Aridic properties (WRB)

Aridic properties is one of the diagnostic properties used, in the WRB system, to discriminate some soils from others.

Description

The term "Aridic properties" (from Latin aridus, dry) refers to a number of properties that are common in surface horizons of soils occurring under arid conditions and where pedogenesis exceeds new accumulation at the soil surface by aeolian or alluvial activity.

Criteria

Aridic properties require :

• an organic carbon content of less than 0.6 percent if the texture is sandy loam or finer, or less than 0.2 percent if the texture is coarser than sandy loam, as a weighted average in the upper 20 cm of the soil or down to the top of a diagnostic subsurface horizon, a cemented layer, or to continuous rock, whichever is shallower

and :

• evidence of aeolian activity in one or more of the following forms :
  • the sand fraction in some layer or in in-blown material filling cracks contains rounded or subangular sand particles showing a matt surface (use a ×10 hand-lens). These particles make up 10 percent or more of the medium and coarser quartz sand fraction
  • wind-shaped rock fragments (ventifacts) at the surface
  • aeroturbation (e.g. cross-bedding)
  • evidence of wind erosion or deposition

and :

• both broken and crushed samples with a Munsell colour value of 3 or more when moist and 4.5 or more when dry, and a chroma of 2 or more when moist

and :

• a base saturation (by 1 M NH₄OAc) of 75 percent or more

Additional remarks The presence of acicular (needle-shaped) clay minerals (e.g. sepiolite and palygorskite) in soils is considered connotative of a desert environment, but it has not been reported in all desert soils. This may be due either to the fact that, under arid conditions, acicular clays are not produced but only preserved, provided they exist in the parent material or in the dust that falls on the soil, or that, in some desert environments, there has not been sufficient weathering to produce detectable quantities of secondary clay minerals.

See also

• The FAO reference text, (2007 version)