August 24, 2018 Soil Degradation Written by Sara Slavikova
erosion
Soil erosion is the most serious precursor of soil

degradation that comes with global implications. Nearly 10 million hectares of arable land are lost to erosion and other forms of soil degradation every year [1].

Countries all over the world are battling with worrisome sight of deepening gullies crisscrossing the landscape and barren fields stripped of the fertile topsoil.

Even the costs of losing such an important resource are incredibly high. According to the estimates, the cost of offsetting erosion effects in the United States ranges between US $100 million to $44 billion per year [1]!

If we will not take necessary steps to prevent the accelerating loss of our precious soils, there might come a time when we will not be able to produce enough food to feed growing population or harvest natural resources that require soils as a medium for growth. Neither will we be able to enjoy the crucial ecosystem services soils perform for us, such as filtration of rainwater to be suitable for drinking and a completion of nutrient cycles.
 

 

What is soil erosion?

Soil erosion is a natural process that has been shaping the face of the earth for millennia and has given rise to many specific landscapes, exposing rocky peaks or curving meandering river channels.

Soil erosion occurs when upper layers of soil are removed from their original location in the process that is either slow and goes unnoticed for long periods of time or can be sudden and cause immediate damage (for example: appearance of a rill in the middle of a crop field after heavy rain).

When erosion takes place, soil particles get loosen by water, wind or gravitational pull and can be easily carried away by the repeated action of these forces.

Water erosion on the beach

Water erosion on the beach

We can see the effect of this after heavy rains when small water streams find their way down a hiking trail. Since water comes with a great destructive power, with every new storm it will cut deeper and deeper into the trail, transforming it completely.

Once a small erosive process takes place, it makes the surface vulnerable to consecutive erosive events. This results in the loss of topsoil and will affect the health of the whole ecosystem as well as our capability to make the use of it – for growing crops, for example.
 
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What causes soil erosion?

Soil erosion is a global issue and we can see its traits everywhere we go. It doesn’t matter whether it is in the rural or urban settings, erosion affects even untouched soils of pristine natural areas.

The reason for this is that erosion is caused by numerous factors, originating from both natural processes and human activities. In some cases, it is the combination of more of these factors that influence soil stability and health.

Let’s have a look at the most common causes of soil erosion.
 

Natural causes of soil erosion

 

#1 Water runoff and rainfall

Water is one of the strongest erosive agents that has a great potential to disturb the soil surface.

Depending on the duration and intensity of a rainfall, soils can exhibit signs of one of the four forms of the damage, including:

  • Splash erosion
  • Sheet erosion
  • Rill erosion
  • Gully erosion

 
Starting with the minor soil disruption caused by the impact of falling raindrops, splash erosion moves around small particles in the upper soil layer.

If soils get saturated with water after prolonged periods of intensive rain, their capacity to absorb water decreases. This leads to the increased surface runoff, which initiates the following forms of erosion.

In some cases, sheet erosion washes away the top layer of soil in one continuous piece (one “sheet”). Or the force of water gives rise to the formation of small rills that can eventually develop into gullies. These types of erosion are a common sight on many agricultural lands.

We can observe the formation of a small channel (rill) created by running water after the storm. If this channel is left as it is, water will cut deeper into the soil, widening and deepening the channel until it will create a gully through which large amounts of fertile topsoil are carried away from the site.

Eroding coastline

Eroding coastline

Soils with a loamy texture, consisting of fine sand and silt particles, are generally more prone to water erosion. These soils are structurally weak because their particles do not stick well together [2].
 

#2 Flowing water

Flowing water bodies such as rivers or streams put continuous pressure on the walls and the bottom of their channel. Flowing water gradually tears down river banks, stripping away parts of surrounding land and undermining its stability.

The most destructive force of a flowing body of water occurs during the period of floods. High amounts of rapidly moving water are capable of tearing and removing large pieces of land and sediments. In some unfortunate cases, tumultuous water even tears down houses, bridges or other otherwise sturdy structures.
 

#3 The slope gradient

Erosion is more likely to take a place on hilly sites.

In general, the susceptibility to erosion increases with the steepness and the length of the slope. This happens because inclined surfaces do not absorb water as well as straight surfaces.

When it rains, most of the rainwater runs down the hill. And as you can imagine, the steeper and the longer the hill is, more velocity and force this water gains. Increased runoff combined with the gravitational force can easily strip off the upper soil layer, especially if it is already damaged in some way, for example by deforestation.
 

#4 Soil erodibility

Soil erodibility is the natural susceptibility of a soil to erosion. Some soils are more prone to erosion than others and the main factors that affect their vulnerability are:

  • Texture: Soil erodibility increases with the amount of silt and fine sand particles. The least likely to erode are clay soils because they hold well together.
  • Organic matter content: Organic matter in the soil has two important functions. The top layer of rotting leaves or other vegetation shield soils from the direct impact of rain and wind. The organic material from deeper layers binds other soil particles together and stimulates the formation of aggregates, which are important structural component of resistant soils.
  • Structure: When individual particles form stable aggregates, they are less likely to be carried away by rain or wind due to their heavier weight and improved cohesion. Good soil structure also supports creation of cavities and large pores that allow easy movement of water and its prompt absorption. Degraded soils with a poor structure are easily erodible because both of these characteristics are lacking.
  • Permeability: Soils with better permeability allow water infiltration, which makes them less likely to suffer a damage from heavy rains [3].

 
It is not a coincidence that all these four characteristics are equally as important for soil fertility as they are for the ability of soils to resist erosion. In general, healthy soils are capable to withstand weather extremities the best. This is another reason why maintaining good soil health should be our priority.
 

#5 Wind

Erosion by wind is a common sight in dry and barren areas where vegetation doesn’t hold soils in place. Sand dunes consisting of fine sand particles are the best example of the wind-induced movement of the upper soil layer. According to a scientific study in Sahara desert, sand dunes can move by 50 to 295 feet throughout one year.

Sand dunes formed by wind

Sand dunes formed by wind

With the same ease and frequency as sand dunes move, light particles of dirt and dust can get carried by wind in many vulnerable areas. These wind-blown particles then deposit somewhere else – very often in the least suitable places like roads, crop fields or private properties.

Wind erosion does not only leave behind nutrient deprived soils that cannot support living vegetation, and thus make the situation even worse, but it also leads to a more serious problem in the long term – desertification.

The Simpson Desert Conservation Park in Queensland, Australia, featuring large sand dunes including the popular ‘Big Red’ dune, is an area that demonstrates the power of long-term wind erosion and gradual particle deposition [4].


 

#6 Local climate

The amount and intensity of precipitation, number of windy days, wind power, droughts, flooding or sudden weather changes. All these factors and their combinations can cause erosion under certain circumstances.

This means that some climates actually create perfect conditions for soil erosion, be it because of exceptionally heavy rains and flooding, or prolonged droughts and strong winds.

For example, researchers from the Plymouth University in the United Kingdom were appointed to investigate alarming levels of soil erosion in Tanzania.

Over the past 10 years, vast areas of Tanzanian landscape suffered extensive damage due to erosion caused by vegetation loss combined with climatic factors – long periods of drought are subsequently followed by heavy rains during the rainy season. The outcome are deep gullies (up to 26 feet!) fragmenting the landscape and making it difficult for people and cattle to move around safely.
 

#7 Loss of vegetation cover

Plants and trees help stabilize soils and protect them from the direct exposure to rain or wind.

In fact, vegetation is one of the greatest tools in the prevention of erosion. It slows down surface runoff, enabling better water infiltration into the soil. It also shields a soil from raindrops and breaks down the wind before it can reach the soil with the full power. And nothing holds soil better together than dense root system of healthy plants.

Dry, sealed soil without any vegetation

Dry, sealed soil without any vegetation

Considering how many protective functions vegetation cover has, it should come as no surprise that changes of land cover can be a primary cause of erosion in areas that previously haven’t suffered of this problem.

You may notice this development after flooding. When an area gets flooded, most of the vegetation is stripped away, which leaves originally covered soils fully exposed to the erosive force of rain. Unless protective measures are taken, the land will suffer even more damage with every new rainfall.
 

Human-induced causes of soil erosion

 

#1 Mining

Mining, one of the activities that enabled the growth of our economies, has severely scared the face of our planet and destroyed many unique habitats without mercy. Some of the world’s largest mines cut as deep as 0.75 miles into the earth’s surface and spread over an area of more than 2,000 acres [5].

These vast surfaces are changed forever. Vegetation and top soil layers are removed and high amounts of waste from the deep layers are brought up to the surface where they often remain deposited without any further management, even though they usually contain various mineral deposits that can create a toxic cocktail after the exposure to water and oxygen.

This creates a direct path for erosive processes to redistribute all this (noxious) material to distant locations and remodel already severely damaged landscape.

Illegal amber mining in Ukraine has left behind thousands of hectares of destroyed land. Land that was previously a home to lush forests and prospering ecosystems is turned into lifeless swaths of mud and post-mining craters filled with water that was pumped there under pressure to help uncover amber deposits underneath. The site of destruction is left behind without anyone taking care of the damage. Only erosion gradually removes the last pieces of soil that could harbor new life.
 

#2 Deforestation

The rate of erosion in forests is naturally very low because complex root systems of trees anchor the soil in place and fallen leaves or other green material offers a protective cover.

But during the deforestation when forests are clear cut or burned down in the ‘slash-and-burn’ practice, soil stability is disturbed. And even though the soil was healthy and resistant prior deforestation, after clearing, it will be easily washed away by rain.

Deforested land

Exposed and vulnerable deforested land

This is a common problem in many countries with rainforests. When rainforests are cut to make space for crops, valuable topsoil erodes away under the pressure of heavy rains and crop yields decline in a very short period of time. In the search for a land that will provide satisfactory yields, local farmers cut more of the forest and the same story repeats again. And again.

This is perhaps an all too familiar story for farmers from Madagascar. The island loses to deforestation-induced erosion around 400 tons of topsoil per hectare [6]. It is so much soil that it doesn’t go unnoticed. How? Soil particles are washed off into rivers, giving water a specific red color and leaving patches of eroded red dirt dispersed all over the land.
 

#3 Agriculture

Numerous cases from the past could help describe how agriculture contributes to soil erosion, but perhaps one of the best examples is the ‘Dust Bowl’ that turn nearly 150 million acres of land into a dry and dusty wasteland in the North America’s Midwest and Southern Great Plains in 1930s [7].

The Dust Bowl was caused primarily by the misjudgment of the local climate and ecology. Inexperienced farmers settled in the area and in the hope of making profit from increased prices of wheat, turn millions of acres of native grassland into heavily tilled fields.

But the area is naturally very dry, and the land couldn’t support crops that were not adapted to the climate like the native grasses were. When the crops failed, barren land remained fully exposed to the erosive forces of wind, which gave rise to massive dust storms.

Storms which were truly disastrous.

One dust storm creating a wall of 2 miles and traveling over 2,000 miles even reached the New York City, giving the first-hand experience of powerful erosive forces to its residents [8].

Overgrazing is another major contributor to erosion. When large groups of animals feed from one area, the vegetation cannot naturally regenerate and will start dying. Without having space large enough, animals also repeatedly trample the vegetation and soil, triggering its further degradation and making the plant recovery more difficult.


 

#4 Urban development

Urbanization changes the landscape in many ways that encourage erosion and make it worse over the time. Many construction projects begin by completely removing vegetation, which affects the ability of soils to absorb water, often leaving soils exposed and vulnerable for many years before the project is finished.

Cities are also characteristic with large amounts of impervious surfaces that do not allow water to infiltrate into the ground at all. This increases the surface runoff which washes off fragmented pieces of soil in between.

In some locations, spaces between buildings create pathways for wind that magnify its erosive power.

Loess Plateau in China is infamous for the world’s highest erosion rates. 1.6 billion tons of soil are washed off into the Yellow River from the plateau every year.

According to scientists who investigated the conditions in the area, rapid urban development with large construction projects is one of the main reasons for these high rates of soil loss. Researchers found out that loosen piles of earth removed from construction projects degrade more than 10 times faster than normal soils.
 

#5 Recreational activities

Tourism and recreational activities often damage ecosystems, especially if the number of tourists in a given area is greater than the ecosystem capacity to deal with it.

Tourism increases vulnerability of slopes to erosion

Tourism increases vulnerability of slopes to erosion

In the most visited places, tourists trample the vegetation around trails, slowly creating larger patches of vegetation free surface. Frequently walked trails become compacted, which leads to the decreased soil permeability and higher surface runoff. The combination of these factors then results in progressively eroding trails and areas around them as people try to avoid slippery or muddy surface of the main trail.

The same scenario happens when off-road biking, horse riding, having fun with ATVs or parking cars on the side of the road.
 

#6 Climate change

Climate change comes with many changes for our planet and the environment.

One of the most talked about effects of climate change is a changing rainfall pattern. With some areas getting more frequently heavy rains and other areas suffering of prolonged droughts, the risk of erosion is expected to rise around the world.

Extreme events accompanied by floods, land-slides and debris flow also take their toll on the health of our soils. Just the global events of the summer 2018 demonstrate the great destructive power of climate change. Strong monsoon rains in Kerala, India, triggered disastrous landslides that killed many people, while the whole Northern Hemisphere boiled under extreme heat waves, sparking deadly wildfires as far as the Arctic circle.

All these events strip soils bare and allow erosion to take place.

Additionally, large-scale erosion accelerated by climate change threatens numerous coastal communities in Alaska. As previously frozen sub-surface layers of permafrost melt, soils easily fall apart and get washed off by more intense rainfall and fierce waves that batter the coast cut deeper into the mainland.

For example, a small village Napakiak has lost in one single storm 50 feet of its shoreline to erosion [9].
 
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Effects of soil erosion on the environment

Soil erosion is an ongoing process that affects each site in a slightly different way. Its effects can be seen directly in the location where erosion takes place, known as on-site effects, and at the site where the eroded soil deposits – off-site.

Let’s first have a look at the problems on-site erosion brings.
 

On-site effects of erosion

 

#1 Impaired soil productivity

Agricultural land is among the most affected lands by erosion worldwide. For example, most cultivated lands in Iowa lose to erosion two to five tons of topsoil per acre per year [10]. The loss of topsoil is what decreases the productivity of soils and negatively affects our capability to grow crops on them.

Topsoil is crucial for plant growth because it contains the majority of organic matter and 50 percent of important nutrients such as phosphorus and potassium [10]. It is in the topsoil where large pores and soil aggregates form, allowing for proper water infiltration and aeration.

When topsoil erodes away, nutrient and organic material is lost and only compacted clayish soils with poor structure remain behind. Due to the unfavorable conditions, plants grown on these soils strive and often do not produce sufficient yields.

A very common practice in intensive agriculture to offset the declining yield from eroded soils is to add synthetic fertilizers which supply some of the lost nutrients. However, this strategy is only a short-term fix to get the last bit of harvest from damaged soils.
 

#2 Desertification

Overexploited lands in dry regions are extremely vulnerable to soil erosion. Soil erosion itself seriously impairs the quality of soil and its natural recovery rate, but it is not the worst problem that can occur.

In many cases, erosion is the first step that starts the irreversible transformation of the landscape into the barren desert. Grain by grain, sand and dirt is picked up by wind from mismanaged lands and pushed farther and farther into new territories, slowly swallowing remaining vegetation and turning the area into the wasteland.

Erosion enhances desertification

Erosion enhances desertification

Once turn into desert, soils cannot support plant life anymore, which leads to crop failures, the disappearance of whole ecosystems and biodiversity declines.

Luc Gnacadja, executive secretary of UN’s Convention to Combat Desertification, said that desertification is “the greatest environmental challenge of our time” and “a threat to global wellbeing” [11].

With previously cultivated lands turning into deserts, many communities across the world are losing their only chance of providing enough food for themselves or their livestock.

Mongolia is one of the countries with a high vulnerability to desertification. In fact, wind erosion has been displacing more than 40 tons of soil from each hectare of cultivated land throughout the past 30 years. Such a high rate of erosion, worsened by climate change, threatens to turn 90 percent of the Mongolian territory into a desert [12].

Where will the famous Mongolian nomads graze their large herds of livestock then?


 

#3 Land degradation

Erosion by wind or water is greatly responsible for nearly 85 percent of soil degradation throughout the world [13]. A new study backed by the United Nations estimates that one third of all the land is severely degraded worldwide [14].

Land degradation is defined as the long-term loss of soil productivity and ability to provide crucial ecosystem services for the proper health and functioning of our planet. This includes services such as decomposition of organic matter and cycling of nutrients, formation of new topsoil and groundwater replenishment.

A common problem of degraded lands is a sealing of the soil surface. This minimizes water infiltration and causes a myriad of changes to local hydrology. Increased runoff, for example, leads to frequent flooding after rain, while soil overall becomes drier because it loses its water holding capacity over the long term.

Degraded land cannot support these essential soil functions or growth of vegetation, and therefore, renders affected land unsuitable for cultivation.

Unfortunately, some countries experience the problem more than others. According to the Food and Agriculture Organization (FAO), erosion degrades 200,000 hectares of land in the Philippines every year and 114 million hectares get destroyed in India.
 

Off-site effects of erosion

 

#1 Water pollution and sedimentation

The major off-site problem caused by erosion is the deposition of eroded soil, along with pollutants it picked up, into watercourses.

This causes two serious problems:

 
Besides introducing new pollutants in the water, soil transported into water bodies disrupts aquatic ecosystems by changing chemical and physical properties of water.

Agricultural fertilizers can cause eutrophication and mass dying of aquatic life. Increasing sediment loads can block rivers and dams, eventually leading to mud floods and further damage to surrounding ecosystems and built structures.

High sedimentation rates also damage hydro-electricity plants and decrease the lifetime of constructed water reservoirs. Additional problem is the pollution of drinking water supply, which requires extra investment into removing impurities and making water suitable for drinking again.

Environmental Protection Agency (EPA) estimates that the yearly cost of fixing the damage caused by sedimentation rises as high as $US 16 billion in the United States. Sedimentation which is out of 70 percent brought about by human-induced erosion [15].
 

#2 Flooding

The deposition of silt in water courses often obstructs their natural path. This increases the risk of flooding and further enhances erosion of water banks, since the water seeks the way to flow around a newly created obstruction.

Small mountain creek slowly erodes its banks

Small mountain creek slowly erodes its banks

Disturbed soils also do not absorb water as much as they naturally would. Deforested areas turn into arable lands or overgrazed pastures lose their ability to effectively capture and retain water. Higher surface runoff then swells water streams to bigger size and floods become more frequent and extensive even in areas where they haven’t occurred ever before.

Rainy years in 2010 and 2011 in Colombia led to extreme flooding of the Magdalena River, whose banks are a home to 80 percent of Colombians. The floods were a tragedy for these people, forcing 2.5 million people to relocate and destroying one million hectares of crops.

After the event, scientists sought after the cause of this unusually strong event and found out that the sedimentation rates in the river are highest they have ever been – all due to erosion happening on large areas of recently deforested land [16].
 

#3 Airborne dust pollution

Just as the story of the Dust Bowl mentioned previously suggests, wind erosion has the power to cause widespread air pollution that can affect places hundreds or even thousands of miles away from the original site of erosion.

There are numerous cases describing its suffocating effects.

In the spring of 2015, the Department for Environment, Food and Rural Affairs warned England’s citizens about “blood rain”- rain mixed with red sand and light dirt particles carried by wind more than 2,000 miles from Sahara Desert to England. The polluted rain was expected to contribute to poor outdoor air quality that might result in respiratory problems [17].

The same problem bothers citizens in China, North and South Korea, and Japan, as the dust blown from the Gobi Desert scourges their territory every spring.

Wind-blown dust particles do not harm only the health of people and animals, they also damage young plants and crops. Wind often picks up fine particles of sand, silt and organic matter and buries or breaks seedlings, while exposing seeds or plant roots in other places. This leaves behind weak and vulnerable crops that cannot provide sufficient yield.


 

#4 Damage to infrastructure

Whether it is a damage caused by flooded rivers, roads and rails torn apart by sliding land or dams cracking under the burden of accumulating sediments, soil erosion can gradually and certainly somewhat sneakily destroy many built structures. In fact, to predict the future rate of erosion when planning a new construction project is extremely difficult, since there are so many possible triggers.

Eroding coastline undermines beach house foundations

Eroding coastline undermines beach house foundations

Despite the access to modern technology and modelling tools, erosion still unpleasantly surprises many project designers.

For example, Indian government has measured the rate of sedimentation in 11 water reservoirs around the country and found out that the amount of deposited material is up to 1,600 percent higher than previously expected. The lifetime of these reservoirs has been therefore significantly shortened [18].
 
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Soil erosion is an unfathomable enemy that often goes unnoticed right in front of our eyes and slowly eat away the most precious resource we have – our soil. Without healthy soils life cannot exist. It is time to start carrying about what happens with this resource and whether it has been managed in a sustainable manner.

So, what are you waiting for?

You can be the one who takes over this task and becomes the facilitator of the change for better soils.

 


References

[1] http://www.fao.org/soils-portal/soil-degradation-restoration/cost-of-soil-erosion/en/
[2] http://users.ictp.it/~pub_off/lectures/lns018/30Salako1.pdf
[3] http://fruitsandnuts.ucdavis.edu/files/68962.pdf
[4] https://www.qld.gov.au/environment/assets/documents/land/soil/wind-erosion.pdf
[5] https://www.mining-technology.com/features/feature-top-ten-deepest-open-pit-mines-world/
[6] https://rainforests.mongabay.com/0903.htm
[7] https://www.history.com/topics/great-depression/dust-bowl
[8] https://www.history.com/news/10-things-you-may-not-know-about-the-dust-bowl
[9] https://www.alaskapublic.org/2018/08/20/napakiak-gets-federal-funding-for-erosion-climate-change-impacts/
[10] https://crops.extension.iastate.edu/soil-erosion-effect-soil-productivity
[11] https://www.theguardian.com/environment/2010/dec/16/desertification-climate-change
[12] http://forum.mn/res_mat/Desertification%20in%20Mongolia.pdf
[13] https://www.tandfonline.com/doi/full/10.1080/24749508.2017.1301053
[14] https://www.theguardian.com/environment/2017/sep/12/third-of-earths-soil-acutely-degraded-due-to-agriculture-study
[15] https://cfpub.epa.gov/npstbx/files/ksmo_sediment.pdf
[16] http://news.trust.org//item/20130423154348-k239i/
[17] https://www.telegraph.co.uk/news/weather/11526891/Saharan-red-dust-and-a-spike-in-air-pollution-prompts-health-warning.html
[18] https://www.internationalrivers.org/sedimentation-problems-with-dams