Soil Formation and Types

Soil is a complex mixture of minerals, organic matter, water, and air, formed from the breakdown of rocks and organic materials over long periods. The way soil forms and its subsequent movement (or lack thereof) determines its classification.

Residual Soil Formation

Residual soil, also known as sedentary soil, is a specific type of soil that is formed directly from the in-place weathering action of the underlying parent rock. The defining characteristic of residual soil is that it continues to remain at the place of formation, meaning it has not been transported away by agents like wind, water, or glaciers.

• The soil's composition is closely related to the mineralogy of the bedrock from which it originated.
• This type of soil is formed through a combination of physical weathering (like temperature changes, frost action) and chemical weathering (like dissolution, oxidation, hydrolysis).
• Examples include laterite soils formed over basalt or granite, and black cotton soils derived from basaltic rocks.

Transported Soil Types

To highlight the unique nature of residual soil, let's examine other soil types mentioned in the options, which are formed elsewhere and moved to their current location:

• Alluvial soil: This soil type is transported and deposited by flowing water, typically rivers and streams. It is common in river valleys and deltas, often highly fertile due to accumulated sediments.
• Marine soil: These are soils or sediments that have been deposited in ocean or sea environments. They are transported by ocean currents or settle out of seawater.
• Aeolian soil: This soil is transported and deposited by wind. Fine particles like silt and clay are carried long distances by wind before settling, forming deposits such as loess.

The key distinction is that alluvial, marine, and aeolian soils are all "transported soils" because they are moved from their original site of weathering and deposition, unlike residual soil which stays put.

Residual Soil Features

The question specifically asks about soil "formed by weathering action on rock and remain at the place of formation." This description perfectly matches the definition and characteristics of residual soil.

Therefore, based on its formation by weathering action on rock and its characteristic of remaining at the place of formation, the correct classification for such soil is residual soil.