Geoscience Reference
In-Depth Information
land degradation costs approximately $40 billion
every year throughout the world as a consequence
of lost soil productivity.
Soil degradation involves a loss of organic
matter, nutrients and water from land. There
are various processes that contribute to soil
degradation. It can, for example, be the result of
forest clearances (see
Chapter 5)
in and through
which soils get cut off from the water, carbon and
nutrients that trees supply. Soil degradation is
also often the result of the over use and poor
management of land within modern agriculture.
The overgrazing of pastures and the over-
cultivation of arable land results in the net loss of
nutrients and water from the soil and a decline in
the ability of the soil to support future farming
endeavours. In the case of forest clearances, water
(in the form of rainfall) transfers nutrients away
from soils (a process referred to as leaching). In the
case of bad agricultural management practices, it
is the crops and animals that remove nutrients
from the soil. Studies indicate that imbalances in
soil nutrient budgets are much higher in less
economically developed countries than they
are in more economically developed countries
(agriculturalists in the latter appear much more
able to apply inorganic fertilizers to their land). A
study of 38 countries in Sub-Saharan Africa, for
example, indicates that between 1983 and 2000
each country had a negative soil nutrient budget
(with more nutrients being taken from soils
than was returned annually) (Dent et al, 2007: 97).
This means that some 950,000km
2
of land in
Sub-Saharan Africa is under serious threat of
long-term land degradation (Dent et al, 2007).
The United Nations Environment Programme
and the FAO now compile data on global levels
of soil and land degradation. This research
programme is called the Land Degradation
Assessment in Drylands (LADA for short). This
study, which is largely based on remote sensing
satellite data, indicates that over the last 25 years
there has been a 12 per cent decline in the net
primary productivity (NPP) of land.
For more information on the UN LADA
programme and its assessment reports
go to:
In the most extreme of cases, forest clearances
and poor land management practices can result
in soil erosion, or the net loss of topsoil from the
land. Soil erosion is, of course, a natural process,
but as we have seen in the case of the Dust Bowl it
can be greatly accelerated by humans. Soil erosion
occurs when exposed soil is moved by wind and/or
water. While soil erosion is a global problem, it
particularly afflicts dry land agricultural regions
such as those found within Africa, Australia and
parts of Asia. In West Africa, for example, estimates
indicate that wind-based soil erosion affects 1.45
million km
2
of land (Dent et al, 2007: 95). Global
estimates suggest that soil erosion results in the loss
of 20,000 to 40,000km
2
of previously productive
land.
The scale of contemporary patterns of soil
degradation, expressed through both net
nutrient loss and soil erosion, make it one of the
defining environmental characteristics of the
Anthropocene. But the effects of soil degradation
are not evenly distributed in geographical terms:
with dryland agricultural communities in less
economically developed countries often being the
most severely affected. The degradation of soil
has a series of local and global consequences. At a
local level, soil degradation is associated with the
onset of human poverty. The large-scale loss of
productive soil also results in local declines in rates
of biodiversity, as the varied species that live in
and depend on soil suffer as a consequence of its
deterioration. In a global context, soil degradation
is also a contributory factor in accelerating patterns
of climate change. Soil degradation contributes
to climate change in two main ways. First, soil
erosion results in the release of carbon (from soil's
organic matter) back into the atmosphere. Second,
degraded soils are less able to support the trees and
