Ultisols are commonly called red clay. The word ‘ultisol’ refers to this soil order being defined as the ultimate product of mineral weathering in a humid, temperate climate without new soil via glaciation.
Ultisols are the common forest soil in warm humid climates.
They are frequent in the southeastern United States, extending westward to East Texas. They also occur in the Pacific Northwest and eastern California. Ultisols have a high acidity and low organic matter content. They have an accumulation of silicate clay in the B horizon and, unlike Alfisols, they have low base saturation caused by more intense weathering.
In the South, fertilization accompanying planting has generally been limited to phosphorus applications on poorly drained clayey Ultisol soils. Ultisols are generally deficient in available phosphorous and require phosphorus fertilizer for optimum crop yield.