What additional requirement must be met for the bottom of a spodic horizon?

To determine the depth requirement for the bottom of a spodic horizon, it’s important to understand the characteristics and classification of this horizon within soils. A spodic horizon is typically characterized by specific features resulting from the leaching and accumulation of organic matter, aluminum, and iron, commonly seen in certain soil types such as Spodosols.

The correct answer indicates that the bottom of a spodic horizon must be at least 12 cm deep. This depth criterion is essential for classification purposes and reflects the soil's ability to exhibit the necessary morphological and compositional traits associated with spodic horizons.

Meeting this depth requirement is crucial because it ensures that the properties associated with the spodic conditions—such as the accumulation of illuviated materials—are present and can be adequately analyzed.

Other depth specifications, like 18 cm, 24 cm, and 15 cm, may describe different horizons or soil characteristics but do not align with the specific criteria for spodic horizons that are set forth in soil classification guidelines. Therefore, understanding the precise depth requirement is key to correctly identifying and classifying soil horizons in the field.