Final answer:
The O horizon is the uppermost soil layer rich in organic matter, while the A horizon is the topsoil essential for plant growth, and the B horizon is the subsoil with accumulated fine material.
Step-by-step explanation:
The uppermost soil horizon that contains a high amount of organic matter is known as the O horizon. This layer is characterized by freshly decomposing organic matter, humus, at its surface, with decomposed vegetation at its base.
The A horizon, which is also referred to as the topsoil, consists of a mixture of organic material with inorganic products of weathering and is critical for plant production due to its high organic matter content and microbial activity. In contrast, the B horizon or subsoil is an accumulation of mostly fine material that has moved downward, forming a more dense layer within the soil profile.