Which horizon is used for determining the depth of soil evaluation pits if it is shallower?

The correct choice for determining the depth of soil evaluation pits when they are shallower is the restrictive horizon. This horizon is significant because it denotes layers in the soil profile that may impede water movement or root growth, effectively limiting the depth to which soil evaluations can be accurately performed. When evaluating soil, particularly for agricultural or construction purposes, it's essential to recognize and assess any restrictive horizons, as they can substantially influence drainage, nutrient availability, and the overall suitability of the soil for specific uses.

The topsoil horizon typically contains the most organic matter and is crucial for plant growth, but it does not indicate limitations in the profile. The subsoil horizon, located beneath the topsoil, may still contain valuable nutrients but often has different physical or chemical characteristics that might not necessarily restrict depth evaluation. Ancillary horizons refer to layers that may not have a significant impact on the primary soil characteristics being assessed for evaluation. Thus, assessing the restrictive horizon is crucial for understanding the limitations of soil depth and making informed decisions based on the evaluation findings.