Which term describes a hard-packed layer of clay that is challenging for tree and shrub roots to penetrate?

The term that accurately describes a hard-packed layer of clay that poses difficulties for tree and shrub roots to penetrate is "hardpan." This term refers specifically to a subsurface layer that has become compacted and can hinder root growth, water infiltration, and overall soil aeration. In many cases, hardpan layers are formed due to processes such as repeated traffic or heavy rainfall, which compress the soil particles tightly together, creating a barrier that roots struggle to pass through.

Subsoil, by contrast, is the layer beneath the topsoil that generally contains minerals and nutrients but may not necessarily be compacted. It serves as a reservoir for nutrients that can support root health. Topsoil is the uppermost layer of soil, rich in organic material and nutrients, where most biological activity occurs; it is not typically hard-packed. Loam is a soil type composed of a balanced mixture of sand, silt, and clay, known for its fertility and good drainage qualities, thus not relevant in the context of a hard-packed layer.