A fragipan is a diagnostic

horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...

USDA soil taxonomy USDA soil taxonomy (ST) developed by the United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters (most commonly their properties) and in sev ...

. They are altered subsurface soil layers that restrict water flow and root penetration. Fragipans are similar to a duripan in how they affect land-use limitations. In soil descriptions, they are commonly denoted by a Bx or Btx symbol. They often form in loess ground. The first record of fragipans is probably that by John Hardcastle in New Zealand in the 1880s.

Characteristics

The Required Characteristics of a fragipan according to the "Keys to

, tenth edition, 2006" are #The layer is 15-cm or more thick; ''and'' #The layer shows evidence of

pedogenesis Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations l ...

within the

or, at a minimum, on the faces of structural units; ''and'' #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'' #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'' #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'' #The layer is not effervescent in dilute HCl. What this means in plain English is #The fragipan has to meet a certain thickness requirement. #There needs to be evidence that the fragipan formed from altered

parent material Parent material is the underlying geological material (generally bedrock or a superficial or drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are ...

, as opposed to being simply undeveloped dense subsoil. #The layer cannot have been altered by plant roots, which would result in granular

soil structure Soil structure describes the arrangement or the way of soil in the solid parts of the soil and of the pore space located between them. It is determined by how individual soil granules clump, bind together, and aggregate, resulting in the arrangem ...

, and plant roots are unable to penetrate the fragipan except along intermittent cracks. #The requirement that a fragipan can slake in water separates it from a Petrocalcic Horizon and a duripan. #Fragipans are very hard, even when thoroughly wet, to the extent that roots cannot penetrate. #A fragipan does not contain calcium carbonate or

magnesium carbonate Magnesium carbonate, (archaic name magnesia alba), is an inorganic salt that is a colourless or white solid. Several hydrated and basic forms of magnesium carbonate also exist as minerals. Forms The most common magnesium carbonate forms are ...

, which would make it a Petrocalcic 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 The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

age. Some fragipans inherit their properties from buried soils called

paleosols In the geosciences, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The precise definition of the term in geology and paleontology is slightly different from its use in soil science. In geolo ...

. It has been proposed that fragipans form by hydroconsolidation, that is the collapse of the soil structure due to loading and wetting.Assallay,A.M.,Jefferson,I.,Rogers,C.D.F.,Smalley,I.J. 1998. Fragipan formation in loess soils: development of the Bryant hydroconsolidation hypothesis. Geoderma 83, 1-16

References

Citations

{{Reflist

General sources

* Ajmone Marsan F., Pagliai M., and R. Pini. 1994. Identification and Properties of Fragipan Soils in the Piemonte Region (Italy). Soil Sci. Soc. Am. J., 58:891-900. * Duncan M.M. and D.P. Franzmeier. 1999. Role of Free Silicon, Aluminum, and Iron in Fragipan Formation. Soil Sci. Soc. Am. J., 63:923-929

. * Grossmann R.B. and F.J. Carlisle. 1969. Fragipan soils of the eastern United. States. Advances Agron. 21:237-279. * Lindbo D.L., Rhoton F.E., Hudnall W.H., Smeck N.E., Bigham J.M., and D.D. Tyler. 2000. Fragipan Degradation and Nodule Formation in Glossic Fragiudalfs of the Lower Mississippi River Valley. Soil Sci. Soc. Am. J., 64:1713-1722

. * Scalenghe R., Certini G., Corti G., Zanini E., and F.C. Ugolini. 2004. Segregated Ice and Liquefaction Effects on Compaction of Fragipans. Soil Sci. Soc. Am. J., 68:204 -214

. * Ugolini F.C. and B.P. Warkentin. 2006. Perspectives on the Relationship Between Soil Science and Geology. Geological Society, London, Special Publications, 266:183-190. * Soil Survey staff. 2006. Keys to Soil Taxonomy, Tenth Edition. USDA-NRCS. Washington, DC. Available online at: http://soils.usda.gov/technical/classification/tax_keys/ * Van Vliet B. and R. Langhor. 1981. Correlation Between Fragipans and Permafrost with Special Reference to Silty Weichselian Deposits in Belgium and Northern France. Catena 8:137–153. * https://web.archive.org/web/20070610013420/https://www.soils.org/sssagloss/?check - Soil Science Society of America Soil Terms Glossary

External links