9 minute read
Healthy soil, healthy places
Soils in urban green spaces and gardens canbe healthier than many agricultural soils. Photo by Gary Butterfield on Unsplash
From flood mitigation and vegetation support to antibiotics and our immune systems, healthy soils underpin healthy people and places. Landscape professionals understand these myriad benefits and are well placed to employ their unique skillset to mitigate risk and promote best practice.
When soil is healthy it provides many functions, or ecosystem services – the benefits that we, as society, get from nature. These include food production, carbon storage, mitigation of flooding and drought, detoxification of pollution, and protection from diseases.
Soil provides this multifunctionality through its physical, biological, and chemical properties. In practice, this means that healthy soil has good structure, a diverse biological community of organisms, and plenty of organic matter and nutrients. Healthy soils support the growth of plants, including crops and, given that 95% of global food production relies on soil, its health plays a vital role in food security. Soil is also a living system; it’s estimated that 59% of all life on earth lives in soil, making it the world’s most species-rich habitat. Its health determines how well it functions as a living ecological entity and, equally, its capacity to provide for our health, wellbeing, and ability to function as a society.
Urban soil multifunctionality and biodiversity
In cities and urban areas, soils still provide important functions; they are simply less visible than in farmland or natural landscapes. Contrary to expectations, soils in urban greenspaces and gardens can be healthier than many agricultural soils, although they still experience pressures, such as high footfall. Urban soils underpin many ecosystem services that are important for both human wellbeing and urban resilience. These include local flood mitigation, buffering the urban heat island effect, supporting urban vegetation that captures air pollution, access to greenspace for health, and urban food growing.
Urban soils are full of biological life. A study of Central Park in New York showed that the diversity of microbial life in the park soil was as broad as that found in soils across the globe, in natural ecosystems from multiple continents.
Soil organisms can be categorised into three groups according to their function: ecosystem engineers, chemical engineers, and biological regulators. Ecosystem engineers are larger soil animals such as earthworms, ants, and small mammals. They ingest or move large amounts of soil and incorporate organic matter into the soil matrix. Chemical engineers are decomposers; these are smaller microorganisms (or microbes) that include bacteria and fungi. Their role is to decompose plant residue and other organic matter, transforming nutrients so that they are available to plants. Biological regulators are microorganisms (protozoa, nematodes) or larger organisms (springtails, mites). They act to regulate the population of other soil organisms, either through predation or by releasing chemicals, such as enzymes, antibiotics or hormones. This also gives them the potential to control soil diseases. These incredible organisms not only perform important functions in soil, but their activities also drive ecosystem processes that govern global systems such as nutrient and carbon cycling, and have a significant influence on human health, society, and the planet.
Healthy urban greenspaces
Ecosystem engineers physically alter soil structure and help to form soil aggregates (clumps of particles that are bound together). In soils with good structure there are numerous pore spaces between aggregates where water and oxygen can be stored. Earthworms and plant roots maintain that structure by forging a network of pathways and tunnels through the soil, creating infiltration channels for hydration and respiration to take place. In urban greenspaces, soils can be less compacted than agricultural soils, with ample pore spaces that support healthy vegetation. It’s well known that access to good-quality greenspace contributes to our wellbeing and is associated with improved mental and physical health outcomes.
Healthy soils in our greenspaces also store carbon in the form of organic matter. Storing more carbon helps contribute to climate change mitigation as well as making our soils more resilient to the impacts of climate change.
Soil compaction, on the other hand, creates a serious problem. It restricts pore spaces so prevents water either infiltrating or being stored. This in turn leads to waterlogging or flooding during wet weather and increased drought during dry weather. Compaction also prevents oxygen from entering the soil, making it inhospitable to many soil organisms, plants, and trees. On construction projects, soil compaction is a major cause of soil degradation. It occurs due to trafficking by heavy vehicles, inappropriate soil stockpiling and reinstatement, and can render the soil incapable of supporting plant growth. Over time, soil ecosystem engineers can improve the soil, but it may take years to recover. So it’s vitally important that we prevent damage to soils during construction in order to deliver successful greenspaces.
Healthy soils in our greenspaces also store carbon in the form of organic matter. Storing more carbon helps contribute to climate change mitigation as well as making our soils more resilient to the impacts of climate change. Organic matter allows soil to hold more water, helping plants survive drought and reducing dust or wildfire risk. Increased organic matter and soil carbon can be encouraged in the landscape by planting trees, shrubs, deep-rooted grasses and perennials, and limiting physical disturbance or compaction of the soil. Healthy and well-managed soils in established greenspaces and gardens act as vital pockets for infiltration, oxygen, and carbon within largely impermeable urban landscapes.
Contamination risks in urban soils
Soil helps to capture contamination, and microbes are then able to biodegrade some organic pollutants. As such, soils and microbes play a part in the transport, bioavailability, and risk posed by contaminants. While it is beneficial for soil to capture contaminants and prevent them reaching groundwater, build-up can lead to soils becoming a health risk. This is particularly the case in urban soils where greenspaces can be a repository for historical and current contamination and are also locations where people have more contact with soil.
In urban food growing, heavy metals, perfluoroalkyl and polyfluoroalkyl substances (PFAS), forever chemicals and other contaminants can pose a risk. For example, although lead can accumulate at low levels in plant tissue, the main risk pathway is considered to be ingestion through aerosols, such as dust inhaled during digging, or soil attached to unwashed produce or hands. Soil is also a sink for micro- and nanoplastics, the latter of which can be taken up by plant roots and leaves, and may find their way into food grown in urban areas. However, there remains a great deal of uncertainty around the long-term impacts of microplastics on human health.
Soil microbes and human health
Soil microbes also have the ability to control human pathogens and over 70% of naturally occurring antibiotics that we use in medicine come from soil organisms. However, the widespread use of antibiotics has led to antibiotic resistance, and this may be increasingly present in soil due to the use of antibiotics in livestock, and through manure or wastewater. To protect the use of soil as a source of antibiotics, we need to maintain its microbial diversity and prevent the loss of microbial species through soil degradation.
Research now suggests that human health is influenced by exposure to natural environments, including soil. It is thought that exposure to soil microbes likely plays a role in the development of the immune system, and that this exposure is linked to microbially driven immune responses that influence mental and physical wellbeing. It has been shown that early life exposure to microbes can promote tolerance to allergens. For example, children in rural areas exposed to microbial endotoxins tend to have fewer allergies than those from urban areas.
Why we Need To Take A Holistic Approach To Landscape And Soil
Working as a landscape architect requires holistic thinking, bringing together many professions and stakeholders to understand a site and deliver what’s needed. The broad nature of this work means landscape architects are ideally placed to understand the role of soils in a project’s success and help prioritise it on the agenda. Soil is often taken into consideration too late in a project and, as such, is not always well planned for.
Talk about soil with clients early in the planning stage, design with the site’s soil health in mind, and treat soil as a living ecosystem. Use Defra’s guidance on the sustainable use of soils on construction sites. You might also bring in soil scientists by using the British Society of Soil Science ‘find an expert’ service.
Working with nature is crucial for sustainable and forward-looking design, so use it to support healthy, functioning soils with rich biodiversity. Not only will it support more successful greenspaces and projects, but it’s also an opportunity to improve the health of people, ecosystems, and society.
Roisin O’Riordan worked as a landscape architect and in an environmental NGO before moving into soil science. She’s now in the soil and peat science team at Defra where she gathers evidence to support policymaking. Her interests are in soil ecosystem services, soil carbon and soils in planning and construction.
