Soil Degradation Processes, Causes, and Assessment Approaches
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- Soil degradation
is an element of the land degradation process and refers to a decrease in soil’s productivity and quality.
- Soil erosion
is a soil degradation process defined as displacement of topsoil from land surface through water, wind, or tillage.
is a soil degradation process that refers to the degradation of land through salt accumulation. It is a natural process or human induced through irrigation and land clearing, in which case it is called secondary salinity.
- Soil contamination
is the chemical degradation caused by presence of harmful substances resulting from activity such as waste disposal, mining, oil extraction, and military or nuclear activities.
- Soil sealing
is a permanent covering of land and its soil with impermeable artificial material, such as asphalt and concrete.
- Soil organic matter
is a complex mixture of organic material (plants, plant tissue, microorganisms or animals) at different stages of decomposition due to both abiotic and biotic processes.
Land degradation refers to a loss of or reduction in the productivity of the land, which arises as a result of various natural processes, often accelerated by an anthropogenic perturbation (Lal 1993). The most significant causes of land degradation are land use, climate change, overpopulation, and urbanization.
Land degradation leads to a reduction in soil quality and a decrease in a future potential for the survival of living organisms (Fitzpatrick 2002). It is a global threat with three distinct categories: natural degradation, human-induced degradation, and desertification. Induced degradation results from inappropriate land use and management and occurs more rapidly than natural degradation (Fitzpatrick 2002). The most severe degradation form is desertification, which occurs in drylands covering about 40% of the world’s land surface (UNEP 1992). Land degradation has both on-site and off-site effects. On-site effects are experienced directly where degradation occurs (a reduction in the productive capacity of land), whereas off-site effects occur in the surrounding areas (flooding, sedimentation, a water quality decline) (FAO 2011; Stringer 2017). Land degradation has a wider scope than soil degradation, which refers to a decline in soil quality and productivity. The speed of the soil degradation process depends on natural factors (characteristics of soil, climate, and vegetation) and anthropogenic factors (land use, soil management, and farming/cropping system) (Lal 2001).
More than 75% of land in the world is degraded (Gibbs and Salmon 2015). The five global assessments of soil degradation carried out between 1977 and 2003 estimated that global degradation ranges between 15% and 63%, whereas for dryland degradation, the range is from 4% to 74% (Safriel 2007).
The global extent of human-induced soil degradation. (Modified from Oldeman et al. 1991)
Land surface (106 ha)
Land surface affected by different types of soil degradation (106 ha)
Chemical soil degradation
Physical soil degradation
According to the world map of human-induced soil degradation, severely degraded countries are mostly located in Africa (Swaziland, Angola, Gabon, Congo, Equatorial Guinea, and Zambia) and Asia (Bhutan, Thailand, Indonesia, Republic of Korea, Laos, and Malaysia) (Bai et al. 2008).
In Africa, land degradation affects 2/3 of the territory, from humid zones to arid and semiarid zones, and about 485 million people (ECA 2007). Oldeman et al. (1991) estimated that 494 million ha of the land in Africa is affected by human-induced soil degradation.
Asian countries face many of the problems of soil erosion, soil salinization, an increasing population and pasture and vegetative degradation. Asia is the continent that is threatened most by human-induced soil erosion, with an affected area of 748 million ha (Oldeman et al. 1991). Together, Africa and Asia account for more than 55% of all global drylands (Reynolds et al. 2007). The Asian countries which are the most vulnerable to desertification are Afghanistan and Pakistan (Eswaran et al. 2001).
Average soil loss rate [t ha−1 y−1]
% of the total soil loss in the EU
aLoss of agricultural land due to urbanization [ha]
The Food and Agriculture Organization and the Intergovernmental Technical Panel on Soils indicate that 14% of global land degradation occurs in Latin America and the Caribbean region, with water erosion, organic carbon losses, and salinization the main drivers of land degradation (FAO and ITPS 2015).
The most widespread processes that lead to the degradation of land and soil resources are physical, chemical, biological, and natural processes (Johnson and Lewis 2007). Physical processes include a decline in the soil structure, leading to crusting, compaction, erosion, desertification, and environmental pollution, whereas chemical processes include a loss of nutrients and/or organic matter, acidification, salinization, and pollution (Oldeman et al. 1991; Eswaran et al. 2001). Important among biological processes are a decline in land biodiversity and a reduction in both the total and biomass carbon levels (Eswaran et al. 2001).
This chapter includes sections related to the causes and types of soil degradation and different assessment methods and provides future guidelines for soil degradation prevention.
Causes of Soil Degradation
There are many factors contributing to soil degradation, among which the most recognized are deforestation, shifting cultivation, overgrazing, monocropping, and the use of agrochemicals.
Deforestation. The main natural causes are fires and floods and among human activities the causes are logging, timber production, the conversion of a forest into agricultural land, and urbanization. The effects of deforestation are numerous: a loss of species, increased carbon emission or increases in the greenhouse effect, flooding, and soil erosion.
Shifting cultivation. It is an old farming practice, where the “slash-and-burn” technique is applied to clear land, followed by a long fallow period important for the restoration of soil fertility. Many studies indicate that crop burning has harmful effects on soil, such as an increased susceptibility to soil erosion and a reduction in nutrients. A better alternative is “chop-and-mulch,” i.e., the cutting of the plants (crops) that are then used as mulch. This significantly increases the concentration of nutrients and the content of organic matter (FAO and UN 2015).
Overgrazing. It is intensive grazing that leads to a significant disturbance of the growth, quality, and composition of vegetation. Grasslands with the high pressure of livestock lose vegetation cover and, therefore, fertility of the soil, which becomes susceptible to erosion. Numerous studies have shown that during overgrazing, there is a change in soil moisture, organic matter, nitrogen content, and microbial activity. The total carbon in the soil is permanently reduced by 12% due to overgrazing over a period of 40 years (Li et al. 1997). This is also the cause of soil erosion and desertification.
Monocropping and use of agrochemicals. Many crops widespread throughout the world (wheat, corn, rice) have been cultivated as a monoculture for many years on the same soil, in the absence of crop rotation. Over a long period of time under the same culture, a soil loses nutrients and its resistance to insects and pests is reduced, so farmers are forced to use pesticides in order to provide the required yield. The use of artificial fertilizers, pesticides, and other chemicals introduces heavy metals and often very toxic chemicals into the land. Their nonselective and excessive use has a permanent negative effect on the quality of soil and represents one of the most significant forms of degradation (Osman 2014).
In addition, other causes of soil degradation include the mismanagement of irrigation, the use of heavy agricultural machinery, mining, war, or indiscriminate waste disposal.
Types of Soil Degradation
Soil degradation is classified in many ways in the literature. The Global Assessment of Soil Degradation (GLASOD) recognizes four major types of soil degradation: water erosion, wind erosion, chemical deterioration (including organic matter decline, salinization, acidification, and pollution), and physical deterioration (such as compaction, sealing, waterlogging, and urbanization).
The global loss of soil due to water erosion amounts to 20–30 billion tons per year, which is equivalent to a loss of land use of 1 m on an area of 13,300 to 20,000 km2 per year (FAO and UN 2015). Soil losses caused by wind erosion are estimated at 2 billion tons annually, whereas soil losses caused by tillage are estimated at 5 billion tons per year. To date, established tolerant land losses have only beset as short-term goals. A long-term goal should be that the degree of degradation of agricultural land leads to a zero level (UN, Sustainable Development Goals – 15.3 Land Degradation Neutrality).
Numerous studies, including FAO, have estimated the costs of soil erosion. According to Fitzpatrick (2002), direct and indirect annual costs due to erosion can be up to 400 billion dollars worldwide. This represents a cost of approximately $80 a year for every person on Earth due to soil erosion.
Organic matter decline occurs due to the inadequate use and treatment of soils. Organic matter is important not only for soil fertility but also for the structure, aeration, infiltration, water retention capacity, and soil biodiversity (Montanarella 2007). It serves as a soil acidity puffer and a source of energy for soil microorganisms as well. Organic carbon is the most important component (about 58%) of organic matter and soil quality indicators (Young et al. 2015). Significant carbon losses arise from activities that lead to the transition from natural to agricultural ecosystems (the destruction of forests, the burning of biomass, etc.) (Lal 1993). The global total of organic and inorganic carbon in soil is estimated at 1,500 Gt. According to research conducted at the Joint Research Centre (Italy), 45% of the land in Europe has a low or very low content of organic carbon. Agricultural production has further aggravated land degradation, which has contributed to an estimated loss of between 42 and 78 billion tons of carbon, mainly emitted into the atmosphere as carbon dioxide and other gasses, with a negative effect on climate change and food production during the past century (Lal 1993). Small changes in organic carbon in soil have major consequences for the concentration of carbon dioxide in the atmosphere, because its volume in soil is three times higher than in the atmosphere. The practice of improving organic matter involves growing cover crops and can have huge benefits for soil (soil erosion reduction and the prevention of leaching nutrients). In addition, the balanced fertilization increases crop yields, and there are increased amounts of organic residues returned to soil (FAO 2005).
Salinization is one of the most widespread forms of soil degradation and occurs in arid and semiarid areas, where the amount of precipitation is small and irrigation is applied without a proper drainage system. However, salinization can occur in all climatic areas if irrigation is irregularly applied and is a result of natural (primary) and human-induced (secondary) processes (FAO and UN 2015). The degradation caused by salinization is believed to have affected a total area of about 62 million hectares, with estimated global losses of around 27.3 billion USD per year, over the last two decades (Qadir et al. 2014). According to the EEA (1995), salinization occurs and negatively affects 3.8 million hectares in Europe, with the most endangered parts in Italy, Spain and Hungary, among others.
According to Montanarella (2007), the most significant consequences of salinization are a loss of soil fertility, a reduction in water infiltration, a loss in biodiversity, damage to the infrastructure, and the weakening of soil, to name but a few. The negative effects of salinization can be significantly mitigated by improved water management (irrigation and drainage), a better use of fertilizers and the application of adaptive cultures. In some areas it is necessary to consider the possibility of land use change and the conversion of soil into cultivated land (Young et al. 2015).
Soil contamination results from industry, mining, illegal landfills and poorly managed landfills, the storage of chemicals, accidental or intentional chemical spills, the disposal of hazardous materials, and military activities. There is no relevant data about the assessment of soil contamination parameters (the total pollution area, the type of pollutant, the number of inhabitants exposed to contamination, environmental damage, etc.) for countries in the world because there is no common methodology for their assessment (Montanarella 2007). According to Montanarella (2007), the most significant consequences of contamination are a risk to health for the people living in the environment of the polluted area, the contamination of surface waters, the contamination of groundwaters, a loss of biodiversity and biological activity, and a loss of soil fertility due to disturbance of the nutrient cycle. The European Environment Agency (EEA) estimates that 60% of Europe’s land is polluted by industrial activities. Among the most common harmful pollutants are heavy metals (37%) and mineral oils (33%). The number of contaminated sites equates to one-third of the countries of the European Union, with the highest number of monitored contaminated sites (422) in Serbia (GIZ 2017). The polluted soil remediation options are classified into biological, chemical, or physical and may be applied either in situ (using barriers on-site in order to prevent the movement of pollutants) or ex situ (treating the excavated soil off-site) (Scullion 2006).
Other Types of Soil Degradation
Acidification, like salinization, is a severe form of degradation that results in a reduction in the agricultural land production potential. The acidification process prevents plants using water, resulting in drying and erosion, and causing an increased leaching of nutrients and an irreversible breakdown of silicate minerals in the soil (Fitzpatrick 2002). Soil acidity is a natural process that can be accelerated by human activity and is particularly pronounced in areas with less precipitation. Acid soil management involves monitoring the soil pH, understanding the tolerance of crops and pastures to acidity and treating surfaces with neutralizers in order to prevent acidity.
Soil compaction is caused by long-term pressure on the surface, brought about by the activity of heavy mechanization in processing agricultural land (Kertész 2009), or due to a high grazing intensity and the resulting grazing pressure of livestock. The effects of soil compaction are a loss of soil fertility due to structural change, a reduction in the infiltration capacity of the land, an increased sensitivity to erosion, and a loss of the biodiversity of the land. Some measures that may prevent or reduce soil compaction are the application of a conservation tillage system, the control of and a reduction in vehicle traffic, and the avoidance of using oversized equipment (Raper and Kirby 2006).
Soil sealing involves covering the soil surface with an impermeable material. The main reasons for soil sealing are urbanization, an increase in the traffic infrastructure and a migration of the population (Montanarella 2007). It occurs to the detriment of agricultural land (Table 2). The highest sealing rate of 16–20% is recorded in European countries (Belgium, Denmark, and the Netherlands) due to population growth and industrial development (Kertész 2009). The consequences of soil sealing are an increase in the risk of floods, the disruption of water and gas flows, a reduction in groundwater, water pollution, and the loss of both land and land biodiversity. One of the most common soil sealing mitigation measures is using highly permeable materials and surfaces, a green infrastructure and water harvesting (European Commission 2012).
Many authors consider desertification an equivalent to degradation in arid and subarid areas, while others identify desertification as a particular type of soil degradation, given the fact that it may appear as a higher form of degradation in moderately humid or humid tropical areas (Eswaran et al. 2001). Both terms mean a permanent loss of land productivity. About 33% of the global area is estimated to be susceptible to desertification. Desertification is present in Africa, several Asian countries, and South America, but it has also become a problem in the United States, Australia, and Southern Europe; in fact desertification is a problem in about 100 countries across all five continents, affecting over 2 billion people. Due to desertification, about 12 million hectares of land for processing are lost each year. The major global problem caused by desertification is a loss of the biological and economic productivity of land (EU 2011). The main cause of desertification is overgrazing. The consequences of desertification are a reduction in yield or crop failure, floods, a reduction in water quality, hunger, and poverty, to name but a few. Based on the United Nations Convention to Combat Desertification (UNCCD), which came into force in December 1996, many recommendations have been made to reduce the desertification process (Safriel 2007). Some of the measures to be applied in order to combat desertification are as follows: introducing policies for changing land use patterns and the methods for cultivating agricultural crops, educating the population, and introducing new soil technologies including improved water management and the application of good practices.
Soil Degradation Assessment
The need for assessing soil degradation by using different methods based on expert opinion, land users’ opinions, modelling, field observation, monitoring and measurement, remote sensing, and GIS has been recognized since the early 1930s (Kapalanga 2008).
The expert-based approach is used because of a lack of reliable data regarding soil and land degradation, some advantages being the best local knowledge is included in the assessment; land degradation causes, types, degrees, and extents can be assessed on multiple scales; and it contributes to raising awareness, thus being a supportive collaboration and a form of information sharing. The limitations of this approach are its subjective nature and the fact that the gathered data may not always be up-to-date.
The remote sensing approach, based on the use of satellite imaginary, attracts huge interest. Compared to the expert-based approach, this approach enables the rapid acquisition of up-to-date information across a large area in a homogeneous manner. A lot of approaches to soil degradation assessment are based on remote sensing (GLASOD, ASSOD, SOUVER), but the reason remote sensing is used for soil degradation assessment on a small scale is due to the unavailability of extensive ground data necessary for reliable estimates (Kniivila 2004).
The modelling approach is widely applied in order to assess different types of degradation. The dominant type of degradation in Europe is water erosion, which is assessed by using different models: models based on the Universal Soil Loss Equation (USLE), the Pan European Soil Erosion Risk Assessment (PESERA), statistical regression-based approaches, and factor scoring methods based on expert knowledge (Mantel et al. 2014). To predict the risk of degradation, these models use the geographical information system (GIS).
Global Assessment of Human-Induced Soil Degradation – GLASOD (1987–1990)
1st edition of World Atlas of Desertification – WAD1 (1992)
Assessment of Soil Degradation in South and Southeast Asia – ASSOD (1995–1997)
The World Overview of Conservation Approaches and Technologies – WOCAT database (1992)
Mapping of Soil and Terrain Vulnerability in Central and Eastern Europe – SOVEUR (1997) 2nd
2nd edition of World Atlas of Desertification – WAD2 (1997)
The Millennium Ecosystem Assessment – MEA (2001–2005)
Land degradation assessment in Drylands project – LADA (2006)
Global Assessment of Land Degradation and Improvement – GLADA (2006–2009)
Global Land Degradation Information System – GLADIS (2009–2011)
Comparison of soil degradation assessment methodologies. (Source: Lynden et al. 2004)
South and Southeast Asia (17 countries)
Central and Eastern Europe (13 counties)
Units loosely defined (physiography, land use, etc.)
Physiography, according to the standard SOTER methodology
Physiography and soils, according to the standard SOTER methodology
Degree of degradation and extent classes
Impact on productivity and extent percentages
Degree(the intensity of the process), impact on productivity, and extent percentages
Rate of degradation
As with ASSOD
No conservation data
Some conservation data
No conservation data
Data not on a country basis
Data available per country
Data available per country
Maximum 2 degradation types per map unit
More degradation types defined, no restrictions on the number of types per map unit
As for ASSOD, but a special emphasis on pollution
The distribution of the main degradation types in South and Southeast Asia. (Modified by Lynden and Oldeman 1997)
% of the total degraded area
One of the newly assessment of global land and soil degradation is the third edition of the World Atlas of Desertification (WAD3), which is being compiled by the Joint Research Centre (JRC) of the European Commission, in partnership with the United Nations Environment Programme (UNEP) (Caspari et al. 2015). This method uses the Normalized Difference Vegetation Index (NDVI) to assess land degradation. A variety of studies conducted until now have used different indicators of land degradation, such as land cover data, NDVI index, net primary production (NPP), soil erosion state, and soil moisture index.
There are many actions and agreements designed to avoid or reduce land degradation, some of them being the United Nations Convention to Combat Desertification (UNCCD), the United Nations Framework Convention on Climate Change (UNFCCC), the Convention on Biological Diversity (CBD), the Convention on Wetlands of International Importance, and the 2030 Agenda for Sustainable Development with Sustainable Development Goals (SDGs) where SDG 15 makes an explicit reference to land degradation neutrality (LDN) (UN 2015). To monitor the realization of SDGs, the Global Indicator Framework (UN 2017) was adopted, which is updated every year. The Global Indicator Framework includes 231 indicators. Within SDG 15, 14 indicators have been identified, including an indicator called “the proportion of the land degraded over the total land area,” which refers to the restoration of degraded land and soil.
Land degradation is a natural or human-induced process affecting more than 75% of the world’s land. It refers to the decline of the entire ecosystem’s ability to provide goods and services and has three categories: natural degradation, human-induced degradation, and desertification.
Soil degradation is a form of land degradation that refers to loss of soil quality and productivity. It can occur as a natural process caused by the inherent characteristics of the soil, climate, and topography. Human-induced degradation develops more rapidly than natural degradation and can be reduced or avoided by regulating human interventions such as deforestations, overgrazing, and mismanagement of agricultural land.
The causes of soil degradation that are human driven include overgrazing, shifting cultivation and monocropping, and the use of agrochemicals, while deforestation can present either as a natural or human-driven loss of trees.
Soil erosion by water is the main and most widespread form of soil degradation globally, leading to soil loss, increased pollution, and sedimentation in rivers. The key soil erosion control measures include the maintenance of a protective cover (trees, mulches, and crops), the selection of optimal land use, and the construction of technical works (e.g., check dams) on the riverbed.
Besides soil erosion, soil sealing and soil contamination are the main problem for EU soils. Soil sealing comprises a permanent covering of land and its soil with impermeable artificial material (asphalt and concrete) and is a result of urbanization and infrastructure development.
Soil contamination is the chemical degradation of soil that involves the presence of harmful substances in the soil as a result of industrial activities, mining, nuclear activities, or improper disposal of waste. Remediation options include a variety of physical and chemical treatments of soil in place (in situ) and after excavation (ex situ).
Salinization is a widely present type of chemical soil degradation that occurs in arid and semiarid areas (mostly in Asia), causing soil infertility, a reduction in water infiltration, loss in biodiversity, and damage to the infrastructure. The expansion of salt-affected soil could be reduced by applying appropriate irrigation practices, better use of fertilizers, or land use change.
Soil organic matter (SOM) is considered an indicator of soil degradation. Soils containing more organic matter have better structure, increased water infiltration, and they are less susceptible to compaction, erosion, and desertification.
To prevent soil degradation processes that lead to the deterioration of soil chemical, physical, and biological properties, it is necessary to monitor and assess soil degradation processes using appropriate methods (expert-based, remote sensing, or modelling). Identification of the degradation type is important to define all consequences and, thus, the expected cost of soil degradation mitigation measures. It is important to be aware that several degradation processes could occur simultaneously, or one type of process could directly cause the occurrence of another (e.g., the occurrence of deforestation could lead to soil erosion and ultimately cause an increased risk of flood). The spatial extent, degree, and rate of degradation type should be a base for the decision-making process regarding the best sustainable land management (SLM) practice that should be applied.
Some challenges are related to the better monitoring and assessment of soil degradation. It is necessary to close data gaps, enable access to data and data comparability, gather more on-the-ground information, and take into account the uncertainty of the future.
The selection of an investment solution and a capacity building approach to support the implementation of EU SDG 15 is crucial. The involvement of different ministries, departments, and agencies with adequate communication and cooperation is required, as is citizen participation in the implementation of the SDGs. There is a need for taking actions at local and sub-national levels, and implementing policies and programs at national and regional levels that can prevent or reverse land degradation. In addition, the global population has to reduce pressure on the environment by reducing its demands and economic activities.
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