Soil is one of the earth’s most important natural resources: it underpins human food production systems, supports the cultivation of vegetation for feed, fibre and fuel, and has the potential to help combat and mitigate climate change. It is also a rich and complex ecosystem, accommodating a staggering array of biodiversity. Healthy soil is crucial for human life and wellbeing, and yet soils across the globe are being threatened and damaged by human activities. Here we discuss the relationship between humans and soil, how it is changing and why it is so important.
“The nation that destroys its soil destroys itself”
Franklin D. Roosevelt, 1937
What is soil?
Soil is loosely defined, by scientists and environmental agencies, as the ‘physically altered top 1.2m of the earth’s crust’.1 Indeed, the British Society of Soil Science calls it ‘the dynamic skin of the earth’2: this dynamism comes from the interaction of the different components of soil, including minerals, organic materials and living organisms. Water and air also form a complex network of channels and chambers between the solid components of soil. These components, or solid ‘grains’, are chemically active: they interact with each other continually, so the composition and properties of soil are constantly changing.3
Though soil may appear homogenous, it is in fact made up of a diverse range of different components. Many of the particles that can be seen with the naked eye come from rocks broken down into fine grains; these grains can be classified as sand, clay or silt, depending on their size. While sand grains are course and feel gritty to the touch, clay is finer and feels sticky. Silt particles, meanwhile, are intermediate in size and have a silky texture.4
Until recently, our knowledge of the components of soil was largely limited to these solid, visible grains of rock. 2010, however, saw the launch of the ‘Earth Microbiome Project’; a multidisciplinary initiative which uses genome sequencing to identify, characterise and analyse microbial communities across the globe.5 The project aims to collect samples from all possible environments on the planet, and to sequence the ‘microbiome’ – the combined genetic material of all the microorganisms in a given environment – for each sample.6 Before the project’s launch, less than 1% of the total DNA present in a gram of soil had been sequenced, meaning that we had little understanding of the living organisms found in soil.7 In the seven years since the project began, however, the microbial communities it has identified in soil samples from across the globe have been nothing short of astounding in their scale and complexity. Far from being inert matter, soil is teeming with life, made up of a vibrant, thriving population of bacteria, archaea and fungi: indeed, it is now thought that 90% of all organisms on all seven continents are soil-dwelling microorganisms8. There are between 10,000 and 50,000 species in a single teaspoon of healthy soil9, and there is more biodiversity in the bacterial community of a single handful of healthy soil than in all the animals of the Amazon basin.10
These microbial communities perform a wide array of ‘ecosystem services’ essential for the health of soil and the organisms it supports. For example, Saprophytic bacteria and fungi – microorganisms that break down dead or decaying matter as a source of nourishment – decompose the complex substrates of dead plant and animal matter to generate humus. Humus is the dark, organic component of soil from which plants take most of their nutrients: the amount of humus in soil determines both the soil’s capacity for water retention and its fertility.
Why is it important?
Soils from around the globe are diverse in terms of their structure, depth, texture and fertility: these differences mean that soils provide a wide range of benefits – environmental, economic and social – to human societies in many different environments across the planet.
Soil fertility: supporting agriculture and forestry
We have already seen that the microorganisms found in samples of soil produce humus, which is the basis for soil’s fertility and therefore for global agriculture and forestry industries. With global demand for crop production predicted to increase by 100-110% between 2005 and 205011 and agricultural land currently covering almost 40% of the planet’s land area12, soil fertility is increasingly a factor of vital importance to human health and nutrition.
In addition to improving soil fertility by breaking down organic matter to produce humus, soil microorganisms can often form symbiotic relationships with plants such that one cannot flourish without the other: nitrogen fixation is a well characterised example of such a relationship. Nitrogen is needed by all organisms for the manufacture of amino acids and nucleic acids, but atmospheric nitrogen is inert and cannot be used by higher organisms such as plants and animals. Some species of bacteria can ‘fix’ atmospheric nitrogen into biologically useful ammonia (NH3), which can be used by plants.13 Nitrogen-fixing bacteria such as Rhizobium invade the root hairs of leguminous plants by inducing specific sets of genes in response to compounds secreted by the host root: once inside the inner root tissue of the plant, the bacteria synthesize the proteins necessary for nitrogen fixation.14 The plant is able to use the ammonia produced by bacterial enzymes known as nitrogenases, while the bacteria can utilise carbohydrates, proteins and oxygen generated by the plant.15 Similar mutualistic relationships exist between other species of plant and bacteria, and between plants and other microorganisms. Microorganisms such as nematodes and arthropods, meanwhile, also play a role in regulating the levels of nitrogen available to plants. These microbes consume certain species of nitrogen fixing bacteria, excreting excess nitrogen and other minerals in a plant-available form.16 Some species are able to consume up to 5,000 bacteria per minute17: they can therefore act as effective regulators of the bacterial population of soil and have great potential as biocontrol agents.
A building material
Soil is still used as a primary building material in many regions across the world: in fact, the British Society of Soil Science estimates that approximately half of the world’s population live or work in buildings constructed using soil.18 While only some types of earth are suitable for use as a building material, the UN Food and Agriculture Association (FAO) suggests that some advantages of using soil for construction are its low cost, its ease of use, its fire-resistant qualities and its high thermal capacity.19
We now know that many soils act as highly efficient carbon sinks: the world’s soils are thought to store approximately 15 gigatonnes (15 thousand million tonnes) of carbon, three times as much as all of the earth’s terrestrial vegetation combined.20,21 27% of this soil carbon is accounted for by a single glycoprotein, glomalin, found in humus.22 Glomalin, an extremely stable, iron-bound molecule, is produced by the spores and hyphae of a group of fungi that form symbiotic relationships with vascular plants. Though not yet well characterised, Glomalin presents exciting new possibilities in carbon management and storage technologies. Indeed, increased carbon levels actually boost glomalin production: in a 3-year study, an increase in CO2 of 300ppm resulted in a five-fold increase in glomalin production by fungal hyphae, which in turn led to increased soil stability and fertility.23 This is an important avenue to explore in a time when the question of climate change is more urgent than ever before.
Just as it acts as a carbon store, soil can store large amounts of water: as well as being important for vegetation growth, this can be a key factor in preventing flooding under extreme weather conditions.24 Soil can also act as a water filtration system: as water drains through soil, soil microorganisms and minerals act upon it to remove pollutants and toxins. This filtration occurs through physical, biological and chemical processes: bacteria, fungi and other microorganisms found in soil interact with pollutants carried by water, while the physical and chemical composition of soil affects how water and other particles are able to move through it.25
Soil is an extraordinarily diverse and complex ecosystem, playing host to some 25% of our planet’s biodiversity. The soil food web involves many trophic levels, with microorganisms, plants and animals interacting and contributing to water, nutrient and nitrogen cycling.26
How are humans impacting soil?
The pressures of population growth, food insecurity and agricultural intensification are leading to widespread soil degradation in many regions. This degradation can take many forms, including:
Soil erosion is a serious concern in many parts of the globe: the term refers to the process by which topsoil is worn away by natural processes – wind and weather effects, for example – or by human activity such as farming and deforestation. Over the last 150 years, some 50% of the planet’s topsoil has been lost in this fashion27: there are few reliable estimates of the extent to which humans have contributed to this loss, but studies have shown that it varies widely across different regions, from 8% in North Africa to 75% in Australia.28 Soil erosion has significant impacts on crop production, as well as off-site effects such as the transport of agricultural inputs and pollutants to waterways.29
Overall, rates of soil erosion are significantly higher than rates of soil formation, representing a substantial long-term threat to world’s soils and – by extension – global food security. 30 Soil sealing
Soil sealing is another physical process, most often associated with the spread of urban land, which leads to a loss of soil fertility and agricultural potential. Sealing occurs when soil is covered by an impenetrable material, effectively causing it to become ‘non-soil’.31 Estimates place the current rate of global soil sealing at 250-300km2 day, though this rate is likely to increase given ongoing trends of rural migration into urban areas.32
Compaction & other physical processes
Soil compaction is another form of soil degradation, occurring when soil is compressed by the use of heavy agricultural equipment: compaction reduces the amount of water and air present in soil, and can impair crop emergence, root penetration and nutrient and water uptake by crops.33
Physical processes such as over-farming, land clearance and deforestation also drastically disrupt the activities and diversity of soil microorganisms. The indiscriminate clearance of land for cultivation – as well as the use of herbicides to create a monoculture of a single plant – reduces the quantity and diversity of plant residues within soil, and therefore the quantity and diversity of microbial habitats and food sources. Intensive tilling of soil, meanwhile, breaks down soil aggregates that are held together by fungal hyphae and reduces the number of fungi in the soil. It is true that these techniques, alongside the use of chemical fertilizers and herbicides, have allowed large increases in crop yields and food production in Europe, Asia and the Americas in recent years: it may be, however, that there is a price to pay for this short-term success.
Use of agricultural chemicals
Many modern agricultural practices have a deleterious effect on microbial soil communities. Chemical fertilizers, fungicides, herbicides, pesticides – those substances on which we have staked our food security – have all been shown have devastating effects on indigenous microbial communities within soil. Global use of agricultural chemicals occurs on an enormous scale:
worldwide, 3×109kg of pesticides are applied to crops each year, with comparable usage figures for herbicides.34 Of these quantities, approximately 0.1% of the applied chemical will reach the target organism: the remaining 99.9% will remain in the soil until it is degraded, a process which can take several weeks or months. Repeated application of agricultural chemicals can therefore cause longterm contamination of the humus and potentially permanent alteration of the soil microbiome.35 Agricultural chemicals can alter the physiological, metabolic and biochemical behaviour of microbiota in the soil, disrupting the relationships between plants and microbes and decreasing nutrient bioavailability.36
Agricultural chemical usage has vastly different effects on different microbes, depending on a huge range of factors, such as the ability of individual microbe species to adsorb, metabolise, detoxify or degrade individual chemicals. As such, the effect of pesticide and herbicide usage on microbial communities is extremely difficult to predict. Certainly, it is not always negative. Some microbial groups are able to use certain pesticides as a source of energy and nutrients, and therefore thrive upon application. In other instances, a pesticide that is toxic to one group of microorganisms will reduce competition and allow a previously less successful species to become established in the ecosystem.37 However, it appears that almost all applications of agricultural chemicals lead to a longterm loss of microbiome biodiversity and soil fertility. In addition to this, agricultural chemicals significantly reduce microbial infection of plant roots, therefore directly reducing nitrogen fixation and overall growth in many plant species.38
Soil acidification occurs at a slow rate as a natural process, but this process is accelerated dramatically by human inputs. There are four key mechanisms by which this accelerated acidification takes place: removal of product, leaching of nitrogen, inappropriate use of certain fertilisers and accumulation of organic matter.
The removal of crops and vegetation from soil drives acidification because most agricultural products are slightly alkaline: their removal lowers the pH of the soil, particularly if they are not replaced. Leaching of nitrogen, meanwhile, occurs when soil nitrate levels exceed the quantity that can be used by plants. When this occurs – usually following excessive or inappropriate usage of nitrogenous fertilisers – the nitrate drains below the plant root zone and into the groundwater system, leaving the soil with a reduced pH. Increased fertiliser usage can also contribute to build-up of organic matter in soil: while this can be beneficial – in terms of impact on soil structure, for example – it also has the effect of accelerating acidification and reducing soil pH.39
Increased soil acidity can limit water and nutrient availability, reduce microbial activity and contribute to aluminium toxicity in soil, with severe implications for fertility and agricultural production. Recent work indicates that topsoil acidity affects approximately 30% of the ice-free land area of the earth, while subsoil acidity is thought to affective up to 75%.40
Salinization can have serious negative impacts on soil fertility and crop yields, but it is often difficult to directly pinpoint its cause. While some soils are naturally saline, salinization can also be driven by inappropriate management practices and poorly delivered irrigation programmes.41 Salinization occurs most often in arid and semi-arid areas, and is thought to be exacerbated by climate change: this frequently occurs when insufficient rainfall drives irrigation schemes, which can lead to salinization when executed without proper planning and expertise. While there is little recent data on salinization, studies conducted in the 1990s suggested that up to 77 million ha of land was affected by ‘secondary’, or human-driven, salinization. 42
Soil contamination is driven by a range of factors: intensive industrial or agricultural activity, inadequate waste disposal, mining and military activities all frequently introduce excessive amounts of contaminants into soils. Soil contamination is difficult to trace, measure and assess: however, it is known that soil contaminants such as heavy metals and organic and inorganic pollutants can reduce soil fertility, negatively impact biodiversity and – in the case of some organic pollutants – facilitate the spread of infectious disease.
Looking to the future
It is evident that our planets’ soils are facing severe challenges at the hands of urbanisation, pollution, unsustainable land use and climate change. The UN’s Food and Agriculture Organization (FAO) has highlighted soil as a serious cause for concern, stating that “the promotion of sustainable soil and land management is central to ensuring a productive food system, improved rural livelihoods and a healthy environment” while also noting that “the current rate of soil degradation threatens the capacity to meet the needs of future generations”.
Although the ecological relationship between humans and soil is all too frequently overlooked, both large organizations and grassroots projects are increasingly showing awareness of the issues that surround it. The FAO declared 2015 the ‘International Year of Soils’ as well as promoting ‘World Soil Day’ on the 5 December every year. National and local governments are also placing a growing emphasis on the importance of soil, while charity and NGO projects working at the grassroots level demonstrate that there is hope for healthier soils worldwide. Explore the links below for some examples of action currently being taken:
- Kiss the Ground, a California-based group prioritising education and healthy soils
- The Red Soil Project: eroding poverty through sustainable farming
- Soil Erosion Modelling by the African Conservation Trust
- Living Soil Training by the Hummingbird Project in India
- SOIL Haiti: using ecological sanitation to tackle Haiti’s toughest challenges
- The Rodale Institute, an independent agricultural research organization conducting research into soil health and related issues
It is only by concerted action on a range of scales that we can hope to achieve healthier soils worldwide: to do so will by no means be an easy task, but there is little doubt that it is a crucially important one for the future of our planet and the human life it supports.
1 ‘What is Soil?’, The British Society of Soil Science, accessed 25 April 2017
2 ‘What are Soils?’, The British Society of Soil Science, accessed 25 April 2017
5 ‘About’, The Earth Microbiome Project, accessed 25 April 2017
7 Gilbert et al (2010): The Earth Microbiome Project: Meeting report of the “1st EMP meeting on sample selection and acquisition” at Argonne National Laboratory October 6th 2010. Standards in Genomic Sciences, December 2010; 3(3): 249 – 253
8 ‘M Amaranthus: Going back to our roots: Agriculture and the Living Soil’, Mycorrhizal Applications, Inc., 27 December 2013 (accessed 25 April 2017)
11 Tilman et al (2011): Global food demand and the sustainable intensification of agriculture, Proceedings of the National Academy of Sciences of the United States of America, 108(50)
12 ‘Agricultural land (% of land area)’, World Bank Data, accessed 25 April 2017
13 ‘M Amaranthus: Going back to our roots: Agriculture and the Living Soil’, Mycorrhizal Applications, Inc., 27 December 2013 (accessed 25 April 2017)
16 Brady and Weil (2009): Elements of the Nature and Properties of Soils. Prentice Hall (3rd Edition): ISBN 9780135014332
18 ‘What do Soils do for us?’, The British Society of Soil Science, accessed 25 April 2017
19 ‘Earth as building material’, Food and Agriculture Organization of the United Nations, accessed 25 April 2017
20 ‘What do Soils do for us?’, The British Society of Soil Science, accessed 25 April 2017
21 Comis and Don (2002): Glomalin: Hiding Place for a Third of the World’s Stored Soil Carbon. Agricultural Research (United States Department of Agriculture, Agricultural Research Service)
24 ‘What do Soils do for us?’, The British Society of Soil Science, accessed
25 April 2017 25 ‘Soils clean and capture water’, Soils.org, accessed 16 May 2017
26 ‘Soils and biodiversity’, Food and Agriculture Organization of the United Nations, accessed 16 May 2017
27 ‘Soil erosion and degradation’, World Wildlife Fund, accessed 2 May 2017
28 ‘Status of the World’s Soil Resources: Chapter 7, the impact of soil change on ecosystem services’, FAO Report, p176
30 ‘Status of the World’s Soil Resources: Chapter 7, the impact of soil change on ecosystem services’, FAO Report, p175
31 ‘Soil sealing’, European Commission, accessed 2 May 2017
32 ‘Status of the World’s Soil Resources: Chapter 7, the impact of soil change on ecosystem services’, FAO Report, p176
33 ‘Agricultural soil compaction: causes and management’, Alberta Agriculture and Forestry, accessed 2 May 2017
34 Gomes: Health impact from pesticide residues in a desert environment. Middlesex University Research Repository, 1998.
35 Ampofo, Tettch and Bello: Impact of commonly used agrochemicals on bacterial diversity in cultivated soils. Indian Journal of Microbiology, September 2009; 49:223–229
37 Gomes: Health impact from pesticide residues in a desert environment. Middlesex University Research Repository, 1998.
38 Potera: Agriculture: Pesticides Disrupt Nitrogen Fixation. Environmental Health Perspectives, December 2007; 115(12): A579.
39 ‘The causes of soil acidity’, Acid Soil Action – NSW Agriculture, accessed 16 May 2017
40 ‘Status of the World’s Soil Resources: Chapter 6, global soil status, processes and trends’, FAO Report, p124
41 ‘Status of the World’s Soil Resources: Chapter 7, the impact of soil change on ecosystem services’, FAO Report, p178-9