Soil Formation
Soil is an unconsolidated material which may comprise of solids, water and air. The process of formation of soil is termed as paedogenesis. Soil is formed due to the erosion/weathering of rocks which may be carried out either physically / mechanically or chemically. Main physical agent unloading, erosion, freezing, and thawing involve in the weathering of rocks include running water, wind, ice & gravity. Chemical agency is involved in the weathering of the rocks include oxidation, reduction & carbonation. Soil formed due to physical weathering means reduction of size without any change in the original composition (same as the parent) of the parent soil/rock and if it is form due to chemical weathering, weather mineral constituent of soil is different from parent rock or soil.
Soils that retained over the parent rock (at the site of weathering) is referred as residual soil. And any soil that has been transported from its place of origin by any of the agency and has been redeposited, is called transported soil.
Geological cycle of formation of soil –
The agency of soil transportation will affect the characteristics of soil such as size of the soil particles, their shapes and smoothness, surface texture and the degree of sorting that takes place in a soil deposit. These characteristics are shown in following table –
Effects of Transportation on Sediments
| – | Water | Air | Ice | Gravity | Organism |
| Size | Reduction through solution, little abrasion in suspended load, Some abrasion and impact in traction load. | Considerable reduction | Considerable grinding and impact | Considerable impact | Minor abrasion effects from direct organic transportation |
| Shape & Roundness | Rounding of sand and gravel | High degree of rounding | Angular, soled particles | Angular, non-spherical | – |
| Surface texture | Sand: smooth, polished and shiny | Impact produces frosted surface | Striated surfaces | Striated surfaces | – |
| Sorting | Considerable sorting | Very considerable sorting | Very little sorting | No sorting | Limited Sorting |
Types of Soil –
Soil Types Common descriptive terms such as gravels, sands, silts, and clays are used to identify specific textures in soils. We will refer to these soil textures as soil types; that is, sand is one soil type, clay is another. Texture refers to the appearance or feel of a soil. Sands and gravels are grouped together as coarse grained soils. Clays and silts are fine grained soils. Coarse grained soils feel gritty and hard. Fine grained soils feel smooth. The coarseness of soils is determined from knowing the distribution of particle sizes, which is the primary means of classifying coarse grained soils. To characterize fine grained soils, we need further information on the types of minerals present and their contents.
Classification of Soil based on transportation –
- Alluvial Deposits – Alluvial soil is the type of soil which is deposited from suspension from running water. Also called fluvial soil this type of soil generally found near the river bank. These have poorly graded / uniformly graded sediment deposited which may consist of gravel / sand / silt / clay.
- Marine Deposits – It is the type of soil which is deposited from suspension in sea water. These are also compressible and have low shear strength.
- Lacustrine Deposits – It is the type of soil which is deposited from suspension in still water of the lakes. These are either silty or plane such have flocculated structure.
- Glacial Deposits – It is the type of soil which is transported by glacier and not transported or segregated by water. Such soil may be well graded
- Aeolian Deposits – It is the type of soil which is transported by wind. Such soil may be poorly graded, cohesionless.
A few of the soils are listed below that formed by these deposits –
Varved Clays – It is consisting of thin alternating layers of silt and clays of glacial origin. The constituents of varved clay were transported into fresh water lakes by the melted ice at the close of the ice age
Colluvial / Talus Soil – It is the type of soil which is transported by gravitational force. This type of soil is generally observed in the bottom of mountain valley.
Marl Soil – It is uniformly graded marine soil which is formed due to the decomposition of bones & dead cell mass of aquatic life.
Loess Soil – It is fine grained wind blown silt which is slightly cemented due to the presence of calcium compounds & montmorillonite. The size of particles ranges between about 0.01 to 0.05 mm. These soils are collapsible due to very uniform grain size, and have high void ratio. Its colour is yellowish light brown and it is found in dry continental regions.
Loam Soil – It is the mixture of clay, silt & sand in definite proportion (not more than 50%), which in some cases also consist of organic matter.
Bentonite Soil – This type of soil formed due to chemical weathering of volcanic ash. These soils generally find its application in drilling operations as lubricants. It has high content of montmorillonite.
Tuff Soil – It is fine grained slightly cemented volcanic ash which may be transported either by wind or water.
Laterite Soil – It is the type of soil which is formed due to leaching (washing of silica compound is termed as leaching) rocks. This type of soil is generally observed in hilly areas having humid climate. These are residual soils that are cemented with iron oxides.
Gumbo Soil – It is highly plastic, black colour, sticky soil.
Peat Soil – It is highly organic soil that almost entirely consist of vegetated matter and different stages of decomposition. Its colour varies from black to dark brown and it poses organic odour. It is highly fibrous & posses highly compressible.
Muck Soil – It is fine particle inorganic soil mix with black, decompose organic matter. This type of soil is generally observed in areas having inefficient sewerage facilities & offer the overflooding of rivers.
