Exposure of lower, hard and rocky layers.
Explanation
The removal of the top layer of soil, a process known as soil erosion, significantly alters the physical and chemical properties of the land. The topsoil (A-horizon) is the most fertile layer, rich in organic matter (humus) and essential nutrients required for plant growth.
Analysis of Options:
- Exposure of lower, hard and rocky layers. is Correct: The topsoil covers the subsoil and bedrock. When this soft, porous layer is removed by agents such as wind or water, the lower, harder, and rocky layers are exposed. This process can eventually lead to desertification, rendering the land barren.
- Increase in soil humus. is Incorrect: Humus is concentrated in the topsoil. Consequently, the removal of the top layer leads to a sharp decrease in soil humus content.
- Increase in soil fertility. is Incorrect: Soil fertility is derived primarily from the nutrient-rich topsoil. Its removal depletes the soil of essential nutrients, thereby decreasing fertility.
- Increase in water holding capacity. is Incorrect: The organic matter and structure of topsoil facilitate water retention. Removing this layer reduces the soil's porosity and water-holding capacity, often leading to increased surface runoff.
Key Takeaway: The erosion of topsoil exposes the underlying hard and rocky layers, resulting in a loss of humus, reduced fertility, and decreased water-holding capacity.