**B. O-horizon**

The O-horizon, also known as the organic horizon, is the topmost layer of soil that consists of organic matter. It is primarily composed of decomposed plant and animal materials such as leaves, twigs, and organic debris. This layer is usually darker in color compared to the underlying layers due to the high organic content.

**Weathering in the O-horizon:**

Weathering is the process of breaking down rocks and minerals into smaller particles over time. It involves physical, chemical, and biological processes. In the case of the O-horizon, weathering primarily occurs through biological processes.

1. **Biological Weathering:** In the O-horizon, the organic matter serves as a substrate for various microorganisms, fungi, and bacteria. These organisms break down the organic matter through decomposition, releasing nutrients and minerals into the soil. This process is known as biological weathering.

2. **Decomposition:** During decomposition, the microorganisms and other decomposers feed on the organic matter, breaking it down into simpler compounds. This breakdown process leads to the release of nutrients and minerals that were originally stored in the organic matter. These nutrients become available for plants and other organisms to utilize.

3. **Humification:** Another important aspect of weathering in the O-horizon is humification. As the organic matter decomposes, it undergoes a transformation into humus, a dark-colored substance that is rich in organic compounds. Humus helps improve soil structure, water-holding capacity, and nutrient availability in the soil.

**Conclusion:**

In conclusion, weathering in the soil primarily occurs in the O-horizon, which is the topmost layer consisting of organic matter. Biological processes such as decomposition and humification play a crucial role in breaking down the organic matter and releasing nutrients into the soil. This process of weathering in the O-horizon creates a rich and fertile layer that supports the growth of plants and other organisms.

O horizons are soil layers with a high percentage of organic matter. Typically within a woodland area there are three distinct organic layers: one of leaves, pine needles and twigs (Oi); underlain by a partially decomposed layer (Oe);and then a very dark layer of well decomposed humus (Oa).