A fragipan is a diagnostic horizon in USDA soil taxonomy. 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.
The Required Characteristics of a fragipan according to the "Keys to USDA soil taxonomy, tenth edition, 2006" are
What this means in plain English is
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 age. Some fragipans inherit their properties from buried soils called paleosols. It has been proposed that fragipans form by hydroconsolidation, that is the collapse of the soil structure due to loading and wetting.