5.10 - Oxisols

Oxisols are the most weathered of the 12 soil orders in the USDA soil classification system. (See Lesson 2 -- Processes of Weathering.) They are composed of the most highly weathered tropical and subtropical soils, and are formed in hot, humid climates that receive a lot of rainfall.  Oxisols are located primarily in equatorial regions.

These soils are extensively leached, and the clay size particles are dominated by oxides of iron and aluminum, which are low in natural fertility (Ca²⁺, Mg²⁺, K⁺) and high in soil acidity (H⁺, Al³⁺).  While Oxisols are typically physically stable, with low shrink-swell properties and good erosion resistance, these soils require extensive inputs of lime and fertilizers to be agriculturally productive.

Key Characteristics: Oxisols

• The most highly-weathered soils
• Form in hot, humid climates with high annual rainfall.
• Commonly occur in equatorial latitudes.
• Highly weathered and leached, dominated by iron and aluminum oxides.
• Low in natural fertility (basic cations, Ca²⁺, Mg²⁺, K⁺) and high in soil acidity (H⁺, Al³⁺)
• Physically stable soils, with low shrink-swell properties.
• Extent of world ice-free land area: 8%