The Environmental Food Crisis

Kenya land use and rain-use efficiency

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).

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

urban cropland grassland woodland water

major decline moderate decline improvement

Yellow Green

Courtesy of ISRIC, Bai ZG and Dent DL (2006)

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.

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