Alluvial soils are immature, azonal soils formed by the deposition of sediments by river systems. In India, these soils are primarily the result of the depositional work of the three major Himalayan river systems—the Indus, the Ganga, and the Brahmaputra—along with their numerous tributaries. They are also formed in coastal regions by marine wave action and river deltas. Because they are formed through ongoing running water deposition rather than the in-situ weathering of bedrock over millennia, these soils lack well-developed vertical soil horizons (profiles).
Geographical Distribution
Alluvial soils constitute the largest soil group in India, covering approximately 43.4% of the country’s total land area.
- The Great Indo-Gangetic-Brahmaputra Plains: This continuous longitudinal belt extends from Punjab and Haryana in the west, through Uttar Pradesh and Bihar, to West Bengal and Assam in the east.
- Coastal Strips and Deltas: Found along the eastern coastal plains, particularly in the deltas of the Mahanadi, Godavari, Krishna, and Cauvery rivers.
- Inland Corridors: Narrow corridors of alluvial deposits extend through Rajasthan into parts of Gujarat, along with the structural valleys of the Narmada and Tapi rivers.
Chronological Classification: Khadar versus Bhangar
Based on the age of deposition and morphological characteristics, the alluvial tract is broadly divided into two major types.
Bhangar (Older Alluvium)
- Age and Position: Represents the older alluvial deposits situated above the flood level of rivers, forming older terraces.
- Texture and Composition: Coarser and clayey in texture. It contains high concentrations of calcareous concretions or calcium carbonate impurities known as Kankar.
- Fertility: Relatively less fertile than Khadar, requiring regular fertilization for high-yield agriculture.
- Regional Variations: In the drier tracts of Punjab and Haryana, the Bhangar soil exhibits saline and alkaline efflorescences, locally known as Reh, Usar, or Kallar.
Khadar (Newer Alluvium)
- Age and Position: Younger alluvial deposits found in low-lying floodplains. These tracks are inundated and replenished by rivers almost every year.
- Texture and Composition: Fine-grained, light-colored, and contains a higher proportion of sand and silt. It is largely free from Kankar formations.
- Fertility: Highly fertile, moisture-retentive, and ideal for intensive cultivation without heavy chemical fertilizer inputs.
Geological Sub-divisions of Alluvial Plains
When moving from the foot of the Himalayas down to the deltaic plains, the alluvial terrain shifts across four distinct geomorphological zones.
- Bhabar: A narrow belt (8 to 10 km wide) running parallel to the Shiwalik foothills. It consists of unassorted, coarse pebbles and gravels. Rivers descending from the mountains disappear underground in this tract due to its extremely high porosity. The soil here is skeletal and unsuited for agriculture.
- Terai: Located south of the Bhabar belt, this is a marshy, damp zone where the underground streams re-emerge. The soil is rich in nitrogen and organic matter but requires drainage management. It supports dense forests and major cultivation of sugarcane and rice.
- Bhangar: The older upland alluvial plains lying between the river valleys (interfluves).
- Khadar: The low-lying, flood-prone river beds subject to fresh silt deposition.
Chemical and Physical Characteristics
- Mineral Deficiencies and Efficiencies: Alluvial soils are generally rich in Potash, Phosphoric acid, and Lime. However, they are highly deficient in Nitrogen and Organic Matter (Humus) across almost all Indian regions.
- Soil Texture: Varies dynamically from pure drift sand in the upper reaches to loamy texturing in the middle plains, and stiff clay in the delta regions.
- Soil Reaction (pH): Mostly ranges from neutral to slightly alkaline (pH ≈ 6.5 to 8.0), making them highly responsive to irrigation and chemical fertilization.
Agricultural Significance
Due to its vast extent, loose textural composition, flat topography, and high water-retention capabilities when clay-rich, alluvial soil is considered the agricultural backbone of India.
- Major Crops (Upper and Middle Plains): Highly suited for the cultivation of Wheat, Sugarcane, Tobacco, Cotton, Maize, and Oilseeds.
- Major Crops (Lower Plains and Deltas): The clayey, water-retentive variations in West Bengal, Odisha, and coastal Andhra Pradesh are heavily utilized for intensive Rice (Paddy) and Jute cultivation.