The black cotton soil of India has been formed due to the weathering of
- (a) brown forest soil
- (b) fissure volcanic rock ✓ UPSC's answer
- (c) granite and schist
- (d) shale and limestone
Why the answer is (b)
• Black cotton soil (regur) forms from the weathering of basalt, the fissure-type volcanic rock that erupted through cracks to create the Deccan Traps about 66 million years ago — option (b).
• It is rich in iron, lime, magnesia and alumina, holds moisture, and swells and cracks with the seasons ('self-ploughing').
• It covers Maharashtra, Malwa, Saurashtra and parts of Telangana–Karnataka, and suits cotton, hence the name.
• Granite and schist (c) give red and laterite soils; shale and limestone (d) give alluvial/calcareous soils; brown forest soil (a) is itself a soil, not a parent rock.
• Hence option (b).
Why the other options are wrong
- (a) brown forest soil
- Brown forest soil is a soil type, not a parent rock.
- (c) granite and schist
- Granite and schist weather into red soils.
- (d) shale and limestone
- Shale and limestone do not yield regur.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2021, held on 10 October 2021. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.