What is soil erosion?

 

Before we start, it is worth pointing out that soil erosion is an entirely natural process that has occurred on planet earth for millions of years.

‘Erosion’ is an all-encompassing term for the geological process where natural substances are gradually worn away or broken down by natural forces.

When relating to soil, erosion refers to the wearing away of the top layer of soil.

Naturally, this may occur by wind, water, snow or ice as with all types of erosion. But thanks to human activity this process has gone from a gradual one, which might have taken place over decades or centuries, to a much faster one.

Each year around 75 billion tons of soil is eroded worldwide and soil is being lost at 13 to 40 times as fast as it is naturally being replaced. (1)

This has therefore led to quite a few environmental problems as a consequence.

So what are the causes, effects and solutions to soil erosion?

 

What causes increased soil erosion?

 

Deforestation

 

Trees are vital in preventing soil erosion for a number of reasons.

1) The roots hold the soil together

Trees, and other vegetation play a key role in holding soils together. The roots of large trees, in particular, can extend for well beyond the point where the branches end.

These roots are there to provide stability for the tree, but also to absorb nutrients and water mostly from the top layer of the soil.

These roots vary in size from large roots for strength to tiny roots for absorption. This network that is created weaves through the top layer of soil and binds it all together, keeping it in place and preventing soil erosion.

2) The leaves provide shelter from the rain and wind

The leaves of a tree slow down rain as it falls preventing it from directly hitting the soil in a process known as ‘interception’. These water droplets are slowed right down and may either gradually make their way to the soil or evaporate off the leaves before getting chance.

All this interception prevents rain from directly hitting the soil which causes erosion as the droplets dislodge soil particles and wash them away

3) Groups of trees in a woodland prevent wind erosion

As a collective, trees act as an effective barrier to wind.

If you’ve ever stood in a dense forest on a windy day you’ll know it feels very sheltered. This prevents erosion of soils further as winds are less powerful and so less likely to blow away any of the top soil layer.

Deforestation is common across the planet and has been for centuries, mainly due to the expansion of agriculture which demands large amounts of deforested land.

Deforestation increases soil erosion for all the reasons mentioned above. Research has shown that erosion rates of soils in deforested land reached over 1500 times greater than those in forested land prior to deforestation! (2)

Studies, in fact, show that just reducing forest cover below 60% is enough to cause serious soil erosion. (3)

 

Tilling and Ploughing

 

Once these agricultural lands are created, the soils are repeatedly disturbed every year before the crops are planted in a process known as tillage.

Tilling soil involves turning the top layer of soil that has become compacted to make it suitable for planting seeds.

However, in breaking up the soils and destroying the structure this accelerates erosion. These smaller particles of soil created by the tilling process are much lighter and can be eroded by wind or washed away by a heavy rain shower.

The amount of soil erosion from farmland can be directly attributed to the level of tillage undertaken on the land. This problem gets worse every year as the valuable topsoil, which contains all the nutrients and microbes needed to grow crops, is eroded. This means farmers increase the level of tilling which leads to more erosion and so on.

 

Climate Change

 

In a warming climate, many areas of the planet are experiencing higher temperatures. This leads to soils drying out more quickly and more frequently. A drier soil is more easily eroded by the wind, as you can imagine through the imagery of dust storms in deserts and other dry regions.

Climate change is also leading to an increase in more intense rainfall events. This combined with the hotter temperatures and drier soils means that when this heavy rain does fall, the erosion rates are much higher as the droplets dislodge the soil and water runs over the surface washing even more away.

But the full interaction between a changing climate and soil erosion is hard to predict because so many factors are at play.

Microbes in the soil may break down organic matter faster due to higher levels of carbon dioxide in the atmosphere. Less organic matter content in the soil also leads to increased rates of erosion.

 

What are the effects of soil pollution?

 

So we know which human activities are leading to this increase in soil erosion, but what effects is it having on the environment?

Loss of farmland

 

It is estimated that around half the worlds topsoil (the nutrient rich upper layer of soil) has been lost to in the past 150 years. This is largely due to erosion but also intensive farming practices. According to the United Nations; this shows no signs of slowing with land becoming degraded and unsuitable for farming at a rate of 24 billion tonnes a year!

This loss of productive farmland is worrying as the global population continues to grow, we are having to feed it on less land. 20% of the world’s croplands are decreasing in productivity as the world’s population grows at 1.1% per year.

Studies in Iowa, USA, which is known for it’s rich fertile soils, found that due to erosion; the average depth of the topsoil decreased from around 40cm to around 20cm in around a century. In particularly bad years when storms combine with dry weather, huge amounts of topsoil can be lost in one year. 2014 was one such year when Iowa lost 14 million tonnes of soil.

This high nutrient topsoil is not easy to replace, it has accumulated over centuries and so once it is lost it won’t be returning within our lifetimes. With over 40% of land already used for agriculture, there aren’t many places left to farm!

 

Need to increase tillage

 

As the soil quality decreases due to erosion, farmers have to resort to trying to get more life out of the dying substance. To do this they turn to the process of tilling, where they use machinery to turn the soil over.

As we have already discussed, this is a vicious cycle. Tilling the topsoil destroys the ecosystem within it and the broken-up structure is far more vulnerable to further erosion.

 

Pollution of watercourses

 
Image: Flickr ( docentjoyce) cc2.0

All that soil that is eroded has to end up somewhere, and like most types of pollution that place is in watercourses such as rivers and streams. Once washed or blown into these waterbodies the soil causes other issues.

Firstly, it makes it difficult for sunlight to penetrate meaning any aquatic plants in the water that need this light for photosynthesis no longer receive it and end up dying.

Secondly, these soils are often high in nutrients, both naturally (as most organic matter is in the topsoil) but also due to the amount of fertilizer that is present in agricultural soils. Once this nutrient-rich soil enters the water it causes some aquatic life such as algae to experience rapid growth. These algal ‘blooms’ cause havoc to other life as they absorb huge amounts of oxygen from the water in a process known as ‘eutrophication’.

Once rivers become polluted with soil sediment they can also become unsuitable for human or animal consumption impacting many communities that rely on this.

 

Air Pollution

 

Erosion from the wind causes particles of soil to become suspended in the air. Although this occurs naturally, degraded, drier soils are more susceptible to wind erosion causing an increase in overall air pollution in certain areas.

Air pollution can lead to human health problems such as asthma which is directly linked to airborne dust.

 

Landslides

 

A landslide is once again a natural process that occurs when soil, rocks or other materials move down a slope due to the force of gravity. Usually, a whole layer of soil moves at once which leads to devastating impacts caused by the weight and speed of the moving solid mass.

The likelihood of this occurring is increased as soil erosion occurs on these sloped areas.  

 

Flooding

 

The top layer of soil holds large amounts of water in the pores and spaces between particles.

Without as much topsoil to absorb water, water runs off the compacted soil or exposed bedrock more quickly causing flash floods in heavy rain events.

Soil which ends up in rivers and other waterbodies can also settle to the bottom and reduce the carrying capacity of the river, this means that in the event of rainfall the water level is more likely to rise above the river banks and cause a flood.

 

Climate change

 

Yes, climate change has already been mentioned as a cause of soil erosion but it is also an effect of soil erosion too.

Soils sequester and store large amounts of carbon. Grasslands, for example, store a total or 343 gigatons of carbon in just the top one meter of soil. (4)

But as these soils become degraded that carbon is lost back to the atmosphere. In the past 30 years, approximately 3.02 gigatons of carbon has been lost from grassland soils. (5)

As you are probably aware, carbon in the atmosphere (in the form of carbon dioxide) is not something we need any more of. Carbon dioxide is a greenhouse gas and acts to trap in heat from the sun causing global temperatures to rise at unparalleled rates. Soil erosion and degradation leads less carbon being stored in soils and activities such as tilling just make the problem worse as this process causes even more carbon to be released from the soil.

 

Solutions to soil degradation and erosion

 

Less plowing and tilling

 

Knowing that soils are being lost at a dramatic rate, and also knowing that it is very difficult to grow food with poor soils, farmers are trying to move towards a low or no-till system of farming. This reduction in tilling allows the soil to maintain its structure and lowers the rate of erosion from wind and rain.

Farmers are also rotating crops more frequently to allow soils a year or two between harvests for soils to recover slightly. When left ‘fallow’ fields are sown with ‘cover crops’ which improve the soil quality and are left to rot adding further to the nutrients in the top layer of soil.

Some companies are taking an even more innovative approach to tackling this problem. The Small Robot Company, a UK-based startup have created a series of small farming robots which can carry out tasks on the farm such as planting crops, inspecting individual plants and spraying targeted pesticides. These smaller machines reduces the requirement for large heavy machinery such as tractors which compact the soil and increase the requirement for tilling. The targeted pesticide application reduces the amount of pesticide that is needed which also improves soil health and structure and reduces the need to turn the soil to remove weeds too.

I interviewed Ben Scott-Robinson, the founder of the Small Robot Company on the podcast, to listen to the episode press play below:

   

Perennial Crop Planting

 

Most crops that we produce globally are ‘annuals’. Meaning that they are only around for one year rather than a ‘perennial’ plant that remains over many years.

The problem with annual plants is that because they are only around for a short time their root systems are very shallow staying near to the soil surface. This does a poor job of holding soils together. Annual crops also have to be re-sown each year which means more disturbance of the soils every year.

One solution to this may therefore be to start looking towards more perennial crops, ones that last longer than a single year. Perennial crops have much deeper, more complex root systems that do a much better job of holding the soil together and helping prevent erosion. They also don’t need to be re-sown every year, meaning the soil is far less frequently disturbed. It also has other benefits for farmers such as weeds establish less easily and this means less pesticide use is required.

This is not a perfect solution as even the perennial crops begin to lose productivity after 3 or 4 years, but it is much better than the current annual system.

 

Reforestation

 

We mentioned that deforestation is one of the leading causes of soil erosion and therefore an easy way to counter this is by reforestation.

Planting new trees will gradually improve soil health as the roots bind the topsoil back together and the leaf litter which slowly rots on the ground will add to the layer of soil every year.

Areas of woodland and even larger hedgerows can also act as useful windbreaks on the edge of farmland. The wind slows down as it hits the mass of vegetation making it less likely to blow away the valuable top soil layer.

New trees will also help reduce erosion from water. The leaves intercept and slow down raindrops meaning they have less momentum when the reach the soil, and secondly the trees absorb water from the soils making them less waterlogged and less prone to landslides.

 

Concluding Remarks

 

Soil erosion is a natural process but one that is being rapidly accelerated due to human activities. Most of these activities are related to agriculture and so to slow them down we must look for more sustainable innovative solutions that can help restore and maintain the health of our precious soils.

 

References

 
  1. Pimentel, David, and Nadia Kounang. “Ecology of soil erosion in ecosystems.” Ecosystems 1.5 (1998): 416-426.
  2. Zheng, Fen-Li. “Effect of vegetation changes on soil erosion on the Loess Plateau.” Pedosphere 16.4 (2006): 420-427.
  3. Haigh, M.J.; Rawat, J.S.; Rawatc, M.S.; Bartarya, S.K.; Rai, S.P. Interactions between forest and landslide activity along new highways in the Kumaun Himalaya. For. Ecol. Manag. 199578, 173–189
  4. Part of Springer Nature 2018 175 K. Lorenz and R. Lal, Carbon Sequestration in Agricultural Ecosystems, https://doi.org/10.1007/978-3-319-92318-5_4.
  5. Chen, Shiping, et al. “Plant diversity enhances productivity and soil carbon storage.” Proceedings of the National Academy of Sciences 115.16 (2018): 4027-4032.

Rob Wreglesworth

Rob is the head writer and podcast producer for The Disruptive Environmentalist. He is on a mission to build a community of people that are passionate about solving environmental problems.
Rob Wreglesworth
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