Soil Orders and Taxonomy

Soil, just like plants and animals, has been broken down by scientists into a hierarchical classification system, which is as follows: orders, suborders, great group, subgroup, family, and series. While there are thousands of types of soil around the world, they can all be classified under 12 major orders.

The Twelve Orders of Soil Geomorphology. Picture compliments of the US Department of Agriculture, National Resources Conservation Service.

Soil orders occur from how the soil changes over centuries, thousands or millions of years. The different orders arise from many factors including:

  • type of rock the soil formed from
  • climate
  • type of vegetation present
  • floods
  • volcanic activity
  • availability of water
  • and many other factors.

The 12 Orders of Soil Geomorphology

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Alfisols are found in in semiarid to moist areas. They formed under forest or mixed vegetative cover and are productive for most crops.

Andisols tend to be highly productive soils. They are common in cool areas with moderate to high precipitation, especially those areas associated with volcanic materials.

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Aridisols are soils that are too dry for the growth of mesophytic plants. They often accumulate gypsum, salt, calcium carbonate, and other materials that are easily leached from soil in more humid environments. Aridisols are common in the world’s deserts.

Entisols occur in areas of recently deposited parent materials or in areas where erosion or deposition rates are faster than the rate of soil development; such as dunes, steep slopes and flood plains.

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Gelisols are soils that have permafrost near the soil surface, have evidence of frost churning, or ice segregation. These are common in the higher latitudes or high elevations.

Histosols have a high content of organic matter and no permafrost. Most are saturated year round, but a few are freely drained. They are commonly called bogs, moors, pears or mucks.

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Inceptisols are soils of semiarid to humid environments that generally exhibit only moderate degrees of soil weathering and development. These occur in a wide variety of climates.

Mollisols are soils that have a dark colored surface horizon relatively high in content of organic matter. The soils are base rich throughout and therefore are quite fertile.

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Oxisols are highly weathered soils of tropical and subtropical regions. They characteristically occur on land surfaces that have been stable for a long time. They have low natural fertility as well as a low capacity to retain additions of lime and fertilizer.

Spodosols formed from weathering processes that strip organic matter combined with aluminum from the surface layer and deposit them in the subsoil. These tend to be acid and infertile.

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Ultisols are soils in humid areas. They are typically acid soils in which most nutrients are concentrated in the upper few inches. They have a moderately low capacity to retain additions of lime and fertilizer.

Vertisols have a high content of expanding clay minerals. They undergo pronounced changes in volume with changes in moisture. Because they swell when wet, vertisols transmit water very slowly and have undergone little leeching. They tend to be fairly high in natural fertility.

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