Final answer:
The A horizon or topsoil is primarily composed of humus and minerals. It contains organic material mixed with inorganic products of weathering and is essential for plant growth.
Step-by-step explanation:
Composition of Soil Layers
The layer of soil that is primarily composed of humus and minerals is known as the A horizon, which is also commonly referred to as the topsoil. The O horizon also contains a significant amount of humus. The A horizon plays a crucial role in plant growth as it blends organic material with the inorganic products of weathering. This horizon has a dark coloration which signifies a high presence of organic matter, making it a vital component for the sustenance of agriculture and ecosystems. Directly beneath it is the B horizon, known as subsoil, which consists mainly of mineral material that has been transported downward, forming a denser layer within the soil profile.
Physical attributes such as the structure and fertility of the soil are greatly influenced by the proportion of humus, sand, silt, and clay present in these layers. Clay, with its attributes of plasticity and cohesion when mixed with water, contains fine particles such as feldspar, silica, and iron, giving it a distinct reddish-brown color and influencing the soil's abilities to retain water and nutrients vital for plant life.