Technical insights from SoilsCon on soil health and management
In February over 150 landscape industry professionals gathered at SoilsCon 2024 to hear key industry speakers share their expertise and highlight the critical importance of soil health.
Landscape architects, landscape contractors, developers, civil engineers, garden designers, and topsoil suppliers, attended the fifth soils conference at Phyllis Court, Henley-on-Thames, organised by soil scientist and landscape consultant Tim O’Hare.
Accredited Supplier, Tim O’Hare Associates, is based in Wallingford, Oxfordshire, and has provided soil science consultancy to some of the highest profile public realm landscape schemes in the UK and overseas, including the Queen Elizabeth Olympic Park, Athletes Village Stratford, Mayfield Park Manchester, Battersea Power Station, and RHS Garden Bridgewater. The practice’s overseas projects are extensive and include Commonwealth Park in Gibraltar, Parque Central in Valencia, Oman Botanic Garden, and Al Muwaiji Fort in Abu Dhabi.
In the four years that have passed since the last SoilsCon in 2019, the marked changes in the UK’s climate have had a major impact on our soils.
Following a brief introduction and welcome, Tim O’Hare talked about soil health and functions in landscapes and how healthy soils are critical to supporting the numerous functions they fulfil. UK soils are extremely diverse in their physical, chemical and biological properties, he explained, and understanding at the planning stage the soil you are working with is crucial.
On greenfield sites, this is determined by undertaking a Soil Resource Survey, which identifies the opportunities for the soil’s re-use. Brownfield sites will also have resources, but also possibly not just ‘contamination’, with the ‘made ground’, demolition materials and recycled aggregates often suitable for re-use, and particularly for more biodiverse, species-rich habitats required for BNG implementation.
Tim continued by stressing the importance of good soil management and correct stockpiling pre-construction to protect on-site soils. Badly managed soils can result in failed planting schemes. Soil compaction is the biggest contributor to soil degradation on construction sites and whilst the NHBC’s requirement is for a topsoil layer of just 100mm over subsoil, Tim argued that the optimum soil profile would be 300mm of topsoil over 300-600mm of properly de-compacted subsoil.
Tim also explained the principles of cover systems for brownfield site remediation and stressed the importance of inter-disciplinary collaboration in designing them to deliver ‘soils for all occasions’ - “we have the resources and technology to deliver soils for a broad range of landscaping requirements” he said.
Next up to the podium was Dr Iain Gould, Associate Professor in Soil Science at the University of Lincoln, who spoke about the fundamental role of organic matter to soil health and quality.
Because organic matter holds carbon in the soil, measuring it has rocketed, Iain explained. It is the ‘glue’ that holds soil aggregates and mineral particles together, providing the soil with its vital stability and resilience and ability to retain moisture in periods of drought.
Without organic matter, soil aggregates are unable to stick together and when water is added the structure falls apart and disperses, releasing carbon and becoming compacted and anaerobic and resulting in poor root growth.
By increasing the levels of organic matter, the workability of the soil improves, nutrients are retained and released slowly to the plants, the soil darkens and retains heat to benefit plant growth, and the biology of the soil improves as micro- and macro-organisms break down the organic matter, increasing the number of earthworms, and extending the food web.
Whilst the role of soil in retaining carbon is important, Iain said, so is food production. Resilient soils, in a year where parts of the East of England have already had 45% of their annual rainfall, are vital for crops. Cultivation of soils results in a loss of carbon to the air through oxidation and therefore adding soil organic matter in the form of compost, manure, digestate, biosolids etc will help to build and maintain resilient soils.
After the morning break, during which a Soil Surgery was held, staffed by soil scientists from Tim O’Hare Associates, the next speaker was Tim White, Senior Associate at Tim O’Hare Associates, who discussed soil considerations for the delivery of Biodiversity Net Gain.
Tim explained that Natural England’s BNG guidance document scores habitats’ biodiversity value according to key parameters. The more distinctive a habitat is, and the better their condition, the greater their value in terms of BNG, although they are more demanding to create, Tim explained. Reference to soils in the guidance says that low fertility soil is good for certain habitats such as grasslands, to reduce competition from aggressive species and grasses.
Of key importance is phosphorous, which is held by soils and is not easy to reduce, whereas nitrogen tends to accumulate and only a proportion of the nitrogen is available to plants. Importantly, organic matter isn’t detrimental to habitats requiring a low fertility soil. Environmental factors such as whether a site is prone to drought, waterlogging, or is steeply sloping need to be considered, as does the prevalence of pernicious weeds.
A Baseline Soil Assessment carried out by soil scientists that doesn’t focus simply on pH and extractable phosphorus is important to identify soil types suitable for habitat creation. Tim ran through several key habitat types listed in the guidance, referring to the BNG ‘metric’, and outlined each habitat’s preferred soil type, hydrology, fertility, pH and carbonate levels. He highlighted that for development sites a Soil Management Plan, which covers the topsoil stripping, stockpiling, and decompaction of subsoil, is necessary to ensure the future soil viability and in conclusion he reminded the audience that, in soil terms, ‘low quality’ is not the same as ‘low fertility’.
Closing the morning session was multi award-winning garden designer Helen Elks-Smith FSGD, who’s talk ‘But it’s just dirt’ highlighted the cultural difference between garden designers and housebuilders in terms of soil. Plants, she said, need healthy soil in which to flourish but the gulf between designers and contractors can sometimes be enormous in how they perceive, and treat, this crucial component of any successful garden project.
Illustrating her talk by referencing two of her domestic schemes, Helen cited examples of poor onsite soil management by contractors, despite her giving clear instructions on what needed to be done to protect the soils. Compaction, contamination with rubbish, and general misuse of soils destined for planted areas confirmed Helen’s experience that housebuilders don’t understand about soils.
Compaction is by far the biggest problem, she said, and so soil protection zones on site are vital. And the language used around soils is important too, with a need to educate domestic clients through the gardening media. She explained how important it is for designers, particularly those operating in the domestic sector, to have tight contracts in place to avoid liability for planting failures caused by poor soil management. The commercial sector is much further ahead in terms of understanding the role of soils in landscape schemes, she said.
After lunch, Jennifer Mui CMLI, Director, MRG Studio, delivered her presentation on low-fertility soils in climate change-resilient landscapes, referencing The Woolbeding Glasshouse and Silk Route Garden project in West Sussex.
This scheme was commissioned by the National Trust who wanted to create a garden that would work with an increasingly dry climate. Jennifer explained that climate change is seeing plants that historically grew further south on the globe now moving to more northern climes and that we need to re-think our attitudes to what plants we now perceive to be ‘native’ to our shores.
The concept of 12 garden zones, each representing geographic areas along the Silk Route, and culminating in a stunning 10-sided Heatherwick Studio-designed glasshouse, required 12 different soil types. Tim O’Hare Associates (TOHA) undertook an initial topsoil and subsoil survey from which they produced a soil strategy, a soil management plan for the stripping of the existing soils, and soil designs and specifications for each of the 12 zones.
It was a complex project requiring drainage across the site, placement of subsoils, tree planting, placement of large rocks, and finally the placing of the low-fertility topsoil prior to planting. The TOHA team supported Jennifer’s team throughout the installation and the National Trust continues to monitor the performance of the species selected for each of the zones.
Kevin Barton CMLI, FLI, Director, Robert Bray Associates, spoke about the challenges faced by the hard-working soils in bioretention raingardens.
He opened by saying that SuDS is all about how we manage water quantity, and that SuDS should not take up space on a development but rather be included within the landscape as a landscape-led initiative.
A successful SuDS scheme, he said, embraces four equal pillars - water quantity, water quality, biodiversity, and, in Kevin’s opinion the most important, amenity. He cited a recent scheme that features a 300mm deep sandy loam rain garden taking run-off from a roof; permeable paving; and a SuDS basin in the centre of the scheme with grasses and boardwalk through which children are encouraged to run and explore. He stressed the need for the area to be useable when it’s not raining.
Kevin also discussed the hugely polluting impact of highway run-off and the SuDS features employed to progressively clean and remove the pollution, i.e. microplastics, rubber, heavy metals etc. Bioretention raingardens are challenging and intensive, he said. Taking run-off from the surface before it goes underground is key to preventing failed raingardens. Kevin also stressed how maintenance impacts their effectiveness – better to build fewer raingardens properly and maintain them well, which could include engagement with local businesses and charities.
Following the afternoon break and the last opportunity for delegates to seek advice from the soil scientists in the soil surgery, the penultimate presentation on the use of biochar and dolerite in the landscape was delivered jointly by Katherine Iles, Senior Geotechnical Engineer, and Ryan Coghlan, Senior Landscape Architect at ARUP. Biochar is a carbon-rich form of charcoal, produced by heating any organic material at very high temperatures. This process also creates biofuel, and the residual heat can be converted to energy.
At the moment of production it sequesters carbon and has a broad range of applications including the removal of contaminants from soils and enhancing soil fertility. Dolerite is a natural, calcium-rich silicate and a by-product of quarrying in Scotland, Northumberland, Durham, and the Welsh borders. Both biochar and dolerite are considered a ‘waste’ by the regulators, who are unsure where to place them in the waste hierarchy. Dolerite is currently used in green roof substrates and to help establish low fertility conditions.
A research and development project – the BEIS Phase 1 Study – hopes to build an evidence-based case for the use of biochar and dolerite at scale to create different habitats, and to demonstrate they are safe to use, thereby closing the policy gap. Katharine and Ryan agree that a multi-disciplinary approach is needed to look at how these materials can be sustainably produced/sourced and utilised more widely.
The day’s final presentation came from Wildflower Turf Ltd’s Helen Simmen, who talked passionately about the soil microbiome and how species-rich habitats promote healthy soil. Whilst our knowledge of soils and their importance has grown in recent years, she said, people think of it as inert, when it is teeming with life. Soil hosts half of life on earth, and only one percent is known to us. There are an unfathomable number of microorganisms in the soil and what they do is crucial to ecosystem processes - our survival depends on them.
The health of the soil, therefore, is fundamental to our planet. Interaction between ‘above ground’ and the soil happens through plants, and nutrient poor soils provide the conditions for plants to thrive. Helen said we must work with nature and not treat soil organisms as inconsequential. Species diversity creates good healthy ecosystems but whilst there is regulation for above ground biodiversity, regulation on soil health is sparse. The varied root structures of different species help percolation and infiltration and reduce excessive flow. Soil life is improved, which in turn helps with carbon storage. 97% of species-rich meadows in the UK have been lost since the 1930s. Life, Helen said, would be a lot easier if we just looked after our soils.
SoilsCon 2024 brought together landscape professionals with a passion for knowledge and a genuine desire to work with nature to protect and enhance our soils. After a four-year break, delegates have been able to hear directly from nine outstanding speakers at the forefront of soil research, product development, and landscape application, and take forward the message that the health of our planet’s soils is absolutely fundamental to our way of life.
Further reading