Soil Erosion Threatens Food Production

Abstract

Since humans worldwide obtain more than 99.7% of their food (calories) from the land and less than 0.3% from the oceans and aquatic ecosystems, preserving cropland and maintaining soil fertility should be of the highest importance to human welfare. Soil erosion is one of the most serious threats facing world food production. Each year about 10 million ha of cropland are lost due to soil erosion, thus reducing the cropland available for world food production. The loss of cropland is a serious problem because the World Health Organization and the Food and Agricultural Organization report that two-thirds of the world population is malnourished. Overall, soil is being lost from agricultural areas 10 to 40 times faster than the rate of soil formation imperiling humanity’s food security.

1. Introduction

The loss of soil from land surfaces by erosion is widespread and reduces the productivity of all natural ecosystems as well as agricultural, forest, and pasture ecosystems [1,2,3]. Concurrently with the growing human population, soil erosion, water availability, climate change due to fossil fuel consumption, eutrophication of inland and coastal marine bodies of water, and loss of biodiversity rank as the prime environmental problems throughout the world.

Currently nearly 66% of the world population is malnourished [4,5], the largest number of malnourished people ever (malnutrition: faulty nutrition due to inadequate or unbalanced intake of nutrients or their impaired assimilation or utilization) [6]. With the world population now over seven billion and expected to reach 9.3 billion by 2050, more food will be needed [7]. Consider at present that more than 99.7% of human food (calories) comes from the land [8], while less than 0.3% comes from the marine and aquatic ecosystems. Maintaining and augmenting the world food-supply basically depends on the productivity and quality of all agricultural soils.

Human induced soil erosion and associated damage to all agricultural land over many years have resulted in the loss of valuable agricultural land due to abandonment and reduced productivity of the remaining land which is partly made up for by the addition of nitrogen and phosphate fertilizers [2,9,10,11]. This loss of cropland to the effects of soil erosion often results in the creation of new cropland out of forestland and pastureland and the need to enrich these new croplands with inputs of nitrogen and phosphate fertilizers [12]. In addition, soil erosion reduces the valuable diversity of plants, animals, and soil microorganisms.

In this paper, the diverse factors that cause soil erosion are assessed. The extent of damage associated with soil erosion is analyzed, with emphasis on the impact these causative factors may have on future human food security as well as on the natural environment.

2. Causes of Erosion

Erosion occurs when soil is left exposed to rain drop or wind energy. The raindrops hitting a hectare of land in the New York State region of the United States provide the energy equivalent of 60,000 kcal (250 × 10⁶ joules) per year with about 1000 mm of rainfall [13]. This 60,000 kcal roughly equals the energy in eight liters of gasoline. The raindrops hitting soil loosen the soil particles and with even a 2% slope start the movement of the soil downhill. Sheet erosion is the dominant type of erosion [3,14]. The impact of soil erosion is intensified on all sloping land, where with each degree of slope more of the surface soil is carried away as the water moves downhill into valleys and streams.

Wind energy also has great power to dislodge surface soil particles and transport them long distances. A dramatic example of this was the wind erosion in Kansas during the winter of 1995–1996 when it was relatively dry and windy. At this time approximately 65 t/ha of soil was eroded from this valuable cropland (Figure 1). Wind energy is sufficiently strong to propel soil particles thousands of kilometers. This is illustrated in the photograph by NASA (Figure 2) which shows a cloud of sand being blown from Africa to South and North America.

Figure 1. About 50 mm of soil blown from cropland in Kansas during the winter of 1995–1996 (E.L Skidmore, USDA, Manhattan, KS. photo spring of 1996).

Figure 1. About 50 mm of soil blown from cropland in Kansas during the winter of 1995–1996 (E.L Skidmore, USDA, Manhattan, KS. photo spring of 1996).

Figure 2. Cloud of sand from Africa being blown across the Atlantic Ocean [15].

Figure 2. Cloud of sand from Africa being blown across the Atlantic Ocean [15].

2.1. Soil Structure

Soil structure influences the ease with which soil can be eroded. Soils with a medium to fine texture, a low level of organic matter content, and weak structural development are most easily eroded [16]. Typically these soils have low water infiltration rates and therefore are subject to high rates of water erosion and are easily displaced by wind energy.

2.2. The Role of Vegetative Cover

Land areas covered by plant biomass, living or dead, are more resistant to wind and water soil erosion and experience relatively little erosion because rain drop and wind energy are dissipated by the biomass layer and the topsoil is held together by the biomass [17]. For example, in Utah and Montana, as the amount of ground cover decreased from 100% to less than 1%, erosion rates increased approximately 200-fold [18]. In forested areas, a minimum of 60% forest cover is necessary to prevent serious soil erosion and landslides [19,20,21]. The extensive removal of forests for cropland and pasture is followed by intensive soil erosion.

Loss of vegetative soil cover is especially widespread in developing countries where populations are large and growing, and agricultural practices are often inadequate to protect topsoils. In addition, cooking and heating in these countries frequently depend on the use of crop residues for fuel. For example, in the 1990s about 60% of crop residues in China and 90% in Bangladesh routinely were removed from the land and burned as fuel [22]. More recent estimates of the amount of crop residues in Bangladesh that could be harvested for biomass energy conversion without negatively impacting future crop yields amount to 50% of all rice crop residues and 80% of non-rice crop residues [23]. More recently crop residues in China are being used less as a domestic fuel source [24] due to the increased availability of fossil fuels. However China has plans to burn about half of the 600 billion tons of straw (from grains) crop residues produced annually to generate electricity [25]. In areas where fuelwood and other biomass are scarce, even the roots of grasses and shrubs are collected and burned [26,27]. All these practices leave the soil barren and fully exposed to rain and wind erosion forces.

2.3. Land Topography

The topography of a given landscape, its rainfall and/or wind exposure all combine to influence the land’s susceptibility to soil erosion. In the Philippines, where more than 58% of the land has a slope greater than 11%, and in Jamaica where 52% of the land has a slope greater than 20%, soil erosion rates as high as 400 t/ha/year have been reported [1]. Erosion rates are high especially on marginal and steep lands which have been converted from forests to crops [1]. In addition under arid conditions with relatively strong winds soil erosion rates as high as 5600 t/ha/year have been reported in an arid region in India [28]. Even in a developed country with abundant farmland such as the United States where there is less need to exploit croplands with steeper slopes, erosion losses as of 2007 average 13 tons/ha/year [29]. In a developed region such as Europe, the measured rates of erosion range between 3 and 40 tons/ha/year with “losses due to individual storms of from 20–40 tons/ha, that may happen every two or three years, are measured regularly in Europe, with losses of more than 100 tons/ha in extreme events” [30,31].

2.4. Other Soil Disturbances

Although world agriculture accounts for about three-quarters of the soil erosion worldwide, erosion occurs whenever humans remove the vegetative cover [1,32]. The construction of roads, parking lots, and buildings are examples of this problem. Although the rate of soil erosion from construction sites may be exceedingly high, the erosion occurs for a relatively brief period. Once the land surface is seeded to grass or covered with other vegetation, the erosion declines [33,34,35].

Natural ecosystems also suffer erosion losses. This is especially evident along stream banks, where erosion occurs naturally due to the powerful action of the adjacent moving water. Increased soil losses occur on steep surfaces (30% or more) when a stream cuts through adjacent land. Even on relatively flat land with only a 2% slope, stream banks are eroded during heavy rains and flooding.

3. Assessing Soil Erosion

Although soil erosion has been taking place very slowly in natural ecosystems throughout geologic time, its cumulative impacts on soil quality over billions of years have been significant. Worldwide, erosion rates range from a low of 0.001 t/ha/year on relatively flat land with grass or forest cover, to rates ranging from 1 to 5 t/ha/year in mountainous regions with natural vegetation [36]. Yet even soil erosion with low rates sustained over billions of years can result in the displacement of enormous quantities of soil. In addition, eroded soil frequently accumulates in valleys forming alluvial plains. The large deltas of the world, such as those of the Nile, Ganges and Mississippi Rivers are the result of millennia of erosion [37].

Worldwide it is estimated that approximately 75 billion tons of fertile soil are lost from world agricultural systems each year [38,39]. An estimate of the total amount of soil eroded by water from the world’s arable land per year for land quality classes I through VI is 67 billion tons [40]. In the 1990s, soil scientists Lal and Stewart [1] and Wen [41] report that 6.6 billion tons of soil per year are lost in India and 5.5 billion tons are lost annually in China. According to a study conducted by the Central Soil Water Conservation Research and Training Institute in Dehradun, India reported in 2010 that the average rate of soil loss due to erosion in India is 16.4 tons per hectare annually with an annual total loss of 5.334 billion tons [42]. A three-year study conducted by researchers associated with the Chinese Ministry of Water Resources, the Chinese Academy of Sciences and the Chinese Academy of Engineering reported in 2009 that all of China’s 646 counties suffer from significant soil and water losses, equivalent to a combined area of 3.75 million km² [43]. A two-year study further reports that if the current rate of soil loss in China continues over the next 50 years, food production will decrease by 40% [44]. Considering these two countries together occupy only 13% of the world’s total land area and have agricultural practices that have sustained agriculture for thousands of years, the estimated 75 billion tons of soil lost each year worldwide is conservative. The amount of soil lost from the United States cropland due to water and wind has decreased from 3.06 billion tons in 1982 to 1.725 billion tons in 2007 [29].

3.1. Loss of Productivity in Managed Ecosystems

Approximately 50% of the earth’s land area is devoted to agriculture: About one-third is planted to crops and two-thirds is grazing land [45]. Forests occupy about 20% of the world’s land area [46]. Of these three areas, cropland is most susceptible to erosion because of the frequent cultivation of soils and that vegetation is often removed before the crops are planted which exposes the soil to wind and rainfall energy. In addition, cropland is often left without vegetative cover between plantings which intensifies erosion on agricultural lands, erosion rates that are estimated to be 75 times greater than erosion in natural forest areas [38].

3.2. Worldwide Cropland

Currently, about 80% of the world’s agricultural land suffers moderate to severe erosion, while 10% experiences slight erosion [47,48]. Worldwide, erosion on cropland averages about 30 t/ha/year and ranges from 0.5 to 400 t/ha/year [2]. As a result of soil erosion, during the last 40 years about 30% of the world’s cropland has become unproductive and much of that has been abandoned for growing crops [49,50].

The nearly 1.5 billion ha of world cropland now under cultivation for crop production are almost equal in area to the amount of cropland (2 billion ha) that has been abandoned by humans since farming began [51,52] (D. Pimentel, Personal Communication, 19 July 2013). Such abandoned land, once biologically and economically productive, now only produces little biomass but also has lost considerable diversity of the plants, animals, and microbes it once supported [11,52]. Each year an estimated 10 million ha of cropland worldwide are abandoned due to lack of productivity caused by soil erosion [53]. Worldwide, soil erosion losses are highest in agro-ecosystems of Asia, Africa, and South America, averaging 30 to 40 tons/ha/year [11]. In developing countries, soil erosion is particularly severe on small farms that are often located on marginal lands where the soil quality is poor and the topography is frequently steep. In addition, poor farmers tend to raise row crops such as corn and beans; row crops are highly susceptible to erosion because the crop vegetation does not cover the entire tilled soil surface [54]. For example, in the Sierra Region of Ecuador, about 60% of the cropland was abandoned because erosion and inappropriate agricultural practices left the land devastated by water and wind erosion [55]. Similar problems are evident in the Amazonian region of South America, especially where vast forested areas have been cleared to provide land for sugarcane and other crops, plus livestock production. Past soil erosion for the African continent as a whole has caused an average annual crop yield decline of 8.2% and 6.2% for sub-Saharan Africa [56] and that if higher soil erosion rates continue unabated, average annual crop yield declines of 16.5% and 14.5% for sub-Saharan Africa may be possible.

3.3. U.S. Cropland

The lowest average erosion rates on cropland occur in the United States and Europe ranging from 10 to 15 t/ha/year [57]; erosion rates in the United States have dropped from an average 16.4 t/ha/year in 1982 to 10.8 t/ha/year in 2007 [29]. However, even these relatively low rates of erosion greatly exceed the average rate of natural soil formation from the parent soil material; under agricultural conditions the soil formation rate ranges from 0.5 to 1 t/ha/year [2,3,9,58]. This means that most U.S. cropland is losing soil faster than soil formation can replace it. Soil erosion is severe in some of the most productive agricultural ecosystems in the United States. For example, one-half the fertile topsoil of Iowa has been lost by erosion during the last 150 years of farming because of erosion [59]. These high rates of erosion in 1982, about 16.6 t/ha/year, have been reduced but remain high at 11.5 t/ha/year as of 2007 in Iowa and surrounding areas because of the rolling topography and the mostly corn and soybean production, row crops where the soil surface between rows is left exposed to wind and rain [29,60,61]. Similarly, 40% of the rich soil of the Palouse region in the northwestern U.S. has been lost during more than 100 years of cultivation [62]. In both of these regions, intensive agriculture is employed and mono-cultural plantings are common. In addition, most of the fields are left without a cover crop in the late fall and winter leaving the soil exposed to further erosion. Yearly many valuable hectares of cropland are abandoned after they have become unproductive due to wind and water erosion [63].

3.4. Pasture and Rangeland

In contrast to the average soil loss of 10.8 t/ha/year from U.S. cropland, pastures lose soil at about 6 t/ha/year [29,64]. However, erosion rates on pastures intensify whenever overgrazing occurs. Even in the United States, about 75% of non-Federal lands require conservation treatments to reduce grazing pressure [65]. More than half of the rangelands, including those on non-Federal and Federal lands, are now overgrazed and have become subject to high erosion rates [66,67].

Although erosion rates on U.S. cropland have decreased during the past three decades, erosion rates on rangelands and pastures remain high (6 t/ha/year) [64]. High erosion rates are typical on most of the world’s pastures and rangelands [50]. In many developing countries, heavy grazing by cattle, sheep, and goats has removed most of the vegetative cover, exposing the soil to severe erosion. In Africa, about 80% of the pasture and rangeland is seriously eroded and degraded [68]. The prime causes for exposed soil are overgrazing and the removal of crop residues for cooking fuel but even by the 1990s researchers realized that these causes are so intertwined with the effects of rainfall variability and the occurrence of drought on the vegetation that it can be difficult to determine how much erosion is due to human activity [69]. Rangeland degradation and resultant soil erosion often occurs in sub-Saharan Africa but according to some researchers only under special circumstances such as when animals are concentrated, due to a restriction of the animals’ movements, will a specific area of rangeland be overgrazed rather than the overall rangeland [70].

3.5. Forest Land

In stable forest ecosystems, where soil is protected by vegetation, erosion rates are very low, ranging from only 0.004 to 0.05 t/ha/year [47,71]. Tree leaves and branches not only intercept and diminish raindrop and wind energy, but leaves and branches also cover the soil under the trees to further protect the soil. However, the situation changes dramatically when forests are cleared for cropland or pastures are developed for livestock production and the soil is exposed to rain and wind energy [55,72].

4. Effects of Soil Erosion on Terrestrial Ecosystems

Soil erosion reduces the general productivity of terrestrial ecosystems [73,74]. In the order of importance, soil erosion increases water runoff thereby decreasing water infiltration and the water-storage capacity of the soil [3]. In addition, during the erosion process organic matter and essential plant nutrients are removed from the soil and soil depth is reduced. These changes not only inhibit vegetative growth but reduce the presence of valuable biota and the overall biodiversity of the soil [3,74]. These factors interact, making it almost impossible to separate the specific impacts of one factor from another. For example, the loss of soil organic matter increases water runoff which reduces the soil’s water-storage capacity, which diminishes nutrient levels in the soil and also reduces the natural biota biomass and the biodiversity of soil ecosystems [73,74,75].

4.1. Water Availability

Water is a prime limiting factor for productivity in all terrestrial ecosystems because all vegetation requires enormous quantities of water for growth and for the production of fruit [76]. For example, 1 ha of corn will transpire about seven million liters of water during the growing season of about three months [77] and lose an additional two million liters of water by evaporation from the soil [76]. During soil erosion by rainfall, water runoff significantly increases with less water entering the soil and less water available to support the growing vegetation.

4.2. Nutrient Losses

Eroded soil carries away vital plant nutrients such as nitrogen, phosphorus, potassium, and calcium. Typically, the eroded soil contains about three times more nutrients per unit weight than are left in the remaining soil [78]. A ton of fertile topsoil averages 1 to 6 kg of nitrogen, 1 to 3 kg of phosphorus, and 2 to 30 kg of potassium, whereas the topsoil on the eroded land has an average nitrogen content of only 0.1 to 0.5 kg per ton [79,80]. To offset the nutrient losses inflicted by crop production, large quantities of fertilizers are often applied. Troeh et al. [3] estimate that lost soil nutrients cost U.S. agriculture several billion dollars annually. If the soil base is relatively deep, about 300 mm, and if only from 10 to 20 tons of soil is lost per hectare per year, the lost nutrients can be replaced with the application of commercial fertilizers and/or livestock manure. However, the replacement strategy is expensive for the farmer and nation and usually poor farmers cannot afford fertilizer. Not only are the fertilizer inputs fossil-energy dependent, these chemicals can harm human health and pollute the soil, water and air [64,81].

4.3. Soil Organic Matter

Fertile soils typically contain 100 tons of organic matter per hectare (4% to 5% of total topsoil weight) [17,58]. About 95% of the soil nitrogen and 25 to 50% of the phosphorus is contained in the soil organic matter [82]. Because most soil organic matter is found close to the soil surface as decaying leaves and stems, erosion significantly reduces the soil organic matter. Both wind and water erosion selectively remove the fine organic particles in the soil leaving behind larger soil particles and stones. Several studies have demonstrated that the soil removed by either water or wind erosion is 1.3 to 5 times richer in organic matter than the soil left behind [1]. For example, the reduction of soil organic matter from 0.9% to 1.4% (assuming a soil organic content of 4 to 5%) lowered the crop yield potential for grain by 50% [58,83].

Soil organic matter is a valuable resource because it facilitates the formation of soil aggregates and thereby increases soil porosity. The improved soil structure in turn facilitates water infiltration and ultimately the overall productivity of the soil [80]. In addition, organic matter aids cation exchange, enhances plant root growth, and stimulates the increase of important soil microbes [75,82,84].

Once the organic matter layer is depleted, the productivity of the ecosystem, as measured by plant biomass, declines both because of the degraded soil structure and the depletion of nutrients that were contained in the organic matter. In addition to low yields, the total biomass of the biota and overall biodiversity of these ecosystems is substantially reduced [85,86].

4.4. Soil Depth

Growing plants require soils of adequate depth in which to extend their roots. Various soil biota, like earthworms, also require a suitable soil depth [2,84]. Thus, when erosion substantially reduces soil depth of from 30 cm for deep soils to even less than 1 cm for thin soils, plant root space can be minimized, and the plants could be stunted.

5. Biomass, Soil Biota and Biodiversity

The biological diversity existing in any ecosystem is directly related to the amount of living and non-living organic matter present [52,84,85,86,87]. As mentioned, erosion, by diminishing soil organic matter, reduces the overall soil biomass and biological activity. Ultimately, this has a profound effect on the diversity of plants, animals, and microbes present in the soil ecosystem. Numerous positive associations have been established between biomass abundance and species diversity [88,89,90,91]. Vegetation is the main component of ecosystem biomass and provides the vital resources required both by animals and microbes for their survival. This is illustrated in Table 1 [92]. Along with plants and animals, microbes are a vital component of the soil and constitute a large percentage of the soil biomass. One cubic meter of soil may support up to 200,000 arthropods, 10,000 earthworms, plus billions of microbes [74,93,94]. A hectare of productive soil may have a biomass of invertebrates and microbes weighing up to 10,000 kg/ha (Table 1). In addition, soil bacteria and fungi add 4000 to 6000 species and in this way contribute significantly to biodiversity especially in moist, organic soils [52,74].

Erosion rates that are 10 to 20 times above the sustainability rate or soil formation rates of 0.5–1 t/ha/year reduce the diversity and abundance of soil organisms [74,95]. In contrast, agricultural practices that control erosion and maintain adequate soil organic matter favor the proliferation of soil biota [74,96,97]. The application of organic matter or manure also enhances the biodiversity in the soil [74,98]. Species diversity of macrofauna (mostly arthropods) increased 16% when organic matter or manure was added to experimental wheat plots in Russia [99]. Similarly, species diversity of macrofauna (mostly arthropods) more than doubled when organic manure was added to grassland plots in Japan [100], and increased 10-fold in Hungarian farm land [101].

Table 1. Biomass of various organisms per hectare in a temperate region pasture [92].

Organism	Biomass (kg fresh weight)
Plants	20,000
Fungi	4,000
Bacteria	3,000
Arthropods	1,000
Annelids	1,320
Protozoa	380
Algae	200
Nematodes	120
Mammals	1.2
Birds	0.3

Table 1. Biomass of various organisms per hectare in a temperate region pasture [92].

Organism	Biomass (kg fresh weight)
Plants	20,000
Fungi	4,000
Bacteria	3,000
Arthropods	1,000
Annelids	1,320
Protozoa	380
Algae	200
Nematodes	120
Mammals	1.2
Birds	0.3

Field experiments using collards confirm the relationship between biomass and biodiversity in which arthropods species diversity rose 4-fold in experimental plots with the highest collard biomass compared with that found in control collard plots [102]. Depending on the soil type, ecological characteristics of the area, and land use practices, one hectare of productive soil can contain as many as 21,000 species [74]. In a study of bird populations, a strong correlation between plant biomass productivity and bird species diversity was reported when a 100-fold increase in plant biomass yielded a 10-fold increase in bird species diversity [87].

Soil erosion has indirect effects on ecosystems that may be nearly as damaging as the direct effects in reducing plant biomass productivity. Tilman and Downing [103] found that the stability and biodiversity of grasslands were significantly reduced when the number of plant species decreased; as plant species richness decreased from 25 species to five or less, the grassland became less resistant to drought. The overall result was that the grassland was more susceptible to drought conditions and required more time to recover its productivity than when a greater abundance of plant species was present.

Sometimes soil erosion causes the loss of a keystone species, and that keystone species’ absence may have a cascading effect on the survival of a wide array of other species within the ecosystem. Species that act as keystone species include the dominant plant type, like oaks, that maintain the biomass productivity and integrity of the ecosystem; predators and parasites that control the feeding pressure of some organisms on major plants; pollinators of various plants in the ecosystem; seed dispersers; as well as the plants and animals that provide habitats required by other essential species like biological nitrogen fixers [72,74]. Thus, in diverse ways, the normal activities within an ecosystem may be interrupted when the populations of keystone species are significantly altered. The damages inflicted can be severe in agro-ecosystems when, for instance, the numbers of pollinators are drastically reduced or even eliminated and there is little or no reproduction in the plants affected [2,74].

Soil biota perform many beneficial activities that improve soil quality and ultimately its productivity [74,96,104,105]. For example, soil biota recycle basic nutrients required by plants for their growth [74]. In addition, the tunneling and burrowing activities of earthworms and other soil biota enhance soil productivity by increasing water infiltration [104]. Earthworms, for instance, may construct up to 220 tunnel channels per square meter in old bush fallow areas in Nigeria while cultivated areas had only 34 channels per square meter and the channel diameters in the cultivated areas were narrower [106]. These channel openings enable the water to infiltrate rapidly into the soil. Other soil biota contribute to soil formation and productivity by mixing the soil components, enhancing aggregate stability, and preventing soil crusting. This churning and mixing of the upper soil redistributes nutrients, aerates the soil, exposes soil to the weather for soil formation, and increases water infiltration rates, thus making soil conditions favorable for increased soil formation and plant productivity. Earthworms bring from 10 to 500 t/ha/year of soil from underground to the soil surface [107,108], while some insects, like ants, may bring 34 t/ha/year of soil to the surface [109,110]. Snails are reported to help the formation of 1000 kg/ha of soil per year [111].

6. Sediments and Wind Blown Soil Particles

Beyond the damage to rain fed agricultural and forestry ecosystems, the effects of erosion reach far into surrounding environments [112]. For instance, large amounts of eroded soil are deposited into streams, rivers, lakes, and other ecosystems. The USDA [60] reports that 60% of water-eroded soil ends up in streams. In China, approximately 1–2 billion tons/year of soil was transported down the Yellow River into the Yellow Sea from 1950–1970 but since the late 1980s the sediment load has decreased due to better soil conservation practices on the Loess Plateau, and the greater use of Yellow River water for irrigation, human consumption and industrial uses [113,114,115]. The most costly off-site damages occur when soil particles enter lake or river systems [116,117]. Of the 75 billion tons of soil lost worldwide, approximately two-thirds become deposited in lakes and rivers [60,118]. In some areas, heavy sedimentation leads to river and lake flooding [38,39]. Some of the flooding that occurred in the midwestern United States during the summer of 1993 was caused by increased sediment deposition in the Mississippi and Missouri Rivers. These deposits raised the level of the waterways, making them more prone to overflowing and flooding [119]. Sediments disrupt and harm aquatic life by contaminating the water with soil particles, fertilizers, and pesticides [120]. Siltation of reservoirs and dams reduces water storage, increases the maintenance cost of the dams, and shortens the life of the reservoirs [2].

Wind-eroded soil also causes off-site damage because soil particles propelled by strong winds act as abrasives and air pollutants [121,122]. Soil particles sand-blast U.S. automobiles and buildings and caused an estimated $8 billion in damage each year during the 1980s [2,123]. A prime example of the environmental impact of wind erosion occurs in the U.S., where wind erosion rates average 13 t/ha/year on cropland and sometimes reach 65 t/ha/year [124]. Yearly off-site erosion costs in New Mexico in 1984, including health and property damage, are estimated to be nearly $500 million [123]. The estimated total damage from wind erosion in the U.S. was estimated to cost nearly $10 billion each year in the 1990s [2].

The long range transport of dust by wind has implications for health worldwide. Griffin et al. [125] report that about 20 human infectious disease organisms, like anthrax and tuberculosis, are easily carried in the soil particles transported by the wind. Inhaled dust can also cause problems such as irritation of the respiratory passages and diseases such as lung cancer and dust can carry harmful materials such as organic chemicals, heavy metals, and radioactive materials into the lungs [81].

Soil erosion contributes to global warming because CO₂ is added to the atmosphere when enormous amounts of biomass in the soil are exposed to the air and oxidized [86,126,127,128,129]. One hectare of soil may contain about 100 tons of organic matter or biomass; if eroded and oxidized, this erosion would contribute about 45 tons of carbon to the atmosphere. A feedback mechanism exists wherein increased global warming intensifies rainfall which, in turn, increases erosion and continues the cycle [128].

7. Conservation Technologies and Research

Estimates are that agricultural land degradation alone can be expected to depress world food production as much as 30% during the next 50 years [49]. This forecast emphasizes the need to implement known soil conservation techniques. These soil conservation techniques include biomass mulches, crop rotations, no-till, ridge-till, added grass strips, shelterbelts, contour row-crop planting, and various combinations of these. Basically all of these techniques require keeping the land protected from wind and rainfall energy by using some form of biomass cover on the land which means either leaving most of the crop residues on the cropland or planting cover vegetation on a harvested cropland [2,3,11].

In the U.S. during the past decade, soil erosion rates on croplands have been reduced over 25% using various soil conservation technologies [57,130]. Yet, even with the decline in erosion, soil is still being lost at a rate 10 to 15 times above sustainability [64]. However, soil erosion rates on pasture and rangelands have not declined during the past 20 to 30 years and still remain six times above sustainability [64].

8. Conclusion: Productive Soils and Food Security

Soil erosion is a disastrous environmental problem throughout the world. Erosion is a slow insidious problem that is continuous. Indeed, 1 mm of soil, easily lost in one rain or wind storm, is so minute that its loss goes unnoticed by the farmer and others. Yet this loss of soil over a hectare of cropland amounts to about 15 t/ha. Replenishing this amount of soil under agricultural conditions requires approximately 20 years, meanwhile the lost soil is not available to support crops. Along with the loss of soil is the loss of water, nutrients, soil organic matter, and soil biota. The soil system is severely harmed when soil erosion is allowed to occur.

Future food security is threatened where cropland degradation is allowed to occur because of significantly reduced crop productivity. Shortages of cropland are already having negative impacts on world food production [131,132]. For example, the Food and Agricultural Organization (FAO) of the United Nations reports that the per capita grain production has been declining for more than two decades, based on the availability of grains (Figure 3). Note, cereal grains make up more than 80% of the world’s food. Although grain yields per hectare in both developed and developing nations are still increasing, these increases are slowing.

Worldwide, soil erosion continues unabated while the human population continues to increase rapidly and 66% of the world population is now malnourished [4]. If soil conservation is ignored and population control is ignored, more malnourished people and more deaths will occur.

Figure 3. Cereal Grain Production per capita in the world from 1961 to 2010 [58,133].

Figure 3. Cereal Grain Production per capita in the world from 1961 to 2010 [58,133].