CSIR NET Dec 2015 Q115: Sedimentology
Major soil orders characteristic of semi-arid tropical climates
In semi-arid tropical regions, seasonal wetting and drying cycles combined with moderate rainfall and often base-rich parent materials favor the development of two characteristic soil orders: Alfisols, which are moderately weathered soils with a clay-enriched (argillic) subsurface horizon and relatively high base saturation, typical of savanna and semi-arid tropical/subtropical woodland environments; and Vertisols, dark, clay-rich (montmorillonite-dominated) soils that undergo pronounced shrink-swell behavior with seasonal wetting and drying, developing characteristic deep cracks and self-mulching structure, common in semi-arid tropical regions with pronounced wet-dry seasonality (e.g., the Deccan black cotton soils of India). This pairing of Alfisols and Vertisols as the dominant soil orders of the semi-arid tropics matches option 3.
Option 1 (Vertisols and Mollisols) incorrectly pairs Vertisols with Mollisols, which are dark, organic-rich, base-saturated soils typically associated with temperate grasslands (e.g., prairies, steppes) rather than the tropics. Option 2 (Oxisols and Histosols) is incorrect because Oxisols are deeply weathered, iron/aluminum oxide-rich soils typical of humid (not semi-arid) tropical rainforest regions, and Histosols are organic (peat) soils typical of waterlogged/wetland environments, neither of which characterizes semi-arid tropical conditions. Option 3 is correct: Alfisols and Vertisols are the classic soil orders of semi-arid tropical climates with pronounced seasonality. Option 4 (Oxisols and Vertisols) incorrectly includes Oxisols, which require intense, prolonged humid-tropical weathering rather than the more limited weathering typical of semi-arid conditions.