Leaching is the

Step 1: Analyze the question The question asks for the definition of leaching in soil science and geography.

Step 2: Explain the concept Leaching is the process by which water, mostly from rainfall, dissolves soluble nutrients and minerals (such as calcium, potassium, and nitrates) in the upper soil layers and carries them downward, out of the reach of plant roots. It is common in areas of heavy rainfall, such as southern Nigeria's rainforest belt, and it often leaves the topsoil poor in nutrients.

Step 3: Evaluate each option

• A. Accumulation of soil mineral in one layer of soil. This describes illuviation or eluviation's deposit zone (for example, the B horizon), or calcification or salinization. It is not leaching itself.
• B. Washing down of soil nutrients. This is correct. Water moves dissolved nutrients downward through the soil profile.
• C. Erosion of soil from steep slopes. This is soil erosion, specifically surface wash or gullying. It involves the movement of soil particles across the surface, not downward through the profile.
• D. Loss of soil moisture. This is evaporation or desiccation, not leaching.

Step 4: Answer The correct option is B, washing down of soil nutrients.

Tip: Think of "leach" like "leak": nutrients leak downward with water. Also remember the pair:

• Eluviation: removal of materials from the upper layer (leaching).
• Illuviation: accumulation of those materials in the lower layer (this is option A).

Don't confuse leaching (downward, within the soil) with erosion (removal over the surface).