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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Soil Erosion and Leaching
Soil is one of our most precious natural resources. It takes hundreds to thousands of years to form just a few centimetres of topsoil, yet it can be washed or blown away in just a few seasons when not properly managed. In this session, we'll explore how human activities impact soil through erosion and leaching and what we can do to protect this vital resource.
Key Definitions:
Soil is essential for life on Earth. It supports plant growth, filters water, cycles nutrients and is home to billions of organisms. Approximately 95% of our food is directly or indirectly produced from soil. When soil is damaged through erosion or leaching, it affects food security, water quality and biodiversity.
Each year, about 24 billion tonnes of fertile soil are lost due to erosion. It can take up to 1,000 years to form 1 cm of topsoil, but this can be lost in just a single heavy rainstorm when soil is exposed. The UN estimates that 33% of Earth's soils are already degraded.
Soil erosion occurs when the top layer of soil is removed by forces like water, wind, or ice. While erosion happens naturally, human activities have dramatically accelerated the process, causing serious environmental problems worldwide.
Occurs when rainfall or irrigation washes away soil particles. Types include sheet erosion (thin layers removed), rill erosion (small channels) and gully erosion (large channels).
Happens when strong winds blow across exposed soil, picking up and carrying away soil particles. Most common in dry areas with little vegetation cover.
Results from farming activities that move soil downslope. Ploughing and other cultivation practices can move surprising amounts of soil over time.
Many human activities disrupt the natural protection soil has against erosion:
During the 1930s in the American Great Plains, a combination of severe drought and poor farming practices led to catastrophic wind erosion events known as the "Dust Bowl." Massive dust storms (sometimes called "black blizzards") carried away millions of tonnes of topsoil. Thousands of families were forced to abandon their farms and the disaster highlighted the importance of soil conservation. This event led to the establishment of the Soil Conservation Service in the USA and the development of many erosion control practices still used today.
While erosion is visible, leaching is a less obvious but equally damaging process. Leaching occurs when water-soluble substances are washed downward through the soil profile, removing nutrients from topsoil and potentially contaminating groundwater.
When water moves through soil, it can dissolve and carry away soluble substances:
Rainfall intensity: Heavy rain increases leaching.
Soil texture: Sandy soils leach more readily than clay soils.
Vegetation cover: Plants reduce leaching by taking up water and nutrients.
Soil pH: Acidic soils often experience more leaching of certain nutrients.
Soil fertility loss: Nutrients needed for plant growth are washed away.
Water pollution: Leached fertilisers can cause eutrophication in water bodies.
Groundwater contamination: Pesticides and other chemicals can reach drinking water supplies.
Soil acidification: Leaching of base cations can increase soil acidity.
Several common practices make leaching worse:
In many parts of the UK, intensive agriculture has led to nitrate leaching into groundwater. In some areas, nitrate levels in drinking water have approached or exceeded the safe limit of 50 mg/litre set by the EU. This has led to the designation of Nitrate Vulnerable Zones (NVZs) covering about 55% of land in England, where farmers must follow strict rules about when and how much fertiliser they can apply. These measures aim to reduce nitrate leaching while maintaining agricultural productivity.
There are many effective strategies to reduce soil erosion and leaching. Many of these practices work by mimicking natural ecosystems, which rarely suffer from severe erosion or nutrient loss.
Plants protect soil in multiple ways. Their leaves intercept raindrops, reducing their erosive impact. Their roots bind soil particles together and create channels that help water infiltrate rather than run off. They also take up nutrients that might otherwise leach away. Maintaining good vegetation cover is one of the simplest and most effective ways to protect soil.
Sustainable farming approaches like organic agriculture, conservation agriculture and regenerative farming all emphasise soil health. These systems typically use minimal tillage, maintain soil cover, incorporate diverse crop rotations and carefully manage organic matter and nutrients. These practices not only reduce erosion and leaching but can also improve crop resilience to drought and floods.
Many countries have implemented policies to combat soil erosion and leaching:
The United Nations has declared soil conservation a global priority through initiatives like the Global Soil Partnership and World Soil Day (December 5th). These efforts recognise that soil is a finite resource that requires careful management for future generations.
Farmers across the UK are increasingly adopting regenerative agriculture practices to combat soil erosion and improve soil health. For example, in East Anglia, an area prone to wind erosion, farmers have formed the Groundswell Agriculture network to share knowledge about no-till farming, cover cropping and diverse rotations. These farmers report reduced erosion, improved water infiltration and increased earthworm populations. Some have been able to reduce fertiliser use by up to 40% as soil health improves, demonstrating that environmental and economic benefits can go hand in hand.
Soil erosion and leaching are serious environmental challenges that threaten food security, water quality and ecosystem health. Human activities have accelerated these natural processes, but we also have the knowledge and tools to address them. By understanding how our actions affect soil and adopting sustainable practices, we can protect this vital resource for future generations. Remember that healthy soil is the foundation of healthy ecosystems and sustainable food production.
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