Soil Taxonomy, a classification system developed by the United States Department of Agriculture’s soil survey staff, categorizes soils into twelve distinct orders based on measurable and observable properties. This system facilitates detailed soil surveys and is widely used worldwide, influencing other soil classification systems. Each soil order represents a grouping of soils with unique characteristics and ecological significance.
Key Facts
- The 12 soil orders are: Gelisols, Histosols, Andisols, Oxisols, Vertisols, Alfisols, Aridisols, Inceptisols, Entisols, Mollisols, Spodosols, and Ultisols.
- Soil Taxonomy, the classification system, was developed by the United States Department of Agriculture’s soil survey staff.
- Soil Taxonomy is widely used worldwide and has influenced other soil classification systems.
- Each soil order represents a grouping of soils with distinct characteristics and properties.
- Gelisols are soils found in very cold climates with permafrost near the surface.
- Histosols are soils composed mainly of organic materials, such as peat and muck.
- Andisols are soils that have formed in volcanic ash or other volcanic ejecta.
- Oxisols are highly weathered soils found in intertropical regions, rich in iron and aluminum oxide minerals.
- Vertisols are clay-rich soils that undergo significant volume changes with moisture fluctuations.
- Alfisols are moderately leached soils with high native fertility, often found in temperate humid and subhumid regions.
- Aridisols are calcium carbonate-containing soils of arid regions with limited leaching.
- Inceptisols are soils with minimal horizon development, more developed than Entisols but lacking features of other soil orders.
- Entisols are soils of recent origin, characterized by great diversity and found in various environmental settings.
- Mollisols are fertile soils of grassland ecosystems, extensively used for agriculture.
- Spodosols are acidic soils with a subsurface horizon enriched in organic matter and aluminum and iron compounds.
- Ultisols are highly weathered soils with a subsurface horizon depleted in clay and nutrients.
The Twelve Soil Orders
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Gelisols
- Found in very cold climates with permafrost near the surface.
- Limited geographical distribution to high-latitude polar regions and localized areas at high mountain elevations.
- Support a small percentage of the world’s population due to extreme environmental conditions.
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Histosols
- Composed mainly of organic materials, such as peat and muck.
- Form in settings with restricted drainage, inhibiting the decomposition of plant and animal remains.
- Ecologically important due to large quantities of stored carbon.
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Andisols
- Formed in volcanic ash or other volcanic ejecta.
- Dominated by glass and short-range-order colloidal weathering products.
- Possess unique chemical and physical properties, including high water-holding capacity and phosphorus retention.
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Oxisols
- Highly weathered soils found in intertropical regions.
- Contain few weatherable minerals and are often rich in iron and aluminum oxide minerals.
- Exhibit extremely low native fertility due to low nutrient reserves and high phosphorus retention.
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Vertisols
- Clay-rich soils that undergo significant volume changes with moisture fluctuations.
- Found primarily in tropical and subtropical regions.
- Characterized by deep wide cracks during dry periods and expansion upon wetting.
- Present engineering challenges and prevent distinct horizon formation.
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Alfisols
- Moderately leached soils with high native fertility.
- Found mostly in temperate humid and subhumid regions.
- Have a subsurface horizon in which clays have accumulated.
- Support a significant portion of the world’s population and are used for agriculture and forestry.
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Aridisols
- Calcium carbonate-containing soils of arid regions.
- Exhibit limited leaching and subsurface horizon development.
- Found in areas with low precipitation and high evaporation.
- Used mainly for range, wildlife, and recreation due to the dry climate.
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Inceptisols
- Soils with minimal horizon development.
- More developed than Entisols but lack the features of other soil orders.
- Found in a wide range of ecological settings, including steep slopes, young geomorphic surfaces, and resistant parent materials.
- Used for forestry, recreation, and watershed.
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Entisols
- Soils of recent origin with usually no genetic horizons except an A horizon.
- Characterized by great diversity in environmental setting and land use.
- Found in steep, rocky settings and large river valleys.
- Provide cropland and habitat for millions of people worldwide.
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Mollisols
- Fertile soils of grassland ecosystems.
- Characterized by a thick, dark surface horizon resulting from long-term addition of organic materials.
- Found primarily in middle latitudes and prairie regions.
- Extensively used for agriculture due to high productivity.
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Spodosols
- Acidic soils with a subsurface accumulation of humus complexed with aluminum and iron.
- Typically form in coarse-textured parent material.
- Have a light-colored E horizon overlying a reddish-brown spodic horizon.
- Often support forests and require lime for agricultural productivity.
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Ultisols
- Strongly leached and acidic forest soils with relatively low fertility.
- Found primarily in humid temperate and tropical areas.
- Have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors.
- Support productive forests but require lime and fertilizers for agriculture.
Conclusion
The twelve soil orders, as defined by Soil Taxonomy, provide a comprehensive framework for classifying soils based on their distinct characteristics and properties. This classification system facilitates detailed soil surveys, aids in understanding soil behavior and processes, and guides land use and management decisions. The twelve soil orders represent the diverse array of soils found across the globe, each with its unique ecological significance and role in supporting various ecosystems and human activities.
References
- Soil Survey Staff. 1999. Soil Taxonomy, 2nd ed. USDA-NRCS, Washington, DC.
- Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12th ed. USDA-NRCS, Washington, DC.
- USDA. 2006. Land Resource Regions and Major Land Resource Areas of the United States, the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook 296. USDA-NRCS, Washington, DC.
- USDA-NRCS. 2018. Official soil series descriptions. Accessed at: https://soilseries.sc.egov.usda.gov
FAQs
What is Soil Taxonomy?
Soil Taxonomy is a soil classification system developed by the United States Department of Agriculture’s soil survey staff. It categorizes soils into twelve distinct orders based on measurable and observable properties.
What are the twelve soil orders?
The twelve soil orders are:
- Gelisols
- Histosols
- Andisols
- Oxisols
- Vertisols
- Alfisols
- Aridisols
- Inceptisols
- Entisols
- Mollisols
- Spodosols
- Ultisols
What is the significance of the twelve soil orders?
The twelve soil orders represent a comprehensive framework for classifying soils based on their distinct characteristics and properties. This classification system facilitates detailed soil surveys, aids in understanding soil behavior and processes, and guides land use and management decisions.
How are the twelve soil orders distributed geographically?
The twelve soil orders are found across the globe, with each order having a distinct geographical distribution. For example, Gelisols are found in very cold climates, while Oxisols are found primarily in intertropical regions.
What are some of the key characteristics of each soil order?
The key characteristics of each soil order vary widely. For instance, Mollisols are characterized by a thick, dark surface horizon resulting from long-term addition of organic materials, while Spodosols have a subsurface accumulation of humus complexed with aluminum and iron.
How do the twelve soil orders influence land use and management?
The twelve soil orders have a significant influence on land use and management. For example, Mollisols are highly productive and extensively used for agriculture, while Aridisols are used mainly for range, wildlife, and recreation due to the dry climate.
What are some of the challenges associated with managing different soil orders?
Different soil orders present unique challenges for management. For example, Vertisols undergo significant volume changes with moisture fluctuations, which can lead to engineering problems. Oxisols, on the other hand, have extremely low native fertility and require careful management to maintain productivity.