In terms of soil horizons, which layer is usually most affected by shrink and swell?

The B horizon is typically the layer most affected by shrink and swell due to its composition, which often includes clay minerals that are prone to volumetric changes in response to moisture fluctuations. This horizon accumulates materials leached from the upper layers (such as the A horizon) and is where clay can build up through processes like illuviation.

The properties of clay make this layer particularly reactive to changes in water content. When the soil dries out, clay particles can move closer together, causing the soil to shrink. Conversely, when water is absorbed, clay swells, leading to an expansion of the soil volume. This cycle of shrinking and swelling can lead to structural changes in the soil, which may influence conditions for plants, soil health, and even contribute to physical land movements over time.

In contrast, the other horizons either do not have the same clay content or are more affected by different processes. The A horizon, being the top layer, is primarily influenced by organic matter and surface conditions, while the C horizon consists of weathered parent material that shows less variability in response to moisture changes. The O horizon, made up mostly of organic matter, doesn’t typically go through shrink and swell processes since it is less composed of soil minerals.