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This is the second part of a much longer article published in the journal Science Progress, and which may be found here: http://stl.publisher.ingentaconnect.com/content/stl/sciprg/2014/00000097/00000002/art00001


3. A Preliminary Overview of Land Degradation and Soil Degradation.

Land degradation is a phenomenon that is becoming more severe in various regions of the world. Remote sensing measurements (http://www.isric.org/projects/land-degradation-assessment-drylands-glada) indicate that more than 20% of all cultivated areas, 30% of forests and 10% of grasslands are undergoing degradation3,4. Land degradation and desertification are thought to affect 2.6 billion people in more than one hundred nations, spanning in excess of one third of the land surface of the Earth5. The global scale of these issues was reinforced at the United Nations Convention to Combat Desertification, the Convention on Biodiversity, the Kyoto protocol on global climate change and the millennium development goal6, all of which are worsened by the activities of humans. Various inappropriate uses of land may cause soil, water and vegetative cover to become degraded, with the loss of both soil and the biological diversity of flora, with impacts on the structure and functions of ecosystems7. Once land has become degraded, it is more vulnerable to the effects of climate change, particularly rising temperatures and droughts of greater severity. The entire regional environment is encompassed by the term land degradation; however, individual aspects of soils, water resources (surface, ground), forests (woodlands), grasslands (rangelands), croplands (rainfed, irrigated) and biodiversity (animals, vegetative cover, soil) are implicit here8. Land degradation is a complex process, and involves a number of interactive amendments in the properties of the soil and vegetation – being physical, chemical and biological, in their nature.

Hence, the definition of land degradation varies from one region to another, according to the emphasis on particular topics, but the effect is most severe in drylands, and thus the 40% of the earth’s surface that contains them9. It has been estimated that around 73% of rangelands in dryland areas, 47% of marginal rain-fed croplands and a significant percentage of irrigated croplands10 have been degraded. 20% of the world’s pastures and rangelands are degraded through overgrazing, and it is estimated that, through erosion and both chemical and physical damage, some 65% of agricultural land in Africa is degraded, along with 31% of the continent’s pasture lands and 19% of its forests and woodlands10. Overgrazing has primarily been brought culpable for land degradation in Africa, i.e. human impacts, but more recent thinking is that climatic factors are those most important - particularly rainfall variability and long-term drought10. It is in Sub-Saharan Africa that land degradation is most extensive, where it impacts on some 20-50% of the land and therefore affects the daily lives of well above 200 million people7.

The definition of land degradation is the reduction in the capacity of the land to provide ecosystem goods and services and assure its functions over a period of time for the beneficiaries of these11. Land degradation is particularly significant in dryland regions, where large areas and populations are impacted upon. An expanding population, and the migration of large numbers of people into drylands during long wet periods tend to maroon significant numbers there in dry periods. Alternative uses of land, e.g. the introduction of irrigated and non-irrigated cash crops, and the use of water for industrial and urban purposes, at the expense of rural agricultural producers, tend to disrupt traditional production chains in dryland regions. Indeed, entire production systems may break down if these effects are not mitigated or compensated for. By removing protective cover, deep ploughing, heavy grazing and deforestation, the soil is left particularly vulnerable to wind erosion when droughts are severe and protracted.

Overgrazing prevents or delays the regrowth of vegetation, or favours only unpalatable shrubs, especially during long droughts or close to water points. Land degradation may cause a reduction in the productivity of land with associated socio-economic difficulties, e.g. lack of certainty over food security, migration of populations, and that ecosystems may be incompletely developed and damaged. It has been estimated that, worldwide, around $40 billion is lost annually to land degradation - if the embodied costs of increased fertilizer use, and the loss of biodiversity and of unique landscapes were accounted for, this figure would undoubtedly be far greater. To reclaim degraded land is very costly, and if severely degraded, it may no longer provide an essential range of ecosystem functions and services, resulting in a loss of the goods and manifold additional potential environmental, social, economic and non-material bestowals that are necessary for the maintenance and development of society12. There is, however, a confusing use of terminology, caused in part by the jargon of different disciplines, but which obfuscates the compilation of an overview of this broad subject, which aims to compare “like with like”, not “chalk with cheese”.

“Land” may be understood11 to refer to an ecosystem, and to include land, landscape, terrain, vegetation, water, and climate, while “soil” is a specific entity and a component of land. Degradation or desertification of land refers to an irreversible decline in its “biological potential”: a term which, in itself, resists definition due to its dependence upon a multitude of interacting factors. Land degradation has no single and simply measured marker, but rather the term points to the fact that one of the land resources (soil, water, vegetation, rocks, air, climate, relief) has altered disadvantageously. As an example, a landslide may be seen as a visible process of land degradation, but the land may eventually recover its productivity. Indeed, the “scars” from old landslides may prove more productive than the neighbouring land. According to the UN/FAO definition13, the term “land degradation” generally signifies the temporary or permanent decline in the productive capacity of the land. Another definition of land degradation is: "The aggregate diminution of the productive potential of the land, including its major uses (rain-fed, arable, irrigated, rangeland, forest), its farming systems (e.g. smallholder subsistence) and its value as an economic resource." The broader connection between degradation (usually a result of land use practices) and the consequences of it in terms of land use is key to most published definitions of land degradation. The use of the term land degradation, in contrast to soil degradation, allows the bigger picture to be seen: to incorporate natural resources, such as climate, water, landforms and vegetation. This definition comprises the productivity of grassland and forest resources, and also the productivity of cropland. Since under different circumstances, land degradation may be reversible or irreversible, some definitions draw a distinction between the two cases.

Given sufficient time, all degradation is reversible, and hence the definition is a matter both of the particular focus and the timescale over which the effect is being considered. Soil erosion is a principal cause of land degradation, but it must be acknowledged that there are additional or simultaneous influences - e.g. lowering of the water table and deforestation - that may impair the productive capacity of cropland, rangeland and forests. Hence, the "productive capacity of land" cannot be determined from any one measure, and instead land degradation is estimated11 using “indicators”, which are potential signals that land degradation has taken place, rather than observations of degradation per se. For example, an “indicator” that land degradation is occurring further upslope may be provided by the accumulation of sediment further down. An indication of a reduction in the quality of soil might be falling crop yields, which may be a result of soil degradation and land degradation. Since the soil mediates collectively (holistically) many essential processes involving vegetation growth, overland flow of water, infiltration, land use and land management, its quality is a prime indicator of land degradation – therefore, when soil is degraded the land is too. Evidence from the soil (mainly soil degradation) and from plants growing on the soil (soil productivity) are prime indicators of land degradation.

In addition, the definitions of “dryland” are variable, which serves further to confound the situation. In regard to the severity of land degradation, two basic schools of thought have emerged. Economists take the view that if the situation were really as serious as others claim, market forces would have resolved it by now, and in support of this, it is argued that land managers (e.g. farmers) would not let their land degrade to a degree that a loss of their profits would ensue. However, this does not take account of an imminent failing supply of plentiful cheap oil14 and potentially one of phosphorus-based fertilizer15 too: commodities without which the current agricultural situation could not exist. In many instances, it is only through such inputs that accepted yields can be maintained, and if a farmer decides to turn his industrial farm into an organic farm, he must initially suffer reduced crop yields. Those “others”, tending to come from backgrounds of ecology, soil science and agronomy, believe that there is a serious threat to feeding the growing global population posed by land degradation, in terms of reduced biomass yields and by a compromised environment.

As a consequence of the variation in definitions and terminologies employed by different workers, the statistics pertaining to both the extent of land degradation and its rate of advance, vary considerably. In addition, statements are often made, particularly in the media, such as, “Globally, we are losing 10 million hectares of fertile soil each year. That is 30 soccer fields per minute...”, or “75 billion tonnes of soil, the equivalent of nearly 10 million hectares of arable land, is lost to erosion, waterlogging and salination; another 20 million hectares is abandoned because its soil quality has been degraded.” It is hard to know what precisely is being described here. A direct translation of a mass of soil to a land area implies that there is a given, average depth of soil physically removed, waterlogged or salinized, but as we shall see, the actual and global situation is less straightforward. If “soil” is being used as a synonym for “arable land”, the loss of 75 billion tonnes would accord with an average soil depth of, 75 x 109 t/10 x 106 ha = 7,500 t/ha; assuming an average soil density of 1.4 g/cm3, this accords with a volume of 5,357 m3. This is distributed over one ha = 10,000 m2 = 1 x 108 cm2, and so we have 5,357 m3 x 1 x 106 (cm3/m3)/1 x 108 cm2/ha = 53.6 cm (i.e. half a metre), which does not seem realistic.

It is probably neither accurate nor appropriate to compare the mass of soil that is thought lost to erosion (blown or washed away) with an area directly of land that has become unproductive. It appears most likely that the “lost” 10 million hectares should not be compared directly with the mass of soil that is allegedly “lost”: the latter being a global phenomenon. Thus, the global area of arable land amounts to 1,387 million ha, and if 75 billion tonnes of soil were being lost over this expanse, it would equate with a soil depth of: (75 x 109 t/1.4 g/cm3) x 1 x 106 (cm3/m3)/1,387 x 106 ha x 1 x 108 cm2/ha = 0.39 cm (4 mm), which might appear more reasonable, while another estimate, that the loss over the world’s arable land is 24 billion tonnes would accord with an average loss of a little over 1 mm, which might be imperceptible. However, as we discuss later, the rate of “loss” of soil occurs quite variously according to climate and location (and also timescale), across the globe, and not all the soil that is moved over an area, is actually removed from it.

4. Degradation of soil.

4.1 Mechanisms of Soil Degradation

Soil degradation is a principal cause of land degradation, and soils may be degraded via several identifiable and different mechanisms, as we now outline under the following headings13.

(1) Water erosion.

This is the removal of soil particles by the physical action of water (Figure 5). This is normally manifested as sheet erosion (a more or less uniform removal of a thin layer of topsoil), rill erosion (small channels in the field) or gully erosion (large channels, similar to incised rivers). One highly significant aspect of water erosion is that it is the finer and more fertile fraction of the soil that is removed selectively.

[Fig 5]

(2) Wind erosion.

This is where soil particles are physically removed by the action of wind, and it is fine to medium sized sand particles that are most affected. This normally is sheet erosion, involving the removal of thin layers of soil, but hollows and other features can also be sculpted.

(3) Loss of soil fertility.

This may be defined as the degradation of the physical, biological and chemical properties of soil, which leads to a loss in its productivity. Additional factors include: (a) the loss of SOM, which reduces the biological activity of soil; (b) a further consequence of declining SOM is the deterioration in the physical properties of soil, e.g. its structure, degree of aeration and ability to hold water and to drain effectively; (c) soil nutrient levels that are vital for the healthy growth of plants may become deficient, or rise to toxic amounts; (d) substances that are toxic substances may accumulate in the soil – e.g. from pollution, or the over-application of fertilisers.

(4) Waterlogging.

When the level of groundwater close to the soil surface rises, or surface water is not adequately drained, the soil may become waterlogged. In this case, the root zone becomes saturated by water, typically resulting in an oxygen deficiency.

(5) Accumulation of salts.

When there is an increase in the concentration of salt in the soil water solution, the effect is termed salinization. In contrast, when the number of sodium cations (Na+) on the soil particles increases, the effect is called sodication, and the resulting soil is termed to be “sodic”. It is poor irrigation management which, more often than not, is to blame for salinization while sodication tends to be a naturally occurring feature, especially in regions with a fluctuating water table.

(6) “Soil burial”.

This is also known as sedimentation, and may occur when fertile soil is buried under less fertile sediments (by flooding); or it can be a consequence of winds which blow sand over fertile grazing

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