The soil crisis is a threat to food security

In this series, we focus on a variety of issues around agriculture and our current food systems to begin exploring how we might solve them. In our previous article, Dr. Anya Perera considered how agriculture and its workforce can be transformed for a more sustainable future.

Here, we continue discussing sustainability through an agronomic lens, examining how current farming practices are accelerating soil erosion with potentially dangerous and irreversible consequences.

What is topsoil?

As the name suggests, topsoil is the upper, outermost layer of soil. Normally around 5–10 inches (10–30 cm) deep, topsoil is highly fertile due to the presence of large quantities of organic matter and structure. It contains trace minerals, as well as clay, silt and/or sand; the respective quantities of these are determined by the geographical location of the soil.

Topsoil plays an important part in nutrient cycling and the regulation of nutrient and water flow, whilst providing a habitat for a diverse array of animals and microorganisms. It is an incredibly important natural resource with 95% of the world’s crops being grown in topsoil.

The processes that generate high-quality, fertile topsoil can take centuries. But the world is ploughing through this finite resource at an alarming rate. About 40% of the world’s land has already been taken over by agriculture, while livestock grazing and expanding urban areas are taking further chunks out of what is left over.

According to The Committee on Climate Change (CCC) report (2015), Britain has lost approximately 84% of its topsoil since 1850, with the erosion continuing at a rate of 1cm to 3cm a year. On a global scale, a study of 82 sites in 21 counties by Iowa State University showed that in the 50 years (from 1959), soil structure and levels of organic matter had degraded while acidity had increased.

Recent studies have shown that soil erosion, in parts of the UK, is above tolerable levels. When erosion is above “tolerable level” it means that it is occurring at an unsustainable rate, which will likely lead to a drop in crop yield. This is likely to impact crop production in the medium to long term, resulting in a loss of soil fertility.

What is soil erosion?

Soil erosion occurs when particles of soil detach from their normal location and are relocated elsewhere due to the effects of wind, water or gravity (mass movement).

In the UK, erosion by water is more of a problem than erosion caused by wind or mass movement. Where water is responsible for soil erosion, it can be splashes from rain, thin flows of water (interrills) or thin sheets of water.

Soil erosion may occur by the tillage of steep hillsides(1) as well as the loss of soil that is attached to root crops like potatoes. Tilling breaks up the soil and destroys its natural structure, displacing and killing many of the vital bacteria and fungi that live there.

De-stoning

De-stoning is also a key element of soil erosion. De-stoning is a post-harvesting process by which stone or contamination is removed from various crops of different size and colour in order to separate all the impure and unwanted materials from the seeds.

Sensitivity to erosion is increased by this removal of stones from soil. In order to cultivate crops such as potatoes, carrots etc., the soil is increasingly de-stoned as a consequence of the loss of organic matter and thus it becomes more susceptible to erosion. Cultivated soil has a reduced surface roughness and an increased moisture content which aids further erosion.

Nutrition loss

Erosion occurs when the physical forces of wind and water are causing detachment and removal of surface soil, which is the most fertile part of the soil. Research shows that long-term erosion causes soil to lose valuable nutrients which results in lower yields of less nutritious crops. Soil nutrients are mainly lost through the runoff many nutrients are dissolving in rain water or lost. Wind erosion may also lead to negligible loss as organic matter, clay partials or soil in sand, silt, clay ratio is disturbed with reduced fertility.

Biodiversity loss

Studies have shown that erosion leads to diversity loss in soil fungal species. The shear forces of runoff dissolve macroaggregates (large particles of soil). This exposes the fungi to the air and depletes the nutrient supplies they need to flourish.

Topsoil is home to many soil nematodes (small, slender worms) and it has been found that the loss of topsoil decreases nematode populations. When conventional tillage (resulting in topsoil loss) occurs, the number of plant-feeding nematodes decreases. However, when zero tillage (or crop rotation) occurs, nematode trophic linkages increase. This suggests that a zero tillage approach which reduces topsoil is important to increase microbial diversity.

Furthermore, Gyphosate-based herbicides (GBHs) are thought to cause a decrease in the thickness of topsoil when applied frequently. The application of GBHs results in a decrease in vegetation cover, which exposes the topsoil to erosion. This is also implicated in reducing earthworm biomass and reproductive fitness. Earthworms play a vital role in degrading organic matter to release nutrients to the soil, therefore anything negatively impacting on them will also impact on the soil quality.

Impact on waterways

Soil erosion also impacts the ecology of waterways when the eroded sediment is often flushed into streams, rivers and reservoirs. This is detrimental in terms of nutrient enrichment (eutrophication) and pesticides, which are washed into waterways along with fine sediment.

Furthermore, fish-spawning grounds, the areas of water where aquatic animals spawn, or produce their eggs, can also be damaged by these pesticide inputs and leads to decreased ecological diversity.

Soil erosion also increases the risk of muddy-flooding due to sediment translocation/displacement that occurs during periods of heavy rainfall. The costs of preventing flooding, or cleaning up post-flood, are very expensive — both in terms of properties and people’s livelihoods.

How does land management affect soil erosion?

Erosion rates are highly affected by land management. Ploughing up and down hills, growing crops on steep hills and leaving fields bare of crops can all contribute to an increased rate of erosion in heavy rainfall.

Although geography and soil type can affect erosion rates, erosion will happen on any land that is farmed and will only be exacerbated by heavy downpours. Extreme weather and climate change is often wholly blamed for soil erosion, but erosion is worsened by socio-economic factors.

Currently, UK farmers are paid subsidies for managing their land. However, this management occurs in a way which increases erosion at an unsustainable rate. Farmers are placed in the difficult position of either planting high value crops on as much land as possible or putting sheep to graze there, even if the land is unsuitable for this purpose.

Such planting of crops results in large areas of land being bare and subsequently subject to erosion, while overgrazing results in the exposure of upland soils, which are further eroded by the weather. Unfortunately, the short-term economic benefits of growing high-value crops in unsuitable locations or overgrazing cattle are deemed to outweigh the long-term risks of soil erosion.

Land management needs to be addressed by policymakers in tandem with growers and experts or soil erosion will continue.

It’s time to address soil erosion

Roughly between one billion and six billion hectares of land are considered degraded, says Ronald Vargas, secretary of the global soil partnership at the FAO. This may have occurred naturally in a handful of regions, however it is undeniable that intensive agriculture, deforestation, industrial activity and climate change have vastly accelerated the soil degradation in almost every country.

Current farming practices are heavily contributing to the loss of valuable, nutrient-rich topsoil. Erosion has historically peaked at times of population pressure on the land. As COVID-19 and other crises demonstrate the difficulties of feeding a growing global population, we must begin to address how we should change the way that we grow food.

It is essential that we provide nutritionally valuable food to meet the demands of a growing global population. In order to do this, we simply cannot ignore the problem of soil erosion. Not only must a variety of farming methods be combined, but there should be more productive regulations regarding soil erosion. To tackle the problem of soil erosion, our current agricultural practices must be combined with more sustainable solutions.

(1) the preparation of land for growing crops