Question

The O$_a$ sub-horizon denotes-

• A. Organic horizon with highly decomposed organic matter
• B. Organic horizon with intermediately decomposed organic matter
• C. Organic horizon with least decomposed organic matter
• D. Organic horizon without decomposed organic matter

Hint:

Remember: “a” stands for advanced decomposition in the O-horizon classification.

Answer 1

The Correct Option is A

The O$_a$ horizon consists of well-decomposed organic matter, indicating advanced stages of decomposition, and typically appears dark and homogeneous.

Answer 2

Comparative reasoning across the O-horizon subdivisions:
Soil scientists split the organic (O) horizon into three subdivisions purely on how far decomposition has progressed. Freshly fallen litter with intact leaf and twig structures, showing almost no decomposition, is designated Oi. Once decay has proceeded far enough that original plant parts are only partly recognisable, it is designated Oe. When decomposition is so advanced that virtually no original plant structures remain visible and the material is dark and granular, it is designated Oa. Since the question asks specifically about the Oa sub-horizon, and Oa is by definition the most fully decomposed of the three, the correct description is highly decomposed organic matter, confirming option 1.