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The Twelve Soil Orders

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College of Agricultural & Life Sciences
Twelve Soil Orders
Oxisols

Oxisols

Oxisols (from French oxide, "oxide") are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in Fe and Al oxide minerals. Most of these soils are characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorus retention by oxide minerals and low cation exchange capacity (CEC). Most nutrients in Oxisol ecosystems are contained in the standing vegetation and decomposing plant material. Despite low fertility, Oxisols can be quite productive with inputs of lime and fertilizers. The Oxisols are divided into five suborders: Aquox, Torrox, Ustox, Perox and Udox.

Oxisols occupy approximately 7.5% of the global ice-free land area. In the U.S., they only occupy approximately .02% of the land area and are restricted to Hawaii.

Oxisols have either: (1) an oxic horizon (low CEC, less than 10 percent weatherable mineral content); or (2) at least 40 percent clay between the mineral soil surface and 18 cm and a kandic horizon that meets the weatherable mineral requirement of an oxic horizon. (USDA-NRCS image)

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A map showing the global distribution of Oxisols. Source: USDA-NRCS

A map showing the percent of land in Oxisols. Source: USDA-NRCS

Diagram and description of Oxisols suborders

Map of Oxisols dominant suborders. Source: USDA-NRCS

Oxisols (Brazil)

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Haplaquox, Federal District, Brazil — This poorly drained Oxisol is found on footslopes and low-lying areas in the Federal District of Brazil. The soil is formed in highly weathered sediments and the subsoil contains very few weatherable minerals. However, because it is saturated for much of the year, the anaerobic conditions do not favor formation of Fe oxides. As a result, dull colors dominate the soil profile in sharp contrast to the bright red and yellow colors exhibited by the Oxisols on the surrounding uplands with better drainage. (Image from http://SoilScience.info)

Haplustox, Federal District, Brazil — This very deep soil exhibits the bright reddish colors associated with hematite, a type of Fe oxide that is common in well-drained Oxisols. Fe oxides are capable of fixing large quantities of phosphorus in forms that are unavailable to plants, often resulting in P deficiencies. The oxic diagnostic horizon (Bo horizons) in this soil is characterized by very low weatherable mineral content and low activity clays. Scale is in meters. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification, and Management of Oxisols)

Hapludox, Manaus, Brazil — The yellow colors seen in this very deep Oxisol are attributable to the presence of goethite, which is another type of Fe oxide that is common in soils. As Fe is released through weathering, goethite formation in soils is favored under conditions that are generally more acidic, cooler, and moister than those under which hematite is formed. Scale is in centimeters. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification, and Management of Oxisols)

Torrox (Hawaii)

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Torrox Landscape, Hawaii — Soils of these old, stable landscapes currently receive approximately 635 mm (25 inches) of mean annual precipitation with a mean annual temperature of 73°F (23°C). Even though the current soil moisture regime is aridic, much of the weathering associated with these soils likely took place under more humid conditions in the past. With irrigation and fertilizers, pineapple (pictured here), coffee and sugarcane can be grown.

Typic Eutrotorrox (very-fine, kaolinitic, isohyperthermic), Hawaii — This is a very deep, well-drained soil that contains an intensely weathered oxic horizon (Bo) characterized by the dominance of kaolinite clay and hematite, an Fe oxide that imparts the bright red colors. Because this soil has formed in material weathered from basic igneous rock, it contains relatively more Ca, Mg and K than do many Oxisols and is slightly acid to neutral with pH values approximately 6.5. This soil is a member of the Molokai series. (Image reproduced from Soils of the Great Plains: Land Use, Crops and Grasses by Andrew R. Aandahl by permission of the University of Nebraska Press. Copyright 1982 by the University of Nebraska Press)

Oxisol structure — This is a 100x magnification of a thin section of an Oxisol from Thailand showing well-structured aggregates. The reddish aggregates are composed of a mixture of kaolinite and hematite, an Fe oxide. These stable aggregates are responsible for high infiltration and permeability, and this makes many Oxisols resistant to runoff and erosion. The dark areas in the image represent pore space; the white areas are probably quartz grains. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification and Management of Oxisols)

Perox (Hawaii)

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Perox Landscape, Hawaii — This Perox landscape near the Kauai Agricultural Research Station receives approximately 2540+ mm (100+ inches) of precipitation annually. Rainfall is greater than evapotranspiration every month of the year. In Soil Taxonomy, this is designated as a perudic soil moisture regime. This condition severely limits use of these soils for agricultural production. (Image from Google Maps).

Anionic Acroperox (fine, ferruginous, isothermic), Hawaii — This dark reddish brown Oxisol is a member of the Halii series. It is characterized by relatively high organic matter content and extremely low cation exchange capacity (CEC). The term Anionic indicates that some soil horizons have a net zero or positive charge. Scale is in feet. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification, and Management of Oxisols)

Plinthite

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Plinthic Haplustox, Kerala, India — This deep exposure shows an Oxisol with a thick plinthite (Bv) layer. Plinthite has a red-yellow-gray/white mottled appearance and is several meters thick here. It typically forms in highly weathered soils that are saturated for some period during the year. Upon repeated wetting and drying, plinthite will harden irreversibly to form petroplinthite, sometimes referred to as ironstone or laterite. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification, and Management of Oxisols)

Plinthite Blocks, Kerala, India — Left: Plinthite is being cut into blocks and removed from an Oxisol. The blocks will be dried and irreversibly hardened into petroplinthite. The hardened petroplinthite blocks (right) can then be used for construction purposes in areas where rock resources are scarce. (Images from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification and Management of Oxisols)

Plinthite as building material, Malacca, Malaysia — Hardened petroplinthite blocks were used in 1511 to construct this Portuguese fort. Although weathered in appearance, they have held up for five centuries in a warm climate receiving approximately 1015 mm (40 inches) of precipitation annually. (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification and Management of Oxisols)

Oxisols and Agriculture

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Udox Landscape, southwestern Nigeria — This is an example of shifting cultivation practices used on some low-nutrient Oxisol landscapes in Africa and South America. In a process sometimes referred to as slash-and-burn, the land in the foreground has been cleared, burned and planted with maize. By burning the vegetation and leaving the ashes on the soil, nutrients contained in the standing biomass are released back into the soil for use by the cultivated crop. The land will be used for rotating crops throughout the next 4-5 years, after which it will be allowed to revert to forest for 5-10 years before being cleared again. (Image from the Soil Science Society of America Marbut Memorial Slide Set)

Humic Rhodic Eutrustox, Rwanda — This is an example of a humus-rich Oxisol. At elevations greater than 1,000 m (3,280 ft) in the humid tropics, Oxisols like this one contain relatively large quantities of soil organic matter. This soil contains more than 16 kg of organic carbon per square meter to a depth of 1 m as evidenced by the dark colors. Because this and similar soils are better-suited for low-input agriculture than those at lower elevations, they support some of the highest human population densities in the world. Maize, bananas and peanuts are growing in this farming system. Each scale division is 10 cm (4 inches). (Image from the Guy D. Smith Memorial Slide Collection. F.H. Beinroth et al., 1993. Properties, Classification and Management of Oxisols)

Amazonian Dark Earth, Brazil — This human-influenced soil is known as Terra Preta, which is Portuguese for “black earth.” These soils occur in patches within Oxisol- and Ultisol-dominated landscapes of the Amazon basin and have been extensively modified by additions of charcoal, domestic wastes and other materials dating back from 500 to several thousand years ago. As a result, Terra Preta soils are much darker and have more organic matter, greater nutrient-retention capacity, and more plant-available P, K and Ca than unaltered neighboring soils. The excavated profile shown here is approximately 1 meter deep and is from the Hatahara archaeological site near Manaus. (Photo by Manuel Arroyo-Kalin, appearing in D. Mckey and S. Rostain, 2015. Farming Technology in Amazonia. 10.1007/978-94-007-3934-5_9893-2.)

Typic Haplustox, Federal District, Brazil — This soil is located on the Brazilian Agricultural Research Corporation (EMBRAPA) research station. The exposed plant roots were painted to illustrate the root proliferation made possible by the additions of lime and fertilizer over the previous 20 years. Prior to these additions, initial plantings of corn and soybeans only grew a few inches. Under native conditions, the soil was very infertile and supported vegetation so deficient in calcium and phosphorus that it did not adequately nourish cattle when grazed. Today with the use of lime and fertilizer, profitable production of soybeans, corn and wheat is possible on thousands of hectares of this and similar soils. (Image and description courtesy of Dr. Stan Buol, North Carolina State University)

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Department of Soil and Water Systems

University of Idaho

Physical Address:
E. J. Iddings Agricultural Science Laboratory, Rm 242
606 S Rayburn St

Mailing Address:
875 Perimeter Drive MS 2340
Moscow, ID 83844-2340

Email: rheinse@uidaho.edu

Web: uidaho.edu/cals/sws

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The Twelve Soil Orders

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Oxisols

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Oxisols (Brazil)

Torrox (Hawaii)

Perox (Hawaii)

Plinthite

Oxisols and Agriculture

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