Chemistry Reference
In-Depth Information
taken into account. In some instances, such as the adoption of reduced tillage
systems, win-wins of decreased inputs and decreased soil physical damage can
be achieved.
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2 Forms and Extent of Soil Physical Degradation
Quantifying the extent of soil physical degradation is not an easy task. The
'Global Land Assessment of Soil Degradation' (GLASOD) was setup in the
early 1990s to attempt to provide expert advice on the current situation.
Human-induced soil degradation has affected 24% of the global inhabited land
area, 1966 Mha. The global range of degradation ranges by continent from as
low as 12% in North America to 31% in Asia. Within the term of soil 'physical
degradation' used by GLASOD, three types exist: (i) compaction, crusting and
sealing; (ii) waterlogging; and (iii) subsidence of organic soils. Compaction,
crusting and sealing have caused an estimated 4% (68.3 Mha) of degradation,
with waterlogging and subsidence of organic soils degrading 11 Mha and 4
Mha, respectively. Of all soil physical degradation, agricultural mismanage-
ment has accounted for 80% and overgrazing 16%.
The large areas of the globe affected by soil degradation are shown in
Figure 1. By far the greatest form of degradation is erosion by wind and water,
with pockets of easily physically degraded soils in isolated regions. The quality
of data used to map soil resources, however, was discussed in Chapter 4. There
is insufficient evidence available to accurately map many of these threats and
the map itself is over 20 years old. Over the past two decades a major negative
change has been increased machinery weight, but positive changes also have
occurred, such as better organic management, reduced tillage adoption in
sensitive regions, and new engineering technologies that decrease damage. An
interesting observation from Figure 1 is that the physically degraded region in
Sweden corresponds to a country where considerable research investment in
soil compaction occurs. Although the threat of soil compaction may have
encouraged greater research investment, it is likely that the soils of other
regions, such as the UK, are also vulnerable, but poorly characterised.
Figure 1 only shows the major form of soil degradation. Different types of
soil physical degradation often interact. Soil compaction, hardsetting and
slumping all can enhance soil erosion. A management practice designed to
decrease one form of soil physical degradation generally has a positive impact
on other types of degradation. Organic matter incorporation, for instance, can
decrease the slumping of seedbeds, erosion by either wind or water, and the
resilience of soil to compaction.
2.1 Soil Compaction
Soil compaction is the loss of porosity through mechanical damage to soil.
This definition can be extended to encompass all forms of mechanical damage
that decrease crop productivity or have a negative environmental impact.
