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Land Degradation: An overview

H. ESWARAN1, R. LAL2 and P. F. REICH3

Published in: Eswaran, H., R. Lal and P.F. Reich. 2001. Land degradation: an overview. In: Bridges, E.M., I.D. Hannam, L.R. Oldeman, F.W.T. Pening de Vries, S.J. Scherr, and S. Sompatpanit (eds.). Responses to Land Degradation. Proc. 2nd. International Conference on Land Degradation and Desertification, Khon Kaen, Thailand. Oxford Press, New Delhi, India.

Land degradation will remain an important global issue for the 21st century because of its adverse impact on agronomic productivity, the environment, and its effect on food security and the quality of life. Productivity impacts of land degradation are due to a decline in land quality on site where degradation occurs (e.g. erosion) and off site where sediments are deposited. However, the on-site impacts of land degradation on productivity are easily masked due to use of additional inputs and adoption of improved technology and have led some to question the negative effects of desertification. The relative magnitude of economic losses due to productivity decline versus environmental deterioration also has created a debate. Some economists argue that the on-site impact of soil erosion and other degradative processes are not severe enough to warrant implementing any action plan at a national or an international level. Land managers (farmers), they argue, should take care of the restorative inputs needed to enhance productivity. Agronomists and soil scientists, on the other hand, argue that land is a non-renewable resource at a human time-scale and some adverse effects of degradative processes on land quality are irreversible, e.g. reduction in effective rooting depth. The masking effect of improved technology provides a false sense of security.

The productivity of some lands has declined by 50% due to soil erosion and desertification. Yield reduction in Africa due to past soil erosion may range from 2 to 40%, with a mean loss of 8.2% for the continent. In South Asia, annual loss in productivity is estimated at 36 million tons of cereal equivalent valued at US$5,400 million by water erosion, and US$1,800 million due to wind erosion. It is estimated that the total annual cost of erosion from agriculture in the USA is about US$44 billion per year, i.e. about US$247 per ha of cropland and pasture. On a global scale the annual loss of 75 billion tons of soil costs the world about US$400 billion per year, or approximately US$70 per person per year.

Only about 3% of the global land surface can be considered as prime or Class I land and this is not found in the tropics. Another 8% of land is in Classes II and III. This 11% of land must feed the six billion people today and the 7.6 billion expected in 2020. Desertification is experienced on 33% of the global land surface and affects more than one billion people, half of whom live in Africa.

Land degradation, a decline in land quality caused by human activities, has been a major global issue during the 20th century and will remain high on the international agenda in the 21st century. The importance of land degradation among global issues is enhanced because of its impact on world food security and quality of the environment. High population density is not necessarily related to land degradation; it is what a population does to the land that determines the extent of degradation. People can be a major asset in reversing a trend towards degradation. However, they need to be healthy and politically and economically motivated to care for the land, as subsistence agriculture, poverty, and illiteracy can be important causes of land and environmental degradation.

Land degradation can be considered in terms of the loss of actual or potential productivity or utility as a result of natural or anthropic factors; it is the decline in land quality or reduction in its productivity. In the context of productivity, land degradation results from a mismatch between land quality and land use (Beinroth et al., 1994). Mechanisms that initiate land degradation include physical, chemical, and biological processes (Lal, 1994). Important among physical processes are a decline in soil structure leading to crusting, compaction, erosion, desertification, anaerobism, environmental pollution, and unsustainable use of natural resources. Significant chemical processes include acidification, leaching, salinization, decrease in cation retention capacity, and fertility depletion. Biological processes include reduction in total and biomass carbon, and decline in land biodiversity. The latter comprises important concerns related to eutrophication of surface water, contamination of groundwater, and emissions of trace gases (CO2, CH4, N2O, NOx) from terrestrial/aquatic ecosystems to the atmosphere. Soil structure is the important property that affects all three degradative processes. Thus, land degradation is a biophysical process driven by socioeconomic and political causes.

Factors of land degradation are the biophysical processes and attributes that determine the kind of degradative processes, e.g. erosion, salinization, etc. These include land quality (Eswaran et al., 2000) as affected by its intrinsic properties of climate, terrain and landscape position, climax vegetation, and biodiversity, especially soil biodiversity. Causes of land degradation are the agents that determine the rate of degradation. These are biophysical (land use and land management, including deforestation and tillage methods), socioeconomic (e.g. land tenure, marketing, institutional support, income and human health), and political (e.g. incentives, political stability) forces that influence the effectiveness of processes and factors of land degradation.

Depending on their inherent characteristics and the climate, lands vary from highly resistant, or stable, to those that are vulnerable and extremely sensitive to degradation. Fragility, extreme sensitivity to degradation processes, may refer to the whole land, a degradation process (e.g. erosion) or a property (e.g. soil structure). Stable or resistant lands do not necessarily resist change. They are in a stable steady state condition with the new environment. Under stress, fragile lands degrade to a new steady state and the altered state is unfavorable to plant growth and less capable of performing environmental regulatory functions.

Effects of land degradation on productivity

Information on the economic impact of land degradation by different processes on a global scale is not available. Some information for local and regional scales is available and has been reviewed by Lal (1998). In Canada, for example, on-farm effects of land degradation were estimated to range from US$700 to US$915 million in 1984 (Girt, 1986). The economic impact of land degradation is extremely severe in densely populated South Asia, and sub-Saharan Africa.

On plot and field scales, erosion can cause yield reductions of 30 to 90% in some root-restrictive shallow lands of West Africa (Mbagwu et al.,1984; Lal, 1987). Yield reductions of 20 to 40% have been measured for row crops in Ohio (Fahnestock et al., 1995) and elsewhere in Midwest USA (Schumacher et al., 1994). In the Andean region of Colombia, workers from the University of Hohenheim, Germany (Ruppenthal, 1995), have observed severe losses due to accelerated erosion on some lands. Few attempts have been made to assess the global economic impact of erosion. The productivity of some lands in Africa (Dregne, 1990) has declined by 50% as a result of soil erosion and desertification. Yield reduction in Africa (Lal, 1995) due to past soil erosion may range from 2 to 40%, with a mean loss of 8.2% for the continent. If accelerated erosion continues unabated, yield reductions by 2020 may be 16.5%. Annual reduction in total production for 1989 due to accelerated erosion was 8.2 million tons for cereals, 9.2 million tons for roots and tubers, and 0.6 million tons for pulses. There are also serious (20%) productivity losses caused by erosion in Asia, especially in India, China, Iran, Israel, Jordan, Lebanon, Nepal, and Pakistan (Dregne, 1992). In South Asia, annual loss in productivity is estimated at 36 million tons of cereal equivalent valued at US$5,400 million by water erosion, and US$1,800 million due to wind erosion (UNEP, 1994). It is estimated that the total annual cost of erosion from agriculture in the USA is about US$44 billion per year, about US$247 per ha of cropland and pasture. On a global scale the annual loss of 75 billion tons of soil costs (at US$3 per ton of soil for nutrients and US$2 per ton of soil, for water) the world about US$400 billion per year, or approximately US$70 per person per year (Lal, 1998).

Soil compaction is a worldwide problem, especially with the adoption of mechanized agriculture. It has caused yield reductions of 25 to 50% in some regions of Europe (Ericksson et al., 1974) and North America, and between 40 and 90% in West African countries (Charreau, 1972; Kayombo and Lal, 1994). In Ohio, reductions in crop yields are 25% in maize, 20% in soybeans, and 30% in oats over a seven-year period (Lal, 1996). On-farm losses through land compaction in the USA have been estimated at US$1.2 billion per year (Gill, 1971).

Nutrient depletion as a form of land degradation has a severe economic impact at the global scale, especially in sub-Saharan Africa. Stoorvogel et al. (1993) have estimated nutrient balances for 38 countries in sub-Saharan Africa. Annual depletion rates of soil fertility were estimated at 22 kg N, 3 kg P, and 15 kg K ha-1. In Zimbabwe, soil erosion results in an annual loss of N and P alone totaling 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 due to soil fertility depletion (Stocking, 1986; UNEP, 1994).

An estimated 950 million ha of salt-affected lands occur in arid and semi-arid regions, nearly 33% of the potentially arable land area of the world. Productivity of irrigated lands is severely threatened by build up of salt in the root zone. In South Asia, annual economic loss is estimated at US$500 million from waterlogging, and US$1,500 million due to salinization (UNEP, 1994). Potential and actual economic impact globally is not known. It is not known either for soil acidity and the resultant toxicity of high concentrations of Al and Mn in the root zone, a serious problem in sub-humid and humid regions (Eswaran et al., 1997a).

It is in the context of these global economic and environmental impacts of land degradation, and numerous functions of value to humans, that land degradation, desertification, and resilience concepts are relevant (Eswaran, 1993). They are also important in developing technologies for reversing land degradation trends and mitigating the greenhouse effect through land and ecosystem restoration. As land resources are essentially non-renewable, it is necessary to adopt a positive approach to sustainable management of these finite resources.

Views on land degradation

Land degradation has received widespread debate at the global level as evidenced by the literature: UNEP, 1992; Johnson and Lewis, 1995; Oldeman et al., 1992; Middleton and Thomas, 1997; Dregne, 1992; Maingnet, 1994; Lal and Stewart, 1994; Lal et al., 1997. At least two distinct schools have emerged regarding the prediction, severity, and impact of land degradation. One school believes that it is a serious global threat posing a major challenge to humans in terms of its adverse impact on biomass productivity and environment quality (Pimentel et al., 1995; Dregne and Chou, 1994). Ecologists, soil scientists, and agronomists primarily support this argument. The second school, comprising primarily economists, believes that if land degradation is a severe issue, why market forces have not taken care of it. Supporters argue that land managers (e.g. farmers) have vested interest in their land and will not let it degrade to the point that it is detrimental to their profits (Crosson, 1997). There are a number of factors that perpetuate the debate on land degradation:

  1. Definition: There are numerous terms and definitions that are a source of confusion, misunderstanding, and misinterpretation. A wide range of terms is used in the literature, often with distinct disciplinary-oriented meaning, and leading to misinterpretation among disciplines. Some common terms used are soil degradation, land degradation, and desertification. While there is a clear distinction between ‘soil’ and ‘land’ (the term land refers to an ecosystem comprising land, landscape, terrain, vegetation, water, climate), there is no clear distinction between the terms ‘land degradation’ and ‘desertification’. Desertification refers to land degradation in arid, semi-arid, and sub-humid areas due to anthropic activities (UNEP, 1993; Darkoh, 1995). Many researchers argue that this definition of desertification is too narrow because severe land degradation resulting from anthropic activities can also occur in the temperate humid regions and the humid tropics. The term ‘degradation’ or ‘desertification’ refers to irreversible decline in the ‘biological potential’ of the land. The ‘biological potential’ in turn depends on numerous interacting factors and is difficult to define. The confusion is further exacerbated by the definition of ‘dryland’ where different definitions are used. It is important to standardize the terminology, and develop a precise, objective, and unambiguous definition accepted by all disciplines.

  2. Extent and rate of land degradation: Because of different definitions and terminology, a large variation in the available statistics on the extent and rate of land degradation also exists. Two principal sources of data include the global estimates of desertification by Dregne and Chou (1994), and of land degradation by the International Soil Reference and Information Centre (Oldeman et al., 1992; Oldeman, 1994). Table 1 shows that degraded lands in dry areas of the world amount to 3.6 billion ha or 70% of the total 5.2 billion ha of the total land areas considered in these regions. In comparison, in Table 2, Oldeman (1994), shows that the global extent of land degradation (by all processes and all ecoregions) is about 1.9 billion ha. The principal difference between the two estimates is the status of vegetation. Although the estimates by Dregne and Chou cover only dry areas, they also include the status of vegetation on the rangeland. Therefore, the estimates in Tables 3 and 4 are not directly comparable. There is also a difference in terminology used to express the severity of land degradation. Dregne and Chou used the terms slight, moderate, severe, and very severe to designate the severity of degradation. Oldeman used the terms light, moderate, strong, and extreme, and these terms may not be comparable to those of Dregne and Chou. Oldeman et al. (1992), on the basis of expert judgment, attempted to differentiate natural from human-induced degradation. Eswaran and Reich (1998) attempted to evaluate vulnerability to land degradation and desertification. Differences in terminology and approaches, and also the areas included in the assessment, mean that the estimates of the three workers are difficult to compare (Tables 1, 2, and 3).

    Table 1. Estimates of all degraded lands (in million km 2) in dry areas (Dregne and Chou, 1994).

     
    Continent Total area Degraded area † % degraded
    Africa 14.326 10.458 73
    Asia 18.814 13.417 71
    Australia and the Pacific 7.012 3.759 54
    Europe 1.456 0.943 65
    North America 5.782 4.286 74
    South America 4.207 3.058 73
    Total 51.597 35.922 70
    † Comprises land and vegetation.


    Table 2. Estimates of the global extent (in million km 2) of land degradation (Oldeman, 1994).

     
    Type Light Moderate Strong + Extreme Total
    Water erosion 3.43 5.27 2.24 10.94
    Wind erosion 2.69 2.54 0.26 5.49
    Chemical degradation 0.93 1.03 0.43 2.39
    Physical degradation 0.44 0.27 0.12 0.83
    Total 7.49 9.11 3.05 19.65

    Table 3. Vulnerability to desertification and wind and water erosion (Eswaran and Reich, 1998). Only arid, semi-arid, and sub-humid areas (in million km 2) are considered according to the definition of UNEP. Estimates of water erosion include humid areas.

     
    Severity Desertification Water erosion Wind erosion
    Low 14.653 17.331 9.250
    Moderate 13.668 15.373 6.308
    High 7.135 10.970 7.795
    Very high 7.863 12.196 9.320
    Total 43.319 55.870 32.373
  3. Land–vegetation relationships: The problem is further confounded by the definition of the term 'vegetation degradation'. It may imply reduction in biomass, decrease in species diversity, or decline in quality in terms of the nutritional value for livestock and wildlife. There is a need for establishing distinct criteria for evaluating vegetation degradation. Table 4 shows an example of vegetation degradation in Australia, but in combination with soil erosion. The quantity and quality of vegetation are not considered.

    Table 4. Vegetation degradation in pastoral areas of Australia (Woods, 1983; Mabbutt, 1992).

     
    Type Area (‘000 km2)
    Total 3.4
    Undegraded 1.5
    Degraded 1.9
    i. Vegetation degradation with little erosion 1.0
    ii. Vegetation degradation and some erosion 0.5
    iii. Vegetation degradation and substantial erosion 0.3
    iv. Vegetation degradation and severe erosion 0.1
    v. Dryland salinity 0.001
  4. Land degradation processes: Different processes of land degradation also confound the available statistics on soil and/or land degradation. Principal processes of land degradation include erosion by water and wind, chemical degradation (comprising acidification, salinization, leaching etc.) and physical degradation (comprising crusting, compaction, hard-setting etc.). Some lands or landscape units are affected by more than one process, of water and wind erosion, salinization, and crusting or compaction. Unless a clear distinction is made, there is a considerable chance of overlap and double accounting. Table 5 shows an example of overlapping degradative processes with increasing risks of double accounting.

    Table 5. Land degradation on cropland in Australia (Woods, 1983; Mabbutt, 1992).

     
    Type Area (‘000 km2)
    Total 443
    Undegraded 142
    Degraded 301
    i. Water erosion 206
    ii. Wind erosion 52
    iii. Combined water and wind erosion 42
    iv. Salinity and water erosion 0.9
    v. Others 0.5
  5. Methods of assessment of land degradation: Global assessment of land degradation is not an easy task, and a wide range of methods is used (Lal et al., 1997). Therefore, data generated by different methods are not comparable. Further, most statistics refer to the risks of degradation or desertification (based on climatic factors and land use) rather than the actual (present) state of the land. Table 3 shows vulnerability to desertification and erosion, and these estimates are much higher than those by Dregne and Chou (Table 1) and Oldeman (Table 2), and are suggestive of the risks of land degradation. The actual degradation may not occur because of judicious land use and advances in land management technologies. Comparison of these data on different estimates shows wide variations because of different methods and criteria used, and highlights the importance of developing uniform criteria and standardizing methods of assessment of land degradation.

  6. Land degradation and productivity: A major shortcoming of the available statistics on land degradation is the lack of cause–effect relationship between severity of degradation and productivity. Criteria for designating different classes of land degradation (e.g. low, moderate, high) are generally based on land properties rather than their impact on productivity. In fact, assessing the productivity effects of land degradation is a challenging task (Lal, 1998). Difficulties in obtaining global estimates of the impact of land degradation on productivity in turn created problems and raised skepticism. Table 6 from the International Board for Soil Research and Management (IBSRAM) indicates the problems involved in relating land degradation by erosion to crop yield. The data from China show that despite significant differences in cumulative soil loss and water runoff, there were no differences in corn yield. Similar inferences can be drawn with regard to the impact of cumulative soil erosion on yield of rice in Thailand. Whereas soil loss ranged from 330 to 1,478 t ha-1, the corresponding yield of rice ranged from 4.0 to 5.3 t ha-1. The lowest yield was obtained from treatments causing the least soil loss. Crop yield is an integrative effect of numerous variables. In addition, erosional (and other degradative processes) effects on crop yield or biomass potential depend on changes in land quality with respect to specific parameters. Table 7 shows that the yield of sisal was correlated with pH, CEC, and Al saturation but not with soil organic C and N contents. Assessing the productivity effects of land degradation requires a thorough understanding of the processes involved at the soil–plant–atmosphere continuum. These processes are influenced strongly by land use and management.

    Table 6. Cumulative soil loss and runoff in relation to crop yield in three ASIALAND Sloping Lands Network countries (Sajjapongse, 1998).

     
    Country Treatment Period Crop Soil loss
    (Mg ha-1)
    Runoff(mm) Cumulative yield
    (Mg ha-1)
    China Control † 1992–95 Corn 122 762 15.3
      Alley cropping 1992–95 Corn 59 602 15.9
    Philippines Control 1990–94 Corn 341 801 5.6
      Alley cropping (low input) 1990–94 Corn 26 43 14.3
      Alley cropping (high input) 1990–94 Corn 15 31 18.7
    Thailand Control 1989–95 Rice 1,478 1,392 4.5
      Hillside ditch 1989–95 Rice 134 446 4.8
      Alley cropping 1989–95 Rice 330 538 4.0
      Agroforestry 1989–95 Rice 850 872 5.3
    † Control = Farmer’s practice

    Table 7. Relationship between yield of sisal and soil fertility (0–20 cm depth) decline in Tanga region of Tanzania (Hartemink, 1995).

     
    Land properties Sisal yield (Mg ha-1)
    Yield levels 2.3 1.8 1.5
      Property value
    pH (1:2.5 in H2O) 6.50 5.40 5.00
    Soil organic carbon (%) 1.60 1.90 1.50
    Total soil nitrogen (%) 0.11 0.16 0.12
    Cation exchange capacity (cmol kg-1) 9.30 7.00 5.00
    Al saturation (% ECEC) 0 20.00 50.00

Issues and challenges

There are sufficient studies and reviews (e.g. Barrow, 1991; Blaikie and Brookfield, 1987; Johnson and Lewis, 1995) that clearly demonstrate the fact that land degradation affects all facets of life. Many issues that confront those working in the domain of land resources include technologies to reduce degradation and also the techniques to assess and monitor land degradation. A number of questions remain unanswered and these include:

  • Is land degradation inevitable?
  • Are there adequate early warning indicators of land degradation?
  • The absence of land tenure and the resulting lack of stewardship is a major issue in some countries that detracts from adequate care for the land; how can this be resolved?
  • Declining soil quality resulting largely from human-induced degradation triggers social unrest; what is the societal responsibility of soil scientists?
  • How can soil scientists better participate in developing public policy?
  • Local actions have global impact; what are the areas for international collaboration?
  • Who pays, who wins in the economics of land degradation?
  • Degradation results in loss of intergenerational equity and the value of bequests. How do we quantify and create awareness?
  • Is there a link between land degradation and the health of humans and animals?
  • Declining soil quality leads to diminishing economic growth in countries where wealth is largely agrarian. How can the rates of resource consumption be quantified?
  • Land degradation often destroys or reduces the natural beauty of landscapes. How might the aesthetic value of land be quantified?
  • How to create greater awareness of the perils of land degradation in society and political leadership?

There are three steps involved in the process of addressing the problem: assessment, monitoring, and application of mitigating technologies. All three steps are in the purview of agriculturists and specifically, soil scientists. The latter clearly have the responsibility for soil science, and over the past decade substantial progress has been made in communicating the dangers of land degradation. However, much remains to be done.

Soil science has made significant contributions to the task of soil resource assessment but its practitioners have shown little or no interest in the additional task of monitoring the resource base (Mermut and Eswaran, 1997). This still remains a new area of investigation requiring guidelines, standards, and procedures. The challenge is to adopt an internationally acceptable procedure for this task. Soil scientists have an obligation not only to show the spatial distribution of stressed systems but also to provide reasonable estimates of their rates of degradation. They should develop early warning indicators of degradation to enable them to collaborate with others, such as social scientists, to develop and implement mitigating technologies. Soil scientists also have a role in assisting national decision-makers to develop appropriate land use policies.

There are many, usually confounding, reasons why land users permit their land to degrade. Many of the reasons are related to societal perceptions of land and the values they place on land. Degradation is also a slow imperceptible process and so many people are not aware that their land is degrading. Creating awareness and building up a sense of stewardship are important steps in the challenge of reducing degradation. Consequently, appropriate technology is only a partial answer. The main solution lies in the behaviour of the farmer who is subject to economic and social pressures of the community/country in which he/she lives. Food security, environmental balance, and land degradation are strongly inter-linked and each must be addressed in the context of the other to have measurable impact. This is the challenge of the 21st century for which we must be prepared.

Desertification

Desertification is a form of land degradation occurring particularly, but not exclusively, in semi-arid areas. Figure 1 indicates the areas of the world vulnerable to desertification. The semi-arid to weakly arid areas of Africa are particularly vulnerable, as they have fragile soils, localized high population densities, and generally a low-input form of agriculture. About 33% of the global land surface (42 million km2) is subject to desertification. Table 8 shows the vulnerability of land to desertification in some Asian countries. Twenty-five percent of the region is affected and if not addressed the quality of life of large sections of the population will be affected. Many of these countries cannot afford losses in agricultural productivity. There are no good estimates of the number of persons affected by desertification nor of the number who directly or indirectly contribute to the process. A recent study by Reich et al. (this volume) provides some estimates for Africa.

Table 8. Estimates of vulnerability to desertification in some Asian countries.

 
Countries Total land area (km2) Vulnerability to desertification
Low Moderate High Very high
Area % Area % Area % Area %
Afghanistan 647,500 2,954 0.46 39,088 6.04 43,838 6.77 436,480 67.41
Bangladesh 133,910 85,163 63.60 0 0 0 0 0 0
Bhutan 47,000 1,407 2.99 0 0 0 0 0 0
Brunei 6,627 0 0 0 0 0 0 0 0
China 9,326,410 262,410 2.81 239,107 2.56 65,638 0.70 72,214 0.77
India 2,973,190 1,277,328 42.96 744,148 25.03 206,317 6.94 165,912 5.58
Indonesia 1,826,440 29,596 1.62 46,290 2.53 5,289 0.29 232 0.01
Japan 374,744 0 0 0 0 0 0 693 0.18
Cambodia 176,520 45,731 25.91 118,155 66.94 0 0 0 0
Laos 230,800 48,963 21.21 35,386 15.33 0 0 0 0
Malaysia 328,550 0 0 0 0 0 0 0 0
Mongolia 1,565,000 26,345 1.68 40,511 2.59 19

0

2,104 0.13
Myanmar 657,740 130,903 19.90 140,387 21.34 20,630 3.14 13,477 2.05
Nepal 136,800 20,131 14.72 8,698 6.36 0 0 228 0.17
North Korea 120,410 0 0 0 0 0 0 0 0
Pakistan 778,720 31,474 4.04 39,605 5.09 17,032 2.19

181,503

23.31
Papua New Guinea 452,860 4,892 1.08 8,175 1.81 27 0.01 0 0.00
Philippines 298,170 20,952 7.03 16,621 5.57 1,708 0.57 0 0.00
Singapore 638 0 0 0 0 0 0 0 0
South Korea 98,190 0 0 0 0 0 0 0 0
Sri Lanka 64,740 6,337 9.79 24,393 37.68 3,421 5.28 0 0.00
Taiwan 32,260 2,902 9.00 201 0.62 277 0.86 0 0.00
Thailand 511,770 90,241 17.63 320,581 62.64 7,265 1.42 0 0.00
Vietnam 325,360 47,516 14.60 59,238 18.21 375 0.12 0 0.00
Total 21,115,069 2,135,245 10.11 1,880,584 8.91 371,836 1.76 872,843 4.13

As shown in Table 9, a high population density in an area that is highly vulnerable to desertification poses a very high risk for further land degradation. Conversely, a low population density in an area where the vulnerability is also low poses, in principle, a low risk. The Mediterranean countries of North Africa are very highly prone to desertification. In Morocco, for example, erosion is so extensive that the petrocalcic horizon of some Palexeralfs is exposed at the surface. In the Sahel, there are pockets of very high-risk areas. The West African countries, with their dense populations, have a major problem to contain the processes of land degradation. Table 10 provides the area in each of the classes of Table 9.

About 2.5 million km2 of land are under low risk, 3.6 are under moderate risk, 4.6 are under high risk, and 2.9 million km2 are under very high risk. The region that has high propensity is located along the desert margins and occupies about 5% of the landmass. It is estimated that about 22 million people (2.9% of total population) live in this area. The low, moderate, and high vulnerability classes occupy 14, 16, and 11% respectively and together impact about 485 million people. Cumulatively, desertification affects about 500 million Africans and though they have relatively good soil resources (Eswaran et al., 1997b,c) their productivity will be seriously undermined by land degradation and desertification.

Table 9. Matrix for risk assessment of human-induced desertification. 1 = low risk; 2, 3 = moderate risk; 4, 5, 6 = high risk; 7, 8, 9 = very high risk. (After Reich et al., 1999.)

 
Vulnerability class Population density (persons km2)
< 10 11–40 > 41
Low 1 3 6
Moderate 2 5 8
High/Very High 4 7 9

Table 10. Land area (1,000 km 2) of Africa in risk classes. (After Reich et al., 1999.)

 
Vulnerability class Population density (persons km2)
< 10 11–40 > 40
Low 2,476 1,005 750
Moderate 2,608 1,180 976
High/very high 2,643 1,074 825

A New Agenda

Although soils are an ecologically important component of the environment, the availability of research and development funds does not match their significance in terms of the cost to society if soils become degraded. The perception that enough is already known about soils, so that generalizations can be made for all soils, is incorrect. There is also a failure to recognize that agriculture is one of the major ‘stressors’ of the environment (Virmani et al., 1994), particularly from a soil degradation point of view (Beinroth et al., 1994). If these attitudes prevail, major catastrophes in the future become more probable.

The purpose of such discussions is to emphasize that soil is virtually a non-renewableresource. It is a basic philosophy that society has an obligation to protect soil, conserve it, or even enhance its quality for future generations. The role of society in sustaining agriculture can be demonstrated, and conversely, the role of soil in sustaining society. The paradigms that brought some countries of the world to agricultural affluence must be evaluated, as well as the policies and practices that have contributed to land degradation and decline in productivity in other countries. We need to look at current concerns, and the urgent need to develop new paradigms for managing soil resources, as proposed by Sanchez (1994) for example, that will carry us through the next few decades. Some of the valid arguments of the past now have little validity in the face of contemporary environmental degradation. New concepts must be defined, the research gaps identified, and the needs that will enable our institutions to meet the challenges and requirements of the next 20 years must be indicated.

Land degradation results from mismanagement of land and thus deals with two interlocking, complex systems: the natural ecosystem and the human social system. Interactions between the two systems determine the success or failure of resource management programs. To avert the catastrophe resulting from land degradation, which threatens many parts of the world, the following concepts from Eswaran and Dumanski (1994) are relevant:

  • Environment and agriculture are intrinsically linked and research and development must address both of them.
  • Land degradation is as much a socioeconomic problem as it is a biophysical problem.
  • Land degradation and economic growth or lack of it (poverty) are intractably linked; (people living in the lower part of the poverty spiral are in a weak position to provide the stewardship necessary to sustain the resource base. As a consequence, they move further down the poverty spiral—a vicious cycle is set in motion).
  • Implementation of mitigation research to manage land degradation can only succeed if land users have control and commitment to maintain the quality of the resources.
  • The focus of agricultural research should shift from increasing productivity to enhancing sustainability, recognizing that land degradation caused by agriculture can be minimized and made compatible with the environment.
  • Land use must match land quality; appropriate national policies should be implemented to ensure this occurs to reduce land degradation; (a framework for evaluation of sustainable land management [Dumanski et al., 1992] is a powerful tool to assess such discrepancies and assure sustainability).

The thrust of a new agenda for resource assessment and monitoring with respect to land degradation (including desertification), has several components. It must be stressed that any research and development activity should be in the larger context of the ecosystem as addressed by Sanchez (1994) and Greenland et al. (1994).

Components of a national strategy to address land degradation (and desertification) comprise:

  • Studies on long-term water needs (quality and quantity).
  • A network of monitoring sites to detect changes in natural resource conditions.
  • Working with farmers by understanding and incorporating indigenous knowledge.
  • Including land degradation aspects in research on cropping and farming systems, and soil and water management.
  • Convincing decision-makers that climate change, desertification, quality of life, and sustainability are all interlinked and addressing one helps the other.
  • Initiating research on a new paradigm that is holistic and focuses on these issues.

A 'scale-sensitive information system' or database on land degradation must be made available for the vertical network of decision-makers to enable them to make effective policies concerning the use and management of resources. Decision-makers at all levels of society should be able to participate in the design and implementation of any tool that affects the social, economic, and ecological well-being. This ensures successful implementation of the program.

Figure 1. Desertification vulnerability.

Conclusions

Agenda 21 (UNCED, 1992, Chapter 12) emphasizes land degradation through desertification, and the international community, particularly through UN organizations, has launched several activities to address it. Other aspects of land degradation only receive a casual mention in Agenda 21, and are briefly considered in Chapter 10 under the general heading, "Integrated Approach to the Planning and Management of Land Resources". In this sense the problem of land degradation has been diluted and as such has not received the global attention that it deserves. Though the stated objective in Agenda 21 is, "to strengthen regional and global systematic observation networks linked to the development of national systems for the observation of land degradation and desertification caused both by climate fluctuations and by human impact, and to identify priority areas for action", we believe that we have yet to mobilize the soil science community to develop a proactive programme in this area. Land degradation remains a serious global threat but the science concerning it contains both myths and facts. The debate is perpetuated by confusion, misunderstanding, and misinterpretation of the available information. Important challenges are:

  • To mobilize the scientific community to mount an integrated programme for methods, standards, data collection, and research networks for assessment and monitoring of soil and land degradation.
  • To develop land use models that incorporate both natural and human-induced factors that contribute to land degradation and that could be used for land use planning and management.
  • To develop information systems that link environmental monitoring, accounting, and impact assessment to land degradation.
  • To help develop policies that encourage sustainable land use and management and assist in the greater use of land resource information for sustainable agriculture.
  • To develop economic instruments for the assessment of land degradation and encourage the sustainable use of land resources.
  • To rationalize the wide range of terminology and definitions with different meanings among different disciplines associated with land degradation.
  • To standardize methods of assessment of the extent of land degradation.
  • To develop non-uniform criteria for assessing the severity of land degradation.
  • To overcome the difficulty in evaluating the on-farm economic impact of land degradation on productivity.

There is an urgent need to address these issues through a multi-disciplinary approach, but the most urgent need is to develop an objective, quantifiable, and precise concept based on scientific principles.

References

BARROW, C.J. 1991. Land Degradation: Development and Breakdown of Terrestrial Environments.Cambridge: Cambridge University Press.

BEINROTH, F.H., ESWARAN, H., REICH, P.F. and VAN DEN BERG, E. 1994. Land related stresses in agroecosystems. In: Stressed Ecosystems and Sustainable Agriculture, eds. S.M. Virmani, J.C. Katyal, H. Eswaran, and I.P. Abrol. New Delhi: Oxford and IBH.

BLAIKIE, P. and BROOKFIELD, H. 1987. Land Degradation and Society. London: Methuen.

CHARREAU, C. 1972. Problemes poses par l’utilization agricole des sols tropicaux par des cultures annuelles. Agronomie Tropicale, 27, 905–929.

CROSSON, P.R. 1997. The on-farm economic costs of erosion. In: Methods for Assessment of Land Degradation, eds. R. Lal, W.E.H. Blum, C. Valentin and B.A. Stewart. Boca Raton: CRC.

DARKOH, M.K. 1995. The deterioration of the environment in Africa’s drylands and river basins. Desertification Control Bulletin, 24, 35–41.

DREGNE, H.E. 1990. Erosion and soil productivity in Africa. Journal of Soil and Water Conservation, 45, 431–436.

DREGNE, H.E., ed. 1992. Degradation and Restoration of Arid Lands. Lubbock: Texas Technical University.

DREGNE, H.E. and CHOU, N.T. 1994. Global desertification dimensions and costs. In: Degradation and Restoration of Arid Lands, ed. H.E. Dregne. Lubbock: Texas Technical University.

DUMANSKI, J., ESWARAN, H. and LATHAM, M. 1992. A proposal for an international framework for evaluating sustainable land management. In: Evaluation for Sustainable Land Management in the Developing World, eds. J. Dumanski, E. Pushparajah, M. Latham, and R. Myers. IBSRAM Proceedings No. 12, Vol. 2, 25–45. Bangkok: IBSRAM.

ERIKSSON, J., HAKANSSON, I. and DANFORS, B. 1974. The Effect of Soil Compaction on Soil Structure and Crop Yields.Bulletin 354. Uppsala: Swedish Institute of Agricultural Engineering.

ESWARAN, H. 1993. Soil resilience and sustainable land management in the context of Agenda 21. In: Soil Resilience and Sustainable Land Use, eds. D.J Greenland and I. Szabolcs, 21–32. Wallingford: CABI.

ESWARAN, H. and DUMANSKI, J. 1994. Land degradation and sustainable agriculture: A global perspective. In: Proceedings of the 8th International Soil Conservation Organization (ISCO) Conference, eds. L.S. Bhushan, I.P. Abrol, and M.S. Rama Mohan Rao. Vol. 1, 208–226. Dehra Dunn: Indian Association of Soil and Water Conservationists.

ESWARAN, H., REICH, P.F. and BEINROTH, F.H. 1997a. Global distribution of soils with acidity. In: Plant-Soil Interactions at Low pH: Sustainable Agriculture and Forestry Production. eds. A.Z. Moniz, A.M.C. Furlani, R.E. Schaffert, N.K. Fageria, C.A. Rosolem and H. Cantarella, 159–164. (Proceedings of the 4th International Symposium on Plant-Soil Interactions at Low pH, Belo Horizonte, Minas Gerais, Brazil.)

ESWARAN, H., ALMARAZ, R., VAN DEN BERG, E. and REICH, P.F. 1997b. An assessment of the soil resources of Africa in relation to productivity. Geoderma, 77, 1–18.

ESWARAN, H, ALMARAZ, R., REICH, P.F. and ZDRULI, P.F. 1997c. Soil quality and soil productivity in Africa. Journal of Sustainable Agriculture,10, 75–94.

ESWARAN, H. and REICH, P.F. 1998. Desertification: a global assessment and risks to sustainability. Proceedings of the 16th International Congress of Soil Science, Montpellier, France.

ESWARAN, H., ARNOLD, R.W., BEINROTH, F.H. and REICH, P.F. 2000. A global assessment of land quality. In preparation.

FAHNESTOCK, P., LAL, R. and HALL, G.F. 1995. Land use and erosional effects on two Ohio Alfisols. Crop yields. Journal of Sustainable Agriculture, 7, 85–100.

GILL, W.R. 1971. Economic Assessment of Soil Compaction. St. Joseph: ASAE Monograph.

GIRT, J. 1986. The on-farm economics of sustainability and public intervention. Canadian Farm Economics, 20, 3–8.

GREENLAND, D.J., BOWEN, G., ESWARAN, H., RHOADES, R. and VALENTIN, C. 1994. Soil, Water, and Nutrient Management Research: A New Agenda. IBSRAM Position Paper. Bangkok: IBSRAM.

HARTEMINK, A.E. 1995. Land fertility decline under sisal cultivation in Tanzania. ISRIC Technical Paper 28. Wageningen: ISRIC.

JOHNSON, D.L. and LEWIS, L.A. 1995. Land Degradation: Creation and Destruction. Oxford: Blackwell.

KAYOMBO, B. and LAL, R. 1994. Response of tropical crops to soil compaction. In: Soil Compaction in Crop Production,eds. B.D. Sloane and C. Van Ouwerkkerk, 287–315. Amsterdam: Elsevier.

LAL, R. 1987. Response of maize and cassava to removal of surface soil from an Alfisol in Nigeria. International Journal of Tropical Agriculture, 5, 77–92.

LAL, R. 1994. Tillage effects on soil degradation, soil resilience, soil quality, and sustainability. Soil Tillage Research,27, 1–8.

LAL, R. 1995. Erosion–crop productivity relationships for soils of Africa. Soil Science Society of America Journal, 59, 661–667.

LAL, R. 1996. Axle load and tillage effects on soil degradation and rehabilitation in Western Nigeria. I. Soil physical and hydrological properties. Land Degradation Review,7, 19–45.

LAL, R. 1998. Soil erosion impact on agronomic productivity and environment quality. Critical Reviews in Plant Sciences, 17, 319–464.

LAL, R. and STEWART, B.A., eds. 1994. Land Degradation. Advances in Soil Science. Vol. 11, New York: Springer.

LAL, R., BLUM, W.E.H., VALENTIN, C. and STEWART, B.A., eds. 1997. Methods for Assessment of Land Degradation. Boca Raton: CRC.

MABBUTT, J.A. 1992. Degradation of the Australian drylands: a historical approach. In: Degradation and Restoration of Arid Lands, ed. H.E. Dregne, 27–98. Lubbock: Texas Technical University.

MAINGNET, M. ed., 1994. Desertification: Natural Background and Human Mismanagement. Berlin: Springer.

MBAGWU, J.S., LAL, R. and SCOTT, T.W. 1984. Effects of desurfacing of Alfisols and Ultisols in southern Nigeria. I. Crop performance. Soil Science Society of America Journal, 48, 828–833.

MERMUT, A.R. and ESWARAN, H. 1997. Opportunities for soil science in a milieu of reduced funds. Canadian Journal of Soil Science, 77, 1–7.

MIDDLETON, N. and THOMAS. D.S.G. 1997. World Atlas of Desertification. 2d ed. 182 pp. Published for UNEP. London: Edward Arnold.

OLDEMAN, L.R. 1994. The global extent of land degradation. In: Land Resilience and Sustainable Land Use,eds. D.J. Greenland and I. Szabolcs, 99–118. Wallingford: CABI.

OLDEMAN, L.R., HAKKELING, R.T.A. and SOMBROEK, W.G. 1992. World Map of the Status of Human-induced Soil Degradation: An Explanatory Note. Wageningen: ISRIC.

PIMENTEL, D., HARVEY, C., RESOSUDARMO, P., SINCLAIR, K., KURZ, D., MCNAIR, M., CRIST, S., SHPRITZ, L., FITTON, L., SAFFOURI, R. and BLAIR, R. 1995. Environmental and economic costs of soil erosion. Science, 267, 1117–1123.

REICH, P.F., NUMBEM, S.T., ALMARAZ, R.A. and ESWARAN, H. 1999. Land resource stresses and desertification in Africa. (This volume.)

RUPPENTHAL, M. 1995. Soil Conservation in Andean Cropping Systems: Soil Erosion and Crop Productivity in Traditional and Forage-Legume Based Cassava Cropping Systems in the South Colombian Andes. Weikersheim: Verlag Josef Margraf.

SAJJAPONGSE, A. 1998. ASIALAND Management of Sloping Lands Network: An Overview of Phase 3.Network Document No. 23, Bangkok: IBSRAM.

SANCHEZ, P.A. 1994. Tropical soil fertility research: Towards the second paradigm. Proceedings of the 15th International Congress of Soil Science, Acapulco, Mexico, 1, 65–88.

SCHUMACHER, T.E., LINDSTROM, M.J., MOKMA, D.L. and NELSON, W.W. 1994. Corn yields: erosion relationships of representative loess and till soils in the North Central United States. Journal of Soil and Water Conservation, 49, 77–81.

STOCKING, M. 1986. The cost of soil erosion in Zimbabwe in terms of the loss of three major nutrients. Consultant’s Working Paper No. 3., Soil Conservation Programme. Rome: FAO Land and Water Division.

STOORVOGEL, J.J., SMALING, E.M.A. and JANSEN, B.J. 1993. Calculating soil nutrient balances in Africa at different scales. I. Supra-national scale. Fertilizer Research, 35, 227–235.

UNCED. 1992. United Nations Conference on Environment and Development. Rio de Janeiro: UN.

UNEP. 1992. World Atlas of Desertification. London: Edward Arnold.

UNEP. 1993. Good news in the fight against desertification. Desertification Control Bulletin, 22, 3.

UNEP. 1994. Land Degradation in South Asia: Its Severity, Causes and Effects upon the People. INDP/UNEP/FAO. World Soil Resources Report 78. Rome: FAO.

VIRMANI, S.M., KATYAL, J.C., ESWARAN, H. and ABROL, I., eds. 1994. Stressed Agroecosystems and Sustainable Agriculture. New Delhi: Oxford and IBH.

WOODS, L.E. 1983. Land Degradation in Australia. Department of Home Affairs and Environment. Canberra: Australian Government Printers.