Soil Formation in India

Soil formation (pedogenesis) in India is a complex evolutionary process governed by the country’s diverse geological structures, varied climatic zones, and distinct topographic features. Understanding these mechanisms is crucial for analyzing India’s agricultural patterns, land degradation challenges, and ecological zones.

Factors Influencing Soil Formation in India

The nature and properties of Indian soils are primarily determined by five interdependent active and passive factors.

• Parent Material (Passive Factor): The geological bedrock determines the mineralogical composition, texture, and chemical structure of the soil.
  • Example: The weathering of ancient crystalline granites and gneishes of the Peninsular Shield forms Red Soils, rich in iron. Conversely, the weathering of Deccan Trap basaltic rocks yields clayey, magnesium-rich Black Soil (Regur).
• Climate (Active Factor): Temperature and precipitation are the most dynamic drivers, regulating the rate of weathering and organic matter decomposition.
  • Example: High temperatures and heavy seasonal rainfall in the Western Ghats and North-East India promote intense leaching, resulting in Laterite soils.
• Topography / Relief (Passive Factor): Elevation and slope determine the runoff of water, rate of erosion, and drainage efficiency, directly impacting soil depth.
  • Example: Steep Himalayan slopes feature thin, immature, and poorly developed skeletal soils due to high erosion, whereas the flat Indo-Gangetic plains accumulate deep, fertile alluvial strata.
• Biological Activity (Active Factor): Soil organisms, including flora, fauna, and microorganisms, break down organic matter into humus and alter soil porosity.
  • Example: The dense deciduous and coniferous forests of Jammu & Kashmir and Himachal Pradesh contribute high organic waste, creating humus-rich Forest/Mountain soils.
• Time (Passive Factor): The duration of the pedogenic process dictates soil maturity.
  • Example: Alluvial soils of the Indo-Gangetic plains are relatively young and lack distinct horizons (Profiles), whereas the soils of the ancient South Indian Peninsula are highly mature.

Fundamental Pedogenic Processes in the Indian Context

Soil formation involves physical, chemical, and biological transformations. In the Indian subcontinent, specific processes dominate distinct geographic zones.

Humification and Eluviation

Humification is the decomposition of organic matter into stable humus. Eluviation refers to the downward transport of dissolved or suspended soil components (clay, iron, aluminum) by percolating water from the upper horizon (A-horizon).

Illuviation

The deposition and accumulation of the eluviated materials into the lower layer (B-horizon). In areas with distinct wet and dry spells, this leads to horizon differentiation.

Calcification and Decalcification

In the arid and semi-arid tracts of Rajasthan, Punjab, and Haryana, high evaporation rates cause upward capillary action, precipitating calcium carbonate (CaCO₃). This leads to the formation of ‘Kankar’ nodules in the sub-soil layers.

Laterization

A process prominent in hot and humid tropical regions. Heavy rainfall leaches away silica and lime, leaving behind insoluble oxides of iron and aluminum. This gives rise to brick-like Laterite soils.

Podzolization

Occurs in the cold, humid Himalayan belt where acidic humus from coniferous vegetation leaches iron and aluminum, depositing them in lower layers and leaving a grey, silica-rich upper horizon.

Salinization and Alkalization

Prevalent in over-irrigated command areas of the Green Revolution belt (Punjab, Haryana, Western Uttar Pradesh) and arid zones. Capillary action brings dissolved salts (Sodium, Magnesium, Calcium) to the surface, forming a white saline crust known locally as Reh, Usar, or Kallar.

Major Soil Types and Geographic Distribution

The Indian Council of Agricultural Research (ICAR) classifies Indian soils based on their genesis, color, composition, and location into eight primary categories.

Morphological Variations: Khadar vs. Bhangar

In the vast alluvial plains of India, chronological differences in soil deposition create two distinct sub-categories of soil morphology.

Khadar (New Alluvium)

• Age and Deposition: Younger alluvium deposited by annual river floods.
  • Texture: Fine-grained, sandy, and highly porous.
  • Fertility: Naturally fertile as it is renewed every year; requires minimal chemical fertilizers.
  • Location: Found in low-lying floodplains and river channels.

Bhangar (Old Alluvium)

• Age and Deposition: Older alluvium representing the floodplains of an earlier geological era.
  • Texture: Coarser, clayey, and contains higher concentrations of calcareous nodules called ‘Kankar’.
  • Fertility: Less fertile compared to Khadar; forms terrace features above the current flood level.
  • Location: Found in interfluves and older river terraces.

Soil Profiles and Horizons in Mature Indian Soils

A typical mature Indian soil profile (such as well-developed Red or Black soils) displays distinct vertical stratifications.

• O-Horizon (Organic Layer): Top layer dominated by fresh and partially decomposed organic matter (leaf litter). Thickest in Himalayan forest soils; virtually absent in Desert and Black soils due to rapid microbial oxidation under high temperatures.
• A-Horizon (Topsoil / Zone of Eluviation): Mineral layer rich in organic matter mixed with weathered parent rock. It is the zone where seeds germinate and plant roots thrive. Nutrients leach downward from this layer.
• B-Horizon (Subsoil / Zone of Illuviation): Accumulation zone containing iron oxides, clay, aluminum, and calcium carbonate washed down from the A-horizon.
• C-Horizon (Substratum / Weathered Parent Rock): Consists of loosely weathered rock fragments. It represents the transition zone between true soil horizons and the underlying unweathered solid bedrock.
• R-Horizon (Bedrock): The unweathered, solid parent rock layer that forms the base of the soil profile.

Key Facts and Trivia for Prelims

• Self-Ploughing Nature: Black soil develops deep, wide cracks during the dry summer season. This allows oxygenation and causes surface soil to fall into the cracks, a phenomenon known as “self-mulching” or “self-ploughing.”
• The Red Color Mechanics: The red tint in Red Soils is due to the high diffusion of iron in crystalline and metamorphic rocks. When these soils occur in a hydrated form (absorbing water), they look yellow.
• Kankar Nodes: The presence of ‘Kankar’ layers in Bhangar and Arid soils acts as a natural barrier, restricting the downward infiltration of water and occasionally creating localized perched water tables.
• Kari Soils: This is the local name given to the highly acidic, black, peaty soils found in the coastal districts of Kerala (Kottayam and Alappuzha), known for high organic accumulation.
• Chos: The seasonal streams flowing down the Siwalik hills in Punjab cause severe gully erosion, stripping away the fertile topsoil of the sub-mountainous tracts.