The topmost layer of a typical soil profile is

Analyze the question: It asks which layer sits at the very top of a typical soil profile. This is the idea of soil horizons.

Explain the concept: A soil profile is a vertical section through the soil, showing layers called horizons. From top to bottom:

1. Humus / topsoil (O and A horizons): The uppermost layer. It is dark and rich in decaying plant and animal matter (organic matter). It holds most of the soil's nutrients, microorganisms, and plant roots.
2. Subsoil (B horizon): Lighter in colour. It contains more clay and minerals that have been washed down (leached) from above.
3. Weathered parent rock (C horizon): Broken-up rock fragments, such as gravel and coarse particles.
4. Bedrock (R horizon): Solid, unweathered rock.

Solution:

• Clay (A) accumulates in the subsoil, below the top layer.
• Coarse sand (B) and fine sand (C) are mineral particles found mainly in the topsoil and subsoil mix, but they are not the topmost layer.
• Gravel (D) is found deeper, near the weathered parent rock.
• Humus (E) is the decomposed organic matter that forms the topmost layer of the soil, which is why topsoil is dark and fertile.

Answer: E. humus.

Tip: Remember that soil goes from "Humus to Rock," or simply "organic on top, rock at the bottom." If a question asks about the most fertile or topmost layer, think humus or topsoil.