Question:
The O$_a$ sub-horizon denotes-
The O$_a$ sub-horizon denotes-
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Remember: “a” stands for advanced decomposition in the O-horizon classification.
Updated On: Jul 2, 2026
- Organic horizon with highly decomposed organic matter
- Organic horizon with intermediately decomposed organic matter
- Organic horizon with least decomposed organic matter
- Organic horizon without decomposed organic matter
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The Correct Option is A
Approach Solution - 1
The O$_a$ horizon consists of well-decomposed organic matter, indicating advanced stages of decomposition, and typically appears dark and homogeneous.
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Approach Solution -2
Elimination using USDA organic horizon nomenclature:
Organic (O) horizons are subdivided by degree of decomposition: Oi (fibric) retains recognisable plant fibres and is the least decomposed, Oe (hemic) is partially or intermediately decomposed, and Oa (sapric) consists of organic material so decomposed that original plant structures are no longer identifiable. Ruling out 'least decomposed' (that is Oi) and 'intermediately decomposed' (that is Oe) leaves the Oa sub-horizon as the one representing highly or well decomposed organic matter, confirming option 1.
Organic (O) horizons are subdivided by degree of decomposition: Oi (fibric) retains recognisable plant fibres and is the least decomposed, Oe (hemic) is partially or intermediately decomposed, and Oa (sapric) consists of organic material so decomposed that original plant structures are no longer identifiable. Ruling out 'least decomposed' (that is Oi) and 'intermediately decomposed' (that is Oe) leaves the Oa sub-horizon as the one representing highly or well decomposed organic matter, confirming option 1.
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