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Manufactured topsoils at the Olympic Park – a review of soil health ten years on
One of the first major urban regeneration projects in the UK to use designed and manufactured soil is assessed – and the results are impressive.
In 2010–2011, 48,000 cubic metres of manufactured topsoil was imported for the first phase (pregames) of the Queen Elizabeth Olympic Park (QEOP). It was one of the first major urban regeneration projects in the UK to embrace the concept of designing multiple soil systems for specific landscape applications and, at the time, the use of this volume of manufactured topsoil to create an urban park was unprecedented.
Early this year, Tim O’Hare Associates (TOHA) completed a follow-up survey of the Park’s manufactured topsoils to assess their current health. The data from this latest survey has now been compared with the data from surveys TOHA carried out in 2010 during the main construction period, and in January 2012 on completion, and the results are exciting.
Background
Healthy soils are a vital component in the delivery of ecosystem services, SuDS, various nature-based solutions and carbon sequestration, and are a key agent in fighting climate change and improving habitat resilience. Comprising a balance of physical, chemical, and biological properties, they are susceptible to rapid deterioration if not managed correctly, and some of the most significant negative impacts result from construction activities.
Remediation operations to deal with the QEOP’s industrial past yielded no reusable soil for landscape purposes and so all topsoil and subsoil had to be imported. The landscape design presented an extremely broad and ambitious range of planting environments, and each had to be catered for from a soil perspective. Requirements for specific soil pH, lime content, fertility status (high and low), organic matter content, drainage capacity, and water retention, meant that more than one soil type was required, as shown in the pie chart below.
Of the soils used on the Park, the multipurpose topsoil, low fertility topsoil and high permeability topsoil were all ‘manufactured’: created by blending carefully selected mineral substrates with various organic ameliorants. The moisture retentive topsoil was created by reconditioning the soil washed from sugar beet crops.
Whilst manufactured soils have been around for at least three decades, knowledge of their behaviour and performance has not been well researched. Some have questioned their long-term sustainability, and even believe they will ‘run out of steam’. Only the growth and establishment of the plants above ground have so far provided some measure of a topsoil’s quality.
Study objective
Prior to installation in QEOP in 2010, each topsoil complied with its respective specification, which included detailed sampling and testing protocols that had to be followed to ensure they were fit for purpose. In January 2012, on completion of the landscape’s construction, TOHA undertook a soil health survey across the Park and in early 2023, a further soil health survey was undertaken. This has provided data on the soils’ current state following ten years of use as a growing medium and following exposure to environmental factors, including weathering and heavy footfall.
The objective of this study was to assess selected physical, chemical, and biological properties of the four topsoil types below and compare the data with the archive laboratory data to provide insight into the longer-term quality and viability of ‘manufactured’ topsoils.
The study soils, and their principal landscape typologies, were:
– Multipurpose topsoil (woodland, shrub and herbaceous planting beds)
– Low fertility topsoil (species-rich meadows)
– High permeability topsoil (amenity grass lawns)
– Moisture retentive topsoil (swales).
Methodology
The soils were assessed by a combination of in situ visual examination and laboratory testing of representative samples. The topsoils in the locations chosen had not received any further organic ameliorants since planting. For the testing, three replicates of each topsoil type were submitted to the laboratory. The test properties included:
– pH (soil reaction)
– Electrical conductivity
– Organic matter
– Major plant nutrients (total nitrogen extractable phosphorus, potassium, and magnesium)
– Cation exchange capacity (a measure of overall nutrient retention capacity)
– Soil carbon (organic carbon, inorganic carbon, active carbon)
– Microbial activity (Solvita CO² burst).
Summary of key findings
Physical condition
In 2012, many of the topsoils were described as ‘slightly compacted to compacted’ with constituent materials, including woody fragments from the composts, easily identified. In 2023, the topsoils had homogenised, and depths were comparable to their previous measurements with no significant settlement.
The multipurpose topsoil, low fertility topsoil and moisture retentive topsoil all displayed moderate to well-defined ‘granular’ and ‘blocky’ structures. The structure of the moisture retentive topsoil was well developed within the upper 100mm or so, and less well defined below this level. The high permeability topsoil was reasonably loose within the upper 90mm and became noticeably compacted below this depth.
Earthworms were recorded within the majority of the trial holes within the multipurpose topsoil, low fertility topsoil and moisture retentive topsoil but rarely recorded with the high permeability topsoil. There were no signs of perched water or restricted aeration.
Chemical parameters
Each of the key chemical parameters were plotted graphically as ‘% change against 2010 values’. This was conducted to see any pattern of change for the chemical parameters against one another. The use of ‘% change’ as a measure enabled parameters with multiple units to be plotted together.
pH value (soil reaction)
The pH values for multipurpose topsoil, low fertility topsoil and moisture retentive topsoil have stayed broadly similar since installation. In contrast, the mean average pH value of the high permeability topsoil declined from pH 9.0 at installation to pH 7.8 now, seemingly caused mainly by leaching out of potassium ions by rainfall and, to a lesser degree, the acidification of the soil by rainfall.
The soils now all display well-balanced levels of organic matter and nutrients, with deficiencies only found in the High Permeability Topsoil.
Electrical conductivity
All soils showed reduced electrical conductivity values (a measure of soluble ions, including plant nutrients) from ‘moderate to high’ values in 2010, to ‘universally low and stable’ in 2023. This reduction is normal and accounts for the loss of excess ions that come with the compost element.
Organic matter and total nitrogen
When compared to their 2010 starting values, in 2023 most of the soils displayed increases to their organic matter content. Total nitrogen contents also increased.
Extractable potassium
The levels of extractable potassium for all topsoil types fell from their 2010 values and again from 2012 to 2023. In the main, however, the potassium values were considered acceptable, with the exception of the high permeability topsoil (average 78mg/l) which is lower than recommended.
Extractable phosphorus and extractable magnesium
Levels of extractable phosphorus and magnesium tended to fall slightly from their starting values and, with the exception of the magnesium content of the high permeability topsoil, would all be considered acceptable for landscape purposes. The magnesium content of the high permeability topsoil was a little low.
Cation exchange capacity –2023 values
Values for the multipurpose topsoil, low fertility topsoil and high permeability topsoil were all ‘low to moderately low’, with levels typical in sandy soils such as these. The ‘moderate’ value for moisture retentive topsoil results from its higher clay content.
Carbon testing
Samples were submitted for the following:
– Soil organic carbon – soil carbon content associated with organic based materials
– Soil inorganic carbon – carbon not relating to living matter
– Active carbon – a measure of the proportion of organic carbon accessible to soil microbes and a useful soil health indicator.
The total carbon stocks for these topsoils ranged from 76 to 116 tonnes per hectare, with good amounts of organic carbon (2.1% to 4.2%) and active carbon from 810 to 1325 mg/kg.
Microbial activity
Results for multipurpose topsoil, low fertility topsoil and moisture retentive topsoil were ‘moderate’ and would be considered acceptable for the time of year the samples were taken. The value for the high permeability topsoil was ‘low’, seemingly due to a combination of factors, including high compaction levels.
Conclusions
The findings have confirmed that the overall health and condition of most of these topsoils is extremely good. They are not only self-sustaining, but most have also improved over time and although they looked man-made when they were installed, they now look, smell, feel, and perform like their ‘natural’ topsoil cousins.
The soils now all display well-balanced levels of organic matter and nutrients, with deficiencies only found in the high permeability topsoil. They have developed in terms of physical condition (soil structure) and, after initial losses of organic matter and nutrients, the soils appear to have matured and stabilised and have since accumulated organic matter, total (organic) nitrogen, and soil organic carbon. All encouraging for carbon sequestration.
The structural condition of the soils has improved from ‘slightly compacted/compacted’ in 2012 to ‘no significant compaction’ in 2023 and they appear to be well drained and aerated. The exception to this is the high permeability topsoil, which was compacted below the upper 90mm. This soil was intended to function under high rates of foot traffic, which would cause this type of soil compaction. Nevertheless, it seems to be supporting the lawns to an acceptable standard with adequate drainage and aeration performances.
The Olympic Park’s soil strategy championed topsoil manufacture as the solution to fulfilling so many of the project’s key sustainability tenets. Ten years on, and with biodiversity net gain so vital to nature’s recovery, the findings of this study should install confidence that sustainable ‘designer’ soil materials (largely derived from recycled, renewable sources) are a viable, long-term solution for major urban regeneration projects in the UK.
Tim O’Hare is Principal Consultant at Tim O’Hare Associates LLP, responsible for the design and implementation of the Queen Elizabeth Olympic Park soil strategy.
