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 by the symbol Bqm. The closest equivalent in the Canadian system of soil classification is called a duric horizon, although it does not mean exactly the same thing as a duripan in the United States. They form almost exclusively in arid or Mediterranean climates, and can be as hard as concrete, which makes plowing very difficult or impossible. Soils that include duripans are generally used for grazing or wildlife habitat, and are seldom cultivated.

The required characteristics for a duripan, according to the "Keys to USDA soil taxonomy, Tenth Edition, 2006" are:

What this means in plain English is:

Duripans occur mostly in arid or semiarid climates, where the soil is usually dry or seasonally dry. Soils with duripans are often geographically associated with areas of volcanic activity, and show evidence of ash or volcanic glass deposition. Volcanic glass weathers rapidly, providing an ample supply of soluble silica to cement the underlying soil. Other common sources of the silica cementing agent are iron-magnesium minerals and feldspars. The parent material of duripan soils usually does not contain a large amount of calcium carbonate, and if there is an abundance of calcium, a Petrocalcic Horizon can form in conjunction with the duripan. The weathered silica is mostly transported by water into the subsoil, where it precipitates and forms microaggregates that gradually grow into a full-fledged duripan. It cannot be considered a duripan until the cracks and gaps that allow roots to penetrate are 10 cm or more apart. A duripan can sometimes be broken up by earthquakes or periods of alternate wetting and drying that cause small volume changes.

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