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Manufactured soils: A user guide
Lightweight manufactured soil profile. © Tim O’Hare
A comprehensive understanding of soils is required throughout the entire design team to ensure appropriate soils are specified to support healthy plant growth and development.
The role of manufactured soils has evolved and expanded hugely over the last few decades to keep up with the imagination and expectations of landscape designers, and especially those working in the urban realm. We expect trees to grow happily in pavements, parks to exist on top of Tube stations and car parks, a garden to flourish on the 10th storey of a building, runoff from roads to flow into and be absorbed by planting beds, and grass lawns to withstand the trampling of millions of pedestrians each year. The soils used to support these challenging landscapes must possess specific properties to meet these demands.
What is a manufactured soil?
Manufactured soil is the widely used term to describe a soil medium that has been created when two or more materials are blended. Other terms, such as soil substitute, man-made growing media, engineered soil, and artificial substrate have also been used over the years, but manufactured soil seems to have now stuck.
In many respects this process is simply a fast-track version of the natural soil-forming process (pedogenesis), whereby soil is formed by the interaction of parent material, topography, climate, and organisms over a long period of time.
The materials used to make a manufactured soil usually consist of two components: inert, mineral-based media (such as subsoil, sands, crushed rock, demolition arisings, soil washings or quarry overburden); and bulky organic ameliorants (such as green compost, spent mushroom compost, composted bark fines, or even natural, as-dug, topsoil). These may be combined with other specific additives where more specialist end-use or targeted functions are required – for example, lightweight media, aggregates, soil conditioners, biochar, and lime.
As the name suggests, the materials are often blended ex situ at dedicated sites using a range of industrial-sized soil processing machinery and equipment such as excavators, loading shovels, screeners, hoppers, and conveyors. Alternatively, it is possible to manufacture the soil in situ at the site using large agricultural machinery to spread and blend the materials on the ground before planting directly into the mix.
The history of manufactured soils
Manufactured soils have been used in the UK commercial landscape industry since the mid-1990s. Before then, the choice of soils for new urban landscape projects was either natural topsoils and subsoils imported from nearby greenfield developments where they were surplus to requirement, or ‘skip waste’ soils. These were the soil fines collected after the contents of skips and other site clearance operations had been screened. They were usually highly alkaline, saline, low in organic matter and plant nutrients, and often contained various sharps (glass, nails, ceramic tile) and chemical contaminants (heavy metals, hydrocarbons, asbestos). In those days the soil supply business was not run by experts in soil, but instead by the haulage industry. Topsoil was simply another commodity that was moved from site to site, with occasional temporary storage in a haulage yard.
There was a welcome initiative by the government of the time to encourage the redevelopment of brownfield sites and discourage greenfield development. Consequently, the readily available supply of natural soils dried up. Around the same time, other schemes meant that it became logistically and commercially viable to manufacture soils for the landscape and construction industry. These included the EU directive for recycling household garden waste, the introduction of a tax on primary aggregates, and changes in contaminated land remediation requirements that ruled out the use of skip waste soils.
Of course, the concept of blending materials together to make a growing medium was nothing new. The horticultural sector had been making potting mixes for many years. For example, in the 1930s John Innes, the renowned British horticulturist, developed a series of standardised potting composts. These were made with varying recipes of peat, sand, sterilised loam, fertilisers and lime, and their introduction revolutionised plant propagation and container gardening.
Sports turf also had a long-established history of using man-made soils in the form of sand rootzones for golf courses, bowling greens, and sports pitches. There was a heavy reliance on highly processed washed sands, peat, and fertilisers for many of these.
The land reclamation sector had also developed methods for remediating old mining sites into country parks, forestry, and agriculture. Here, existing spoil materials were improved with various additives including sewage sludge, paper pulp, and waste lime.
All these previous activities presented useful precedents for making landscape soils. The principal difference between them and manufactured soils was their application, and the types of materials that were commercially available and technically appropriate.
Certainly, the use of peat has never been entertained as a component of manufactured soils, and sewage sludge has its difficulties with regard to heavy metals, odour, and handling ability.
There were a few false starts, where poor-quality manufactured soils were dumped onto the market, and landscape schemes drastically failed. This led to a bad reputation that was difficult to eradicate. As far as I am aware, the first landscape project where manufactured soil was completely relied on and highly successful was Bluewater Park in Dartford, Kent. A total of 60,000m3 of topsoil was manufactured for this project, a retail park positioned in the base of a former chalk quarry. George Longmuir of Freeland Horticulture is credited with producing this topsoil, and Freeland Horticulture went on to become the first fully dedicated topsoil manufacturer in the UK. Once the demand became apparent, others started to emerge.
The soil manufacturing industry has evolved to now service the entire UK, with some companies strategically setting up regional hubs. In the past, the soil supply industry was effectively run by waste and haulage companies, with little knowledge or appreciation of their products. However, the bar has now been significantly raised. Many soil companies work to recognised quality-assurance standards, employ technically competent staff, invest in product development and enhancement, and run independent, quality controlled, compliance testing programmes. Most of the materials used to make these soils are derived from recycled or repurposed resources.
Designer soils
It is widely recognised that soils fulfil several essential functions and support ecosystem services and nature-based solutions that are central to social, economic, and environmental sustainability. This is particularly the case in urban environments, for example:
– Provision of habitat for flora and fauna
– Promotion of biological diversity
– Water management through infiltration, retention, filtration, and groundwater recharge
– Carbon capture and storage
– Temperature control through thermal insulation from green roofs
– Support for infrastructure development
– Contaminated land remediation
By selecting soil materials with the required properties and at the correct mixing ratios, it is possible to manufacture soils that deliver most, if not all, of these requirements.
Manufactured soils come into their own on projects that do not have an existing soil resource to reuse, for example, a podium landscape scheme built over a slab, or where the existing ground is contaminated. In these instances, the required soils can be specified, sourced, tested, and imported to suit the project’s construction programme. If a site has existing soils, but they do not possess the exact properties required by the landscape scheme, it may be possible to manufacture more suitable soils through on-site amelioration.
Soil properties (e.g. pH value, drainage rate, fertility status, weight) can be tailored to suit the needs of the project, with more than one soil used where the requirements are diverse. With the arrival of Biodiversity Net Gain (BNG), it is even more important that the soils have the necessary characteristics to support the target habitats. For instance, recycled sands and demolition arisings are now more viable materials to create the necessary environmental stresses for open-mosaic habitats.
Most manufactured soils are easier to handle and spread without impacting their drainage and aeration characteristics, which is a great benefit on construction sites. If the materials used to formulate the soil have no seed bank, the new landscape should have a clean slate to establish into. This is another advantage, particularly for BNG implementation.
The following table lists examples of soils that are routinely manufactured today. Of course, some can also be sourced from reserves of naturally occurring soils.
The soils used for sustainable drainage systems (SuDS) may derive from several of these soil types. It is not always the case that the soil needs to be fast draining as some SuDS designs can accommodate slowerdraining soils that will be more suited to the planting palette and climatic conditions.
Soil design process
Integrating soil design within any soft landscape design is essential, given the soils’ influence on the success or failure of the scheme. This should cover the protection, recovery, manufacture and re-use of site soils and, if necessary, the sourcing and testing of any imported soils, whether natural or manufactured.
From RIBA Stage 2: Baseline Soil Assessment
The decision to reuse site soils should start early, and ideally by RIBA Stage 2 when other surveys are conducted. If the site has existing soils, some form of baseline assessment is carried out by a soil scientist. This is covered by a Soil Resource Survey in the Defra guidance document Code of Practice for the Sustainable Use of Soils on Construction Sites5 and is relevant for both greenfield and brownfield sites. It is intended to specifically consider the site’s soils with respect to their landscape, ecology, and reuse potential rather than checking for contaminants or geotechnical properties. The resulting report should confirm the types of soil present and explain their strengths and weaknesses with respect to recovery and reuse. This information will be useful to the client design team, including the landscape architect, ecologist, engineer, arboriculturist, and cost consultant.
From RIBA Stage 3 Soil Strategy
Armed with the baseline soils information, the soil scientist can develop a Soil Strategy during RIBA Stage 3. This determines what specific types of soil will be required for the project, and how these can be derived (on-site and/or imported).
Other topics covered usually include considerations for soil amelioration, soil management, soil depths, land drainage, tree pit design, and aftercare.
From RIBA Stage 4 Soil Specification
A Soil Specification is prepared to set out the specific qualities that the soils should possess, and how the soil supplier and landscape contractor should achieve them.
Tim O’Hare is the Founder and Principal Consultant of Tim O’Hare Associates LLP. Tim has designed soil systems for many of the UK’s largest urban regeneration projects and is the founder of the annual soils conference, SoilsCon (26 September 2025).
