What is a layer of hard soil that roots cannot grow through called?

A layer of hard soil that roots cannot grow through is referred to as hardpan. This term specifically describes a dense, compacted layer of soil that inhibits root penetration and water movement. Hardpan typically forms due to processes such as soil compaction from heavy machinery or natural factors, leading to the accumulation of materials like clay. As a result, this layer can create challenges for plant growth, as roots often struggle to access moisture and nutrients beyond this layer.

In contrast, bedrock is the solid rock that lies beneath the soil layers and is not typically described as a layer of soil itself; it represents a much deeper geological formation that cannot be penetrated by plants at all. A clay layer is found in various soil profiles but does not inherently indicate that it prevents root growth, as clay soils can still be conducive to root systems under certain conditions. Subsoil is the layer beneath the topsoil, which can contain a variety of materials and does not necessarily block roots from growing through it; in fact, roots often access this layer for additional nutrients and moisture.