Page 38 - Soil Degradation, Conservation and Remediation

1.6 Classification of World Soils 23

Udults: These are more or less freely drained Ultisols that have a udic soil moisture
regime. They develop in humid areas with well-distributed rainfall. Most of these
soils have a forest vegetation, but some have a savanna.

Ustults: These are freely drained Ultisols that have an ustic soil moisture regime and
a relatively low content of organic carbon. These soils are generally found in
semiarid and subhumid climates. The vegetation commonly consists of forest or
savanna plants.
Xerults: Xerults are freely drained Ultisols that have a xeric soil moisture regime.

They are found in areas with very dry summers and moist winters typically of
Mediterranean or temperate climates. Natural vegetation consisted mostly of
coniferous forest plants.
1.6.12 Vertisols

Vertisols are clayey soils that have deep, wide cracks for a considerable time of the
year and have slickensides (a shiny surface of the cracks produced in soils contain-
ing a high proportion of swelling clays) within 100 cm of the mineral soil surface.
They shrink when dry and swell when moistened. They are generally sticky in the
wet season and hard in the dry season. Most Vertisols have an ustic soil moisture
regime; some have an aridic and a udic regime. Vertisols generally have 50–70 %
clay with a relatively large proportion of fine clay in the clay fraction. The clays in

Vertisols consist predominantly of 2:1 and 2:2 layer clay minerals, but some have
considerable amounts of other clay minerals. The natural vegetation is predomi-
nantly grass, savanna, open forest, or desert shrub. Most Vertisols are well suited to
farming if there is plenty of rainfall or irrigation water and if suitable management
practices are followed. Because of the low permeability and tendency to remain
waterlogged for long periods, Vertisols are often considered as problem soils
(Chap. 11 ). Vertisols are extensive in some parts of the world. They were known

as black cotton soils in India. Vertisols occupy 2.4 % of the global ice-free land sur-
face. (Vertisols have the same name in WRB.) Vertisols have six suborders. They are:
Aquerts: Aquerts are the Vertisols that have aquic soil moisture regime. They have a
water table at or near the surface for much of the year but are also dry enough for
periods for cracks to open. They are found in low areas such as glacial lake
plains, floodplains, stream terraces, and depressions.

Cryerts: Cryerts are the Vertisols that have a cryic soil temperature regime. They are

soils of the cold climate. They are fine-textured soils and periodically shrink and

swell, forming cracks that commonly open in late summer. Cryerts occur on the
cold prairies of Canada where they are commonly derived from lacustrine depos-
its. They also occur in the US Rocky Mountains.
Torrerts: Torrerts are the Vertisols of arid climates. Their cracks commonly stay

open for most of the year but may close for at least a few days during rains. Many
of these soils are found in closed depressions that may be ponded from time to