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fertility were estimated at 22 kg nitrogen (N), 3 kg phosphorus
(P), and 15 kg potassium (K) per ha. In Zimbabwe, soil erosion
alone results in an annual loss of N and P totalling US$1.5 billion.
In South Asia, the annual economic loss is estimated at US$600
million for nutrient loss by erosion, and US$1,200 million from
soil fertility depletion (Stocking, 1986; UNEP, 1994).
Erosion is very significant in land degradation. On a global
scale, the annual loss of 75 billion tonnes of soil costs the world
about US$400 billion/year (at US$3/tonne of soil for nutrients
and US$2/tonne of soil for water), or approximately US$70/
person/year (Lal, 1998). It is estimated that the total annual
cost of erosion from agriculture in the US is about US$44 bil-
lion/year or about US$247/ha of cropland and pasture (Lal,
1998). In Sub-Saharan Africa it is much larger; in some coun-
tries productivity has declined in over 40% of the cropland area
in two decades while population has doubled. Overgrazing of
vegetation by livestock and subsequent land degradation is a
widespread problem in these regions.
The productivity of some lands has declined by 50% due to soil
erosion and desertification (Figure 16). Yield reduction in Afri-
ca due to past soil erosion may range from 2–40%, with a mean
loss of 8.2% for the continent. Africa is perhaps the continent
most severely impacted by land degradation (den Biggelaar
et
al
., 2004; Henao and Baanante, 2006), with the global aver-
age being lower, possibly in the range of 1–8%. With increasing
pressures of climate change, water scarcity, population growth
and increasing livestock densities, these ranges will be prob-
ably conservative by 2050.
Sub-Saharan Africa is particularly impacted by land degradation.
In Kenya, over the period 1981–2003, despite improvements in
woodland and grassland, productivity declined across 40% of
cropland – a critical situation in the context of a doubling of the
human population over the same period (Bai and Dent, 2006).
In South Africa, production decreased overall; 29% of the coun-
try suffered land degradation, including 41% of all cropland
(Bai and Dent, 2007a); about 17 million people, or 38% of the
South African population, depend on these degrading areas.
(Source: Bai and Dent, 2007).
Kenya land use and rain-use efficiency
Trend in biomass in 1981–2003
(left) and in rain-use efficiency
(RUE) in 1981–2002 (right).
De-
creases in RUE could be due to
various factors, including degra-
dation and run-off, soil evapora-
tion, increasing depleted soils,
overgrazing by livestock or other
forms of range degradation.
Left map
Red
Yellow
Green
Purple
Blue
Right map
Red
Yellow
Green
urban
cropland
grassland
woodland
water
major decline
moderate decline
improvement
Courtesy of ISRIC, Bai ZG and Dent DL (2006)