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  3. Soil Orders/
  4. Ultisols

Ultisols (from Latin ultimus, "last") are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes. Intense weathering of primary minerals has occurred, and much Ca, Mg and K has been leached from these soils. Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors resulting from the presence of Fe oxides. The "red clay" soils of the southeastern United States are examples of Ultisols. They are divided into five suborders: Aquults, Humults, Udults, Ustults and Xerults.

Because of the favorable climate regimes in which they are typically found, Ultisols often support productive forests. The high acidity and relatively low quantities of plant-available Ca, Mg and K associated with most Ultisols make them poorly suited for continuous agriculture without the use of fertilizer and lime. With these inputs, however, Ultisols can be very productive. They occupy approximately 8.1% of the global ice-free land area and support 18% of the world's population. They are the dominant soils of much of the southeastern U.S. and occupy approximately 9.2% of the total U.S. land area.

Ultisols soil layers
Ultisols have: (1) a subsurface zone of clay accumulation — either an argillic or kandic horizon; and (2) base saturation of less than 35% in the subsoil. This second criterion is what distinguishes Ultisols from Alfisols. (USDA-NRCS)

Soil Orders

  • Alfisols
  • Andisols
  • Aridisols
  • Entisols
  • Gelisols
  • Histosols
  • Inceptisols
  • Mollisols
  • Oxisols
  • Spodosols
  • Ultisols
  • Vertisols

Aquults, Udults (Southeastern USA Coastal Plain)

Aquults — Ultisols with a water table at or near the surface for much of the year; Udults — Ultisols of humid climates

Kanhapludults (Southeastern USA Piedmont)

Kanhapludults are a type of soil found in the Piedmont region of the southeastern United States

Hapludults

A subtype of soils that are characterized by being deep, well-drained, and moderately permeable, typically found in humid subtropical climates

Humults, Ustults

Humults — well-drained ultisols that have high organic matter content; Ustults — Ultisols of semiarid and subhumid climates 

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Soil Orders

  • Alfisols
  • Andisols
  • Aridisols
  • Entisols
  • Gelisols
  • Histosols
  • Inceptisols
  • Mollisols
  • Oxisols
  • Spodosols
  • Ultisols
  • Vertisols

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Department of Soil and Water Systems

Email: rheinse@uidaho.edu
Location: E. J. Iddings Agricultural Science Laboratory, Rm 242; 606 S Rayburn St
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875 Perimeter Drive MS 2340
Moscow, ID 83844-2340

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