Page 27 - Soil Degradation, Conservation and Remediation

12 1 Soil Resources and Soil Degradation

Ustalfs: Ustalfs have an ustic soil moisture regime and a frigid, mesic, isomesic, or
warmer temperature regime. Ustalfs are the Alfisols of subhumid to semiarid

regions. They occur in the USA, Africa, India, South America, Australia, and
Southeastern Asia. Sorghum, wheat, and cotton are commonly cultivated with
irrigation.

Xeralfs: Xeralfs have xeric soil moisture regime common of regions that have

Mediterranean climate. They are found in South Africa, Chile, Western Australia,
Southern Australia, and the Western USA. They are dry for extended periods in
summer, but enough is available in winter. Small grains and other annuals are
common crops without irrigation. Grapes and olives are also common crops
where the climate is thermic. With irrigation, a wide variety of crops can be
grown.

1.6.2 Andisols

Andisols are characterized by andic materials. Andic materials include volcanic ash,
pumice, and cinders deposited during volcanic eruptions. These materials undergo
transformation to amorphous or poorly crystallized silicate minerals, including allo-
phane, imogolite, and ferrihydrite. Andisols are young soils and have not had
enough time to be highly weathered. These soils are fine-textured and have a high

content of fresh weatherable minerals and a high cation exchange capacity. They
may also contain considerable organic matter as aluminum–humus complex. They
have low bulk densities. These soils are widely distributed in all geographical
regions near sources of volcanoes. Andisols are generally fertile and are used for
agriculture unless restricted by slope, altitude, soil moisture and temperature
regimes, etc. One of the most important characteristics of Andisols is their high

capacity to fix phosphorus on the surface of the amorphous minerals (Cordova et al.
1996 ). This is perhaps the principal chemical constraint of Andisols. Some Andisols
are left under tundra and forests. Andisols cover more than 124 million hectares, or
approximately 0.7 % of the earth’s surface. Major areas of Andisols include Chile,
Peru, Ecuador, Columbia, Central America, the USA, Kamchatka, Japan, the
Philippines, Indonesia, and New Zealand. (The WRB equivalent of Andisols are the
Andosols.) Andisols have seven suborders. They are:
Aquands: Aquands are the Andisols with aquic conditions at or near the surface.

These soils have dark-colored surface horizons that meet the requirements for a
histic, umbric, or mollic epipedon. Aquic conditions result in redoximorphic
features. Aquands occur locally in depressions and along fl oodplains where
water tables are at or near the soil surface for at least part of the year.
Cryands: Cryands are defined as Andisols with cryic soil temperature regimes.

These soils are the Andisols of high latitude (e.g., Alaska, Kamchatka) and high
altitude (e.g., Sierra Nevada in the USA). They are usually occupied by cold
tolerant forests.