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?

A. amount and type of clay accumulation in the B horizon

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.

B. climate and rainfall patterns where they are formed

Incorrect: No, although all the soil forming factors play a part in determining soil properties, Ultisols have accumulations of clay minerals in the B horizon that give them a nutrient retention capacity greater than that of Oxisols.

C. native vegetation at location where they form

D. topography or drainage issues

E. time since deposition of parent material

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.

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