Spodosols are characterized by which unique feature?

Spodosols are recognized for their distinct spodic horizon, which is a layer rich in organic matter and iron and aluminum oxides. This characteristic spodic layer typically develops under acidic forested conditions, often in sandy parent material. The presence of this layer indicates leaching processes where organic acids have dissolved and mobilized minerals, leading to the accumulation of the spodic horizon below the surface layer.

The thick spodic layer distinguishes Spodosols from other soil types, which do not have this specific buildup of organic matter along with metals. This feature is crucial for understanding the soil's formation process, fertility, and its typical vegetative cover. Other choices presented do not accurately capture the defining characteristics of Spodosols. While soils may exhibit high nutrient retention or heavy clay content, these traits are not defining for Spodosols specifically.