The big picture

Soil is vital for our human well-being. It is required for food production and is essential in the nutrient cycle. It can help to filter water and act as a major carbon sink. It is a dynamic living system comprising of millions of micro-organisms of bacteria and fungi. However, human activity can cause increase rates of soil loss and pollute the soil. The loss of vegetation leaves the soil surface exposed and allows soil to be swept away by forces of water and wind. Soil loss and degradation is a global issue affecting both developed and developing countries all around the world.

Use the following document to watch the video and make Cornell notes as an overview of the topic.

Figure 1. Dust storm in Elkhart, Kansas during the 1930s.

During the 1930s in the USA, dust storms swept across the Midwestern states of the USA. This period is commonly referred to as the ‘Dust Bowl’ and brought the issue of soil loss to global attention.

Figure 2. Main areas damaged by the dust storms 1930-1940.

The Dust Bowl was caused by poor farming methods, drought and windy conditions. Farming had resulted in removal of large areas of prairie grass that anchored the soil. The land was regularly ploughed after harvest and left fallow for a period. Overgrazing also led to the loss of vegetation cover. Prolonged drought in the 1930s dried out the soil. When wind storms occurred, large amounts of topsoil were swept into the air causing vast dust storms. The cost was immense with no harvests and damage to property. Many people starved or died from dust penetrating into their lungs causing dust pneumonia. Thousands of farmers lost their livelihoods and migrated to urban areas seeking employment. Unfortunately there were few opportunities in the cities and towns during this period of the Great Depression.

Figure 3. Farming equipment is buried by a dust storm.

The Dust Bowl prompted investment into the study of soils to find solutions to the problem. In 1935 the US Soil Erosion Act came into force to encourage soil conservation practices. Although our understanding of soils and soil conservation has improved since the Dust Bowl, soil loss and degradation continues to be a problem around the world. The Food and Agriculture Organization of the United Nations (FAO) declared 2015 as ‘International Year of the Soils’ with the aim of ‘raising awareness of the importance of soils for food security and essential eco-system functions’.

Fertile soil and succession

Soils provide a medium for plants to anchor themselves and grow. Fertile soils also contain nutrients and water necessary for healthy plant growth. Soils are important in recycling matter and are integral to the nutrient cycles (e.g. the carbon and nitrogen cycle).

Ecological succession occurs over time and results in changes to the soil fertility. Although new soil is constantly formed it is considered to be a non-renewable resource.

Fertile soils

Figure 1. Fertile soils provide the conditions required for seed germination and growth.

Soils which provide a good growing medium for plants contain:

• Organic matter which provides sufficient soil moisture holding capacity and good soil structure. The latter is needed to provide sufficient drainage to prevent water logging.
• The presence of a healthy soil community that breaks down organic matter and returns nutrients back into the soil.
• Essential nutrients and minerals. Nutrients include nitrates, phosphates and potassium compounds. Minerals include sulphur, calcium, magnesium, iron, manganese, boron, copper, zinc, and molybdenum compounds.
• A suitable pH. The pH affects the availability of nutrients and minerals that are available for plant uptake. Many main crops prefer a soil pH of between 5.5 and 7.5. If the soil is too acidic it will release aluminum ions that are toxic.

Figure 2. Soil pH affects nutrients that are available for crop growth.

Role of succession on fertility

As discussed in an earlier section, primary succession involves the development of a community from bare rock with no soil to the development of a climax community with mature soil containing organic matter, possessing good water and nutrient retention capacity and good structure. The climax community and associated soil ecosystem will vary from one place to another and will be dependent on the bedrock and the climate.

Figure 3. Succession process in which soil ecosystem develops and supports plant life. The plant litter contributes to the organic matter that enters the soil.

Biological activity within the soil contributes to mineralization of dead organic matter (waste matter and dead organisms) which increases nutrient levels.

This decomposition process involves:

• Invertebrates e.g. earthworms and woodlice which:
  • Mix some of organic matter into the soil, which makes it more readily available to other organisms including residential bacteria and fungi. This process also helps to aerate the soil.
  • Feed off and digest some of the organic matter. The resulting waste products can be further broken down by bacteria.
• Fungi and bacteria break the organic matter down and releasing nutrients into the soil.

In addition nitrogen fixing bacteria absorb nitrogen gas from the air and transform it into nitrates (discussed the Nitrogen cycle).

In agriculture we often enhance soil fertility by adding fertilizers to increase soil nutrient levels.

Figure 4. Simplified model illustrating inputs and outputs of nutrients in soil.

Non-renewable resource

If soil can be replenished why is it considered as a non-renewable resource?

The rate of soil formation is slow, the FAO estimate that it takes around 1000 years to develop up to 5cm of soil. This figure varies considerably from one place to another and is dependent on climate conditions. Soil formation is fastest under sunny, warm and wet conditions when there is maximum plant growth. This contributes to high levels of plant litter and other dead organic matter which is broken down to form soil. This process of soil formation is much slower under cold and dry conditions, where soil formation may occur at a rate of 1mm per 1000 years.

The current rate of global soil degradation and loss is occurring at a much faster rate than the rate of soil formation. Soil is considered to be a non-renewable resource because it is not replaceable within a human lifespan or at a pace faster than that at which it is used. For soils to be a more sustainable resource, we need to dramatically reduce loss and degradation rates and improve rates of soil formation.

Theory of Knowledge

The loss of soil could be viewed as a part of natural geological and biological change, how do our perceptions influence whether we view this as positively or negatively.

Threats to soils: processes

As discussed in the previous section soil is not considered to be a renewable resource. The rates of loss and degradation are greater than the rate of soil formation. About a third of the world’s soils are degraded. Processes involved with soil loss and degradation include:

• Water erosion.
• Wind erosion.
• Chemical degradation e.g. salinization, acidification, nutrient depletion and chemical pollution.
• Physical degradation e.g. soil compaction.

The majority of global soil degradation occurs as a result of erosion.

Figure 1. Contribution of main processes to soil degradation.

Erosion

Soil particles can be transported form one place to another by either water or wind. Erosion typically removes the fertile topsoil. The loss of organic matter also leads to a reduction in water retention capacity. The eroded soil can enter watercourses which can cause additional problems:

• The resulting sediment can clog up ditches, reduce the capacity of water courses and increase the risk of flooding. Removing the sediment by dredging can be an expensive process.
• The soil particles may be rich in nutrients and pesticides causing water pollution.

Water erosion

Water erosion comprises of three main phrases:

1. Detachment: When the raindrops hit the soil, it frees some of the soil particles. Then run-off detaches more soil particles as it flows.
2. Transport: The flow of the water carries the soil particles.
3. Deposition: When the water slows down, the soil particles are deposited.

Types of soil erosion that occur as a consequence of the action of water include sheet erosion, rill erosion and gully erosion.

Figure 2. Sheet erosion - when a fairly thin even layer of soil is removed by run-off.

Figure 3. Rill erosion - when the run-off scours channels into the soil called rills.

Figure 4. Gully erosion - when the force of water is greater enough to create a deep channel.

Wind erosion

Dry regions with exposed soil surfaces are most vulnerable to wind erosion. The wind picks up the soil particles and carries them through the air. Particles that are light and loose (e.g. sand particles) are more easily picked up by wind currents. The wind velocity can increase along large flat areas making large open areas more vulnerable than smaller areas with trees and shrubs that provide soil coverage and act as wind breaks.

In addition to the impacts of soil erosion outlined above wind-blown soil particles also damage plants within their path, reduce visibility and can cause build-up of soil deposits on roads and other surfaces. As demonstrated in the USA by the Dust Bowl during the 1930s, wind erosion can occur on a vast scale and can impact on human health.

Figure 5. Wind erosion can occur along large areas.

Chemical degradation

Soil fertility can be reduced in a variety of different ways, for example:

• Salinization: This can occur when water evaporates and leaves behind salt. The salt can accumulate and the soil becomes saline. Many types of vegetables and fruit are unable to grow in saline soil. Salinity problems tend to be most severe in arid and heavy irrigated areas due to high levels of evaporation.
• Acidification: Occurs when there is an increase in hydrogen ion concentration lowering the pH. This can be caused by acid deposition, leaching and removal of nutrients from the soil ecosystem and use of ammonium based fertilizers. The latter is converted by bacteria in the soil to form nitrates and hydrogen ions. The nitrates can be either taken up by plants or leached from the soil.
• Nutrient depletion: Occurs due to over exploitation e.g. continual cropping without replacing lost nutrients in the soil. This loss of fertility reduces the capacity of the soil to support further plants. Artificial fertilizers do not always contain all the nutrients required for healthy plant growth.
• Chemical pollution: For example the accumulation of toxic metals within soils from the use of pesticide e.g. Bordeaux mixture a fungicide used on fruit can result in the accumulation of copper within soils.

Figure 6. Soil polluted with oil.

Physical degradation

This can include soil compaction from use of heavy farming machinery and animals. When the soils become compressed, air spaces between the particles are lost. This reduces porosity and the soil may become more easily waterlogged and be difficult for plant root systems to penetrate. A crusty flaky surface may develop that is more vulnerable to water and wind erosion.

Threats to soils: human activities

Human activities can accelerate soil loss and degradation processes. Urbanization leads predominately to loss of soil cover. Overgrazing, deforestation and mismanagement of arable land contribute significantly to soil degradation which can also increase the risk of desertification.

Figure 1. Major causes of soil degradation.

Urbanization

Each year increasing growth of our cities results in loss of soil cover. The quality of soil within urban areas varies greatly due to soil movement from one place to another. Urban soil frequently suffers from compaction and soil pollution (e.g. lead from leaded petrol or buried rubbish). Water and wind erosion can undercut urban structures and damage buildings and roads.

Figure 2. Soil erosion following heavy rainfall has resulted in a landslide blocking a road.

Livestock overgrazing

Excessive vegetation removal by grazing livestock can leave soil exposed to the processes of water and wind erosion.

Deforestation

When trees are removed, the soil is left exposed. The lack of vegetation to intercept rainfall reduces water infiltration into the soil and increases the amount of water run-off. Water erosion of the soil results in the transfer of organic matter and nutrients from the forest into nearby watercourses causing water pollution. Organic matter can contribute to lower dissolved oxygen levels and nitrates can cause cyanaobacteria blooms.

Figure 3. Water in the Ecuadorian Amazon is brown due to sediment from deforestation activity.

Farming

Some farming practices cause soil degradation by exposing the soil to the processes of water and wind erosion or through enhancing chemical or physical degradation. For example:

• Tillage which involves ploughing the land and clearing any debris can leave the area bare and vulnerable to water and wind erosion.
• Monoculture farming can extract specific nutrients leaving the soil nutrient poor.
• Growing multiple crops per year can remove nutrients at a faster rate than they are being replaced. This can result in nutrient deficiencies within the soil.
• Excessive irrigation with poor drainage can cause water erosion or salinization.
• Use of chemicals such as particular pesticides can damage the soil microbial community.
• Cultivation of steep slopes can encourage loss of topsoil to water and wind erosion.
• Use of marginal areas that are not typically suitable for farming without high levels of inputs such as fertilizers. The productivity is often limited for a variety of reasons such as low levels of soil nutrients, poor drainage, difficulty in accessing the area e.g. on a slope and unfavourable weather conditions for crop production. Farming these areas can increase the rate of soil degradation, particular if the above techniques (i.e. ploughing) are used. Overtime, crop yields will decline and the farmer will require more land to survive.

Figure 4. Positive feedback cycle of land clearance and degradation.

Farming methods employed by large scale commercial farming often cause greater soil degradation than small subsistence farming. There is great variation from one farm to another and will be also dependent on the soil conservation practices employed.

Desertification

In relatively dryland regions soil loss and degradation can contribute to desertification, the transformation of arable land to desert. The risk of desertification is further increased by climate change and associated incidents of drought.

Figure 5. Factors contributing to desertification.

Desertification leads to loss of food production that could threaten food security and result in famine.

Watch the following the video ‘Desertification’ by GoodPlanet and consider the factors contributing to desertification, it effects and potential solutions.

Theory of Knowledge

Soil loss and degradation is a global issue, what defines this concept of a global issue?

Soil Conservation

Around 95% of global food sources are dependent on soils. Hence soil loss and degradation can seriously threaten food production and food security. For farming to become more sustainable, a variety of practices that prevent soil loss and degradation need to be employed. This can involve reducing water erosion, wind erosion, salinization, managing available nutrient levels and limiting grazing. The use of marginal land that is unsuitable for agriculture and prone to high levels of erosion should if possible be avoided.

Reducing water erosion

Limiting the process of water erosion reduces loss of fertile land. It also decreases the risk of flooding downstream and the clogging and pollution of aquatic ecosystems.

Vegetation cover can be used to intercept rainfall and reduce soil erosion. Controlling runoff can be used to limit water erosion. This involves:

• Capturing water as it flows using techniques such as:
  • Terracing along steep hillsides. This can be expensive to construct and maintain but is effective at reducing erosion.

Figure 1. Use of terracing in Vietnam.

• Furrow diking which involves creating furrows with small ridges to capture the water.

Figure 2. Furrows are channels in which water collects

• Contour tillage where ploughing occurs across the slope and along the natural contours of the land.

Figure 3. Contour tillage in which strips reduce water flow.

• Strip cropping along steep slopes follow the contour of the land using alternating strips of crops e.g. corn and hay or soybean and alfalfa.

Figure 4. Strip cropping of corn and soybean.

• Buffer strips that are permanent vegetation, they can be located at edge of a field (e.g. grassland or shrub strips that intercept runoff) or within the field (e.g. grassed waterways which are shallow ditches that collect and divert the water).
• Increasing infiltration of water into the soil by improving soil structure and moisture retention capacity through:
  • The addition of organic matter e.g. manure.
  • Mulching using materials such as straw, grass and woodchips to cover the soil surface which also reduces evaporation.
  • Avoiding compaction.
  • Conservation tillage in which residue from the previous crop is left on the soil surface. This also reduces wind erosion.

Conventional tillage vs conservation tillage method

Tillage is used to prepare the soil for sowing seeds. In conventional tillage the soil is physically broken up by ploughing. This results in an open and loose soil structure which is well aerate and moist. It also helps to reduce weeds. Any crop residues are ploughed into the soil and the surface of the land is cleared of all debris.

Figure 5. A conventionally tilled field.

In comparison in conservation tillage, crop residue is left as a mulch on the soil surface. This increases water infiltration, reduces run-off and associated water erosion. No-till is a form of conservation tillage in which no ploughing occurs.

Reducing wind erosion

Techniques used to reduce wind erosion include:

• Wind breaks e.g. trees or large shrubs to reduce wind velocity and capture the blowing soil.

Figure 6. Wind break of poplar trees

• Shelter belts are blocks of trees or shrubs planted at right angles to the prevailing wind to deflect the wind and reduce its velocity.

Wind breaks and shelter belts also potentially provide a habitat for wildlife and pollinators. Some of the techniques used to limit water erosion, also reduce wind erosion:

• Conservation tillage in which the crop residues on the soil surface also provide protection from the wind.
• Vegetation cover which protects the soil.
• Buffer strips that also act as wind breaks.

Watch the following video which incorporates the use of no-till into conservation agriculture ‘No tillage agriculture prevents soil erosion’ by the World Bank.

Reducing salinization

The risk of salinization can be reduced by:

• Avoiding over watering.
• Not watering at certain time of the day. Watering at night or in the late afternoon avoids the heat of the day and evaporation.
• Incorporating good drainage.

Excess salts can be removed from the soil by flushing them out using plenty of water.

Managing soil nutrient levels

Reducing water and wind erosion using the above techniques helps to maintain soil nutrient levels. In addition, nutrients lost to plants need to be replaced and the soil pH may need to be amended to maintain conditions suitable for crop growth. This can be achieved by:

• Addition of organic matter e.g. manure. This increases both organic matter and nutrient levels and improves soil structure.
• Growing of green manure such as leguminous plants e.g. alfalfa, clover and vetch. Green manure also increases the organic content of the soil.
• Addition of synthetic fertilizers.
• Liming, the addition of calcium carbonate (limestone) or calcium hydroxide (hydrated lime) to raise the soil pH and improve the soil capacity to support crops. Low pH levels can reduce available nutrients and mobilize toxic chemicals such as aluminum ions.
• Crop rotation involves changing the crops grown on a plot each season on a 3 or 4 year cycle to prevent depletion of particular nutrients and to maintain soil fertility.

Figure 7. Example of crop rotation system in which legumes help to replenish lost nitrates.

Control grazing

Overgrazing can be reduced by restricting the number of animals and the time spent in one area. Areas should not be allowed to be stripped of vegetation cover. Sufficient time should be given for the vegetation to recover before returning livestock. Fertilizers can be used to increase the growth rate of vegetation. In addition, techniques such as grassed waterways (shallow ditches to collect run-off) and wind breaks can be used to minimize any potential water erosion and wind erosion.

International-mindedness

In some cultures a large number of cattle is seen as a status symbol. Consider how these cultural attitudes impact soil conservation.

Case studies

Soil conservation strategies can be adopted at any scale, from a small kitchen garden to large commercial farms. Our large case study is the Loess Plateau in China. Work through the document using research to expand with SPECIFIC evidence. The video forms an extremely good starting point:

Here are two different examples which are not presented in as much detail but provide a contract case study:

• Quesungual farming system, which adopts various soil conservation measures used by subsistence farmers in Honduras.
• Soil conservation measures adopted by large commercial farmers in South Australia.

Quesungual system in Honduras

The Quensungual system is named after the remote village where it was developed in the 1990s near Lempira in southwest Honduras.

Figure 1. Lempira in Honduras.

Shifting agriculture employing ‘slash and burn’ has been traditionally practiced by subsistence farmers. This method is only sustainable when the population density is low and the forest has time to recover. With a growing population, shifting agriculture causes environmental damage and is unsustainable. In addition, the majority of subsistence farms in Honduras are located on hilly terrain increasing the risk of soil erosion.

Figure 2. Hilly landscape of Honduras.

In Quesungual an agroforestry scheme suitable for cultivation on hilly terrain was used to replace shifting agriculture. This involved clearing the area by hand and maintaining some of the trees and shrubs. In a plot of between 1 and 3 hectares, between 20 to 30 larges trees, including fruit trees, were kept together with lots of small trees and shrubs. A range of crops (polyculture) were planted such as maize, sorghum and bean. Conservation tillage involving no-till was practiced, allowing plant residues to cover the soil surface. Trees and shrubs were regularly pruned to ensure enough light reaches the crops and to encourage healthy growth. This wood could be used as firewood, as timber or for mulching. The tree roots also help to anchor the soil and limit soil erosion.

The adoption of the Quesungual system has led to:

• Increased organic content in the soil.
• Improved soil structure resulting in higher soil moisture retention.
• Increase infiltration of rainfall.
• Increase in soil nutrient levels.
• Dramatic reduction in soil erosion.
• Increase in food production and improved nutrition.
• Increase carbon dioxide absorption (sequestration).
• More sustainable agriculture.
• Increase in food security.

Overtime the Quesungual system has been proven to improve resilience against the effects of climate change including extreme weather conditions. The Quesungual system has been expanded to other regions in Honduras and could be adopted in other countries.

Watch the following video that discusses this case study ‘Honduras Quesungual System’ by FAO.

Large commercial farming systems in South Australia

A wide range of food products are grown commercially in South Australia ranging from wine to grain and beef. However the topsoil in South Australia is relatively shallow and has low fertility. It is the driest state in Australia with an arid and semi-arid climate.

Figure 3. Vineyards in South Australia cover vast areas.

The adoption of practices such as ploughing and leaving fields fallow together with drought has led to severe soil erosion. Overgrazing has also led to wind erosion and sand drifts. To combat erosion the government has promoted:

• Use of no till farming.
• Use of crop rotation.
• Use of cover crops.
• Use of wind breaks and shelter belts.

In addition, improvements have been made to irrigation methods to reduce high levels of soil salinization in the area. The number of farmers undertaking no-till practice increased from 16% in 2000 to 66% in 2011. Soil monitoring programmes demonstrate that farmers that have adopted more sustainable farming practices have seen significant improvements in soil conditions.

Watch the following video ‘The story of soil conservation in South Australia’ by Primary ProducersSA.It covers the problems faced by farmers of soil degradation and the change of farming practices from ploughing the fields to minimum tillage to no tillage method. This involves retaining crop residues (also referred to as stubble) on the soil surface to increase organic content, reduce soil erosion and soil compaction.

Which is better – large or small scale farm systems?

Figure 4. Small scale verses large scale farmers are sometimes likened to the story of David and Goliath. Small scale farmers are often perceived to have a lower environmental impact.

There is often great debate over the value of large scale verses small scale farms. The following video discusses the merits of each ‘The future of farming, food security’ by SAB-Miller.