Sourcing Good
Topsoil
Topsoil is the ‘foundation’ of any soft Important landscape scheme. It is responsible for the establishment and sustained growth of trees, shrubs, grasses, etc. that have been carefully selected, approved and planted. If you get the wrong topsoil, the plants can quickly show their disapproval and the consequences for all concerned can be considerable. Each landscape scheme is unique, so the topsoil for each scheme must be selected with thought and care and not by automatically using the site’s topsoil or the first source of imported topsoil offered by the contractor. question that should be answered by the Soil Resource Survey are: Site Soils Before even considering importing topsoil, any on‐site topsoil needs to be properly assessed. This is achieved as part of a Soil Resource Survey, which evaluates the quality, quantity and suitability of a site’s soil resources (topsoil and subsoil) for the intended landscape scheme. This is undertaken by a Soil Scientist, and involves a review of the intended landscape scheme, a site investigation, and laboratory analysis of soil samples. The findings are presented in an interpretive report, with recommendations for the recovery and re‐
use of the site soils. 1 Tim O’Hare Associates
¾ What are the needs of the planting Above:
scheme? Topsoil for
¾ Does the site drain – i.e. what are the use in
drainage properties of the site subsoil? landscaping
¾ Will the site provide sufficient topsoil for the landscape scheme? ¾ Is there space to store the topsoil? ¾ Will the topsoil be stripped and stored in winter or summer? ¾ Will the topsoil be respread in winter or summer? ¾ Is the topsoil suitable for re‐use? Not all natural topsoils are suitable for landscape construction, especially when their re‐use involves their temporary stripping and storage. Natural topsoil is created by the interaction of a site’s surface geology and the prevailing environment (climate, topography, vegetation, human activity). Given the UK’s varied geology, climate, vegetation and topography, the resultant natural topsoil is very changeable. There is no such thing as an ‘average topsoil’ or ‘typical topsoil’. Topsoil can range from an acid, infertile sand, to a weakly structured silt, to a strongly alkaline, gravely clay. As the soil properties alter, so does the potential to re‐use the topsoil for landscaping purposes. Whilst certain topsoils are suitable for many landscape applications, those with more extreme characteristics are not. For example, silty clay topsoils are excellent for arable crop production, but they make very poor landscaping soils due to their poor handling ability. This is a key factor that is often overlooked by both topsoil buyers and even topsoil suppliers. If the site has re‐usable topsoil, this precious natural resource should be protected and handled with care so that it can be re‐used effectively for the new landscape scheme. Any surplus topsoil can be re‐sold to other suitable landscape projects. If the site topsoil is unsuitable for re‐use, or there is a shortage of topsoil, imported topsoil will need to be sourced. This is usually achieved by including a Topsoil Specification within the Landscape Architect’s Soft Landscape Specification tender document. Topsoil Specifications The quality of imported topsoil used on a scheme very often comes down to a project’s Topsoil Specification. It sets out what should and should not be in the topsoil. The topsoil specification therefore needs to include the technical elements that will ensure an effective growing medium is provided. 2 Tim O’Hare Associates
The most functional specifications are those that list which properties the topsoil should possess prior to planting, turfing or seeding. These normally include: ¾
Visual examination – soil structure consistency, foreign matter, etc. ¾
Particle size analysis (texture) and stone content ¾
pH and salinity values ¾
Major plant nutrients ¾
Organic matter content ¾
Potential contaminants (e.g. heavy metals, hydrocarbons, cyanide, phenols) The values set against each property will vary depending on the required performance of the topsoil. There is a tendency to use the same specification for every landscape project regardless of climate, plant species, programme or construction constraints. Usually some Once you have issued a specification, stick to it and do not be persuaded to change it on price alone. Above:
Wildflowers
growing on
low-nutrient
topsoil
modification to a ‘template specification’ is required for each project and often it is appropriate to have more than one specification if a variety of different ‘landscape habitats’ are to be established e.g. tree pits and wildflower grassland. Natural Topsoil or Topsoil Substitute In today’s landscape industry, there is a choice of Natural Topsoil (soil derived from the upper layer of Greenfield sites) and Manufactured Topsoil (a topsoil substitute made from blending mineral and organic components). There is an on‐going debate about the pros and cons of topsoil substitutes, and whether they are better or worse than natural topsoils. There is no definitive answer to this, as both ‘natural’ and ‘manufactured’ topsoils vary in composition, depending on the site’s geology, etc. (natural topsoil) or the components used to make a topsoil substitute. However, it has to be said that there are a lot of topsoil substitutes in the market place that are excellent landscaping topsoils, which are preferable to a lot of natural topsoils. The revised BS3882 reflects this as it puts Manufactured Topsoil on a par with Natural Topsoil. Topsoil Substitutes In many instances, topsoil substitutes (manufactured topsoils) provide as good as, if not better, alternative to natural topsoil as long as the correct components have been selected and blended in the correct manner. The attributes of topsoil substitutes include a consistent composition, control over the mix, a greater availability, and an absence of weeds and potential contaminants. However, the exact opposite can be the case where inappropriate soils or composts are used, or the blending process does not have the necessary control measures to ensure a consistent mix. For example, ‘Skip waste soil’, which is derived from Brownfield site clearance operations, is not usually suitable as a component for manufactured topsoil. This material is usually extremely alkaline, saline, and deficient in organic matter and plant nutrients. It can often have elevated levels of zootoxic and phytotoxic contaminants and contain ‘sharps’ e.g. glass and metal. Below: Topsoil
manufacture
Natural Topsoil At first glance, it may appear as if nothing can beat the real thing. However, we have already discussed the variable composition of natural topsoils, and why some of these are unsuitable for landscaping. Another reason why natural topsoils are not as preferable as the manufactured alternative is inconsistent availability. Natural topsoil is derived as a by‐product of ‘Greenfield’ development. It is not quarried like sands and gravels. When Greenfield sites are developed for housing, business parks, highways, etc, the topsoil is stripped off before construction commences, and any surplus topsoil is taken off site for re‐use on other landscape projects. As a consequence, the availability of natural topsoil can be inconsistent, as it generally only becomes available in any area at the time of a topsoil strip. At the end of the day, the most important aspect is that the topsoil selected meets the functions of a ‘growing medium’ and the specific requirements of the proposed landscape scheme. 3 Tim O’Hare Associates
Potential Contamination Questions to ask Topsoil Suppliers Topsoil with excessive concentrations of Potentially Toxic Elements (P.T.E.s), can be hazardous to human health and the environment. It is therefore essential to ensure that the soil is safe, particularly if it is to be used for sensitive end uses, such as domestic gardens, public open spaces or allotments. Your topsoil supplier should be in a position to satisfy you that their topsoil is fit for purpose. Choosing a Topsoil Supplier Apart from ensuring that an appropriate Topsoil Specification is used to source topsoil, it is also sensible to choose a good topsoil supplier who can supply what you need, when you need it and in the format you need it in. In order to do this there are several ‘questions’ to be asked. The golden rules when speaking to suppliers of topsoil are: ¾
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Where does your topsoil come
from?
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Natural or Manufactured?
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Agricultural land? Forestry?
Playing fields?
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If manufactured, what
components are used to make it,
are they consistent and are they
sustainable?
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Is it screened demolition waste
or skip waste?
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Has it come from a sustainable
source?
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Will all of my topsoil be coming
from the same source?
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Is the topsoil protected from the
weather?
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Is your topsoil free of Japanese
Knotweed?
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If yes, can I have this in writing?
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Can I come and view the
topsoil?
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How is the topsoil supplied?
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Is the topsoil available in bags
as well as loose?
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Are rigid tippers and articulated
tippers available?
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How far will you go to deliver the
topsoil?
To remember that NOT everything described as topsoil will actually function as a topsoil. Samples can be misleading. Ask to visit the suppliers site to view and sample the topsoil yourself if unsure. Any laboratory analysis certificates supplied should be checked for validity. A Certificate that is 12 months old is highly unlikely to represent the topsoil currently on offer. Likewise, a Certificate that appears to have been faxed 20 times didn’t probably originate from that supplier. If unsure, always ask to see the top copy. If offered soils recycled from demolition or skip waste, be cautious. Such soils can be contaminated with toxic elements, glass and stones, etc. and they usually have low fertility levels. If Natural topsoil is offered, check that there is sufficient volume for your contract and that it is all from one site, so that it is consistent. 4 Tim O’Hare Associates
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Has the topsoil been
independently tested?
Is a sampling protocol used to
ensure a representative sample
is analysed?
Does the testing include
analysis of P.T.E.s (Potentially
Toxic Elements)?
Tim O'Hare Associates LLP
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How often do you analyse?
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Who conducts your analysis?
Howbery Park
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Does the laboratory conducting
the analysis have UKAS and
MCERTS accreditation?
Wallingford
Can I have a copy of the whole
analysis report and does it
include an interpretive section?
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Can I have a copy of the
certificate?
Oxfordshire OX10 8BA UK
T: +44 (0)1491 822653
F: +44 (0)1491 822644
E: [email protected]
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Is the topsoil available all year
and will it be workable when
delivered?
Tim O'Hare Associates LLP is an
independent soil and landscape
science consultancy that supports the
commercial landscape, sports amenity
and construction industries.
Specialising in:
Through asking the topsoil suppliers the above questions you will establish who is a quality supplier and help ensure that the topsoil supplied meets the needs of your scheme. The copyright of this article is vested in the author and no part shall be reproduced in part or in full without the author’s written consent. 5 Tim O’Hare Associates
Topsoil Quality Assessments
Soil Surveys and Landscape Evaluation
Sports Pitch Construction, Drainage and
Agronomy
Topsoil Manufacture Feasibility and
Implementation
Agricultural Land Classification (ALC)
Tree Pit Design and Construction
Plant Failure and Turf Disease Diagnosis
CPD Seminars and Training Courses
Project Management, Specification and
Coordination
Technical Research and Product
Development