Parks, street trees, and shop front shrubberies; splashed across the canvas of our grey urban landscape are little specks of green. They may be dwarfed by high-rise office blocks and fast-moving traffic, but they are some of the most important cubic metres in our towns and cities.
Urban Green Infrastructure (UGI) plays a pivotal role in enhancing the well-being and resilience of urban environments. As cities continue to expand and grapple with the challenges of rapid urbanisation, UGI offers a powerful solution to counteract the adverse impacts of urbanisation on both human populations and the natural environment.
Ensuring the resilience of UGI in the ever-changing global landscape poses a significant and complex challenge. The capacity of UGI to effectively contribute to achieving the UN Sustainable Development Goals hinges greatly upon the resilience of its natural capital.
Among the crucial components of UGI, soils stand out as one of the most pivotal materials in the construction and landscaping industry. Acting as a growth medium for plants, soils serve as the fundamental basis for various green infrastructure projects. Moreover, soils play a vital role in providing numerous essential ecosystem services, such as water storage and filtration, supporting diverse above-ground and below-ground biodiversity, nutrient cycling, and carbon sequestration to combat climate change.
As the demand to green the grey landscapes of our towns and cities continues to grow, the construction and landscaping sector faces mounting pressure to ensure that their infrastructure schemes not only maintain but also enhance the delivery of these ecosystem goods and services.
Sourcing a sustainable soil supply
In UGI projects, the need for substantial quantities of soil within a limited timeframe poses a significant challenge for the sector. Natural rates of soil formation from bedrock are remarkably slow, ranging from as slow as 0.03 to 0.2 mm/year, as evidenced in measurements conducted across the UK (and previously featured in Issue 63 of Air Water Environment International). Such sluggish rates of soil generation equate to the width of an average human hair, rendering them impractical for meeting the demands of UGI schemes. Consequently, alternative approaches for the formation of soils are imperative to provide the necessary materials to facilitate the successful implementation of UGI.
As Dug soils
One approach to address the urgent need for soil in UGI projects is the utilisation of ‘As Dug’ soils. This method involves the removal, or stripping, of soils from greenfield sites, such as farmland. As Dug soils offer several advantages, including being single-sourced and unscreened, which ensures the preservation of their composition and structure, notwithstanding the possibility that soils can lose some of their structure in transit. Since these soils have already undergone natural formation processes, vital properties like organic carbon and a rich microbial community are already established, and emerging properties such as resilience may also be present. These inherent characteristics can contribute significantly to the success of UGI projects by providing a solid foundation for sustainable and thriving green spaces within urban environments.
“sluggish rates of soil generation equate to the width of an average human hair, rendering them impractical for meeting the demands of UGI schemes”
However, there are certain issues associated with As Dug soils. One significant concern lies in the relatively non-renewable nature of natural topsoils. Stripping the uppermost 30 cm of soil from farmland, for instance, represents the removal of approximately 10,000 years' worth of soil formation, considering the slow natural rates of soil generation. Such extensive soil removal can have severe implications for agricultural productivity and long-term soil fertility. The depletion of topsoil from greenfield sites can adversely impact the ecosystems and biodiversity that depend on these natural habitats.
This raises questions about the sustainability and resilience of the As Dug strategy. What might be deemed sustainable and resilient for UGI purposes – meeting the immediate need for soil in urban settings – may not necessarily align with the sustainability objectives for their source locations, and the preservation of natural ecosystems. The increasing demand for specified and performance-based soils further exacerbates this challenge. There simply isn't enough As Dug topsoil being generated to meet the growing requirements for high-performance topsoils in UGI schemes. Mixed waste topsoil or "muck away" soils, often available as an alternative, are also not appropriate for most UGI schemes.
Therefore, the need to blend soils to precise specifications becomes paramount. In doing so, UGI projects can ensure that the generated soils can meet the specific demands of the intended green infrastructure, maximising their environmental benefits and minimising potential adverse impacts.
Manufactured soils, as an alternative to As Dug soils, are a useful form of natural capital extensively utilised in Urban Green Infrastructure, globally. These comprise a blend of organic and inorganic material, sourced and combined off-site, and later transported to site for their incorporation into an UGI design. Distinguishing between the formation of soils naturally and the engineered generation of manufactured soils reveals several key disparities. Firstly, manufactured soils, as opposed to natural soils, are composed of 'parent materials' that are typically sourced from multiple locations and combined off-site. This amalgamation process sets them apart as a 'process industry,' involving various >
chemical, physical, and mechanical steps to produce what we refer to as 'formulated products' – in this case, the manufactured soil. This stands in contrast to natural soils, which form in-situ through geological processes below existing soil profiles, without any intervention from human industrial activities.
The second notable distinction stems from the fact that manufactured soils are generated above-ground and result from
a deliberate manufacturing process. This attribute grants them the advantage of being tailored for specific and bespoke projects. For instance, when utilised in UGI, the composition and volumes of soil required can be precisely specified before the 'soil formation' process commences, allowing for optimised outcomes.
The capacity to engineer manufactured soils according to project-specific requirements sets them apart from their natural counterpart. This adaptability ensures that manufactured soils are adept at fulfilling the unique demands of diverse construction and landscaping projects. Whether it involves enhancing water retention, promoting certain plant species, or bolstering carbon sequestration, manufactured soils can be intentionally designed to meet these objectives effectively.
Designing to a standard
How do we ensure that manufactured soils will deliver to the requirements of the UGI project? In the UK, the BS3882 standard is one of the most actively used British Standards providing guidelines and specifications for topsoil quality. Specifically, it focuses on the requirements for topsoil for use in landscaping schemes, including parks, gardens, sports fields, and other amenity areas. The standard aims to ensure that topsoil used in these projects meets certain quality criteria to support healthy plant growth, sustainable land use, and environmental protection. These criteria are vast, but some of the key parameters include:
- Particle Size Distribution: The standard sets limits for the content of different particle sizes (e.g. sand, silt, clay) to ensure good drainage, water retention, and aeration.
- Organic Matter Content: Topsoil is required to have a minimum organic matter content to support microbial activity, nutrient cycling, and plant growth.
- pH: The pH of the topsoil is specified to be within a certain range to support optimal nutrient availability for plants.
- Nutrient Content: The standard defines the minimum nutrient levels (e.g. nitrogen, phosphorus, potassium) that the topsoil should contain to support healthy plant growth.
- Contaminant Limits: BS3882 sets limits on the concentration of harmful substances such as heavy metals and other pollutants to ensure the topsoil's safety for use in amenity areas.
By adhering to BS3882, landscape designers, contractors, and horticulturists can ensure that the topsoil applied in UGI projects meets consistent quality standards and contributes to sustainable and successful landscaping initiatives. However, the BS3882 is not without its limitations. Whilst it provides a framework for quality control and assurance, an issue lies in the fact that there is only BS3882 standard to serve thousands of green infrastructure projects. This is analogous to relying on one recipe to create a triple-layered chocolate strawberry cake in the morning, a Victoria sponge at lunchtime, a farmhouse apple cake in the afternoon, and a double chocolate and violet Bundt in the evening. One recipe may serve the creation of a specific cake very well, but not even the most proficient pâtissier could rely on a single pre-defined list of pre-weighed out ingredients to generate their entire window display.
One year on: a case study of manufactured soils in London
During the Summer of 2022, I had the opportunity to lead a research project in London, where we conducted a comprehensive study of several sites that had implemented manufactured soils designed to comply with the BS3882 specification. The soils at
these sites were relatively young, having only been placed on site
a year before. Our aim was to study their condition one year
on from installation, and gain understanding about their long-
term effectiveness and suitability for different urban green infrastructure schemes.
We gleaned two interesting insights. First, the opportunity to revisit sites and assess the long-term health of plants and the overall success of supplied materials is a rarity in the field of green infrastructure. Unlike the typical workflow in the sector, where focus shifts swiftly to ‘the next project’, this project afforded us the chance to return, reflect and appraise the performance and maturation of manufactured soils.
Second, the manufactured soils studied showcased notable physical, chemical, and biological transformations, even within a relatively short span of one year. At one of the sites, the soil organic matter content showed a relatively concerning decline of 10% over the course of twelve months. This decrease can be attributed to the absence of additional organic inputs during the year. Organic matter is essential: it plays a pivotal role in water retention, which became especially critical during the heatwave and drought that gripped the UK and Europe last summer.
The soils under scrutiny, characterised by an 88% sand content, posed a significant challenge in terms of water retention. With
such a high sand content, the soils naturally exhibited poor water-holding capacity, making them susceptible to rapid drainage and reduced moisture retention. Given the prevailing adverse climatic conditions and increasing water scarcity, addressing this issue became paramount. The ability of these soils to retain water effectively plays a pivotal role in sustaining the health and vitality
of the green infrastructure and vegetation they support.
In regions facing water scarcity, ensuring adequate water retention capabilities in soils becomes even more critical. The lack of
sufficient water retention can lead to increased irrigation demands, exacerbating water stress in already water-scarce areas. Moreover, inadequate moisture retention can compromise the survival and growth of planted vegetation, potentially leading to increased maintenance costs and reduced overall effectiveness of green infrastructure projects.
In addition to the alterations in organic matter and water-holding capacity, we also observed an escalation in the concentration of certain metals, including Chromium, Zinc, Lead, and Barium. These elevations in metal content are likely a consequence of airborne contaminants present in the surrounding environment. For instance, Barium is commonly used in vehicle brake linings, as well as in everyday materials like paint and glass.
Though the soils still complied with the BS3882 standard values after the first year, the continued trajectory of soil change, particularly with respect to the decline in organic matter, raises concerns about potential non-compliance in the years ahead.
Continuous monitoring and careful, adaptive, evidence-based management is imperative.
Designing soil properties, functions, or something else?
The preliminary findings from our study emphasise that a 'BS3882 soil' serves as a fundamental starting point, but it should not be regarded as the final solution. Moving forward, it is crucial to adopt a more holistic approach, considering the specific needs of plant communities and tailoring soil design and manufacturing accordingly. By prioritising the requirements of plant life, we can create soils that are optimised to support their growth and overall health, thus enhancing the performance of urban green infrastructure.
This raises an intriguing question: should we design soils for their properties, their functions, both, or neither? Thumb a traditional soil science textbook and you are likely to see prescribed a ‘typical’ soil composition of 25% air, 25% water, 45% mineral matter, and 5% organic matter. Basic soil science training emphasises ‘key indicators’ or ‘properties’ of soil quality, such as bulk density, porosity, organic carbon, available P, pH, and microbial biomass, among others. These indicators have coalesced into the seemingly all-encompassing notion of "soil health," the vagueness of which has sparked considerable debate in recent years. However, measuring soil health through soil properties alone provides only a snapshot of the soil's condition at a single time point, leaving out the dynamic interactions between properties and their functions.
Perhaps, then, the focus should shift towards observing and monitoring how properties work together to deliver key ecosystem goods and services, or in other words, soil functions. Soil functions encompass a wide range of processes, including nutrient cycling, organic matter decomposition, water infiltration and storage, carbon sequestration, and bioturbation by mesofauna. Ensuring the capacity of soils to deliver these functions is crucial in a world where we increasingly depend on our soils for numerous benefits.
Monitoring the provision and performance of multiple soil functions is valuable, but it fails to reveal how soils react and recover from stress events or perturbations. Understanding soil resilience, the ability of soil to respond and bounce back after disturbance, has emerged as a prominent topic in the soil science community. Consequently, various projects involve extracting soils from the field and subjecting them to controlled environments with standalone or compound disturbance events to study their resilience.
Therefore, while the BS3882 serves as a useful starting point, the need to manufacture sustainable and resilient soils for UGI necessitates a deeper exploration of the principles underpinning soil formation. By considering the dynamic interactions between soil properties and functions and understanding soil resilience, we can enhance the design of soils to better suit the needs of UGI and support a more sustainable and resilient urban environment.
Moving forwards, we need to go back
Returning to the site after sign-off is vital. The continuous monitoring and management of soils deployed in UGI schemes is needed just as much as our rural farmland soils. Recognising that manufactured soils may undergo changes over time, it becomes necessary to develop effective management strategies, which may include the periodic addition of organic inputs to maintain adequate soil organic matter levels. This practice will contribute to the soil's ability to retain water, particularly during challenging climatic conditions like heatwaves and droughts, thereby ensuring the sustained well-being of the vegetation they support.
“a critical aspect of advancing soil management practices involves proactive monitoring and evaluation after soil installation”
A critical aspect of advancing soil management practices involves proactive monitoring and evaluation after soil installation. Stakeholders should unite in a concerted effort to revisit the sites and assess the quality and performance of the soils over time. This ongoing evaluation is pivotal in making informed decisions about any necessary adjustments or improvements to ensure that these soils continue to deliver essential ecosystem goods and services.
By integrating the knowledge gained from monitoring with insights from plant communities' requirements, we can refine soil design, manufacturing processes, and post-placement management techniques. This iterative approach will contribute to the creation of more resilient and effective soils, which in turn will bolster the sustainability and resilience of UGI solutions.