The soil horizon that has the least nutrients is the "Hard Rocks" horizon, also known as the bedrock.

Explanation:
Soil is composed of different layers, or horizons, each with its own characteristics. These horizons are formed over time through weathering and the accumulation of organic matter. Each horizon has different characteristics and amounts of nutrients.

1. Humus horizon:
- Humus is the topmost layer of soil, composed of decomposed organic matter.
- It is rich in nutrients, such as nitrogen, phosphorus, and potassium, which are essential for plant growth.
- Humus improves the soil's fertility, water-holding capacity, and overall structure.
- It is dark in color and crumbly in texture.

2. Topsoil horizon:
- The topsoil horizon is located just below the humus layer.
- It is also rich in nutrients and organic matter, although to a lesser extent than humus.
- This layer is crucial for plant growth as it provides the necessary nutrients for root development and nutrient uptake.
- Topsoil is typically darker in color than the underlying horizons.

3. Small rocks horizon:
- This horizon consists of small rocks or gravel that have weathered from the underlying bedrock.
- It is usually located below the topsoil horizon.
- While this layer may contain some nutrients, it is not as fertile as the humus or topsoil horizons.
- The small rocks or gravel create a porous structure that allows water and air to move through the soil.

4. Hard rocks horizon:
- The hard rocks horizon, also known as the bedrock, is the deepest layer of soil.
- It consists of solid rock that has not undergone significant weathering or decomposition.
- Since it is composed of solid rock, it has the least amount of nutrients compared to the other horizons.
- Plant roots cannot penetrate this layer, and it does not contribute to plant growth directly.
- The bedrock provides the foundation for the soil profile and influences the characteristics of the overlying horizons.

In summary, the hard rocks horizon has the least nutrients because it consists of solid rock and has not undergone significant weathering or decomposition. The humus and topsoil horizons, on the other hand, are the most nutrient-rich layers and are vital for plant growth.