below ground | city sylviculture and the art and science of city soils

bel ow ground | cit y sy lv iculture and the art and science of cit y soils

J & L Gibbons | Tim O’Hare Associates

bel ow ground | cit y sy lv iculture and the art and science of cit y soils

Š J & L Gibbons LLP / Tim O’Hare Associates 2014. All rights reserved. The moral rights of the authors and photographer have been asserted.

forword

Johanna Gibbons and Tim O’Hare have worked together on numerous projects over the last 20 years, many with challenging sites and soils, and clients with high expectations.

Jo and Tim both recognize the challenges of the urban landscape and the importance of urban tree soils, particularly related to the establishment of trees for the long-term.

The essay touches on the interests of both disciplines; landscape architecture and soil science in a discussion on the art and the science of soil, reflected in the poetry of Steven J. Fowler, Poet in Residence at J & L Gibbons.

table of contents

p. 01

forword

p. 03

introduction

p. 04

fissured bark

p. 05

the great storm of 1987

p. 06

photographic evidence

p. 07

who we are and why we care

p. 08

19 swan yard

p. 09

what are soils?

p. 10

photographic evidence

p. 11

the rooting environment

C Horizon (regolith)

p. 13

mile end park green bridge, london

Take in hot pectin

p. 14

angel building, london

Allen Fisher

p. 15

chobham manor phase 1, london

p. 16

foresight

p. 17

works by steven j. fowler

p. 19

soil analysis

p. 24

glossary

p. 25

references

p. 26

biographies

p. 27

imprint

—

whatever buries man inevitably gets to know him? might say quietly see you on saturday whatever might ask of under earth public conversations, held loudly so all others can hear are the opposite of this earth’s depths which is private what inherited thoughtful tradition of burial is more necessary than the forgotten, or which, methods burning? melting :- ash spreads over the volume of boulangeries in paris built over catacombs & so cannot rise more than one storey —

Steven J. Fowler

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introduction

lichen encrusted bark

I pick the tree-lined streets in my cycle commutes across London. The air is cooler, the glare is less, and if there’s a breeze, the ear can tune into a gentle rustle of leaves over the humdrum of the city. I know certain groups of trees well - some now for over half a century - having lived in the borough all my life. They are majestic friends, heroic in their resilience to almost all weathers, with lofty canopies and textured or fissured bark encrusted with lichen.

These trees are London’s living heritage, some older than the buildings around them.

Their health, vigour and stature is directly related to the soils that sustain them. What graces the eye relies on what lies beneath our feet.

Planted by our forefathers and with the benefit of their foresight, London’s large trees give natural scale to the city. They are our urban forests, planting vast volumes of biomass in amongst the inertia of built form, rich in biodiversity and providing a free service by enhancing our quality of life. I cannot think of any man-made innovation that matches the performance of a large specie tree(1) in terms of cooling the city, carbon sequestration, oxygen giving, storm water interception, biodiversity or property uplift.

The existing resource of street trees is in a continuous state of either growth or decline. We need to perpetuate a better age balance of the overall large specie tree canopy to ensure that there is continuity into the future. The mass planting of the Victorian era means that many of the London Planes, for instance, are of a similar age.

Today’s public realm is complex, no more so than under the pavement. In this hidden and congested below ground world of the city, we wish to highlight the nature of city soil, a living substrate and a precious resource often ignored that should instead be the primary consideration in advocating sustainability of London’s urban forest.

To exist as a nation, to prosper as a state, and to live as a people, we must have trees.

— Theodore Roosevelt

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the great storm of 1987

I remember walking to the studio in Shoreditch in the aftermath of the hurricane in October 1987, when 15 million trees were blown over overnight in the Southern counties(2).

We couldn’t cycle – there were trees down everywhere across the roads. We lamented the loss of many wonderful specimens, leaving what seemed like huge raw bald patches in the urban fabric.

However in Kew Gardens, this sad aftermath led to some positive critical observations: many trees that had not fallen completely began to sprout vigorous new branches. It was as if they had been given a shot of oxygen. The more popular the tree, the more it had suffered from the trodden ground of millions of admiring visitors, and the on-going compaction had starved the roots and stunted the tree’s growth.

Another revelation was new evidence that about 90% of the root plate of most trees grew in the top 600mm of soil, as trees all over the country toppled by the wind now presented their root systems for all to see.

The more frequent occurrence not only of high wind events but also storm water surges, increasingly in summer when the soil is dry and hard-baked, has confirmed the need for critical drainage strategies; smart urban water management(3) that slows the flow from sky to sea despite the impermeability, not just of our streets, but also of London’s parks and green spaces.

The fashion of a traditional grit joint means the engineered profiles of our ‘hard’ landscape have the potential to perform some degree of porosity, accepting and holding water within the construction. Rather this than a sterile sealed surface that gives no opportunity for tree roots with little exposure to direct precipitation, to access moisture and maintain vigour.

photogr aphic ev idence

Visible root plate

London's urban forest

London’s soils need to be able to perform naturally as life giving super-sponges: — — — —

Absorbing Attenuating Supporting Sustaining Living heritage

Geostrophic wind scale, 16.10.87

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who we are and why we care

j & l gibbons 19 swan yard london n1 1sd

Soil is the foundation of all life, of all biodiversity. It is a dynamic living medium, complex physically, chemically and biologically. Soil is our unsung hero, so often unnoticed, unappreciated, neglected. Even the words ‘dirt’ and ‘soiled’ conjure up negative connotations.

In cities, soil needs particular protection and nurture, especially during the construction process. Thus over the course of a project, we take on stewardship of the soil. We’ll call on Tim - a sharp-eyed soil scientist - to join us on site with mini auger in hand. There is no denying what he draws out of the ground - a deep plug of soil that presents a narrative of care or disregard that would otherwise remain concealed beneath the surface.

We are Landscape Architects working on regeneration and heritage projects mostly in London. We have worked with Tim and his soil consultancy for many years. His scientific expertise reveals, analyses and assesses potential fragility, uniqueness and f loristic potential. We are equally passionate about the hidden asset that lies below ground. We are both enamoured with the design and science of growing mediums that can provide for city sylviculture while supporting the intensity of use and loadings to which London’s public realm is typically subjected everyday.

We’d like to see the symbiotic and dynamic relationship between trees and soils being more widely appreciated. As the Biopilic Cities(4) movement picks up pace, acknowledging that we are hardwired to nature (it’s just that we have forgotten), and as nearly 50% of the world’s population is urban, then the role of urban forestry, our street trees, parkland trees and groups of trees around where we live and work, becomes even more important. We have a duty to plan for and manage our city sylviculture imaginatively, ambitiously and generously, looking ahead way beyond political cycles, and instead in terms of hundreds of years - periods of time equivalent to the full life of a tree.

Below ground is our preoccupation. Steven Fowler, Poet in Residence at our studio, is exploring a mutual fascination. The poem sequence is about the specificity of the soil itself, and how its character shapes the city. It is about minutia and immensity describing the cultural mysteries and the hidden biology of a medium we generally take for granted, the health of which is directly related to our own.

We know more about the movement of celestial bodies than about the soil underfoot.

— Leonardo DaVinci

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what are soils

Soils are sensual. They have texture, scent and colour. They are the living skin of the planet. When Tim grabs a handful and lifts it to his nose with his eyes closed to take in the health or otherwise of a sample, he is reading a living dynamic, with more microorganisms in that one handful than people on earth(7). He observes how the material rolls, whether it binds into a cohesive joint or a crumbling one, his fingers registering any granular quality and moisture.

Samples provide evidence from site that are verified at the laboratory (8) to confirm what a good soil scientist already knows. The trial pit is preferably dug by hand, or if suitable mechanical plant is on site, Tim will take advantage of a deeper excavation, deep enough to expose a profile of humus, topsoil, subsoil(9) and possibly even the parent material. The characteristics of each illuminate the resource as an asset or a liability, a below ground narrative of physical and human geography. He interprets the science into horticultural potential. A recipe that reflects the specific needs of the plant species selected and prevailing environmental conditions, that measures and balances soil composition; assesses the texture of the soil which has a strong bearing on how the soil behaves; considers soil amelioration to enable the slow-release of nutrients and trace elements, beneficial soil microbes, water-retention, nutrient retention, structural development; analyses soil porosity and structural stability to ensure the potential for proper growth and function of root systems.

In London, there are few places that have never been developed. These areas, termed ‘brownfield,’ tell a story of occupation and land-use, a cultural history of a place, and the soils are very much anthropogenic in composition. Often the ecosystem is surprisingly diverse, as it has been left to its own devices, more than so-called ‘greenfield,’ where land management prescriptions typically exert a control over nature. These unprepossessing soils may actually provide a good basis for a tree rooting medium or urban tree soil, able to be engineered to give the correct balance of air, water, plant nutrients and microbes while providing stability to support overlying pavement constructions.

There is also a plethora of ducts, cables, mains, pipes and tunnels. There is no reason that London’s street trees can’t continue to coexist within this context, but there is a pressing need for a more creative and equitable cross disciplinary collaborations as tree roots compete for space with the services that make the city tick.

photogr aphic ev idence

Brownfield ruderal vegetation

Crushed brick and concrete substrate

Compacted site soils

Soils are developed; they are not merely an accumulation of debris resulting from decay of rock and organic materials ... In other words, a soil is an entity - an object in nature which has characteristics that distinguish it from all other objects in nature.

Anthropogenic soils

— C.E.Millar & L.M.Turk, 1943

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the rooting env ironment

Roots require the right below ground environment to thrive, and every situation is different – different geology, site history, microclimate, and hydrology and design intent.

Pits that provide for a more equitable condition for the coexistence of roots and the often historic tangle of services, each with their own demands. Tree pits should be big, wide rather than deep, combined into a whole rooting zone rather than an isolated pit, giving at least 25m3 of soil or substrate allowing lateral growth. Pits need to allow air and water to the root zone, and must drain freely, but not too quickly, so that trees have the opportunity to take up moisture and nutrients before outflow to the sewer.

The natural environment for trees is woodland, so a city’s microclimate and pavements present relatively hostile conditions for growth and pedogenisis. This is the challenge facing the makers of city soils. For trees, the important components of a rooting medium are a good subsoil or substrate with the right structural and drainage characteristics allowing adequate gaseous exchange.

There is much innovation in the field notably in Netherlands(11), Sweden(12), the USA(13) and the UK developing load-bearing substrates that provide air and water for root growth while supporting pavements above. These ‘engineered’ soil profiles, or urban tree soils can be a recipe of mineral and organic components, including recycled composts, sands, crushed brick and concrete. A reconfigured and reconstructed ‘soil’ or rooting medium, the subject of long terms trials; or they can be an open cellular system that take the full weight of highway loadings rather than the soil they are filled with, to recreate a kind of below ground forest floor scenario. Trials now over many decades show the benefit of these systems in terms of enhanced health, stature, performance value and longevity of street trees, providing mounting evidence through the investment in long-term monitoring.

We maintain an on-going interest in projects we’ve delivered to ensure that we continue to keep abreast of research and that we contribute in a modest way to innovative practice.

Three such projects illustrate our focus on city soils and our preoccupation with the advocacy of city sylviculture in London.

Dirt is designed. It is uniquely composed, site specific, and innately intelligent.

— Dirt, viaBooks, a PennDesign Publication

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mile end park green bridge, london

angel building, london

This fascinating retrofit was originally a millennium project to maintain continuity of the parkland over the A11 using a green bridge. Ten years later, nothing much had survived, apart from a thin layer of compacted soil and one or two surviving pines. Replanting was part of the High Street 2012 fringe project in anticipation of the London Olympics.

We viewed the project afresh as a technical challenge with TOHA and together evolved an approach to retrofit that referenced an upland landscape in terms of tree specification, procurement and soil profiling, mimicking a pioneer forests that exist happily in many cases on thin soils over bedrock. Instead of replanting the failed semi-mature trees, we procured a froth of densely-planted, young, resilient pioneer bare-root birch and alder seedlings in a mass of broom. The idea was that seedlings adapt quickly and require little nurture in doing so.

In tandem, Tim specified decompaction and amelioration with sharp sand to inject air into the soils and improve drainage characteristics. Specific mechanical plant was required to lightly mix the soils in an operation that took required diligent site supervision. Just two years later, the micro upland forest, without irrigation or staking, has exploded with growth.

The project was managed and designed as series of precise interventions. New semimature plane tree plantings were grafted into an existing tree resource, and legacy of a 1980's scheme, both landscape and architecture, were retrofitted to retain the essential structure of the existing while injecting new life.

The real challenge related to the soils, using a strategy evolved with TOHA for a temporary construction road through existing mature trees to enable the £70m project to be built from the front, while protecting soils within root zones. We took a proactive approach to satisfy the planning authority on this, setting up a regular monitoring procedure that took readings of the soils around existing trees to detect any soil compaction, register moisture content, and provide automatic irrigation to reduce stress. This was combined with visual inspections by an arboriculturalist on a monthly basis through 18 months of construction and periodic below ground inspections by Tim armed with his mini hand auger. The methodology we proposed was instrumental in securing planning approval from Islington Borough Council, and subsequently cited as best practice by CIRIA(15) in relation to BS 5937, Trees and Construction.

The result of this care is a thriving group of trees, a surprisingly verdant urban landscape, and a building almost entirely let prior to completion with a population of 3000 all with natural ventilation that allows the tree-top leafiness of the limes and planes to permeate the office space, now home to the Design Council and Cancer Research amongst others.

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chobham manor phase 1, london

A new British Standard was published in February 2014, developed by a team of experts in the nursery trade and construction industries, concerned with the nurturing of trees from nursery through procurement to establishment as a continual process. This is the context for the Chobham Manor project – the first development platform of the London Legacy Development Corporation – to be instigated. We took the opportunity early in the design process to forecast the value of the proposed tree resource in terms of energy saving, improved air quality, increased property value and carbon sequestration. The purpose of the valuation exercise being to underpin cost planning, help secure the right budgets, and ensure that the development vision of tree lined streets was actually achievable.

We persuaded Taylor Wimpey to commission the use of i-tree, a sophisticated software package to predict (for the first time we believe in the UK) the value of the proposed tree planting design, in pure monetary terms.

The evaluation is made based on a number of factors including tree species and the soil volume provided, key to longevity. It is a persuasive tool in the promotion of the benefits of best practice tree pit design and investment to those in the development team who are better convinced by cost and revenue balance sheets.

foresight

It is time that the hidden investment in city soils was evaluated as a matter of course, and those who have the foresight be applauded, to encourage higher standards and to secure into the future a larger and more specie diverse urban forest for London.

Trees and Design Action Group have recently published comprehensive guidance on urban trees co-authored with the Institute of Civil Engineers and the Chartered Institute of Highways & Transportation amongst others. An enlightened Environment Team at the Greater London Authority is currently undertaking the whole-sale valuation, through the i-Tree methodology, of London’s entire tree resource. Through the RE: LEAF partnership and mobilisation of an army of the capital’s greenest volunteers an unprecedented mission to measure, schedule and value the city’s sylvicultural asset will be undertaken this year.

London has a wonderful climate for growing, and also contains foremost institutions of research, science and design. As professionals in the field we have a duty to protect, retain and enhance city soils, soils that are being rapidly and extensively compacted, eroded, and excavated under intensifying use for increasingly deeper basements and more massive tunnelled infrastructure. We need to exert influence, and support a stronger mandate for, and greater priority to, large specie trees and our urban forest so fundamental to London’s identity.

It’s been 350 years since the publication of John Evelyn’s book titled Sylva in 1664 by the Royal Society in London. The hefty tome inspired the notion of sustainable resource management, and provided a seminal reference on the history, science and art of forestry practice and our relationship with trees and forests. Furthermore the anniversary is celebrated with a new publication titled The New Sylva, A discourse of Forest and Orchard Trees for the Twenty-First Century .

What better time to emphatically express the economic and ephemeral value of the majestic street tree, the urban forest, and the soils on which it all depends. The street is, after all, the social conduit for city life. This matter concerns, therefore, the sustainability of communities, the economic success of the metropolis, and the fundamental health of Londoners.

Since there is nothing which seems more fatally to threaten a weakening, if not a dissolution, of the strength of this famous and flourishing nation, than the sensible and notorious decay of her wooden walls…

— John Evelyn

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works by stev en j. fowler

works by stev en j. fowler

A Horizon (topsoil) Myst the gyst in toil and mystery thought it was simply a riddle —

O Horizon (humus) And said, ‘Student, are you asleep? Do you see these people Business brings them amazement?

—

Allen Fisher

the collection of bodies on the earth’s surface in places modified or even made by man when asphyxiation was further down the fearful containing living matter, its upper limit air or shallow water where dissolution of the shelf searches for a way of being shared with others because of that kind of margin grades to deep water or to barren areas, included the horizons at where it puts her hands to her face to sedate from fingers, from forehead beneath them from her enclosed mouth finally the cash-machine poem of the underearth nearer the surface, where living organisms parent materials, and relief is in the few places where it contains thin horizons that are impermeable to roots at where it puts her hands to her face to embody failing earth beneath them.

In some cases these sediments were reworked by wind like f lats going up soil being as cheap to ruin as the horizon.

—

Allen Fisher

the old institution of sediment constitution is no longer viable < magnetic fields miniature blackholes, more necessary sewage drilling the Whole Earth Catalogue apologises & like the First Primary Rus Chronicle bound in skin as a necrocomicon, turns black still surface enough to discern pity to form low earth speculative astrophysics in clods the worm fec bubble ghost puck Reich’s “towards the earth” machine breaks the Pesci hypothesis man < baseballbat all might is ought without ground to think on springbok of earth - hiding death no less all magic body juices & profound turning of monkey from solid to liquid, I await your harvest —

Steven J. Fowler

Steven J. Fowler

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soil analysis

soil analysis

Imported Topsoil

Imported Subsoil

Quantity: Provide as necessary to complete the work. Make due allowance for settlement after laying. The topsoil shall comply with the following lower and upper limits:

Quantity: Provide as necessary to complete the work. Make due allowance for settlement after laying. The subsoil shall comply with the following lower and upper limits:

parameter clay (<0.002mm) silt (0.002-0.05mm) sand (0.05-2.0mm) stones (2-20mm) stones (20-50mm) stones (>50mm) ph value electrical conductivity (1:2.5 water extract) electrical conductivity (caso4 extract) exchangeable sodium % calcium carbonate organic matter total nitrogen carbon: nitrogen ratio extractable phosphorus extractable potassium extractable magnesium

unit

lower limit

upper limit

%

5

18

%

5

35

%

55

85

% dw

--

15

% dw

--

20

% dw

--

0

unit µs/cm

5.5

8.5

--

1500

µs/cm

--

2800

%

--

15

%

--

2

%

4.0

8.0

%

0.20

--

-mg/l mg/l mg/l

--

20:1

26

100

120

1200

50

600

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parameter clay (<0.002mm) silt (0.002-0.05mm) sand (0.05-2.0mm) stones (2-20mm) stones (20-50mm) stones (>50mm) saturated hydraulic conductivity ph value electrical conductivity (1:2.5 water extract) electrical conductivity (caso4 extract) exchangeable sodium % calcium carbonate organic matter

unit

lower limit

upper limit

%

0

18

%

0

35

%

60

90

% dw

--

35

% dw

--

15

% dw

--

0

mm/hr

10

--

unit

5.0

µs/cm

--

µs/cm

--

%

--

15

%

--

2

%

--

1.5

8.5 1500 2800

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soil analysis

soil analysis

pH Value

Nutrients

The pH value of a soil indicates the degree to which it is either acid or alkaline. pH is measured on a scale from 0-14 where pH 7.0 is neutral. Where the soil is acid, values are less than 7.0 whilst if the pH is above 7.0 it is alkaline.

Where soils are strongly acid, aluminium and other potentially toxic elements are more soluble and therefore become more available to the plant. This may affect the plant by reducing its growth. An indication of strong soil acidity is often seen by stunted root growth.

Where soils are strongly alkaline, nutrients (eg. phosphorus, copper, zinc, boron, manganese and iron) tend to become increasingly less available to the plant. Soils containing concrete fragments or building waste contain high levels of calcium ions, which lock up plant nutrients into insoluble forms eg. calcium phosphate.

Where soils become very acid or alkaline, microbial activity is retarded. This reduces the synthesis and release of nutrients from organic sources eg. organic nitrogen.

The three major plant nutrients supplied by the soil are nitrogen, phosphorus and potassium.

Magnesium, calcium and sulphur are also regarded as important secondary nutrients, whilst the micro-nutrients chloride, iron, manganese, copper, zinc, boron and molybdenum are required by the plant in smaller amounts.

Electrical conductivity (salinity) value The electrical conductivity value is a general measure of the soluble salt content or salinity of a soil. Units of electrical conductivity are measured in Siemens (S) or micro Siemens (ÂľS) per centimetre. There is some confusion that the word â&#x20AC;&#x2DC;saltâ&#x20AC;&#x2122; refers only to sodium chloride; however in the context of our analysis and recommendations, salts may include any soluble component of the soil including sulphates, nitrates, phosphates etc. Excess of soluble salts can adversely affect plant life by causing premature wilting, scorching and by affecting the ability of a plant to take up nutrients and water.

Organic matter Soil organic matter consists of a wide range of organic substances that are generally beneficial to the soil. The positive effects of soil organic matter can largely be seen by its retention and gradual supply of nutrients to plants. By breaking down organic matter in the soil, micro-organisms make nutrients (especially nitrogen) available to the plant whilst also improving by-products which improve the soils structural stability.

Organic matter is also effective at retaining moisture. This ability to retain moisture may be either direct or indirect (by maintaining structural stability, the soils water retention is improved). Soils that are well supplied in organic matter are less susceptible to drought during the summer months. Other positive effects or organic matter may be shown by its ability to bond with phytotoxic metals and pesticides thereby reducing their availability.

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Nitrogen Nitrogen is the main growth nutrient, and is required for the growth of leaves and stems.

Deficiencies manifest themselves through weak, stunted growth, and yellowing of the older leaves. Excess nitrogen may however be detrimental to the plant. Effects of excess nitrogen include disproportionate leaf growth at the expense of other parts of the plant e.g. flowers.

Phosphorus Phosphorus is the principal nutrient concerned with plant growth and development.

Deficiencies may be seen through stunted root systems whilst the leaves of some plants may also exhibit a dull green or purple colouration.

Potassium Magnesium performs an important photosynthetic function within the plants, whilst also promoting flower and fruit development. Deficiencies may be seen through the yellowing and death of marginal (and older) leaf tissue, whilst low levels also make the plant more susceptible to diseases.

Magnesium Magnesium is a constituent of chlorophyll, the green pigment which enables plants to photosynthesise magnesium deficiencies may be noted by the interveinal yellowing of the plants older leaves.

Potentially toxic elements These include phytotoxic and zootoxic heavy metals, Cyanide, Phenol, Polyaromatic Hydrocarbons (PAHs), Total Petroleum Hydrocarbons (TPHs) and BTEX compounds.

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soil analysis

glossary

Texture

Urban forest

Soil texture is a term used to describe the relative proportions of sand, silt and clay sized particles that make up the soil. Sand (0.063-2mm), silt (0.002-0.063mm) and clay (<0.002mm) are differentiated from each other solely by their size. Particles larger than 2mm are not determined in the soil texture as they are classed as ‘stones’.

The texture of the soil has a strong bearing on how the soil behaves. Heavier soils (i.e. those soils with a higher proportion of clay and/or silt sized particles) tend to have high moisture and nutrient retention capabilities. Once moist these soils become structurally weaker, as frictional and cohesive forces between soil particle is reduced. Plastic soils are weaker the drier friable soils, making them more prone to structural damage if handled or compacted by heavy machinery. Lighter soils (i.e. those soils with a higher proportion of sand sized particles) tend to exhibit good drainage and aeration, although they are more prone to drought and nutrient deficiencies.

A collections of trees within a town or city, as a critical part of the urban infrastructure, that have an important role in the urban ecology in terms of filtering air, water, sunlight, biodiversity, enhanced wellbeing and spatial quality.

Stones Stones are those soil materials which are larger than 2mm in diameter. Stones or gravels often reflect the solid or drift geology on which the soils are found. Although stones may not be directly detrimental to plant growth, stony soils are more susceptible to water and nutrient deficiencies. Bands or horizons of stones may also form barriers to root growth.

Soil biodiversity The variety of life below ground. European Commission Joint Research Centre: The Convention on Biological Diversity (CBD) defined the soil biodiversity as "the variation in soil life, from genes to communities, and the ecological complexes of which they are part, that is from soil micro-habitats to landscapes."

Topsoil The upper layer of a soil profile. It is usually darker in colour and more organic and fertile than the subsoil layer below.

Subsoil The soil layer between the topsoil above and the little weathered parent material below.

Subsoil usually has lower concentrations of organic matter and available plant nutrients.

Structure

Engineered Topsoil

Structure refers to the aggregation of soil particles (sand, silt, clay and organic matter) into compound units or peds. These peds are characterised and classified on the basis of their size, shape and degree of development.

Between (and within) these structural units exist pores. The occurrence of these pores not only supplies pathways for root growth but results in a more permeable and more oxygenated soil. Soil texture has a large inf luence on soil structure and soil porosity and is one of the major determinants on whether the pores in the soils are large (tend to contain air) or small (tend to contain water). Clay soils usually have well developed strong structural units with a large number of small pores, whilst sandy soils tend to have weaker, looser structures with larger pores. Structural stability is maintained by the byproducts of microbial activity, and the bonding ability of inorganic soil components.

A material produced by combining mineral matter and organic matter in the correct proportions, in order to function as a growing medium. It therefore provides the same function as a natural topsoil in a landscape environment.

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Urban Tree Soil A material that is used for tree planting in hard landscape environments. It is required to serve two functions; 1) to act as a growing medium for tree roots to extend into, with the correct balance of air, water, plant nutrients and soil microbes, and 2) when consolidated, it will also provide the necessary stability to support the overlying hard standing.

Anthropogenic soil: Originates as a result of human activity. It may be created deliberately, as in the case of ‘engineered topsoil’ and ‘sports pitch rootzones’, or inadvertently, by other human activities, such as urban development, demolition, mining, waste disposal.

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references

biogr aphies

Large specie trees: Tree species that mature into large scaled trees www.forestry.gov.uk/thegreatstorm (3) Sustainable Urban Drainage Systems (SUDS): Water Senstivie Urban Design (WSUD) in the UK-Ideas for Build

johanna gibbons | Johanna Gibbons is a Landscape Architect and founding partner J & L Gibbons established in 1986 in London. She was trained at Edinburgh College of Art and is a Fellow of the Landscape Institute a member of several advisory panels for English Heritage, the Forestry Commission, and the Mayor of London. Her practice concerns heritage, green infrastructure and urban regeneration, developing ways to deliver meaningful landscapes through a deliberative planning process of continual feedback, research and engagement. Her particular passion is urban forestry and soil biodiversity.

(1)

(2)

Environment Practitioners, published by CIRIA London 2013 Biophilic Cities: Biophilia is a term popularised by Harvard University myrmecologist and conservationist E.O. Wilson to describe the extent to which humans are hard-wired to need connection with nature and other forms of life.

(6) Anthropogenic soil: soil that originates as a result of human activity (7) European Commission Joint Research Centre : Institute for Environment and Sustainability: Soil biodiversity (8) Lab report unpacked: p19-23 (9) Soil definitions see Glossary (10) Substrate: Various forms of (often) recycled material, typically crushed brick and concrete (11) Amsterdam Tree Sand: a mix of washed semi- rounded particle silica sand and organics blended to a formula developed following research by Dutch Universities in the 1970's (12) Stockholm Structural Soil is rock based tree planting medium developed in the 1990’s; a mix of angular recycled substrates with a specified soil mix and water (13) Cornell University’s Urban Horticulture Institute’s CU-Structural Soil developed in the 1990’s: a mix of angular rock, clayey loam, peat, sulphur, hydrogel, fertilizer, water (14) Bartlett Tree Research was published in Arboriculture & Urban Forestry, July 2006, by E.Thomas Smiley, Lisa Calfee, Bruce R. Fraidrich, Emma J. Smiley Titled ‘Comparison of Structural and Non-compacted Soils for Trees surrounded by Pavement’ (15) CIRIA: The benefits of large species trees in urban landscapes: a costing, design and management guide (C712): Armour, T, Job, M, Canavan, R, March 2012: Large species trees convey the greatest financial, social and environmental benefits, and make a fundamental contribution to the well-being of almost 80 per cent of the UK population. The guide aims to highlight the vital importance of large species trees, including the retention and enhancement of existing trees and carrying out of new plantings in streets, squares and parks for new and existing developments.

(16) BS 8545: 2014 Trees: From Nursery to Independence in the Landscape: Recommendations: The aim is to establish a guide to best practice which recognises the transplanting of young trees from the nursery into the landscape as a continuous process rather than a series of unrelated operations, as represented by current British Standards and other guidelines/ recommendations (17) Treeconomics: A social enterprise and UK market leader in measuring and valuing the benefits derived from our Urban Forests (18) i-Tree: a state-of-the-art, peer-reviewed software suite from the USDA Forest Service released in 2006 that provides urban forestry analysis and benefits assessment tools. The i-Tree Tools help communities of all sizes to strengthen their urban forest management and advocacy efforts by quantifying the structure of community trees and the environmental services that trees provide. i-Tree users can link urban forest management activities with environmental quality and community livability: www.treeconomics.co.uk (19) Eco services: the important benefits for human beings that arise from healthily functioning ecosystems, notably production of oxygen, soil genesis, and water detoxification (Collins English Dictionary) (20) The Trees and Design Action Group (TDAG) is an open collaborative facilitating cross-sector and cross-disciplinary dialogue and projects promoting the role of the urban forest throughout the United Kingdom. The group shares the collective vision that the location of trees, and all the benefits they bring, can be secured for future generations by influencing the planning, design, construction and management of our urban infrastructure and spaces. Established in 2007 as a not-for-profit and apolitical collaborative forum, TDAG incorporated as a charitable trust in 2013. Its membership, online publications and information are free. This approach enables TDAG to assimilate ideas and knowledge independently of organisational hierarchy, profit or commercial interests.

(21) Trees in Hard Landscape A Guide for Delivery, TDAG 2014 (22) Valuing London’s Urban Forest: GLA / www.london.gov.uk/priorities/environment/greening-london/re-leaf/whywe-want-more-trees/valuing-londons-urban-forest (23) The New Sylva, A discourse of Forest and Orchard Trees for the Twenty-First Century, Gabriel Hemery & Sarah Simblet, Bloomsbury Publishing 2014

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tim o’hare | Tim O'Hare has been a professional Soil Scientist for over 20 years, working predominantly in the commercial landscape, sports amenity and regeneration sectors. His particular areas of expertise include topsoil assessment and manufacture, soils for urban tree planting, and brownfield site restoration. Tim is a member of the British Standards Institute working group for Topsoil and Subsoil and he was the project Soil Scientist for the London Olympic Park and Athletes’ Village. He has spent many years investigating tree-planting failures and from this insight has developed practical ideas for successfully establishing trees of all sizes in urban environments. christina m. morra | Christina Morra is an American writer, researcher and campaign coordinator with a degree in Environmental Policy and Municipal Management. She has a strong communications background and thrives in fast-paced and varied environments. She has worked in the field of sustainable construction and with policy makers, elected officials as Director of the Rhode Island branch of Fair Vote campaigning for electoral reform. Christina has been involved in many issue-based campaigns related to environmental and social challenges. steven j. fowler | Steven Fowler is a British poet working in the avant garde, or vanguard tradition. He has published six collections of poetry and been commissioned by the Tate, the London Sinfonietta, Liverpool Biennial, Composite Feedback in Manchester and Reel Arts amongst others. Beginning in 2014, he will be poet in residence at J & L Gibbons. Soil, a poem sequence, mirrors the ongoing discussion with Jo and Neil. It is about the underneath, the stepped upon of the city, the invisible ground, drawing inspiration from the remarkable oeuvre of British poet Allen Fisher. eve izaak | Eve is a Graphic Designer and Illustrator based in London. She studied Visual Communication at University of The Arts London, and now works as a freelancer and in-house designer at Rapha Racing ltd. She takes paticular interest in publiction design, print and cycling.

Clients include: Google, The RSC, Howies, Huck Magazine and Hackney Council. scrub hewitt | Scrub Hewitt is the fourth generation Managing Director of Generation Press who are one of only a few producing their print to the rigorous requirements and standards of the Eco-Management and Audit Scheme (EMAS). As such they are one of the leading environmental printers in Europe, based on the edge of the South Downs in the middle of a working farm.

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imprint

Publishing | J & L Gibbons Authors | Johanna Gibbons | Tim O'Hare | Steven J. Fowler Photography | Eve Izaak | Johanna Gibbons | Sarah Blee Editor | Christina M. Morra Graphic Design | Eve Izaak Printing | Generation Press

Acknowledgements: Particular thanks for enthusiastic support and assistance in the production of this publication are due to Christina Morra for editing with great energy, Scrub Hewitt for his expertise in printing specifications and process as a natural extension of the topic, Eve Izaak for her design eye in the presentation of the material, Steven Fowler, our Poet in Residence, for his lucid and probing interpretations of our ongoing discussions. We would also like to acknowledge design collaborators muf architecture / art on Miles End Bridge and Chobham Manor project work featured, and clients Derwent (Angel) and Taylor Wimpey (Chobham Manor). j & l gibbons 27

j & l gibbons

j & l gibbons

Soil: From Middle English soile, soyle, sule (“ground, earth”), partly from Anglo-Norman soyl (“bottom, ground, pave-ment”), from Latin solium (“seat, threshold, place”), mistaken for Latin solum (“ground, foundation, earth, sole of the foot”); and partly from Old English sol (“mud, mire, wet sand”), from Proto-Germanic *sulą (“mud, spot”), from Pro-to-Indo-European *sūl- (“thick liquid”). Cognate with Middle Low German söle (“dirt, mud”), Middle Dutch sol (“dirt, filth”), Middle High German sol, söl (“dirt, mud, mire”) Danish søle (“mud, muck”). See also sole, soal.