the breakdown of rocks through complex processes. Organisms such as earthworms, termites also play an important role in the breakdown of rocks into simple or smaller substances that constitute soil. Rodents e.g. rats, moles, squirrels etc physically breakdown rocks as they dig holes into the ground. Man also influences soil formation through activities like mining/Quarrying and digging. As a result masses of rock are physically weathered by man to produce soil.

4. Relief

Relief influences soil formation through erosion and deposition. The nature of the relief influences the rate and nature of the soils formed. Steep slopes are easily eroded and this implies that the weathered material on the steep slope soils tend to be skeletal because of erosion. However the rate of soil

formation is high because erosion exposes the parent rock to further weathering.

On the other hand in the gentle slopes, soils tend to be deep, mature and with a well developed profile. In the lowlands or flatlands where rainfall is high leaching takes place and may lead to the formation of Laterite soils that are poor in terms of plant nutrients.

5. Time

This refers to the duration of the interaction of soil forming processes and factors. Soil formation requires adequate time, time is important in that the nature of the soils depends on how long the processes and factors have been interacting.

If a parent rock has been exposed to the weathering processes for a long time, soil formation will complete as compared to a parent rock exposed for a relatively shorter period.

PROCESSES OF SOIL FORMATION AND SOIL PROFILE DEVELOPMENT

1. Weathering

This is the physical disintegration or chemical decomposition of rocks. Rocks break down through various physical and chemical processes leading to soil formation.

High rate of weathering result into deep and mature soils meaning that with high rates of weathering, a mature soil profile is easily reached. In the tropics weathering under hot humid conditions has led to deep Latosols and tropical black earths.

2. Laterisation:

This involves weathering under hot and damp conditions followed by leaching within the tropics to form Laterite. Silica is leached out of the A- Horizon leaving behind iron and alluminium compounds. Laterisation leads to the formation of a stratified profile consisting of an indurated zone known as duricrust.

3. Leaching

Leaching refers to the removal of solvable mineral nutrients by water from the upper layers of the soil profile to the underlying ones i.e. minerals like salt and carbonates dissolve in water in the top soil and move in solution form to the sub soil. Leaching results into an impoverished A-horizon.

4. Eluviation

This involves the movement of soil material in solution or suspension form from one place to another within the soil. This movement can be horizontal or vertical. Eluviation is commonly referred to as the movement in form of colloids or suspension. Eluviation generally leads to the development of an impoverished A- horizon i.e. the top soil is impoverished of soil nutrients.

5. Illuviation

This is the precipitation and accumulation of the leached and eluviated material in the B- horizon of the soil profile i.e. eluviated material and leached materials concentrate in the B- horizon. Illuviation is responsible for the B2 layer (darker colour zone) where maximum accumulation of colloids takes place.

6. Humification

Is the process through which organic matter is decomposed to form humus that is then mixed with the inorganic content. It is common in wet, warm and densely vegetated areas such as equatorial or moist tropical regions. This process influences the development of the AO and A1 layers of the A- horizon or the O-Horizon.

7. Mineralisation

Occurs under extreme conditions in which decomposition of organic matter may extend further than Humification. Organic matter is further broken down into basic parts or components e.g. carbon dioxide, water and Silica. This greatly influences the formation of the A-horizon.

8. Calcification

Is a soil forming process, which takes place in dry or arid areas as a result of upward movement of capillary water with dissolved nutrients through capillary attraction moves from the lower parts of the soil the top or surface. This process is common in areas of limestone geology on East Africa. The soils formed comprise of a lot of calcite or calcium compounds. Calcification leads to the development of shallow soil profiles.

9. Gleization/ Gleying

This soil forming process occurs in climatic environments where there is impeded drainage i.e. where there are swamps or wetlands. Usually the process leads to the development of hydromorphic soil. In addition, the oxygen deficient conditions of water- loggedness leads to the reduction of ferric oxide to ferrous oxide giving a grey colour. Such soils are intrazonal hydromorphic soils with poorly developed profiles.

SOIL PH

This refers to the measure of the concentration of hydrogen ions in a solution. It is therefore the degree of acidity or alkalinity of soil. High concentration of hydrogen ions means the soil is acidic and low concentration means the soil is alkaline. The decay of organic matter increases the acidity of soils. On the other hand bases like calcium, sodium, potassium, make the soils more alkaline. The PH scale that measures from 1- 14 is used to estimate the degree of acidity or alkalinity. A PH of 7 is neutral. If it is below 7 the soil is said to be acidic and if it is above 7 the soil is said to be alkaline.

Soil Erosion

Soil forms over thousands of years but can be washed away quickly due to the carelessness of mankind. The removal of the fertile top soil by agents of erosion like rain water, rivers, wind and waves is termed as soil erosion. Sheet erosion and gully erosion are the two most common types of erosion.