Wednesday, July 23, 2014

Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 3.


This is the third 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


5. Climate change
.

As noted later, the effects of climate change must be considered in the context of soil erosion and land degradation: in particular a more vigorous hydrological cycle is anticipated as a result of rising global temperatures, with much harder rainfall in some regions19. Hence, in the absence of mitigating measures, the future rate of soil erosion can be expected to increase on the global scale. There are a number of prevailing factors at play, the most significant being the erosive force of rainfall; however, we must also consider the following: (a) the changing moisture regime might alter patterns of biomass growth, and affect the canopy layer; (b) the latter may amend both the plant residue decomposition rates (since the underlying processes are driven by temperature and by moisture and are connected with the activity of soil microbes) in addition to the biomass production rates; (c) in consequence of varying rainfall and evapotranspiration rates - which changes infiltration and runoff ratios - the soil moisture content might be affected; (d) a fall in the SOM content may weaken the structure of some soils, rendering them more susceptible to erosion, and the amount of runoff could be increased as a result of surface sealing and crusting; (e) non-erosive winter snowfall might turn into erosive rainfall as the winter temperatures increase; (f) when permafrosts melt, a previously non-erodible soil state can be converted to a highly erodible form; and (g) land use changes, e.g. to grow more cereals, as the global climate and local weather patterns change, may lead to further erosion.

It has been estimated that a ca 1.7% change in soil erosion is likely for each 1% change in total precipitation resulting from climate change20. It is common practice to deal with a loss in soil fertility from erosion by applying greater quantities of artificial fertilizers, which actually compounds the problem [since the soil food web (Figure 4) dies back further2] and incurs yet more water and soil pollution, as opposed to simply giving the land sufficient time to regenerate naturally. The relationship between the mycorrhizal fungi in the soil and, for example, phosphate fertilizer can be thought of as pseudo-addictive, since the fungi act symbiotically with the roots of plants, delivering to them phosphorus (and other nutrients) drawn from the soil, in return for carbohydrate bestowed to the fungi from the plant (formed by photosynthesis). The over-application of artificial phosphorus discourages the growth of the fungi, with the result that the plants become increasingly dependent on artificial phosphorus inputs.

6. Monitoring, measuring, and modelling erosion.

To model erosion accurately is difficult, because of the complexity of the detailed processes of erosion, which involve aspects of climatology, hydrology, geology, chemistry, and physics, etc. Due to the non-linear nature of erosion models, they tend to be difficult to use numerically, and it is accordingly difficult or impossible to make predictions about large scale events on the basis of results taken from plots on much smaller areas. Erosion models are either process-based or empirically based. The former models are physically based and provide a mathematical description of the processes of detachment, transport, and deposition: by solving the equations that describe them, estimates are obtained of soil loss and sediment yields that occur from a given land surface area. The science of erosion is not sufficiently developed that some input of empirical data can be avoided. The fundamental difference between process-based and other types of erosion models is that the sediment continuity equation is used, as is discussed later. Empirical models relate management and environmental factors directly to soil loss and/or sedimentary yields on the basis of statistical methods21. A detailed review22 of process-based and empirical erosion models has been published, including a discussion of conceptual models, which are a kind of intermediary stage between the process-based and entirely empirical models. The most usual model that is employed to assess the degree of erosion and to permit an outlook toward conservation strategies is the Universal Soil Loss Equation (USLE), which remains under improvement and development.

6.1 The USLE.

The USLE was developed using a comprehensive range of data taken from erosion plot and rainfall simulator experiments: in all, over 10,000 plot-years worth of actual data, taken from 50 different locations in 24 U.S. states was used to calibrate the input parameters23. The USLE contains six factors, according to which the long-term average annual soil loss (A) is estimated, which are: the rainfall erosivity factor (R), the soil erodibility factor (K), the topographic factors (L and S) representing length and slope, and the cropping management factors (C and P). The equation has the simple linear structure:

A = RKLSCP

The unit plot concept is important in the context of the USLE, and is defined as the standard plot condition to determine the soil's erodibility, i.e. when the LS factor = 1 (slope = 9% and length = 72.6 feet) where the plot is fallow and tillage is up and down slope and no conservation practices are applied (CP = 1). Under these conditions:

K = A/R

Wischmeier et al.24 have devised a more straightforward means for the estimation of K, the soil erodibility factor, which involves the particle size of the soil, its SOM content, soil structure and profile permeability. If sufficient information is available, K can be approximated from a nomograph. By knowing the length and gradient of the slope, the LS factors can be determined from a slope effect chart. The cropping management factor (C) and conservation practices factor (P) are determined empirically from plot data, and are described in soil loss ratios [i.e. (C or P with) /(C or P without)]. Erosion is measured and analysed using e.g. the micro-erosion meter (MEM) and the traversing micro-erosion meter (TMEM). The MEM has been used successfully to measure bedrock erosion in a range of ecosystems across the globe, and is able to determine both terrestrial and oceanic erosion. There is also the Revised Universal Soil Loss Equation (RUSLE), which is an extension of the USLE, and other related variants. A highly informative and practical description of the use of the USLE can be found at: http://www.asu.edu/clas/shesc/projects/medland/documents/soilerosion.pdf

7. Validity of universal soil loss estimates.

Pimental et al. have asserted25 that globally, soil erosion rates are lowest in the U.S. and Europe, “averaging about 17 metric tons ha-1 year-1”. Boardman26 has investigated the origin of this figure and concluded that it is actually derived from an uncritical extrapolation of data taken from just 12 experimental plots in three small areas of central Belgium, reported by Bollinne in his Ph.D thesis at the University of Liege. [A range of soil loss of 10─25 t ha-1 year-1 (which “average” to ca 17 t ha-1 year-1) has been claimed from Bollinne’s work25]. Hence it is of some interest to know how much erosion is actually occurring across Europe, according to a more extensive compilation of measurements, taken over a larger and more representative area of the continent. Indeed, the available data indicate that the process is quite variable in its extent26, both in space and time, and ranges from (assuming a soil density of 1.4 g cm-3, since some data are quoted in units of m3 ha-1 year-1) < 1 t ha-1 year-1, over a 90 km2 area in southern Sweden, to ca 16 t ha-1 year-1 in northern France, but taken from just 33 small catchments. Arden-Clark and Evans quote27 that erosion rates in the U.K. are 1─20 t ha-1 year-1, while noting that those in the higher part of the range are rare and localised events, and typical rates of erosion are 1─2 t ha-1 year-1. Measurements made over a 709 km2 total area of localities in England and Wales produce a mean28 of 3.2 t ha-1 year-1.

Ryszowski29 reckoned the soil erosion rate in Poland to be 0.52 t ha-1 year-1. Hence, overall, it might appear that the average value of 17 t ha-1 year-1 quoted25 by Pimental et al. represents a considerable overestimate of the rate of soil erosion in Europe. Crosson too has taken issue with the figure30, this time as applied to soil erosion in the U.S. According to the USDA in 1989, an average of 17 t ha-1 year-1 is moved from U.S. croplands as a result of the combined effect of wind and water erosion; however, Crosson cites more recent data showing that by 1992, the rate had fallen to 13 t ha-1 year-1, and refers to a paper by Lal and Stewart31 in which it is stated that, annually, some 36 billion tons (metric tons = tonnes is meant) of soil are eroded worldwide: 10 billion tons from natural phenomena and 26 billion tons as a result of human activities. Crosson notes that Lal and Stewart cite a paper by Brown32 as a source for the 26 billion ton estimate, but this is based on erosion measurements for the United States, which it would appear are higher than those for Europe. At any rate the 36 billion ton figure is considerably less than the 75 billion metric ton value assessed elsewhere to be removed by wind and water erosion, and mostly from agricultural land.

In their response to Crosson, Pimental et al. assert33 that since the United States has about 11% of the world’s arable land (and approximately the same amount of pasture land), and an estimated total soil loss of 4.5 billion tons per year, assuming that the rest of the world suffers similar rates of soil loss, a total global soil loss of 40 billion tons per year is indicated. They further stress that the rates of soil erosion in Asia, Africa and South America are about double those in the U.S., and hence the 75 billion ton annual global loss of soil appears reasonable. They defend too, their contention that some 80% of the world’s agricultural land has been degraded, though Crosson criticised30 this figure as a more than three-fold overestimate, based on the GLASOD34 [Global Assessment of Soil Degradation] study by Oldman et al. which reports that about 1.03 billion ha of agricultural land has suffered moderate-to-strong erosion as a result of wind and water, or less than 25% of the roughly 4.5 billion ha global land-base, under crops, pasture and range. In a later paper, Trimble and Crosson point out the considerable disparity in quoted values for the annual soil losses through erosion from croplands (442 million acres = 179 million ha) across the United States, which vary from 2 billion to 6.8 billion tons, which suggests an average lost soil depth of ca 1─3 mm35. Some are of the opinion that the recent rates of soil erosion are as high as those in the 1930s, though others disagree35.

On the basis of on-farm productivity effects, it was concluded that should the rates of erosion that prevailed in 1982 continue for the next 100 years, the crop yields (output per hectare) would be reduced only by around 2─4%, and hence increased federal funding to reduce the erosion is not justified. Most of the estimations made of soil erosion are based on models, particularly the Universal Soil Loss Equation (USLE) although this itself has been developed and “calibrated” using more than 10,000 plot-years worth of actual data, taken from 50 different locations in 24 U.S. states. Its successor, the Revised Universal Soil Loss Equation (RUSLE), has been improved using further experimental data and in two studies of measured soil loss rates, taken from over 1,700 plot-years worth of data on 205 research plots in 20 locations across the U.S., it was shown that the USLE and RUSLE actually predict rates of soil loss reasonably well, even for post-1960 conditions23. A limitation of USLE and RUSLE is that they predict the amount of soil moved on a field, which is not necessarily the same as the amount of soil physically removed from the field. To estimate the latter, the sediment delivery ratio (SDR) is determined. The SDR is given by the amount of sediment delivered from an area divided by the gross erosion of that same area. Expressed as a percentage, SDR reflects how efficient the watershed is in moving soil particles from an area where erosion is occurring, to the location at which the sediment yield is measured. The model assumes that a relatively small amount of eroded soil leaves a field or stream basin, and some sediment is presumed to be deposited by wind on the field, or along streams as alluvium.

However, it is often assumed that the USLE measures soil actually removed from the land, and the variance in SDR according to particular conditions is not taken account of. As an example, in the 1970s, the sediment delivery to streams from a 3 km2 area in Wisconsin amounted to just 8% of the USLE prediction35, with the remainder thought to be stored as colluvium (loose, unconsolidated sediments that have been deposited at the bottom of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a combination of these processes). In contrast, in the 1930s, the sediment delivered was 123% of the USLE value, due to the then frequent effect of gullying downslope from agricultural fields. Also in the 1930s, it was common that the skies over the eastern United States were filled with enormous clouds from the Dust Bowls, which moved out over the Atlantic Ocean, due to severe wind erosion. It is thought that, more recently, the erosion process amounts more to local redistribution than wholesale loss. To predict the latter phenomenon, the Wind Erosion Equation (WEE) has been used, and as with the USLE there is a mass discontinuity problem: namely that even though soil may be eroded from one area, most of the particles are simply moved onto other fields, and so the net soil loss may be overestimated. It has been claimed that large areas of the U.S. have annual erosion rates of >25 t ha-1, and yet the sediment yields were often in the range 0.5─2.0 tonnes ha-1, including a contribution from significant erosion of the banks and stream channels35. Thus, the total sediment delivered to streams has been reckoned at 2.7─4.0 billion tons, but the actual amount measured at this destination is nearer 0.5 billion tons, with the inference that a large quantity of the sediment must be stored in the watershed. Alternatively, as Trimble and Crosson have averred35, the rate of soil removal may be far less than is apparent using the USLE, calling for more field studies and monitoring of sediment mass, a view endorsed by Nearing23.

Cerdan et al. have used erosion plot data to compile a comprehensive database with which to investigate the rates and spatial variations in sheet and rill erosion across Europe36. It was demonstrated that land use is overwhelming in its influence on erosion rates, which may be greater by an order of magnitude on conventionally tilled arable land, as compared with those surfaces that are permanently covered by vegetation. The erosion rates tend to be lower in the Mediterranean, due to the protective effect of the rocks in the stony soils there. However, because these soils are already very thin, any further loss of soil could be highly detrimental. The average rate of sheet and rill erosion was determined to be ca 1.24 t ha-1 year-1 for the entire area covered by the CORINE database (essentially the whole of Europe - both eastern and western – plus Turkey), and 3.6 t ha-1 year-1 for arable land. Evidently, there are “hot spots” for erosion, but these are masked by quoting a figure for an average erosion rate for Europe. A review22 has been published of the USLE and its family of related models, as applied to the determination of event soil loss, and runoff. It is concluded that the predictive power of the method works well in some locations but poorly in others. One problem in the use of the USLE to predict event erosion is that it was originally designed to model long-term average soil loss, and the event rainfall-runoff factor contained in it does not consider runoff explicitly.

When runoff is measured or estimated reasonably accurately, the prediction may be improved, but in incorporating the direct runoff in the rainfall-runoff factor an impact on some of the other factors used in the model is incurred. Parsons et al. have attempted to predict the travel distance of different particle sizes to provide a model for the erosive impact of specific rainfall/runoff events37: this is of particular relevance to the European situation, in terms of the off-site impact of runoff and erosion. The model only considers simple cases of erosion under a spatially uniform rainfall and on slopes of uniform infiltration and gradient, but it is independent of measurement area since it rests upon the notions of entrainment and travel distances of particles, and so on sediment flux. The results resolve the “paradox” of sediment-delivery ratio, and some of the recent discussion over the validity of erosion rates made from USLE erosion plots; potentially, erosion rates can also be reconciled with the known life-spans of continents. It is concluded that some of the accepted estimates of erosion rates are fallacious, and those that are based on measurement of inter-rill erosion (short runoff plot) are likely to be much larger than reality (for entire hillsides). That noted, the significance of travel distance increases the importance of rill and gully erosion, and of the movement of fine particles to which nutrients are pollutants adhere preferentially. It is clear that far more actual experimental data must be garnered - taken on a range of scales, from small plots to hill-slopes and catchments – in order to evaluate the predictions of the model, and indeed to provide a true image of global soil erosion and land degradation.


8. Measures and realities of global soil erosion.

Results from remote sensing measurements show that the total amount of biomass over the earth’s surface has increased3,4. This may be a result of carbon dioxide fertilization, where photosynthesis and hence plant growth is stimulated by elevated levels of CO2 in the atmosphere. Globally, the amount of biomass was measured to increase by 3.8% during the years 1981─2003. However, it was also determined that 24% of the global land area suffered some degree of degradation during the same period. Hence, there are regions of “greening”, while elsewhere “browning” has occurred. According to a recent study made by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Australian National University, in some of the earth’s driest regions (arid regions of Australia, North America, the Middle East and Africa) foliage has increased by 11% during the past three decades, as a consequence of CO2-fertilization38. Due to different definitions and terms, as noted earlier, there is a wide variance in the estimates of both the degree and rate of global land degradation. Most of the policy decisions over land have been made on the basis of two major sources: the extent of global desertification by Dregne and Chou39, and that of global land degradation by the International Soil Reference and Information Centre by Oldeman et al.34, termed Global Assessment of Human-induced Soil Degradation (GLASOD).

The two sets of figures are not strictly comparable, because while Dregne and Chou considered only dry areas they also included the status of vegetation on the rangeland. The terminology also differs between the two studies: Dregne and Chou used the terms38 slight, moderate, severe and very severe to denote the degree of degradation, while Oldeman et al. used the terms34 light, moderate, strong and extreme, and hence the degree of correspondence between the levels of designation in the two studies is uncertain. Oldeman et al. tried to separate natural degradation from that which had been caused by human activities. Eswaran and Reich40 attempted to determine the vulnerability of land to degradation and desertification, but only considered arid, semi-arid, and sub-humid regions, according to the definition of UNEP; their estimates of water erosion also include humid areas. According to GLASOD34, of the different erosion mechanisms, it is water erosion that is the most important, and afflicts some 1,094 million ha (56%) of the total area that is impacted upon by human-induced soil degradation. Globally, wind erosion affects 548 million ha (38%) of the terrain that is degraded. Chemical soil deterioration affects 239 million ha (12%) of the total, and physical soil deterioration occurs over 83 million ha (4%). The most important subtype of displacement of soil material is through the influence of water or wind. An area of 920 million ha is affected by water erosion (365 million ha in Asia, and 205 million ha in Africa), and 454 million ha by wind erosion. The principal chemical deterioration of soils involves the loss of nutrients and this affects 135 million ha worldwide, of which 68 million is in South America. Salinization follows next in order of its impact, and afflicts some 76 million ha globally, of which 53 million ha is in Asia. An area of 22 million ha is affected by pollution, of which 9 million ha is located in Europe. The most significant subtype of physical soil deterioration is compaction, and occurs over an area of 68 million ha, of which 33 million ha is in Europe, and 8 million ha is in Africa.

GLASOD categorises four degrees of soil degradation. “Light” refers to a somewhat reduced productivity of the terrain, but which is manageable in local farming systems, applies to 38% of all degraded soils (749 million ha). “Moderate” requires improvements which are often more than can be achieved by local farmers in developing nations, and accounts for 46% of the Earth’s degraded soils. Thus 910 million ha of the Earth’s surface has a greatly reduced productivity: >340 million ha of these moderately degraded lands are in Asia and >190 million ha in Africa. There are 296 million ha globally of “strongly degraded” soils (124 million ha in Africa, and 108 million ha in Asia), and it is these that it is not possible to reclaim at the farm level and which may therefore be regarded as lost land. These terrains can only be recovered through major engineering work and/or international assistance. Finally, soils that are “extremely degraded” are regarded as irreclaimable and beyond restoration, amounting to a global total of 9 million ha (>5 million ha in Africa).

GLASOD is not without its critics41, and indeed its authors were well aware of, and the first to indicate, its limitations: principally that it was based on the perceptions of experts, rather than being a direct measure of land degradation. More recently3,4, methods of remote sensing have been applied to determine the extent of global land degradation: LADA (Land Degradation Assessment in Drylands). These aim to determine the degree and trends of land degradation in drylands, degradation hotspots and bright spots (both actual and potential), using changes in net primary productivity (NPP) as a proxy measure of land degradation. [Net primary productivity (NPP) is defined as the net flux of carbon from the atmosphere into green plants per unit time. NPP refers to a rate process, i.e. the amount of vegetable matter produced (net primary production) per day, week, or year]. The most heavily degraded regions are identified to be Africa: south of the equator (13% of the global degrading area and 18% of lost global net NPP), South-East Asia (6% of the degrading area and 14% of lost NPP), south China (5% of the degrading area and 5% of lost NPP), north-central Australia and the western slopes of the Great Dividing Range (5% of the degrading area and 4% of lost NPP), the Pampas (3.5% of degrading area and 3% of lost NPP) and swaths of the high-latitude forest belt in Siberia and North America, directly affecting the livelihoods of the 1.5 billion people who live there. The results indicate that 24% of the total global land surface has suffered degradation during the past quarter century, and may be compared with the 15% of the world’s soil (not land) being degraded, according to the GLASOD study.

Much of the degradation identified by GLASOD33 does not overlap with the areas newly highlighted by LADA, demonstrating that land degradation is both cumulative and global. The authors stress that land-use changes which reduce NDVI [remotely sensed Normalised Difference Vegetative Index (Figure 6)], e.g. from forest to cropland of lower biological productivity, or an increase in grazing pressure, may or may not be accompanied by soil erosion, salinity or other symptoms of land degradation that are of concern to soil scientists. They note further that while long-term trends in NDVI derivatives are only broad indicators of land degradation, taken as a proxy, the NDVI/NPP trend is able to yield a benchmark that is globally consistent and to illuminate regions in which biologically significant changes are occurring. Thus attention may be directed to where investigation and action at the ground level is required, i.e. to potential “hot spots” of land degradation and/or erosion.

[Fig 6]

Montgomery42 has made a global compilation of studies which confirms the long held contention that the erosion rates from conventionally ploughed soils are 1─2 orders of magnitude greater than the background rates of soil production, of erosion under native vegetation, and long-term geological erosion. He concludes that on a global basis, hill-slope soil production and erosion evolve to balance geologic and climate forcing, whereas agriculture based on conventional ploughing increases the rates of erosion to unsustainable levels. At a rate close to 1 mm/year of soil loss (amounting to around 14 t ha-1 year-1), net erosion rates in conventionally ploughed fields can erode a typical hill-slope profile on a timescale of major civilizations, whereas no-till methods of farming cause rates of erosion that are nearer to those of natural creation rates of soil, and hence might set the cornerstone of a system of sustainable agriculture.

Monday, July 21, 2014

Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 2.


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 lands, or catastrophic events, e.g. eruptions from volcanoes.

The above are the primary causes of land degradation, but we may also note:

(7) Water table lowering.

This is often a result of groundwater extraction at a greater rate than the water table can be recharged by natural processes.

(8) Loss of vegetative cover.

Leaving soil bare is a principal route to erosion and degradation. By applying vegetative cover, the soil is protected from erosion both by wind and by water. In addition, organic material is supplied, which assists in maintaining sufficient nutrients to serve healthy plant growth. The roots of plants roots contribute to a good structure of the soil and aid water infiltration.

(9) Increased stoniness and rockiness.

When the levels of soil erosion are extreme, stones and rock may be left bare of soil and exposed. While it is helpful to identify the different kinds of soil degradation, as we have under the foregoing headings, it is a fact that there is frequently a synergy between them. Thus we may note that at the front of a storm there are strong winds, and so wind erosion and water erosion may occur simultaneously. In a “thin end of the wedge” manner, once a soil has undergone some degree of erosion, by whatever means, it is left more vulnerable to further and more rapid erosion than another soil that has suffered a lesser degree of erosion, although the latter may be similar in all other respects. The level of SOM is a pronounced indicator of erosion susceptibility, and soils in which the concentration has become less than ca 2% are particularly prone to erosion, because the soil aggregates are less tightly bound, meaning a greater likelihood of individual soil particles being removed. Slopes that are steep, regions where the rainfall is heavy and how much SOM there is, are significant determinants as to whether and how fast degradation of the soil will occur. While less severe types of erosion can be reversed through changes in management practices (e.g. growing cover crops), at a more serious level and over a long time, the process is effectively irreversible, because any remedial strategy must be measured against the relatively slow processes by which soils are created, taking perhaps hundreds or thousands of years (particularly in cold dry climates) to form a mere few centimetres of soil.

4.2 Human influences

Human activities can influence profoundly the evolution of soils. The construction of “tarmac” roads increases the area of impermeable surface, causing streaming and ground loss. Soil erosion is accelerated by particular farming practices, including an increase of the size of fields, with the removal of hedges and ditches, while meadows are converted to ploughed fields, and farming spring cultures (e.g. sunflower, corn, beet) is on the increase, leaving the ground naked over the winter, when conditions of rain and wind are at their most forceful. Unsustainable methods of modern agriculture are the single major contributor to global soil erosion. When agricultural lands are tilled, their soil is broken into finer particles, a problem that has been accentuated through the use of mechanized farm machinery that permits deep ploughing16. The latter increases the area of soil that is exposed to water erosion. Mono-cropping, farming on steep slopes, the use of pesticides and artificial fertilizers (which destroy the soil food web and hence those microbes whose exudates bind soil together), row-cropping, and surface irrigation methods all take a further toll on the soil and contribute to its erosion. The loss of nutrients from soils is quite specific and it is in the finer soil that the loss, e.g. of phosphorus, occurs more greatly. Tillage increases wind erosion rates, because the exposed soil becomes dehydrated and breaks up into smaller particles that are easily swept away by the wind. The latter situation is worsened when trees are removed from agricultural fields, so that the wind can travel over greater distances and build-up to higher speeds. The rates of erosion are increased by overgrazing, which both removes vegetative cover and causes the soil to become compacted.

4.3 Deforestation

When a forest is undisturbed, its floor soil is covered by layers of leaf litter and humus, which between them form a resisting shield against the impact of raindrops. Both layers are porous and allow rainwater to percolate into the soil beneath them, rather than washing over the surface to form runoff. Before they impact the ground, the raindrops are reduced in their kinetic energy by striking the foliage (canopy) above. Having fallen through around 8 metres (26 feet) the raindrops achieve their terminal velocity, and since forest canopies are usually higher than this, the terminal velocity may again be met lower down, even after striking the canopy. However, the intact forest floor, with its layers of leaf litter and organic matter, is still able to absorb the impact of the rainfall and so it is this, principally, that resists soil erosion more than the overlying foliage17. The rates of erosion are increased when deforestation takes place, because the humus and litter layers are lost from the soil surface, thus exposing it to rainfall. Through the loss of the vegetative cover, and severe soil compaction from logging equipment, the process of erosion is exacerbated. The occurrence of fires can lead to appreciable further erosion, especially if followed by heavy rainfall. The slash and burn method, as applied to tropical forests, is one of the main contributors to global soil loss through erosion, which has rendered complete regions of some counties unproductive. The Madagascar high central plateau has become devoid of vegetation, which amounts to around 10% of the nation’s land area, where there are furrows caused by gully erosion more than 50 metres in depth and one kilometre in length.

4.4 Urbanization and road building.

The process of urbanization denudes the land of vegetative cover, which changes patterns of drainage, and the construction phase itself causes the soil to become compacted. The application of a layer of asphalt or concrete, which is impermeable, both raises the volume of surface runoff water and allows faster wind speeds to be developed over the surface. Both these effects enhance the degree of soil erosion, and as an additional consequence, the sediment in runoff water from urban environments is frequently tainted with fuel, oil, and other toxic materials. Neighbouring watersheds are impacted upon because the volume and rate of water that flows through them is changed, and they become filled with chemically polluted sedimentation. In addition to degrading the land that it flows over, the increased flow of water through local waterways also makes the rate of bank erosion more severe18.

Friday, July 18, 2014

Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 1.


This is the first 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


Abstract.

An overview is presented of the determined degree of global land degradation (principally occurring through soil erosion), with some consideration of its possible impact on global food security. Most determinations of the extent of land degradation (e.g. GLASOD) have been made on the basis of “expert judgement” and perceptions, as opposed to direct measurements of this multifactorial phenomenon. More recently, remote sensing measurements have been made which indicate that while some regions of the Earth are “browning” others are “greening”. The latter effect is thought to be due to fertilization of the growth of biomass by increasing levels of atmospheric CO2, and indeed the total amount of global biomass was observed to increase by 3.8% during the years 1981─2003. Nonetheless, 24% of the Earth’s surface had occasioned some degree of degradation in the same time period. It appears that while long-term trends in NDVI (normalized difference vegetation index) derivatives are only broad indicators of land degradation, taken as a proxy, the NDVI/NPP (net primary productivity) trend is able to yield a benchmark that is globally consistent and to illuminate regions in which biologically significant changes are occurring. Thus, attention may be directed to where investigation and action at the ground level is required, i.e. to potential “hot spots” of land degradation and/or erosion. The severity of land degradation through soil erosion, and an according catastrophic threat to the survival of humanity may in part have been overstated, although the rising human population will impose inexorable demands for what the soil can provide. However, the present system of industrialized agriculture would not be possible without plentiful provisions of cheap crude oil and natural gas to supply fuels, pesticides, herbicides and fertilizers. It is only on the basis of these inputs that it has been possible for the human population to rise above 7 billion. Hence, if the cheap oil and gas supply fails, global agriculture fails too, with obvious consequences. Accordingly, on grounds of stabilising the climate, preserving the environment, and ensuring the robustness of the global food supply, maintaining and building good soil, in particular improving its SOM content and hence its structure, is highly desirable. Those regions of the world that are significantly degraded are unlikely to support a massive population increase (e.g. Africa, whose population is predicted to grow from its present 1.1 billion to 4.2 billion by 2100), in which case a die-off or mass migrations might be expected, if population control is not included explicitly in future plans to achieve food security.


Keywords.

Soil organic matter, global warming, climate change, global food security, land degradation, soil degradation, soil erosion, population.


1. Introduction.

There are particular instances in history, where publication of a book has endowed a critical shift in human thinking as its legacy. Man and Nature; Or Physical Geography as Modified by Human Action is one such example, written by George Perkins Marsh. Published1 in 1864, this is probably the first time that the effects of human actions on the environment were documented, hence auguring-in what we now think of as the conservation movement. It was Marsh’s thesis that ancient Mediterranean civilizations collapsed as a result of land degradation: deforestation caused soils to become eroded, so declining in their productivity. Marsh observed that the same trends were occurring in the United States, as he describes in the following words:

“With the disappearance of the forest, all is changed... The face of the earth is no longer a sponge, but a dust heap, and the floods which the waters of the sky pour over it hurry swiftly along the slopes, carrying in suspension vast quantities of earthly particles which increase the abrading power and mechanical force of the current, and augmented by the sand and gravel of falling banks, fill the beds of the streams, divert them into new channels and obstruct their outlets...

From these causes, there is constant degradation of the uplands, and a consequent elevation of the beds of the water-courses and of lakes by the deposition of the mineral and vegetable matter carried down by the waters...

The washing of soil from the mountains leaves bare ridges of sterile rock, and the rich organic mould which covered them, now swept down into the dank low-grounds, promotes a luxuriance of aquatic vegetation that breeds fever and more insidious forms of mortal disease by its decay, and thus the earth is rendered no longer fit for the habitation of man.”

Soil is the fragile, living skin of the Earth. Consensus of opinion is that land degradation is a major threat to the environment and a potentially significant limitation to the likely success of feeding a human species that is becoming not only more populous (9.6 billion by 2050), but more affluent. The details, however, are complex, disputed and multifarious. In particular, estimates of the extent of soil erosion (by both water and wind), thought to be a principal cause of land degradation, and the provision of a global view of the situation are debatable quantities, since in reality, the process varies according to region, areal dimension and timescale. At issue too is determining the degree to which land is degraded and an exact relation of this to the loss of its agricultural productivity, since there are often insufficient hard data from which firm conclusions can be drawn. At one extreme, it has been proposed that around one third of the world’s available arable land has fallen victim to soil erosion in a 40 year period, and that globally soil is “lost” at a mean rate of 17 tonnes per hectare per year (t/ha/year). Others argue that the latter is an extravagant claim, being based on data derived from a limited number of measurements made in a small region of central Belgium, and cannot be taken to represent an “average rate of soil loss for Europe”, let alone for the wider world. A detailed compilation, based on data from a large number of measurements taken across the European continent, suggests that a more realistic value for the rate of soil erosion is nearer 1─5 t/ha/year, which is nonetheless greater than the natural rate of formation of new soil. It should be noted that “soil erosion” is a technical term and refers to soil that is moved around over an area, but which is not necessarily removed from it and deposited elsewhere. In addition, soil which is lost may be replaced by soil that has been removed from some other location, and so the net loss might be mitigated. Extreme events (e.g. floods, tornadoes) will move far more soil in a short period than is implied in quoting units such as t/ha/year, which is merely an average rate of loss over a far longer timescale (usually of several years or more).

The use (or misuse) of the Universal Soil Loss Equation (USLE) and modified versions of it, is also the subject of some debate regarding just how valid its predictions are of the rate of soil loss at the behest of particular influences that are ascribed by its parameters. The results seem to vary according to exactly where and under what circumstances the USLE is being applied, and in many instances it works very well but in others it fares less adequately. Determinations of the amount of soil being removed are often made on the basis of the amount of sediment that is deposited from some ascribed source. Discrepancies may be identified, however, since not all of the soil that is removed may end up at a single destination (e.g. a river delta), but some of it can become deposited/trapped in additional locations. That the presence of soil organic matter (SOM) is essential for the fertility of a soil has been known since the dawn of agriculture, and that this is impaired by the loss of SOM. The term soil organic carbon (SOC) refers to the elemental carbon content of a soil: this is distributed in different pools. Many of the world’s soils are depleted in SOC, some of them severely, a situation that is exacerbated by industrialized agricultural practices, and restoring the SOC, ideally through the products of photosynthesis, yields many separately identifiable but interconnected benefits. Thus, by increasing the SOC, the structure of the soil is improved, which increases its holding capacity for water, and allows better drainage, hence there is more groundwater and less flooding, while droughts are mitigated. The agricultural productivity of the soil is heightened, enlarging crop yields, and soil erosion is attenuated, especially if the ground is also covered by mulching or with cover crops. Degraded lands may be restored in their production through increasing their SOC content, giving better soil and water quality. The improved soil structure leads to a better retention of water and nutrients, and smaller inputs onto the farm of oil-based fuels, fertilizers, pesticides and herbicides, while biodiversity is enhanced. The term “biodiversity” does not only refer to what is visible above the ground, but also to the root systems of plants, and their accompanying fungal rhizosphere which is an essential part of the soil food web. The importance of rebuilding the soil food web – the ecosystem of microbes, and larger creatures that dwell in soil – is central to maintaining food production in perpetuity, i.e. achieving a system that is truly sustainable.

Of potentially great environmental significance is the prospect that climate change might be mitigated through the removal of atmospheric carbon, taken up by plants through photosynthesis, which is then stored in the soil. Afforestation/ reforestation is considered a key action in storing carbon in biomass (trees) and in soils. Sound management practices offer the potential to mitigate and adapt to climate change, and it is the latter that threatens to increase the potential for soil erosion, diminished soil quality, lower agricultural productivity, with expectedly adverse impacts on food security and global sustainability. Hence this provides one of the more severe tests that might be imposed on humans during the remainder of the 21st century: not only must we mitigate climate change but accept the reality of it and adapt our behaviour and practices to best effect. Relevant management practices are those pertaining to carbon, nitrogen, manure, in low-input systems (also known as sustainable agriculture) and grazing land. Management choices over conservation practices such as no-till, conservation agriculture, and returning crop residue (rubble) to the field to improve nutrient recycling can influence positively carbon sequestration and assist the mitigation of and our adaptation to climate change. Additionally, management of grasslands, restoration of degraded or desertified lands, nitrogen management, to reduce greenhouse gas emissions, precision conservation management on a field and/or watershed scale, along with other management choices can also aid in this cause. Management for climate change mitigation and adaptation is essential for environmental conservation, sustainability of cropping systems, improving the quality of soil and water, and ensuring food security.

According to some estimates, some 2─3 billion tonnes of carbon per annum might be stored in soils for the next 50 years. This should be compared with a current anthropogenic carbon emission level of ca 9 billion tonnes/year, of which around 50% is absorbed by existing terrestrial sinks, and hence around half the remainder could be drawn from the atmosphere and stored in soils, should prevailing emissions continue. However, it is practically certain that they will not, because our use of the fossil fuels (gas, oil and coal) will fall dramatically during this century, meaning that a potential draw-down of CO2 from the atmosphere is possible, perhaps by 50 ppm. While it is derived from biomass, biochar (the name specifically applied to charcoal when it is employed as a soil amending agent) requires energy to run the pyrolysis units that produce it. Biochar is most effective in improving the quality of poor soils, in which it seems to increase nutrient and water retention, and stimulates the growth of microbes in the soil, including mycorrhizal fungi, which both improves the soil structure and fertility and aids the decomposition (or immobilization) of pollutant materials as a bioremediation strategy. Permaculture may offer a host of advantages, including the production of food using reduced inputs of fuels, water and fertilizers, and an absence of pesticides and herbicides, while simultaneously building SOC. Though more readily applicable on smaller areas than are employed in most contemporary farming, such an approach taken by billions of individuals could prove of great significance in ensuring future food security and community resilience. It would be an oversight too, should “seed saving” not be mentioned in a general discussion of future food production. A consideration of these interrelated matters provides the essential substance of this review.

2. Soil.

Civilizations, throughout history, have thrived or fallen according to the goodness of their soils2, and our ability to feed ourselves and our animals depends on a sufficient access to high quality, fertile soil. Jethro Tull (1674-1741) introduced an improved seed-drill that enabled an efficient and consistent planting of seeds, such that the latter were used less wastefully. It was Tull, however, who conceived the erroneous belief that weed-seeds were introduced from manure, and that fields should be heavily ploughed in order to pulverize the soil and release nutrients from it. Guided by this line of thinking, in the 20th century, farmers ploughed fields well beyond the degree necessary to control weeds, and by a combination of such over-ploughing and drought, the Dust Bowls were created in the prairie regions of the Central United States and Canada. The social consequences of the latter were famously dramatised by John Steinbeck in his 1939 novel, The Grapes of Wrath.

Soil2 is made up of layers (soil horizons) which mainly consist of minerals that differ from their primary materials in texture, structure, colour, porosity, consistency, reactivity (pH, redox behaviour), and in chemical, biological and other physical characteristics. Soil is the final result of the consequences, in combination, of climate (temperature, precipitation), relief (slope), organisms (flora and fauna), primary materials (original minerals), and timescale. The material we know as soil (Figure 1) consists of rock particles that have been altered by chemical and mechanical processes, including weathering (disintegration) and accompanying mechanisms of erosion (movement). Soil forms a porous structure and may be envisaged as a three-state system (Figure 2): solid(s) (minerals – clay, silt and sand), liquid (water), and gas (air). The density of most soils lies in the range 1─2 g/cm³.

[Figs 1 and 2]

A good quality soil contains (by volume) 45% minerals, 25% water, 25% air, and 5% of organic material. In a given soil, the mineral and organic components are considered to be constant, while the percentages of water and air may vary, such that the increase in one is balanced by the reduction in the other, i.e. air may be driven out by water, or water be replaced by air as the soil drains. The simple mineral mixture of sand, silt, and clay will evolve, as time passes, into a soil profile that contains two or more horizons, which differ in certain properties, as indicated above. The depth of the horizons can vary considerably from one to another and the boundaries between them are rarely sharply defined. Since the pore space of soil contains both gases and water, the aeration of the soil influences the health of the flora and fauna it contains, but also the emission of greenhouse gases. The colour of soil depends principally on the minerals it contains. Many of the colours are owed to the presence of various iron minerals (Figure 3), and the development and distribution of colours in a soil profile are a consequence of chemical and biological weathering of the primary minerals present, particularly through redox reactions.

[Fig 3]

The soil-evolution process is most strongly influenced by the presence of water, since this medium can promote the growth of plant-life, the leaching of minerals from the soil profile, and the transportation and immobilization of various constituent components. Clay and humus are colloidal particles (< 1 micron in size) present in soil, both of which act as a repository for nutrients and moisture, and serve to buffer the variation in nature and concentration of cations and anions that are present in the soil. Thus, the contribution provided by these materials to the health and properties of soil is far in excess of what might be deduced from their relative proportion by mass of the soil. Colloids are able to solubilise, initially immobile, ions in response to changes in soil pH, and plant root behaviour. The availability of nutrients is also influenced by the soil pH. Most nutrients originate from minerals and are stored in organic material, both living (e.g. bacteria) and dead, and on colloidal particles as ions. The action of microbes on organic matter and minerals may release nutrients, render them immobile, or cause them to be lost from the soil by leaching when they are converted to soluble forms, or by their conversion to gases. Most of the available nitrogen in soils originates from the “fixation” of atmospheric nitrogen gas by bacteria. Of all the components, it is water that has the greatest influence on the formation and fertility of soil, even more so than soil organic matter (SOM)2.

2.1. Organisms

The activities of plants, animals, insects, fungi, bacteria, and humans too, all play a part in the formation of soil2. Fauna - e.g. earthworms, centipedes, beetles, etc. and microbes - mix soils by forming burrows and pores, which allow moisture and gases to diffuse through the soil matrix. As plant-roots grow in soil, channels are also created. Plants with deep taproots can penetrate the different soil layers by many metres and draw-up nutrients from considerable depths. Organic matter is contributed to the soil by plant-roots that extend near the surface, where they are quite readily decomposed. Micro-organisms, including fungi and bacteria, facilitate chemical exchanges between roots and soil and act as a reserve of nutrients. Soil-erosion may arise from the mechanical removal, by human activities, of plants that provide natural surface cover. The different soil layers may be mixed together by microorganisms, a process which stimulates soil formation, since less-extensively weathered material is mixed with more well-developed layers closer to the surface. Some soils may contain up to one million species of microbes per gram (most of these species being unclassified), making soil the most abundant ecosystem on Planet Earth. It is thought that one teaspoonful of soil may contain up to a billion organisms, which form the Soil Food Web (Figure 4).

[Fig 4]

Vegetation can prevent soil-erosion caused by excessive rain and resulting surface runoff. Plants are also able to shade soils, keeping them cooler and reducing the loss, by evaporation, of soil moisture; yet conversely, through transpiration, plants may also cause soils to lose moisture. Plants can synthesise and release chemical agents (including enzymes) – “exudates” - through their extended root-systems, which are able to decompose minerals and so improve the structure of the soil. Dead plants, fallen leaves and stems begin their decomposition on the surface, where organisms feed on them and mix the organic material into the upper soil layers; these additional organic compounds become part of the soil formation process. In addition to the essential characteristics of a particular soil – e.g. its density, depth, chemistry, pH, temperature and moisture - the precise type and quantity of vegetation that may be grown at a particular location depends on a combination of the prevailing climate, land topography, and biological factors.

2.2. Time

Soil formation is a time-dependent process that depends on the interplay of various different and interacting factors2. Soil is a continuously evolving medium, and it requires around 200─1,000 years to form a layer of fertile soil 2.5 cm (one inch) thick. Fresh material, e.g. as recently deposited from a flood, shows no trace of soil development because insufficient time has passed for the material to form a structure that may be later defined as soil. Rather, the original soil surface is buried, and the new deposit must be transformed afresh. Over a period ranging from hundreds to thousands of years, the soil will develop a profile that depends on the nature and degree of biota and climate. Soil-forming mechanisms continue to proceed, even on “stable” landscapes that may endure sometimes for millions of years. In a relentless process, some materials are deposited on the surface while others are blown or washed from the surface. At the behest of such additions, removals, and alterations, soils are always subject to new conditions. It is a combination of climate, topography and biological activity that decides if these changes are rapid or protracted.