What mineral characteristic defines the presence of an argillic horizon?

The presence of an argillic horizon is defined by a significant increase in clay content compared to the horizon above it; specifically, this increase must exceed 1.2 times the clay content of the overlying horizon. This characteristic is a crucial indicator of the argillic horizon, which is typically associated with clay illuviation, where clay particles are transported downwards from upper soil layers and accumulate in the argillic horizon, leading to higher clay content. This process affects soil properties such as its texture, structure, and nutrient-holding capacity, which are essential for understanding soil profiles and their behavior.

In contrast, high pH does not specifically indicate the presence of an argillic horizon, as soils with varying pH levels can contain or lack this horizon. A lower clay content would contradict the definition of an argillic horizon, which requires increased clay accumulation. High organic matter, while significant for soil fertility and structure, does not uniquely identify the transition to an argillic horizon, as organic matter can be present in various horizons independent of clay content changes. Thus, the defining characteristic of an argillic horizon is indeed the clay content exceeding the specified threshold compared to the overlying layer.