1. Soil Types

Soils as they are found in different regions can be classified into two broad categories: (1) Residual soils (2) Transported soils

Soils can broadly be divided into two main categories based on their formation and location:

1. Residual Soils: These soils form and remain in the same area where the parent rock has weathered and broken down. They build up over time.
2. Transported Soils: These soils are moved from their original location by natural forces like water, wind, or glaciers and then deposited in new areas.

Think of residual soils like a cake baking in an oven. The cake stays in one place like the soil remains where it formed. In contrast, transported soils are like slices of that cake being shared at a party. They are taken from the oven and placed on different plates, representing how soils move from one location to another.

Residual soils are found at the same location where they have been formed. Generally, the depth of residual soils varies from 5 to 20 m. Chemical weathering rate is greater in warm, humid regions than in cold, dry regions causing a faster breakdown of rocks. Accumulation of residual soils takes place as the rate of rock decomposition exceeds the rate of erosion or transportation of the weathered material. In humid regions, the presence of surface vegetation reduces the possibility of soil transportation.

Residual soils form in their original location, primarily through the processes of weathering. They can be found at depths ranging from 5 to 20 meters. Factors affecting their formation include:

• Climate: In warmer and humid areas, rocks break down more quickly due to higher chemical weathering rates compared to colder, dry areas.
• Vegetation: Plants growing on the soil help hold it in place, which decreases erosion and soil movement. Therefore, residual soils build up where rocks decompose more than they are carried away.

Imagine a sponge soaking up water. In a humid environment, the sponge retains more moisture (analogous to the faster weathering of rocks and formation of residual soils). A sponge in a dry area, however, doesn’t soak up as much water and dries faster (like slower soil buildup in cold, dry climates). Plants are like those small caps that keep the liquid in the sponge from spilling out.

As leaching action due to percolating surface water decreases with depth, there is a corresponding decrease in the degree of chemical weathering from the ground surface downwards. This results in a gradual reduction of residual soil formation with depth, until unaltered rock is found. Residual soils comprise a wide range of particle sizes, shapes and composition.

As we dig deeper into the soil, the effect of water washing away nutrients (leaching) diminishes. This means that the top layers of soil are more chemically weathered and richer in nutrients compared to deeper layers. Eventually, you reach the unweathered rock, which shows no signs of breakdown. Additionally, residual soils can have a variety of particle types, from fine dust to large pebbles, and diverse mineral compositions, contributing to their overall complexity.

Think of a layered cake again, where the frosting represents the rich, nutrient-heavy topsoil that has been affected by leaching. As you cut deeper into the cake, less frosting is found until you reach the unfiltered cake at the bottom, which represents the unweathered rock.

Weathered rock materials can be moved from their original site to new locations by one or more of the transportation agencies to form transported soils. Transported soils are classified based on the mode of transportation and the final deposition environment.

Transported soils form when previously weathered rock material is relocated by natural forces like rivers, wind, glaciers, or ocean currents. The type of transportation method determines the specific classification of the transported soils, which can be vital for understanding soil characteristics in different areas.

Consider a river carrying sand downstream to form a beach. Just like how the river reshapes parts of the landscape by carrying the sand away, similar natural processes move weathered rock to form different soil types elsewhere.

(a) Soils that are carried and deposited by rivers are called alluvial deposits. (b) Soils that are deposited by flowing water or surface runoff while entering a lake are called lacustrine deposits. Alternate layers are formed in different seasons depending on flow rate. (c) If the deposits are made by rivers in sea water, they are called marine deposits. Marine deposits contain both particulate material brought from the shore as well as organic remnants of marine life forms. (d) Melting of a glacier causes the deposition of all the materials scoured by it leading to formation of glacial deposits. (e) Soil particles carried by wind and subsequently deposited are known as aeolian deposits.

There are several types of transported soils based on how they were moved and deposited:

• Alluvial Deposits: Created by river action, rich in nutrients.
• Lacustrine Deposits: Formed at the edge of lakes, characterized by seasonal layering.
• Marine Deposits: Created in marine environments, combining terrestrial and marine materials.
• Glacial Deposits: Result from glaciers melting and depositing debris.
• Aeolian Deposits: Formed by wind action, such as dunes in deserts.

Think of a nature trail along a river, which showcases all the different types of deposits. The river (alluvial) brings in fresh soil, while winds on the nearby dunes (aeolian) sculpt the land. Each layer tells a unique story of how and when the soil was deposited.

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