Which soil horizon is least likely to interact directly with living organisms?

The C horizon, also known as the parent material layer, is the least likely to interact directly with living organisms. This horizon consists primarily of weathered rock and mineral material that has not undergone significant soil formation processes. It serves as the foundational layer from which the upper soil horizons develop. Because it lacks the organic matter and biological activity present in the upper horizons, the C horizon does not provide the nutrients or habitat that living organisms, such as plants, animals, and microorganisms, typically rely on.

In contrast, the O horizon, which is primarily made up of organic material like decomposed leaves and other plant matter, has a high level of interaction with living organisms. Similarly, the A horizon, or topsoil, contains a significant amount of organic matter and nutrients, making it a vital zone for plant growth and microbial activity. The E horizon is characterized by leaching and is often lighter in color due to the loss of minerals, but it still has some interaction with organisms as roots can penetrate this layer.

Thus, the C horizon stands out as being much less biologically active and engaging with living organisms compared to the other horizons.