5.9 - Ultisols

Ultisols are intensely weathered soils of warm and humid climates. They are typically formed on older geologic locations in parent material that is already extensively weathered.

Ultisols have accumulated clay minerals in the B horizon. While generally low in natural fertility (basic cations, Ca2+, Mg2+, and K+) and high in soil acidity (H+, Al3+) the clay content of Ultisols gives them a nutrient retention capacity greater than that of Oxisols, but less than Alfisols or Mollisols.  Ultisol soils can be agriculturally productive with inputs of lime and fertilizers.

Large areas of Ultisol are found in the southeastern USA, China, Indonesia, South America, and equatorial regions of Africa.

 

Soil order - Ultisols. Image courtesy of USDA-NRCS

Profile example - Cecil Series. Image courtesy of USDA-NRCS

Cecil Series, North Carolina State Soil

USDA details

More information from the University of Idaho

 

Key Characteristics:  Ultisols

  • Intensely weathered soils of humid areas
  • Form on older geologic locations in weathered parent materials
  • Contain subsurface clay accumulations (claypans).
  • Low in natural fertility (Ca2+, Mg2+, and K+) and high in soil acidity (H+, Al3+)
  • Can be agriculturally productive with inputs of lime and fertilizers 
  • Occur extensively in the southeastern USA, China , Indonesia, South America, and equatorial regions of Africa
  • Extent of world ice-free land area: 8%

U.S. Order Distribution Map for Ultisols. Image courtesy of USDA-NRCS

Further Detail from the NRCS

Quiz

Question

Oxisols and Ultisols are both intensely weathered soils. What is a major difference between them that impacts their potential agricultural productivity?

Looks Good! Correct: Yes! Although all the soil forming factors play a part in determining soil properties, the amount and type of clay accumulation in the B horizons of Ultisols gives them a greater ability to retain plant nutrients (i.e., basic cations, Ca2+, Mg2+, and K+) than the clay minerals present in Oxisols.