Here's why:

* Humid Climate: Ultisols form in areas with high rainfall and warm temperatures, which are typical of the eastern US.

* Weathering: The high rainfall causes significant leaching of nutrients, especially calcium and magnesium, from the soil. This results in low base saturation (less than 35%).

* Clay Accumulation: Ultisols have a distinct layer of clay accumulation in the subsoil (Bt horizon). This clay layer is often dense and difficult for roots to penetrate.

* Iron and Aluminum: The leaching of bases also leads to the accumulation of iron and aluminum oxides, giving Ultisols their distinctive red, yellow, or orange color.

Other Soil Types:

While Ultisols are the most common in the humid eastern US, other soil types can be found depending on specific factors like elevation and parent material:

* Alfisols: Found in areas with slightly less rainfall or better drainage, Alfisols have higher base saturation than Ultisols.

* Inceptisols: Younger soils in development, often found in mountainous areas, may have characteristics of both Ultisols and Alfisols.

Importance of Ultisols:

Ultisols are generally fertile soils, but their low base saturation and high clay content can make them challenging to manage. They require careful fertilization and management practices to maintain their productivity for agriculture or forestry.