Page 26 - Soil Degradation, Conservation and Remediation

1.6 Classification of World Soils 11
United Nations (FAO 1998 ) in conjunction with the International Union of Soil
Sciences and the International Soil Reference and Information Center (ISRIC) at
Wageningen University, the Netherlands. Soil Taxonomy was developed through

several revisions over the past five decades by the US Department of Agriculture
(USDA 1975 ). In this hierarchical system of Soil Taxonomy, soil order is the high-
est category which is divided into suborders, suborders into great groups, and so on.
There are 12 orders, 64 suborders, 319 great groups, 2,484 subgroups, 8,000 fami-
lies, and an unknown number of soil series. Characteristics of the soil orders and

their suborders are briefly described below.

1.6.1 Alfi sols

Alfisols are fine-textured soils with high content of exchangeable bases. They
have mostly developed in the humid temperate and also in the humid tropical
regions under deciduous forests. They have accumulation of clay in the B hori-
zon to form argillic, kandic, or natric horizon, with BSP >35 % in the lower part
or below the argillic or kandic horizon. They may contain petrocalcic horizons
and duripan, fragipan, and plinthite. There is enough moisture for plant growth
for three consecutive months during the growing seasons. There is relatively lit-
tle accumulation of organic matter in mineral horizons. These soils are highly
fertile and are extensively cultivated with widely diverse cropping patterns as
favored by climatic conditions. Some are used for hay, pasture, range, and for-
ests. Alfisols comprise 9.6 % of the ice-free land of the earth. (The WRB equiva-

lents of Alfisols are Albeluvisols, Planosols, Luvisols, and Lixisols.) Alfi sols

have five suborders. They are:
Aqualfs: Aqualfs are Alfisols that have aquic conditions (shallow groundwater table

that saturates soil with water) for some time in most years within 50 cm of the
mineral horizon and redoximorphic features in the upper 12.5 cm of the argillic,
natric, or kandic horizon. Aqualfs are abundant in humid regions and are primar-
ily used for rice cultivation. They are fairly fertile, and other crops including

corns (maize) and soybeans can be grown if artificially drained. Nearly all
Aqualfs are believed to have supported forest vegetation in the past.

Cryalfs: Cryalfs are more or less freely drained Alfisols of the cold regions (cryic
soil temperature regime) and occur mostly at high elevations, as in the Rocky
Mountains in the Western USA. They normally have a udic moisture regime.
Most of the Cryalfs are used as forest because of their short, cool growing
season.
Udalfs: Udalfs are the more or less frequently drained Alfisols that have udic soil

moisture regime and a frigid, mesic, isomesic, or warmer temperature regime.
Udalfs are very extensive in the USA and in Western Europe. Most Udalfs with
a mesic or warmer temperature regime have or had deciduous forest vegetation,
and many of the frigid temperature regimes have or had mixed coniferous and
deciduous trees.