Land degradation

Land degradation leads to a significant reduction of the productive capacity of land. Human activities contributing to land degradation include unsuitable agricultural land use, poor soil and water management practices, deforestation, removal of natural vegetation, frequent use of heavy machinery, overgrazing, improper crop rotation and poor irrigation practices. Natural disasters, including droughts, floods and landslides, also contribute. A Global Assessment of Soil Degradation (GLASOD) was undertaken in the early 1990s (Oldeman, Hakkeling and Sombroek 1990, UNEP 1992) and a land degradation assessment of drylands (LADA) was initiated by GEF and UNEP in 2000 and is now being developed with FAO.

Extent and causes of land degradation
Degradation extent	Cause
580 million ha	Deforestation - vast reserves of forests have been degraded by large-scale logging and clearance for farm and urban use. More than 220 million ha of tropical forests were destroyed during 1975-90, mainly for food production.
680 million ha	Overgrazing - about 20 per cent of the world's pasture and rangelands have been damaged. Recent losses have been most severe in Africa and Asia.
137 million ha	Fuelwood consumption - about 1 730 million m3 of fuelwood are harvested annually from forests and plantations. Woodfuel is the primary source of energy in many developing regions.
550 million ha	Agricultural mismanagement - water erosion causes soil losses estimated at 25 000 million tonnes annually. Soil salinization and waterlogging affect about 40 million ha of land globally.
19.5 million ha	Industry and urbanization - urban growth, road construction, mining and industry are major factors in land degradation in different regions. Valuable agricultural land is often lost.
Source: FAO 1996

It has been estimated that 23 per cent of all usable land (excluding mountains and deserts, for example) has been affected by degradation to a degree sufficient to reduce its productivity (UNEP 1992, Oldeman, Hakkeling and Sombroek 1990). In the early 1990s, about 910 million ha of land were classified as 'moderately degraded', with greatly reduced agricultural productivity (see illustrations). A total of 305 million ha of soils ranged between 'strongly degraded' (296 million ha) and 'extremely degraded' (9 million ha, of which more than 5 million ha were in Africa). 'Extremely degraded' soils are beyond restoration (Oldeman, Hakkeling and Sombroek 1990).

Despite these compelling statistics on land degradation, some studies are beginning to question the data, arguing that degradation estimates are overstated. A major reason suggested for the overestimation of land degradation has been underestimation of the abilities of local farmers (Mazzucato and Niemeijer 2001). These authors argue that ' . experts need to discriminate more carefully between a naturally bad state, a temporary bad state and a degraded state of land'.

Soil erosion is a major factor in land degradation and has severe effects on soil functions - such as the soil's ability to act as a buffer and filter for pollutants, its role in the hydrological and nitrogen cycle, and its ability to provide habitat and support biodiversity. About 2 000 million ha of soil, equivalent to 15 per cent of the Earth's land area (an area larger than the United States and Mexico combined), have been degraded through human activities. The main types of soil degradation are water erosion (56 per cent), wind erosion (28 per cent), chemical degradation (12 per cent) and physical degradation (4 per cent). Causes of soil degradation include overgrazing (35 per cent), deforestation (30 per cent), agricultural activities (27 per cent), overexploitation of vegetation (7 per cent) and industrial activities (1 per cent) (GACGC 1994).

Approaches to soil conservation have been greatly modified since the 1970s. Work used to concentrate on mechanical protection, such as bunds and terraces, largely to control surface run-off. This has been supplemented by a new approach (Shaxson and others 1989, Sanders and others 1999) which calls for greater attention to biological methods of conservation, and the integration of water conservation with soil protection, through improved management of soilplant- water relationships, including reduced disturbance by tillage (University of Bern and others 2000). Within the international agricultural research system, the Consultative Group on International Agricultural Research, there is now a commitment to natural resource management, and explicit recognition of degraded land and desertification as environmental problems (Shah and Strong 1999).

Despite these developments, there is no clear indication that the rate of land degradation has decreased. As yet, there are no continuously monitored indicators of soil condition that would permit quantitatively based assessments of changes over time, comparable to the monitoring of deforestation.

It has been suggested that soil monitoring should become a basic task of national soil survey organizations (Young 1991) but this proposal has yet to be widely adopted. An international programme was set up to develop a set of land quality indicators (Pieri and others 1995), comparable to those used to monitor economic and social conditions. The programme continues on a modest scale under the Global Terrestrial Observation System.