soil resources

• USDA


  United States Department of Agriculture
• The 12 orders of soil taxonomy

  • ALFISOLS
  • ANDISOLS
  • ARIDISOLS
  • ENTISOLS
  • GELISOLS
  • HISTOSOLS
  • INCEPTISOLS
  • MOLLISOLS
  • OXISOLS
  • SPODOSOLS
  • ULTISOLS
  • VERTISOLS
• Alfisols


  • In semiarid to moist areas
  • Result from weathering processes that leach clay minerals and other constituents out of the surface layer and into the subsoil
  • Formed primarily under forest or mixed vegetative cover
  • Productive for most crops
• ALFISOLS MAKE UP ABOUT 10% OF THE WORLD'S ICE-FREE LAND SURFACE
• Andisols


  • Form from weathering processes that generate minerals with little orderly crystalline structure
  • Have an unusually high water- and nutrient-holding capacity
  • Highly productive soils
  • Common in cool areas with moderate to high precipitation, especially those areas associated with volcanic materials
• ANDISOLS MAKE UP ABOUT 1% OF THE WORLD'S ICE-FREE LAND SURFACE
• Aridisols


  • Too dry for the growth of mesophytic plants
  • Lack of moisture greatly restricts the intensity of weathering processes and limits most soil development processes to the upper part of the soils
  • Often accumulate gypsum, salt, calcium carbonate, and other materials that are easily leached from soils in more humid environments
  • Common in the deserts of the world
• ARIDISOLS MAKE UP ABOUT 12% OF THE WORLD'S ICE-FREE LAND SURFACE
• Entisols


  • Show little or no evidence of pedogenic horizon development
  • Occur in areas of recently deposited parent materials or in areas where erosion or deposition rates are faster than the rate of soil development
  • Occur in many environments
• ENTISOLS MAKE UP ABOUT 16% OF THE WORLD'S ICE-FREE LAND SURFACE
• Gelisols


  • Have permafrost near the soil surface and/or have evidence of cryoturbation (frost churning) and/or ice segregation
  • Common in the higher latitudes or at high elevations
• GELISOLS MAKE UP ABOUT 9% OF THE WORLD'S ICE-FREE LAND SURFACE
• Histosols


  • Have a high content of organic matter and no permafrost
  • Most are saturated year round, but a few are freely drained
  • Commonly called bogs, moors, peats, or mucks
  • Form in decomposed plant remains that accumulate in water, forest litter, or moss faster than they decay
  • If drained and exposed to air, microbial decomposition is accelerated and the soils may subside dramatically
• HISTOSOLS MAKE UP ABOUT 1% OF THE WORLD'S ICE-FREE LAND SURFACE
• Inceptisols


  • Soils of semiarid to humid environments that generally exhibit only moderate degrees of soil weathering and development
  • Have a wide range in characteristics and occur in a wide variety of climates
• Mollisols


  • Have a dark colored surface horizon relatively high in content of organic matter
  • Base rich throughout and therefore are quite fertile
  • Characteristically form under grass in climates that have a moderate to pronounced seasonal moisture deficit
  • Extensive soils on the steppes of Europe, Asia, North America, and South America
• MOLLISOLS MAKE UP ABOUT 7% OF THE WORLD'S ICE-FREE LAND SURFACE
• Oxisols


  • Highly weathered soils of tropical and subtropical regions
  • Dominated by low activity minerals, such as quartz, kaolinite, and iron oxides
  • Tend to have indistinct horizons
  • Characteristically occur on land surfaces that have been stable for a long time
  • Have low natural fertility as well as a low capacity to retain additions of lime and fertilizer
• OXISOLS MAKE UP ABOUT 8% OF THE WORLD'S ICE-FREE LAND SURFACE
• Spodosols


  • Formed from weathering processes that strip organic matter combined with aluminum (with or without iron) from the surface layer and deposit them in the subsoil
  • In undisturbed areas, a gray eluvial horizon that has the color of uncoated quartz overlies a reddish brown or black subsoil
  • Commonly occur in areas of coarse-textured deposits under coniferous forests of humid regions
  • Tend to be acid and infertile
• SPODOSOLS MAKE UP ABOUT 4% OF THE WORLD'S ICE-FREE LAND SURFACE
• Ultisols


  • Soils in humid areas
  • Formed from fairly intense weathering and leaching processes that result in a clay-enriched subsoil dominated by minerals, such as quartz, kaolinite, and iron oxides
  • Typically acid soils in which most nutrients are concentrated in the upper few inches
  • Have a moderately low capacity to retain additions of lime and fertilizer
• ULTISOLS MAKE UP ABOUT 8% OF THE WORLD'S ICE-FREE LAND SURFACE
• Vertisols


  • Have a high content of expanding clay minerals
  • Undergo pronounced changes in volume with changes in moisture
  • Have cracks that open and close periodically, and that show evidence of soil movement in the profile
  • Transmit water very slowly and have undergone little leaching
  • Tend to be fairly high in natural fertility
• VERTISOLS MAKE UP ABOUT 2% OF THE WORLD'S ICE-FREE LAND SURFACE
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