Residual soil is the material resulting from the in situ weathering of the parent rock. Residual soils are distributed throughout many regions of the world, such as Africa, South Asia, Australia, Southeastern North America, Central and South America, and considerable regions of Europe.

Definition of Residual Soil:
Residual soil refers to the type of soil that is formed directly from the weathering and decomposition of the underlying bedrock. It is typically found in situ, meaning it remains in the same location where it was formed, without being transported by erosion or other geological processes. Residual soil is often characterized by its composition, which reflects the parent rock from which it was derived.

Formation Process:
The formation of residual soil starts with the weathering of the bedrock. Weathering refers to the physical and chemical processes that break down rocks into smaller particles. There are three main types of weathering: physical, chemical, and biological.

1. Physical Weathering: Physical weathering involves the disintegration of rocks without changing their chemical composition. Examples of physical weathering include freeze-thaw cycles, abrasion, and exfoliation.

2. Chemical Weathering: Chemical weathering occurs when rocks are broken down through chemical reactions. Water, oxygen, and acids are common agents of chemical weathering. This process can result in the alteration of minerals and the formation of new compounds.

3. Biological Weathering: Biological weathering involves the action of living organisms, such as plants and animals, in breaking down rocks. Plant roots, for example, can penetrate cracks in rocks, causing them to break apart over time.

As the bedrock undergoes weathering, the resulting particles accumulate over time to form a layer of residual soil above the solid rock. The composition and characteristics of the residual soil are influenced by factors such as the type of parent rock, climate, vegetation, and the duration and intensity of weathering processes.

Characteristics:
Residual soil exhibits several distinct characteristics:

1. Composition: The composition of residual soil reflects the mineralogy and chemistry of the parent rock. For example, if the bedrock is rich in quartz, the residual soil will also contain a significant amount of quartz.

2. Depth: The depth of residual soil can vary depending on the rate of weathering and erosion. In some cases, it may be several meters deep, while in others, it may be only a few centimeters thick.

3. Structure: Residual soil often has a well-developed structure, with distinct layers or horizons. These layers can vary in terms of their physical and chemical properties.

4. Nutrient Content: The nutrient content of residual soil is typically low, as the weathering process tends to remove or leach away essential nutrients. This makes it less fertile compared to transported soils.

5. Stability: Residual soil is generally stable and less susceptible to erosion compared to transported soils. This is because it is firmly attached to the underlying bedrock and is not easily displaced by wind or water.

Overall, residual soil plays a crucial role in supporting plant growth and providing a habitat for various organisms. Its formation and characteristics are influenced by a range of factors, making it a unique and important component of the Earth's surface.