Page 39 - Soil Degradation, Conservation and Remediation

24 1 Soil Resources and Soil Degradation

time by runoff from higher areas. Some Torrerts are found in the southwest of the
USA. These soils are commonly used for rangeland.

Uderts: Uderts are the Vertisols of humid areas. They have a udic soil moisture
regime. The cracks may not open completely some years due to high precipita-
tion. In the USA, the soils occur on gentle slopes and are derived dominantly
from marine shales, marls, and alluvium. Many of these soils supported grass,
but some support hardwood or pine forests.

Usterts: These are the Vertisols in temperate areas that do not receive high amounts

of rainfall during the summer. They have an ustic soil moisture regime. Cracks
open and close once or twice during the year. They are found extensively in the
USA, Australia, Africa, and India. If irrigated, Usterts can be used intensively,
but large areas are used for grazing due to a lack of machinery to till soils.

Xererts: Xererts are the Vertisols of Mediterranean climates, which have xeric soil
moisture regime. These soils have cracks that regularly close and open each year.
In the USA, most of the soils supported grasses.

1.7 Soil Degradation

Land degradation is said to have taken place when the land within an ecosystem is
no longer able to perform its environmental regulatory functions of accepting, stor-
ing, and recycling water, energy, and nutrients and when the potential productivity
associated with a land-use system becomes non-sustainable (Oldeman et al. 1991 ).
Again, soil degradation is considered as the measurable loss or reduction of the
current or potential capability of soils to produce plant materials of desired quantity
and quality. According to some authors (Blaikie and Brookfi eld 1987 ; Chisholm
and Dumsday 1987 ; Blum and Eswaran 2004 ), land degradation is a broader term
than soil degradation. But for synonymous use of the terms land and soil in most soil
management literature, land degradation and soil degradation will be used inter-
changeably in the following sections.
Several physical, chemical, and biological processes are responsible for the
degradation of soil (Lal 1994 ; Eswaran et al. 2001 ). The physical processes include
deterioration of soil structure, crusting, hardsetting, compaction, erosion, and

desertification. The chemical processes include leaching, fertility depletion, acidifi -
cation, salinization, and pollution. The biological processes of soil degradation
include reduction in carbon and decline in soil biodiversity. According to Beinroth
.
et al ( 1994 ), land degradation results from a mismatch between land quality and
land use.
Yield reductions of 30–90 % by erosion in some root-restrictive shallow soils of
West Africa were reported by Mbagwu et al ( 1984 ) and Lal ( 1987 ). Erosion reduced
.
20–40 % yield of row crops in Ohio (Fahnestock et al 1995 ) and elsewhere in
.
Midwest USA (Schumacher et al. 1994 ). In the Andean region of Colombia,
Ruppenthal ( 1995 ) have observed severe losses due to accelerated erosion on some
lands. The productivity of some lands in Africa has declined by 50 % (Dregne 1990 )