The uppermost horizon of soil is known as the topsoil or the A horizon. It is the most important layer of soil for plant growth because it contains a high concentration of organic matter, minerals, and nutrients. The topsoil is typically dark in color, rich in humus, and abundant in minerals, making option D ("All of the above") the correct answer.

Here is a detailed explanation of each characteristic of the uppermost horizon of soil:

Dark in color:
- The topsoil is usually darker in color compared to the lower layers of soil. This darkness is due to the presence of organic matter, such as decomposed plant and animal materials.
- The dark color indicates a higher concentration of organic compounds, which are essential for soil fertility and plant growth.

Rich in humus:
- Humus is a dark, organic material that forms as plant and animal residues decompose over time. It is a key component of the topsoil and plays a vital role in soil fertility.
- Humus improves soil structure, water-holding capacity, and nutrient retention. It provides a favorable environment for earthworms, microorganisms, and other soil organisms that aid in nutrient cycling and decomposition.

Rich in minerals:
- The uppermost horizon of soil contains a wide range of minerals that are essential for plant growth. These minerals are derived from weathering of rocks and are gradually incorporated into the soil profile.
- Some common minerals found in the topsoil include calcium, potassium, phosphorus, nitrogen, and micronutrients such as iron and zinc. These minerals are essential for plant metabolism, growth, and development.

Importance of the topsoil:
- The topsoil is crucial for agriculture and gardening because it provides the necessary nutrients and organic matter for plant growth.
- It is the layer where most of the plant roots are located, allowing them to access water, nutrients, and oxygen.
- The topsoil also acts as a storage reservoir for water, preventing excessive runoff and erosion. It helps regulate the water balance in the soil, providing moisture to plants during dry periods and draining excess water during heavy rains.

In conclusion, the uppermost horizon of soil, or the topsoil, is dark in color, rich in humus, and abundant in minerals. These characteristics make it a vital layer for plant growth and soil fertility.