Ch 6: Soils
Anchors all core physical geography questions on the composition, geographical distribution, and chemical deficiencies of Indian soils, alongside soil degradation patterns.
Soil
This section covers soil profile layers (Horizons A, B, and C) and the basic factors of soil formation. UPSC often tests structural and functional differences between horizons. Horizon A is the topmost zone where organic matter is incorporated; Horizon B is a transition zone containing mineral and organic matter; Horizon C consists of loose parent material. Bedrock lies below this. Note that parent material dictates chemical properties and mineral structure, while climate determines weathering rates. Skip overly simplistic definitions but do not skip the sequence of layers or the interplay of factors.
Horizon 'A' is the topmost zone where organic materials have got incorporated with mineral matter, nutrients, and water. Horizon 'B' is a transition zone containing organic matter and mineral matter.
Classification of Soils
Focuses on the ICAR classification of Indian soils based on their character and USDA soil taxonomy. Crucial data to memorize is Table 6.1 (Soil Orders in India): Inceptisols are the most abundant order (39.74%), followed by Entisols (28.08%) and Alfisols (13.55%). Vertisols (black soils) account for only 8.46%. A classic UPSC trap is to state that Vertisols are the most dominant soil order in India, or to confuse the properties of Inceptisols with Aridisols. Memorize the descending order of these taxonomy classes.
According to the USDA Soil Taxonomy, Inceptisols comprise 39.74% of Indian soils, while Vertisols account for only 8.46%.
Major Soil Groups of India
The most high-yield section of this chapter. It details eight soil types: Alluvial, Black, Red & Yellow, Laterite, Arid, Saline, Peaty, and Forest. Key facts: Alluvial soils are rich in potash but poor in phosphorus; Bhangar is older alluvium with kankar (calcareous) deposits, while Khadar is newer silt. Black soil (Regur) is rich in iron, lime, calcium, potassium, magnesium, but poor in nitrogen, phosphorus, and organic matter; it is self-ploughing and highly moisture-retentive. Red soils develop on crystalline igneous rocks and look yellow when hydrated. Laterite soils are heavily leached, rich in iron and aluminium oxides, but poor in nitrogen, calcium, and phosphate. Saline soils (Usara) occur in dry climates and are induced by capillary action in canal-irrigated zones. Peaty soils are acidic and have high organic content (40-50%).
Black soils are highly argillaceous, swell and become sticky when wet, and shrink when dry, leading to self-ploughing. They retain moisture for long periods, reducing irrigation needs.
Saline soils are also known as Usara soils. In Punjab and Haryana, farmers are advised to add gypsum to solve the problem of soil salinity and alkalinity.
Peaty soils occur in areas of heavy rainfall and high humidity. They are characterized by a large quantity of dead organic matter (40-50 percent) and are highly acidic.
Soil Degradation and Erosion
Examines structural, physical, and chemical decline of soil quality. Focus on water and wind erosion. Sheet erosion is common on steep slopes and goes unnoticed but is highly destructive as it removes the fertile topsoil layer. Gully erosion cuts deep ravines, forming badland topography typical of the Chambal basin. Anthropogenic soil degradation is dominated by salinity issues in Punjab, Haryana, and western UP due to excessive canal irrigation and flooding methods. Traps include confusing sheet erosion with rill erosion, and assuming wind erosion is confined strictly to Rajasthan deserts.
Soil Conservation
Focuses on engineering and agronomic solutions to combat soil loss. Key methods include contour bunding, terrace farming, strip cropping, shelterbelts, and agro-forestry. The Central Soil Conservation Board (established in 1953) is a critical institutional detail. Note the specific regional applications: contour bunding and terracing are used in hilly terrains (Himalayas and Western Ghats), while shelterbelts and sand dune stabilization using windbreaks are critical in western Rajasthan. Avoid generalist assumptions that any technique works universally; UPSC questions test specific geographical pairings.