Soil Formation – Definition, Types, Process, and Factors Affecting

What is Soil Formation?

Soil formation is the process of origin of soil, sometimes referred to as pedogenesis, which is controlled by the effects of geography, environment, and history. Anisotropy (order) in soils can be created or destroyed by biogeochemical processes. These changes cause the formation of layers known as soil horizons, which can be identified by variations in chemistry, colour, structure, and texture. These characteristics appear in distribution patterns of soil types and form in response to variations in soil forming factors.

Further, let us discuss the rock formation, physical weathering, chemical weathering, and geological steps involved in soil formation.

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Factors Affecting Soil Formation

The major factors of nature and structure of the soil that affect soil formation are – parent rock, weathering process, transportation agents, environment, and subsequent conditions.

• Parent rock – Constituents and nature of parent rock.
• Weathering process – Physical and chemical weathering.
• Transportation agents – Wind, running water, glaciers, and gravity.
• Environment – Topography and environment of the location where the soil is deposited.
• Subsequent conditions – Such as drainage, surcharge, leaching, etc.

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Soil Formation – Rock Classification

The nature and structure of the soil are greatly influenced by the type of parent rock that leads to its formation. Therefore, looking into the major rock types is imperative before understanding the soil formation process. There are three major types of rock:

• Igneous Rock: They are regarded as the primary rocks created by the cooling of molten magmas. Example – Basalt, granite, etc.
• Sedimentary Rock: These types of rocks are formed when the sediments/particles resulting from disintegration and decomposition of any rock type are transported, deposited, and cemented into a new rock type. Example – sandstones, limestones, shales, etc.
• Metamorphic Rock: These rocks are formed by partial or full recrystallization of igneous or sedimentary rocks under high temperature and pressure. Example – slate, marble, etc.

Soil Formation – Physical Weathering

In physical weathering, the parent rock is disintegrated into smaller particles without changing the original composition of the rock. The agents involved in soil formation through physical weathering are temperature changes, frost action, erosion by wind, water, and ice. Significant variation in the temperature can cause changes in the volume of rocks in terms of expansion and shrinkage. Such volume changes can result in the development of temperature stresses in the rock, which can lead to its fracture.

Water can enter the fissures present in rock and, if subjected to alternate freeze-thaw cycles, can further break down. Slow and continuous rock erosion due to the action of wind, rain, moving water or glacier can also cause weathering of the rocks. Soil formation through physical weathering generally results in the formation of coarse-grained soil.

Soil Formation – Chemical Weathering

Chemical weathering changes the nature of the original parent rock in addition to fracturing the rock into smaller particles as new compounds are formed due to chemical change. The principal chemical processes that result in soil formation through chemical weathering are hydration, oxidation, carbonation, and leaching. The product of chemical weathering is generally fine-grained soil.

Oxygen and carbon dioxide in the air combine with rock constituents in the presence of water to form hydrated oxides, sulfates, and carbonates. The soil formation of these compounds can lead to volume increase which can cause disintegration of the rock. Water-soluble substances are washed away by leaching, and rocks lose their cementing qualities, breaking them into smaller particles.

Soil Formation – Geological Steps

The 4 geological steps involved in soil formation are weathering, transportation, deposition, and upheaval. The rocks are disintegrated into smaller particles (soil particles) through erosion/weathering (physical or chemical). The soil particles are then transported to a different location by agents such as wind, water, glacier, and gravity, where they get deposited. These soil deposits combine partially or fully to form rocks (upheaval). These rocks again undergo weathering, and the cycle is hence repeated.

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