Final answer:
Clay, iron, and aluminum from overlying horizons mainly accumulate in the B horizon or subsoil, where fine materials leached from topsoil are deposited. The B horizon's composition affects soil properties such as drainage and nutrient retention.
Step-by-step explanation:
Accumulation of Materials in Soil Horizons
Clay, iron, and aluminum brought down from overlying horizons accumulate predominately in the B horizon of soil, which is also referred to as subsoil. This layer is characterized by the accumulation of leached materials such as clay, iron oxides, and various aluminum compounds, enriching it in fine materials compared to the layers above. The process that results in the enrichment of the B horizon involves water movement through the soil profile where some components of the A horizon or topsoil (such as clay particles and minerals) are dissolved and transported downwards (a process known as leaching), and are then deposited in the B horizon. Over time, soil profiles develop distinct layers, with the B horizon generally forming below the A horizon (topsoil) and above the C horizon (soil base).
The B horizon often has a denser structure due to the accumulation of these fine materials, and it may contain nodules or a layer of calcium carbonate depending on the geographic region and the local soil environment. The characteristics of a B horizon can significantly influence the physical properties of the soil, including drainage, aeration, and its capacity to hold nutrients.