Soil health: a prized asset?

The earth beneath our feet is key to sustaining life. Healthy soil is vital for food security, human health, wildlife, biodiversity and cultural heritage. It is also essential for tackling climate change. As a global engineering, management and development consultancy, Mott MacDonald can help, say Collette Patterson and Andrew May.

Globally, soil contains more carbon than is in the atmosphere and all plant life combined – 2500bn tonnes compared with 1360bn tonnes. In the UK alone, soil contains about 10bn tonnes of carbon. Its Environment Agency says this is equal to 80 years of annual UK greenhouse gas emissions. Carbon is also the main component of the organic matter in soil, which is fundamental to its water-holding capacity and structure, and important for resilience to flooding and drought and for water quality.

Urbanisation, intensive and poor farming practices, deforestation and erosion has seen a loss of soil and the decline in soil health across the world. Converting a forest into farmland, for example, not only removes important nutrients and organic material it also significantly reduces the amount of carbon the soil can store by up to 75%. It also makes soil much more susceptible to erosion by wind and water.

Levelling the land

Building often consumes green field agricultural land, disturbing existing soils through stripping back topsoil, forming foundations and excavation for underground services and reprofiling earthworks. Development on brownfield sites follows the same process and frequently involves remediating contaminated soils. More than half of the soil carbon in England is contained within the top 30cm of the soil, while large quantities of excavated soil – clean and contaminated – also end up at landfill and are wasted. The Environment Agency report cited earlier reveals that 28M tonnes of soil was sent to landfill in 2016, more than half (55%) of the waste received in the UK by tonnage. Changing how soil is classified in waste regulations and treating soils as an asset would help reduce the amount going to landfill and the loss of this valuable resource.

Covering soil with water-impermeable surfaces, such as asphalt and concrete, seals soil and effectively shuts off its ecological functions. In England, 10% of soil in urban areas is subject to high levels of sealing. This increases surface water run-off to surrounding land, removes the potential for soil to store carbon and reduces its capacity to both filter atmospheric pollutants and cycle nutrients.

Over-compacting soil using heavy machinery or poor agricultural land management also causes problems. Compacted and poorly structured soils have less capacity to hold water, reducing infiltration rates, aeration and nutrient flow. It also increases the risks of flooding and soil erosion. Compacted soils may absorb more heat, leading to higher urban soil temperatures and leads to a reduction in soils’ carbon sequestration potential. Soil erosion leads to sedimentation, increasing the likelihood of blocked drains or gullies and streambeds. Sediment itself is a valuable resource, often enriching soils with nutrients in floodplains under natural conditions, so it is important to reduce sediment losses in addition to preventing the problems it can cause in watercourses.

Good practice, good outcomes

Maintaining and enhancing current levels of soil carbon through good soil management and restoring degraded and eroded land will increase the capacity of soil to store carbon as well as deliver food production and improve flood resilience. Soil science supports the development of buildings and infrastructure that prioritise sustainable soil management and soil health.

Although the impact on soil is part of the typical environmental assessment process, there is no specific direct planning control in the UK on the sustainable use and management of soil resources on construction sites or a requirement for the monitoring of soil protection and sustainable reuse.

However, good practice can deliver good outcomes. Soil surveys prior to construction provide an understanding of the physical, chemical and biological characteristics of site soils and the results enable us to develop appropriately tailored construction practices. The surveys are effective tools for guiding the protection, management, reinstatement and reuse of soils on construction projects, with the findings used to develop soil management plans (SMP) that are increasingly a common condition of planning permissions for developing greenfield sites. These plans set out measures to protect soil resources from loss or degradation.

To ensure soil is treated as a prized asset, we need clients to be progressive and to set goals for returning land to pre-development conditions as well as adopt modern methods of construction to support the reuse of excavated soil, which can both mitigate risk and provide carbon and cost reduction.

Mott MacDonald is working on projects to raise the importance of soil and is acting to minimise damage and maintain soil health. As part of a joint venture, we delivered regional and farm specific SMPs for phase one of the HS2, the UK’s new high-speed rail line between London and the West Midlands, to support the client’s goal of returning the land to pre-development conditions. Where excavated soil is compatible it can be reused in specific locations, including for embankments, landscaping and creating habitats. On the Avonmouth Severnside Enterprise Area ecology mitigation and flood defence scheme, soil dug up to create two new wetlands totalling 80ha for migratory birds is being used elsewhere on the project, including for the construction of embankments. The project is also importing natural material from local housing developments on greenfield sites around Bristol and Somerset, preventing soils potentially reaching landfill.

Runoff from construction sites is a potential major source of sediment in areas under development. Removal of vegetation and bare soil increases the likelihood of erosion, and typically, increases the volume and runoff rate. Nature-based solutions, including swales, ponds, sediment traps and reedbeds to filter runoff and keep soil and sediment close to source, could be installed prior to construction.

Our catalogue of nature-based solutions contains measures to help improve and restore soils, including carbon-rich peatlands. These range from river catchment management practices to manage water resources and land use to reduce flood risk, soil erosion, sediment build-up and pollution to planting hedgerows and small patches of trees on farmland and near watercourses to stabilise soils and runoff systems to intercept water and trap soil and sediment before it leaves a field. These measures apply equally to developed and developing nations.

Leeds is sited in a catchment where river levels can rise rapidly in response to rainfall and in December 2015 Storm Eva brought torrential rain and flooding to the city and surrounding area. The Leeds flood alleviation scheme will help reduce future flood risk and natural flood management (NFM) is a core element of the scheme. The main NFM focus is on planting trees, and soil and land management.

Every year, in the worst-hit erosion areas of the Shire River Basin in Malawi three truckloads of soil per hectare were washed away. As part of phase one of the Shire River Basin management programme, we supported the Government of Malawi to rehabilitate four catchment areas totalling 130,000ha. To protect their communities and livelihoods, local people planted bamboo and trees to reduce flood risk and retain fertile topsoil, and vegetation along the riverbanks to act as a buffer zone. More than 25,000ha of soil and water conservation and almost 10,000ha of forestry work was completed, and over 4000km of streams and riverbeds rehabilitated.

At Mott MacDonald, we put social outcomes at the forefront of our projects and use our influence to deliver positive change. It is our responsibility to:

• raise awareness of climate-related issues with clients, contractors and designers
• challenge the brief and shape the project from its earliest stages, including how we can promote good soil health and maximise the carbon sequestration potential
• calculate carbon and present carbon reduction opportunities at each stage of the design

It can take several hundred years for 10mm of topsoil to form. Its preservation is essential for food security and a sustainable future. Let’s ensure we help to improve soil management and soil health through our projects.