Page 28 - Soil Degradation, Conservation and Remediation

1.6 Classification of World Soils 13

Torrands: Torrands are more or less well-drained Andisols of dry regions. They
have an aridic/torric soil moisture regime and a frigid or warmer soil tempera-
ture regime. Natural vegetation is mostly desert shrubs. These soils are not
extensive occurring mostly in the western part of North America, Hawaii,
or other Pacific regions. Most of the soils formed under grass or shrub

vegetation.
Udands: Udands are more or less well-drained Andisols of moist regions. They have

a udic soil moisture regime. Udands are Andisols of the humid climates. They are
the most extensive Andisols. These soils are moderately extensive on the Pacifi c
Rim, including Washington, Oregon, and Hawaii in the USA. Most Udands
formed under forest vegetation.
Ustands: Ustands are defined as Andisols with ustic soil moisture regimes. These

soils are distributed in intertropical regions that experience seasonal precipita-
tion distribution. They are found mostly in Mexico, Western USA, Pacifi c
Islands, and the eastern part of Africa. Most Ustands are formed under grass,
shrub, or forest vegetation.
Vitrands: Vitrands are relatively young Andisols that are coarse-textured soils

and are dominated by volcanic glass. Most Vitrands are found near volcanoes.
Vitrands are abundant in Oregon, Washington, and Idaho of the USA where
they form mainly under coniferous forest vegetations. They are the Andisols
that have a frigid or warmer soil temperature regime. They have a low water-
holding capacity. Vitrands are restricted to ustic and udic soil moisture
regimes.

Xerands: Xerands are more or less well-drained Andisols that have a xeric soil

moisture regime and a frigid, mesic, or thermic temperature regime. They are
temperate Andisols with very dry summers and moist winters. Most Xerands
formed under coniferous forest vegetation and some formed under grass or shrub
vegetation.

1.6.3 Aridisols

Aridisols are soils of the arid regions including cold polar, cool temperate, and warm
deserts. Aridisols may also occur in semiarid areas outside the zones broadly classi-

fied as arid, for example, in local conditions imposing aridity such as steep, south-
facing slopes in the northern hemisphere and in soils whose physical properties limit
water infiltration or favor excessive drainage. Aridisols are classified on the basis of

their soil moisture regime which is dry in all parts >50 % of the time in most years
and not moist for as much as 90 consecutive days when the soil is warm enough
(>8 °C) for plant growth. In an aridic/torric soil moisture regime, potential evapo-
transpiration greatly exceeds precipitation during most of the year. In most years,
little or no water percolates through the soil. This hydrologic regime has a distinctive
influence on the development of such soils. However, shifting sands of deserts are not

included in Aridisols.