Which soil layer is typically the deepest?

The R horizon, also known as the bedrock layer, is typically the deepest soil layer. This layer consists of solid rock that is beneath the soil horizons that are formed from the weathering and breakdown of the rock. Unlike the other horizons, which are composed of loosely packed soil, organic matter, or mineral material, the R horizon is distinguished by its intact rock structure.

The A horizon, or topsoil, is primarily composed of organic matter mixed with minerals and is located near the surface. The E horizon is a leached layer, often found beneath the A horizon, where minerals and nutrients have been washed away, making it less fertile. The O horizon consists mostly of organic material, such as decomposed leaves and other plant material, and is typically the topmost layer.

In contrast to these other horizons, the R horizon provides the foundational bedrock that influences the development of the soil profile above it, but it does not interact directly with soil processes like those occurring in the upper horizons. This makes the R horizon distinctly the deepest layer in the soil profile.