A fragipan is a diagnostic horizon in USDA soil taxonomy

USDA soil taxonomy

USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...

. They are altered subsurface soil layers that restrict water flow and root penetration. Fragipans are similar to a duripan

Duripan

A duripan is a diagnostic soil horizon of the USDA soil taxonomy that is cemented by illuvial silica into a subsurface hardpan. Similar to a fragipan, petrocalcic horizon and petrogypsic horizon, it is firmly cemented and restricts soil management. In soil descriptions, they are most often denoted...

 in how they affect land-use limitations. In soil descriptions, they are commonly denoted by a Bx or Btx symbol.

Characteristics

The Required Characteristics of a fragipan according to the "Keys to USDA soil taxonomy

USDA soil taxonomy

USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...

, tenth edition, 2006" are

1. The layer is 15-cm or more thick; and
2. The layer shows evidence of pedogenesis
   Pedogenesis
   Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
   
    within the horizon
   Soil horizon
   A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
   
    or, at a minimum, on the faces of structural units; and
3. The layer has very coarse prismatic, columnar, or blocky structure of any grade, has weak structure of any size, or is massive. Separations between structural units that allow roots to enter have an average spacing of 10 cm or more on the horizontal dimensions; and
4. Air-dry fragments of the natural soil fabric, 5 to 10 cm in diameter, from more than 50 percent of the layer slake when they are submerged in water; and
5. The layer has, in 60 percent or more of the volume, a firm or firmer rupture-resistance class, a brittle manner of failure at or near field capacity, and virtually no roots; and
6. The layer is not effervescent in dilute HCl.

What this means in plain English is

1. The fragipan has to meet a certain thickness requirement.
2. There needs to be evidence that the fragipan formed from altered parent material
   Parent material
   In soil science, parent material is the underlying geological material in which soil horizons form...
   
   , as opposed to being simply undeveloped dense subsoil.
3. The layer cannot have been altered by plant roots, which would result in granular soil structure
   Soil structure
   Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
   
   , and plant roots are unable to penetrate the fragipan except along intermittent cracks.
4. The requirement that a fragipan can slake in water separates it from a Petrocalcic Horizon
   Petrocalcic Horizon
   A petrocalcic horizon is a diagnostic horizon in USDA soil taxonomy. They are formed when secondary Calcium Carbonate or other carbonates accumulate in the subsoil to the extent that the soil becomes cemented into a hardpan. Petrocalcic horizons are similar to a duripan and a petrogypsic horizon...
   
    and a duripan
   Duripan
   A duripan is a diagnostic soil horizon of the USDA soil taxonomy that is cemented by illuvial silica into a subsurface hardpan. Similar to a fragipan, petrocalcic horizon and petrogypsic horizon, it is firmly cemented and restricts soil management. In soil descriptions, they are most often denoted...
   
   .
5. Fragipans are very hard, even when thoroughly wet, to the extent that roots cannot penetrate.
6. A Fragipan does not contain calcium carbonate
   Calcium carbonate
   Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
   
    or magnesium carbonate
   Magnesium carbonate
   Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals...
   
   , which would make it a Petrocalcic Horizon
   Petrocalcic Horizon
   A petrocalcic horizon is a diagnostic horizon in USDA soil taxonomy. They are formed when secondary Calcium Carbonate or other carbonates accumulate in the subsoil to the extent that the soil becomes cemented into a hardpan. Petrocalcic horizons are similar to a duripan and a petrogypsic horizon...
   
   .

Genesis

The genesis of fragipans is open to considerable debate. Soil scientists often argue over the definition of fragipans and whether or not fragipans exist in certain landscapes. Their formation can be attributed to compacting of soils by glaciers during the last ice age, physical ripening, permafrost processes, or other events that occurred in the pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 age. Some fragipans inherit their properties from buried soils called paleosols.

See also

• USDA soil taxonomy
  USDA soil taxonomy
  USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...
  
  
• Duripan
  Duripan
  A duripan is a diagnostic soil horizon of the USDA soil taxonomy that is cemented by illuvial silica into a subsurface hardpan. Similar to a fragipan, petrocalcic horizon and petrogypsic horizon, it is firmly cemented and restricts soil management. In soil descriptions, they are most often denoted...
  
  
• Petrocalcic Horizon
  Petrocalcic Horizon
  A petrocalcic horizon is a diagnostic horizon in USDA soil taxonomy. They are formed when secondary Calcium Carbonate or other carbonates accumulate in the subsoil to the extent that the soil becomes cemented into a hardpan. Petrocalcic horizons are similar to a duripan and a petrogypsic horizon...
  
  
• Petrogypsic Horizon
• Hardpan
  Hardpan
  In soil science, agriculture and gardening, hardpan or ouklip is a general term for a dense layer of soil, usually found below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water...
  
  

More information on soils in specific areas of the United States can be found on Web Soil Survey at http://websoilsurvey.nrcs.usda.gov/app/