Agriculture Reference
In-Depth Information
2.4 EXTENT OF SOIL DEGRADATION
Soil degradation is extensive throughout the world. It is a major threat to agricultural
sustainability and environmental quality and is particularly serious in the tropics
and subtropics (Lal 1993). For example, Nyssen et al. (2009) reported that nearly
all of the tropical highlands (areas above 1000 m asl covering 4.5 million km 2 ) are
degraded due to medium to severe water erosion. Zhao et al. (2007) evaluated the
change from original pasture to cropland in the Horquin sands and found significant
changes in crop yield and soil properties after conversion to cropland. In their study,
soil parameters exhibited a change away from optimal values after the conversion.
Kidron et al. (2010) suggested that the increasing pressure for food alleviating the
traditional practice of 10 to 15 years of cultivation followed with 10 to 15 years of
fallow with a continuous cropping practice has increased the rate of soil degrada-
tion. They found that SOM content showed the strongest relationship to soil deg-
radation and practices which accelerated the removal of SOM increased the rate of
degradation.
In the subhumid and semiarid Argentinean Pampas region, Buschiazzo et al.
(1998) observed that intensive cultivation for over 50 years resulted in soil degra-
dation leading to moderate to severe erosion. A similar conclusion was reached by
dos Santos et al. (1993) for southern Brazil, in which they attributed the severe soil
degradation to the widespread use of wheat ( Triticum aestivum L.) - soybea n ( Glycine
max L. Merr.) or barley ( Hordeum vulgare L.)-soybean double cropping systems
coupled with intensive tillage. Krzic et al. (2000) observed in the maritime climate
of the Fraser Valley in British Columbia, with over 1200 mm of annual rainfall,
that conventional tillage over a number of years contributed to poor infiltration, low
organic matter content, and poor soil structure.
In southern Brazil and eastern Paraguay, Riezebos and Loerts (1998) observed
that mechanical tillage resulted in a loss of SOM, leading to soil degradation across
this region. The conversion of semideciduous forests to cultivated lands has the
potential for soil degradation, and proper management will be required to avoid deg-
radation. Degradation of the soil resource occurs in many different forms, and in
Nepal, Thapu and Paudel (2002) observed that watersheds are severely degraded
from erosion. They found that erosion has impacted nearly half of the land area in the
upland crop terraces. This degradation was coupled with depletion of soil nutrients,
which in turn is continuing to affect productivity in this area. This is similar to the
observation in Ethiopia by Taddese (2001), where severe land degradation caused by
the rapid population increase, severe soil erosion, low amounts of vegetative cover,
deforestation, and a lack of balance between crop and livestock production will con-
tinue to threaten the ability to produce an adequate food supply for the population.
Wang et al. (1985) observed that differences in soil structure and saturated
hydraulic conductivity were related to cropping systems and that degradation of soil
structure in the profile led to corn yield reductions as large as 50%. Yield declines
were related to the shallow root growth and limitations in water availability to the
growing plant. Impacts of poor soil structure on plant growth and yield can be large,
and continued degradation of the soil resource will have a major impact on the abil-
ity of the plant to produce grain, fiber, or forage.
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