Page 35 - Soil Degradation, Conservation and Remediation

20 1 Soil Resources and Soil Degradation

several thousand years ago. Most of these soils occur in the eastern part of the
Great Plains.

Ustolls: Ustolls are Mollisols of semiarid and subhumid climates having an ustic
soil moisture regime. Rainfall occurs mainly during a growing season, often in
heavy showers, but is erratic. Drought is frequent and sometimes may be severe.
Natural vegetation in Ustolls may be grass in the Great Plains and forest in the
mountains of Western USA.

Xerolls: Xerolls are the temperate Mollisols with very dry summers and moist win-
ters within a Mediterranean climate. Xerolls have a xeric soil moisture regime.
Xerolls are extensive in parts of Turkey, northern Africa near the Mediterranean
and in some of the southern republics of the former USSR, and in several states
in the USA.

1.6.9 Oxisols

Oxisols develop under a climate characterized by small seasonal variation in soil
temperature and no seasonal soil freezing and high annual precipitation. They may
have a wide range of soil moisture regimes from aridic to perudic. Oxisols with
aridic soil moisture regimes are often considered as paleosols. Usually, Oxisols
develop under climatic conditions where precipitation exceeds evapotranspiration
for some periods of the year to facilitate the removal of soluble weathering prod-
ucts and favors the residual concentration of kaolinite and sesquioxides, which are
essential to form an oxic horizon. Oxisols have the upper boundary of an oxic
horizon and no kandic horizon within 150 cm or 40 % or more clay by weight in
the fine-earth fraction. Oxisols do not have either of the following: (a) permafrost

within 100 cm of the soil surface, or gelic materials within 100 cm of the soil sur-
face, and (b) permafrost within 200 cm of the soil surface. Oxisols consist mainly
of quartz, kaolinite, oxides of Fe, Mn, and Al, and organic matter. Oxisols are
poorly fertile weathered soils occurring on gentle slopes of geologically old surfaces
in tropical and subtropical regions. The natural vegetation ranges from tropical
rainforests to desert savannas. Although many Oxisols are extremely infertile,
some Oxisols may be made productive when cultivated with appropriate management.
Oxisols comprise 7.5 % of the global ice-free land surface. (The WRB equivalents
of Oxisols are Ferralsols, Plinthosols, and Nitisols.) Oxisols have fi ve suborders.
They are:

Aquox: Aquox are the Oxisols that have a water table at or near the surface for much
of the year in shallow depressions and in seepage areas at the base of slopes.
There is a tendency to accumulate iron in the form of secondary nodules, concre-
tions, and plinthite.
Perox: Perox are well-drained Oxisols with a perudic soil moisture regime. They are

found in continuously humid climates, where precipitation exceeds evapotrans-
piration in all months.