• Soil and natural vegetation play crucial roles in regulating life on Earth. Soils can be transported by running water, wind, or other erosion agents or may remain in their original position.

Soil

• Soil is a highly valuable natural resource, serving as a growth medium for plants, filtering water, and hosting millions of microorganisms. Comprising both mineral and organic matter, soil is susceptible to erosion from human activities, and its slow formation process renders it a non-renewable resource.
• Various soils exhibit distinct chemical properties, with acidity levels determining classifications—soils with lower lime content are termed acidic, while those with higher lime content are alkaline. When soil remains in its original position (in-situ), it undergoes modification influenced by climate, moisture supply, plant growth, and bacterial activity.
• In 2015, the United Nations declared it the International Year of the Soil to highlight its global significance. World Soil Day is observed annually on December 5th.

Structure and Composition of Soil

• Soil structure is determined by the arrangement of individual particles of sand, silt, and clay. When these particles assemble, they form aggregates, resembling larger particles in the soil.
• Soil character is expressed in terms of its aggregation or the binding properties of individual particles to form a secondary unit. Structure, affecting soil properties like porosity, has a significant impact on soil fertility.
• Soil texture and structure determine porosity and permeability. Sandy soil, with large particles, has decreased porosity and increased permeability, leading to higher water percolation and dryness.
• Loam soil, with a balanced mix of sand, silt, and clay, maintains permeability and porosity, making it highly fertile for agriculture.

Soil Texture

• Particle size characteristics include gravel (2 mm diameter), coarse sand (2 to 0.2 mm), fine sand (0.2 to 0.02 mm), silt (0.02 to 0.002 mm), and clay (< 0.002 mm).

Soil Acidity

• Acidity, measured on the pH scale (0 to 14), influences crop growth. A pH of around 6.5 is considered optimal. Lime-deficient soils are acidic, forming in high rainfall areas.

Soil Salinity and Alkalinity

• Saline/alkaline soils have top layers infused with efflorescences. Over-irrigation leads to salt percolation, while good drainage washes salts away. Poor drainage causes stagnant water with high salt concentration, leading to salt deposits.
• Sandy soils are prone to alkalinity, and loamy soils are susceptible to saline-alkalinity.

Factors Influencing Soil Formation

Some factors influencing soil formation are described below.

Parent Material

• These rocks from which soils are formed are called parent materials. Generally, the parent material determines the colouration, mineral composition and texture of the soil.
• In some cases, the soil formed may or may not have the same physical properties of the parent rock. The surface rocks are exposed to the process of weathering. 
• In this process, the rocks are converted into fine grains and provide a base for the soil formation.
•  In Indian context, parent material is generally categorized into different types such as
• Ancient Crystalline and Metamorphic Rocks These red soils and red colour of the soils is due to the presence of iron oxides.
• Cuddapah and Vindhyan Rocks These Calcareous and argillaceous soils, as the rocks are ancient sedimentary rocks. These are devoid of metalliferous minerals.
• Gondwana Rocks It is less mature soils with low fertility, because the rocks are younger sedimentary rocks.
• Deccan Basalts These are regur or black soils because basalts are rich in titanium, magnetite, aluminium and magnesium giving the soil its dark colour.

Climate

• Hot summer and low rainfall develops black soil as is found in some parts of Tamil Nadu irrespective of the parent rock.
• In arid and semi-arid regions, evaporation always exceeds precipitation. There is little vegetation and the soils badly lack humus content.
• Hence, the soils are invariably of light colour. Here, the alkaline elements dissolved with water move from the lower layers to the upper layers by capillary action due to which the soil becomes more alkaline. Alkaline soil is rich in sodium. especially sodium carbonate.

Vegetation

• In latitudinal shifts, despite increased vegetation density from high to low latitudes, humus levels first rise and then decline.
•  In high latitudes, low temperatures hinder organic matter decomposition, while in low latitudes, humus dissolves and leeches to lower layers.
• The roots and leaves of vegetation contribute to humus production, enhancing soil fertility and formation.

Terrestrial Topography

• Topography influences soil formation and erosion. Steep, rocky surfaces reduce soil-forming substance accumulation, resulting in thin soil layers, termed Residual soil. Contour bunding mitigates erosion in such areas.
• The Southern Indian plateau exhibits shallower soil due to heterogeneous relief, while deeper soil is found in river valleys and coastal deltaic areas.

Time

• Rock types affect soil formation time; porous rocks form faster than impervious or massive rocks.

Soil Fertility

• Fertile soil sustains plants with organic substances and clay minerals. Soil fertility is gauged by properties like colour, texture, consistency, porosity, mineral presence, moisture, and chemical processes. Soil horizons, visible in the field, classify soil based on these characteristics.

Soil Profile and Horizons

• The vertical series of soil horizons from the surface to the parent rock is the soil profile. Profiles vary based on the parent rock's location. Soil horizons, identified as O, A, E, B, C, represent different layers.

Organic Horizon

• Marked with the letter O, organic horizons lie above mineral horizons. Animal and bacterial activity influences soil content. O horizons are subdivided into O1 and O2.
• O1 contains decomposing organic matter.
•  O2 consists of humus broken down beyond recognition.

Mineral Horizons

Some mineral horizons are as follows

A-Horizon

• The topsoil horizon primarily comprises minerals (sand, silt, and clay) along with significant organic matter.
• Predominantly found in grasslands and agricultural lands, this surface layer is crucial for plant growth.

E-Horizon

•  A subsurface horizon subject to heavy leaching, resulting in a light color. Typically located between A and B horizons, it represents an older layer.

B-Horizon

• An accumulation of material from layers above, the B-horizon serves as a deposition site for minerals leached from A and E horizons.
• Functions as a zone of illuviation.

C-Horizon

• A subsurface horizon representing the least weathered layer, also known as saprolite. Composed of unconsolidated loose parent material.
• Acts as a zone of accumulation for soluble inorganic compounds percolating down from upper layers.

R-Horizon

• Unweathered parent material or parent rock, consisting of unbroken rock strata at the bottommost layer of the soil profile.

Soil Forming Processes

• Weathering: Breakdown and chemical changes of parent rocks in situ.
• Translocation: Material movement within the soil body.
• Eluviation: Downward transport of fine particles, especially clays and colloids, from the uppermost soil part.
• Illuviation: Accumulation of materials brought downward, typically from E-horizon to B-horizon.
• Decalcification: Leaching of calcium carbonate from the entire soil, usually in moist climates.
• Calcification: Occurs in dry regions where the soil accumulates significant amounts of soluble calcium carbonate and magnesium in the B-horizon.
• Salinization or Alkalization: Extreme evaporation leads to the accumulation of calcium or sodium salts on the soil surface due to capillary action from a water table.
• Desilication: Silica is more mobile than iron and aluminum oxides, resulting in desilicified soils known as ferralsols.
• Podzolization: Iron and aluminum oxides become more mobile, enriching the upper horizons with silica. Typically associated with coniferous and heathland vegetation.
• Gleying: Common in waterlogged or permanent water areas.

Stages in Soil Development

• Initial Stage: Unweathered parent material.
• Juvenile Stage: Weathering has commenced.
• Virile State: Weathered material with significant decomposition, increased clay content.
• Senile State: Decomposition in the final stage, most weathering has occurred.
• Final Stage: Soil development completed, weathered under prevailing conditions.

Classification of Soil

• Based on the global distribution of soils, various attempts have been made by soil taxonomists worldwide and global institutions. 
• Russian pedologist V. Dokuchaev made the first soil classification attempt, and later, in 1938, CF Marbut of the United States Department of Agriculture (USDA) introduced the 'USDA System.'
• The soil was broadly classified into three groups: Zonal soil, Intrazonal soil, and Azonal soil.

Azonal soils

• These soils are developed through the deposition process by erosion agents, consisting of fine rocky particles transported from distant regions. They are immature soils, lacking well-developed profiles, either due to insufficient time for full development or location on steep slopes hindering profile formation. Azonal soils are categorized into the following types:
  • Alluvial Soil: Little or no profile development, found on active floodplains.
  • Regosols: Composed of dry and loose dune sands or loess.
  • Lithosols: Accumulations of imperfectly weathered rock fragments on steep slopes.

Intrazonal Soil

• Soils found within a climatic belt but different due to the dominance of local factors such as relief, parent material, climate, and vegetation. These soils are semi-mature and not continuous.
  • Calcimorphic: Develop on limestone, e.g., Rendzina, Terra rossa, and Terarokes soils.
  • Hydromorphic: Devel