SOIL EROSION

Soil erosion by water, wind and tillage affects both agriculture and the natural environment. Soil loss, and its associated impacts, is one of the most important (yet probably the least well-known) of today's environmental problems.

"The threat of nuclear weapons and man's ability to destroy the environment are really alarming. And yet there are other almost imperceptible changes - I am thinking of the exhaustion of our natural resources, and especially of soil erosion - and these are perhaps more dangerous still, because once we begin to feel their repercussions it will be too late." (p144 of The Dalai Lama's Little Book of Inner Peace: 2002, Element Books, London)

It isn't easy to find comprehensive information on erosion, however. To a large extent this is because soil erosion does not fit neatly under any one heading: it is studied by geomorphologists, agricultural engineers, soil scientists, hydrologists and others; and is of interest to policy-makers, farmers, environmentalists and many other individuals and groups.

The Soil Erosion Site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.

What is soil erosion?

Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Earth's surface, just as has happened on the planet Mars. In general, background erosion removes soil at roughly the same rate as soil is formed. But 'accelerated' soil erosion — loss of soil at a much faster rate than it is formed — is a far more recent problem. It is always a result of mankind's unwise actions, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil may be detached, transported, and (possibly travelling a long distance) deposited. Accelerated soil erosion by water or wind may affect both agricultural areas and the natural environment, and is one of the most widespread of today's environmental problems. It has impacts which are both on-site (at the place where the soil is detached) and off-site (wherever the eroded soil ends up). More recently still, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: this is so-called tillage erosion.  Soil erosion is just one form of soil degradation. Other kinds of soil degradation include salinisation, nutrient loss, and compaction.

Soil Erodibility

Soil erodibility is an estimate of the ability of soils to resist erosion, based on the physical characteristics of each soil. Generally, soils with faster infiltration rates, higher levels of organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam textured soils tend to be less erodible than silt, very fine sand, and certain clay textured soils.

Tillage and cropping practices which lower soil organic matter levels, cause poor soil structure, and result of compacted contribute to increases in soil erodibility. Decreased infiltration and increased runoff can be a result of compacted subsurface soil layers. A decrease in infiltration can also be caused by a formation of a soil crust, which tends to "seal" the surface. On some sites, a soil crust might decrease the amount of soil loss from sheet or rain splash erosion, however, a corresponding increase in the amount of runoff water can contribute to greater rill erosion problems.

Past erosion has an effect on a soils' erodibility for a number of reasons. Many exposed subsurface soils on eroded sites tend to be more erodible than the original soils were, because of their poorer structure and lower organic matter. The lower nutrient levels often associated with subsoils contribute to lower crop yields and generally poorer crop cover, which in turn provides less crop protection for the soil.

Slope Gradient and Length

Naturally, the steeper the slope of a field, the greater the amount of soil loss from erosion by water. Soil erosion by water also increases as the slope length increases due to the greater accumulation of runoff. Consolidation of small fields into larger ones often results in longer slope lengths with increased erosion potential, due to increased velocity of water which permits a greater degree of scouring (carrying capacity for sediment).

Vegetation

Soil erosion potential is increased if the soil has no or very little vegetative cover of plants and/or crop residues. Plant and residue cover protects the soil from raindrop impact and splash, tends to slow down the movement of surface runoff and allows excess surface water to infiltrate.

The erosion-reducing effectiveness of plant and/or residue covers depends on the type, extent and quantity of cover. Vegetation and residue combinations that completely cover the soil, and which intercept all falling raindrops at and close to the surface and the most efficient in controlling soil (e.g. forests, permanent grasses ). Partially incorporated residues and residual roots are also important as these provide channels that allow surface water to move into the soil.

The effectiveness of any crop, management system or protective cover also depends on how much protection is available at various periods during the year, relative to the amount of erosive rainfall that falls during these periods. In this respect, crops which provide a food, protective cover for a major portion of the year (for example, alfalfa or winter cover crops) can reduce erosion much more than can crops which leave the soil bare for a longer period of time (e.g. row crops) and particularly during periods of high erosive rainfall (spring and summer). However, most of the erosion on annual row crop land can be reduced by leaving a residue cover greater than 30% after harvest and over the winter months, or by inter-seeding a forage crop (e.g. red clover).

Soil erosion potential is affected by tillage operations, depending on the depth, direction and timing of plowing, the type of tillage equipment and the number of passes. Generally, the less the disturbance of vegetation or residue cover at or near the surface, the more effective the tillage practice in reducing erosion.

Conservation Measures

Certain conservation measures can reduce soil erosion by both water and wind. Tillage and cropping practices, as well a land management practices, directly affect the overall soil erosion problem and solutions on a farm. When crop rotations or changing tillage practices are not enough to control erosion on a field, a combination of approaches or more extreme measures might be necessary. For example, contour plowing, strip cropping, or terracing may be considered.

Erosion processes

Soil may be detached and moved by water, wind or tillage. These three however differ greatly in terms of: where and when they occur what happens to the area that is being eroded (on-site impacts) how far the eroded soil is moved, and if the soil is moved away from the place where it was eroded, what happens as a result (off-site impacts).

Rainsplash

Rain may move soil directly: this is known as 'rainsplash erosion' (or just 'splash erosion'). Spash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a short distance.

Because soil particles can only be moved a few centimetres at most by this process, its effects are solely on-site. Although considerable quantities of soil may be moved by rainsplash, it is all merely redistributed back over the surface of the soil (on steep slopes, however, there will be a modest net downslope movement of splashed soil). Thus a more descriptive term might be 'rainsplash redistribution'.

Because rainsplash requires high rainfall intensities, it is most effective under convective rainstorms in the world’s equatorial regions. Rainsplash is relatively ineffective where rain falls with a low intensity (e.g. because the rainfall is of frontal origin), such as in the north-west of the USA or in northern Europe.

Rill and gully erosion

Rainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion.

That fraction of the rainfall which does not infiltrate (soak into) the soil will flow downhill under the action of gravity; it is then known as runoff or overland flow. Runoff may occur for two reasons. Firstly, if rain arrives too quickly (i.e. with too high an intensity) for it to infiltrate: the runoff which results is then known as infiltration excess runoff, or Hortonian runoff. Secondly, runoff may occur if the soil has already absorbed all the water it can hold (i.e. because it is fully saturated, or if the soil is frozen). Runoff which results from this situation is known as saturation excess runoff.

As runoff moves downhill, it is at first a thin diffuse film of water which has lost virtually all the kinetic energy which it possessed as falling rain. Thus it moves only slowly, has a low flow power, and is generally incapable of detaching or transporting soil particles.

The microtopography (i.e. small-scale pattern of irregularities) of the soil’s surface tends to cause this overland flow to concentrate in closed depressions, which slowly fill: this is known as ‘detention storage’ or ‘ponding’. Both the flowing water, and the water in detention storage, protect the soil from raindrop impact, so that rainsplash redistribution usually decreases over time within a storm, as the depth of surface water increases. There are, however, complex interactions between rainsplash and overland flow.

If rain continues, the increasing depth of water will eventually overtop the microtopographic depressions. Overland flow that is released in this way is likely to flow downhill more quickly and in greater quantities (i.e. possess more flow power as a result of its kinetic energy), and so may be able to begin transporting and even detaching soil particles. Where it does so, the soil’s surface will be lowered slightly. Lowered areas form preferential flow paths for subsequent flow, and these flow paths are in turn eroded further. Eventually, this positive feedback results in small, well-defined linear concentrations of overland flow (‘microrills’ or ‘traces’).

In many cases, individual microrills become ineffective over time due to sedimentation. A subset, however, grow further to become rills; and a smaller subset may go on to develop into gullies. This process of ‘competition’ between microrills and rills leads to the self-organized formation of networks of erosional channels (dendritic on natural soil surfaces; constrained by the direction of tillage on agricultural soils), which form efficient pathways for the removal of water from hillslopes. It is in such erosional channels that water erosion also operates most effectively to detach and remove soil by its kinetic energy. In most situations erosion by concentrated flow is the main agent of erosion by water.

The flow-dominated erosional channels are separated by interrill areas where the dominant processes are rainsplash and diffuse overland flow; however, boundaries between rill and interrill areas are both ill-defined and constantly shifting.

In some circumstances subsurface flow may be important in determining where channel erosion will begin and develop (e.g. at the base of slopes, and in areas of very deep soils such as tropical saprolites). Meltwater from thawing snow operates in a broadly similar way to rain-derived overland flow, detaching and transporting unfrozen soil in areas of concentrated flow. Snowmelt erosion is, though, less well studied and less well understood.

As erosional channels increase in size (i.e. grow to become large rills and gullies), processes such as gravitational collapse of channel walls and heads increase in importance. Runoff and sediment from rills and gullies may be moved into ditches, stream and rivers, and so transported well away from the point of origin. However, sediment may also be deposited within the rill or gully, or beyond the rill or gully’s confines in a depositional fan, at locations where the gradient slackens. Here it may be stored for a variable period of time, possibly being reworked by tillage activity, until a subsequent erosion event is of sufficient size to re-erode the stored sediment. It may then be redeposited further downstream, or make its way into a permanent watercourse and thence to lake or ocean.

Soil erosion in the past

Erosion of soil by water and wind has been occurring naturally since the first land plants formed the first soil, during the Silurian Period. Accelerated erosion is, from a geological perspective, of very recent origin; yet on a human timescale, accelerated erosion is old. There is considerable archaeological evidence from many parts of the world that accelerated erosion by water (in particular) is often associated with early agriculture. In a scientific context, water erosion’s association with unwise agricultural practices was first noted within during the early decades of the 20th century by pioneers of soil conservation such as Hugh Hammond Bennett in the USA, and subsequently by workers in other parts of the globe. During the period of colonialism, the imposed adoption of European agricultural methods frequently led to accelerated erosion in developing countries. There, the problem often continues to the present day. In the last few decades of the 20th century, there was a worlwide move towards intensive agricultural technologies. These frequently leave the soil bare during times of heavy rainfall. As a result, previously problem-free areas of the world, such as north-west Europe, began to experience notable increases in water erosion.

The extent of soil erosion

Despite the global nature of the problem of erosion by water, even today we do not have good information regarding the global extent of erosion by water. Data on the severity of erosion is also often limited. The GLASOD study estimated that around 15 per cent of the Earth's ice-free land surface is afflicted by all forms of land degradation. Of this, accelerated soil erosion by water is responsible for about 56 per cent and wind erosion is responsible for about 28 per cent. This means that the area affected by water erosion is, very roughly, around 11 million square km., and the area affected by wind erosion is around 5.5 million square km. The area affected by tillage erosion is currently unknown. Because soil is formed slowly, it is essentially a finite resource. The severity of the global erosion problem is only now becoming widely appreciated.

Ways of Preventing Soil Erosion 1.   Prevent soil erosion by planting vegetation, trees, ground cover, shrubs and any other plants. The roots from these plants will help hold the soil in place. Soil will not be easily blown away by wind, or washed away by the rain. 2.   Create windbreaks, which are Hedges or fences of trees designed to reduce erosion, especially wind erosion. Plant them on different plots of lands. 3.   Grow crops on farm lands. When land is not being used, use cover crops because they help prevent soil erosion by wind or rain. Beans are often used as cover crops.4.   Apply mulch, which is; a protective covering of rotting vegetable matter spread to prevent soil erosion. The topsoil is will not be likely washed or blown away, when it’s covered by mulch.  Soil Erosion Prevention Contour farming is another method that’s useful in preventing and controlling soil erosion by water runoff. It’s done by planting along the slope of a hill, following the natural contours of the land, instead of straight up and down or across. Another method is that you could plant a cover crop when your land is not in use. Besides providing protection for your land, many cover crops are nitrogen-fixers, which mean they absorb nitrogen from the air and deliver it back to the land. If you have a problem with wind erosion, try planting a windbreak. A windbreak can be a row of trees, bushes or even a plastic snow fence. Anything that will keep high winds from sweeping across your land can help prevent wind erosion. Keeping your soil healthy is a very important step to take in preventing soil erosion. Soil that is rich in organic matter has better structure and is less susceptible to being washed or blown away. To keep your soil healthy, add plenty of compost each year and don’t over-till when you are planting. Preventing soil erosion is always preferable to attempting to control or reverse it later. Once an area of land has been eroded, it’s sometimes impossible to correct it. Soil Erosion Prevention Methods There are many ways to prevention soil erosion from occurring on your land. There are also many products to assist you with this process. Before home or land owners choose a soil erosion prevention method they should accurately asses the soil erosion carefully before deciding on using a prevention product or methods. We will now discuss 4 top soil erosion prevention methods which will definitely help solve all soil erosion problems at a low cost. Prevention Method 1: Planting Vegetation One of the most common ways to prevent soil erosion on residential landscaped gardens and vegetation and crop properties is to plant flowers, trees and crops over the affect soil. Plants act as protective shields to the soil lessening the impact of rainfall, wind, excessive watering and ice melt. The plants will also help stabilize the soil and prevent it from becoming prone to soil erosion. Some popular soil erosion prevention plants are: wild flowers, crop veggies, small trees and herbs. Plants which crawl up and spread instead of growing upwards are also great soil erosion prevention plants. Prevention Method 2: Matting Soil erosion prevention products are available in many styles. One of the most common products which are most commonly used on residential properties, vegetation crops and vacant land is matting. Matting is available in wood fibres which make it environmentally friendly and biodegradable. The matting will be placed on the soils surface and prevent erosion from occurring. The matting will allow plants, crops and trees to grow through it and the soil will be healthy and stabilized. Matting can be cut to size to suit your property. Prevention Method 3: Mulch/Fertilizer Another soil erosion prevention method which is beneficial to the soil and plants which live in it, is applying a layer of mulch and fertilizer over the soil. The mulch and fertilizer layer will assist the soil to soak in water slowly and it will also lessen the impact of rainfall as it penetrates through to the soil. The mulch and fertilizer layer will also stabilize the affect soil by regaining its PH levels to be healthy and neutralized. Any type of mulch or fertilizer can be used to prevent soil erosion. Prevention Method 4: Retaining Walls/Edging Wet patches or mud puddles on your driveway, or any hardscaped area are a sign of soil erosion runoff. This is generally caused by water erosion. Water erosion is affecting the soil and making it expand and travel to different areas of the yard such as the driveway. You can prevent this from occurring by building a small retaining wall around your garden beds. The retaining wall will act as a shield for the soil and prevent soil erosion from occurring. The wall will also keep water retained in the garden bed so that the soil will slowly soak it in. if used in conjunction with other soil erosion prevention methods this method can be very rewarding to your property. A GIS-based model of soil erosion and transport Soil erosion is a natural process that occurs when the force of wind, raindrops or running water on the soil surface exceeds the cohesive forces that bind the soil together. In general, vegetation cover protects the soil from the effects of these erosive forces. However, land management activities such as ploughing, burning or heavy grazing may disturb this protective layer, exposing the underlying soil. The decision making process in rural catchment management is often supported by the predictive modelling of soil erosion and sediment transport processes within the catchment, using established techniques such as the Universal Soil Loss Equation [USLE] and the Agricultural Nonpoint Source pollution model [AGNPS] Wind Erosion Control Management practices to control wind erosion are critical on sandy, muck, or peat soils, and should also be considered on clay or silty soils. Maintaining good soil structure and residue cover provides good resistance to wind erosion. Where little or no residue is left on the soil surface, (e.g., corn silage), a cover crop of winter rye may be sown to protect the surface of wind-susceptible soils until spring. Fencerows and snowfencing also provide good protection. Strip cropping, or even planting crops at right angles to prevailing winds is a method of controlling wind erosion on land susceptible to strong winds. Tree windbreaks should be planted along the north and west boundaries of fields, and may be planted all around fields where wind erosion is a particular problem. On very steep slopes or areas where blowouts or rills/gullies frequently occur, permanent sod or tree cover should be maintained, and may in fact provide better financial returns. Coral Reef Tells The History Of Soil Erosion Coral reefs, like tree rings, are natural archives of climate change. But oceanic corals also provide a faithful account of how people make use of land through history, says Robert B. Dunbar of Stanford University.  As per a research findings reported in the Feb. 22 issue of Geophysical Research Letters, Dunbar and colleagues used coral samples from the Indian Ocean to create a 300-year record of soil erosion in Kenya, the longest land-use archive ever obtained in corals. A chemical analysis of the corals revealed that Kenya has been losing valuable topsoil since the early 1900s, when British settlers began farming the region.  "We observed that soil erosion in Kenya increased dramatically after World War I, coinciding with British colonialism and a series of large-scale agricultural experiments that provoked a dramatic change in human use of the landscape," said Dunbar, a professor of geological and environmental sciences. "Today, the Kenyan landscape continues to lose topsoil to the Indian Ocean, primarily because of human pressure".  Erosion is a serious threat, he noted, because the loss of fertile soil often is accompanied by a decrease in food production. As per one recent study, soil erosion is a global problem that has caused widespread damage to agriculture and animal husbandry, placing about 2.6 billion people who are at risk of famine. "This is especially worrisome in East and sub-Saharan Africa, where per capita food production has declined for the last half-century," Dunbar said.  Coral bands  For the study, Dunbar and colleagues donned scuba gear and dove to the Malindi coral reef near the mouth of the Sabaki River, the second longest river in Kenya. Draining about 11 percent of Kenya's landmass, the Sabaki transports sediments to the sea.  The scientists took core samples from two large coral colonies, each more than 12 feet tall and about 15 feet wide. To find out how sediment flux has varied over the years, Dunbar's team measured the ratio of two elementsbarium and calciumin the coral skeleton, which is composed of calcium carbonate. "It turns out that there is a lot of barium in soils," Dunbar said. "So whenever something changes the landscape and causes the soil to erode and wash into the rivers, the soil is delivered to the sea. And with that soil comes the barium". The corals then incorporate the barium in well-developed bands that provide a record of annual growth, similar to tree rings, he added. To measure barium levels in the corals, Dunbar's team applied an innovative technique that quickly vaporizes the carbonate, then analyzes its chemical composition with a mass spectrometer. "In the past we used a dentist drill," Dunbar said. "We drilled out a little bit of powder, and then we dissolved the powder and took it to the lab, where we measured the barium with a wet chemical technique. It was a very slow process, very painful. It took forever to get data." The new method, developed by scientists at the Australian National University, "increased the speed at which we could collect data by a factor of 50," he noted.

Agricultural Soil Erosion Is Not Adding to Global Warming

Agricultural soil erosion is not a source of carbon dioxide to the atmosphere, as per research published online today (October 25) in the journal Science. The study was carried out by an international team of scientists from UC Davis, the Catholic University of Leuven in Belgium, and the University of Exeter in the U.K. 

Carbon emissions are of great concern worldwide because they, and other greenhouse gases, trap heat in the Earth's atmosphere and are a major cause of global climate change. 

"There is still little known about how much carbon exactly is released, versus captured, by different processes in terrestrial ecosystems," said Johan Six, a professor of agroecology at UC Davis and one of the study's authors. "We urgently need to quantify this if we are to develop sensible and cost-effective measures to combat climate change". 

In their new study, the scientists observed that erosion acts like a conveyor belt, excavating subsoil, passing it through surface soils and burying it in hollows downhill. During its journey, the soil absorbs carbon from plant material; when the soil is buried, so is the carbon. 

Erosion, therefore, creates what can be described as a "sink" of atmospheric carbon. 

The team improved prior estimates of the amount of carbon being sunk. Said lead author Kristof Van Oost of the Catholic University of Leuven, "Some academics have argued that soil erosion causes considerable emissions of carbon, and others that erosion is actually offsetting fossil-fuel emissions. Now, our research clearly shows that neither of these is the case". 

They observed that erosion captures the equivalent of about 1.5 percent of annual fossil-fuel emissions worldwide. Earlier studies suggested a broad range of erosion's effects, from a sink equaling 10 percent of fossil-fuel emissions, to a source equaling 13 percent. 

Even without major carbon impacts, the scientists said, erosion is a problem that must be addressed, because it has a detrimental effect on agricultural productivity and the surrounding environment. 

Soil erosion: what we still don't know

Greater understanding of the occurrence, processes and impacts of soil erosion by water, wind and tillage is needed. Why? Both directly, in order to enhance mankind's ability to tackle the resulting environmental problems; and indirectly, in order to learn more about the processes of erosion and the conditions under which it occurs.