Final Report
De-mystifying the use of PAS100
compost in horticultural growing
media
Project code: OMK005-002
Research date: December ‘12 – March ‘13
Date: March 2014
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(2013) De-mystifying the use of PAS100 compost in horticultural growing media. (WRAP)
Project OMK005-002) Litterick, AM.; Holmes, S.; Daly, M.; Becvar, A. and Wood, M.,
Earthcare Technical Ltd.
Written by: Audrey Litterick, Susie Holmes, Mike Daly, Anna Becvar and Martin Wood
Earthcare Technical Ltd.
Front cover photography: Healthy plants grown in compost based growing media at Golden Acre Nursery in Dorset
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Executive summary
The Sustainable Growing Media Task Force (SGMTF) was established in 2011 and initially
aimed to explore how to overcome barriers to further reducing peat use in horticulture. It
has since adjusted its remit to that of putting the horticultural sector on a long-term
sustainable footing by ensuring that all choices of growing media (or substrate) used for
amateur gardening and professional horticulture are sustainable. It concluded that green
compost will play an important role in sustainable growing media manufacture but that
further work is required in order to improve confidence in using compost for this purpose.
This project examined nine key issues reported by the SGMTF as representing barriers to
increased use of composts as components of growing media. Evidence was gathered from
scientific literature, relevant guidance/product specifications and from interviews with
stakeholders including seven compost producers (of whom six also produced growing
media), an additional three growing media manufacturers, eight growers and two growing
media retailers. The extent to which each of the nine issues have been resolved was
discussed and recommendations were made as to how to further improve PAS100 composts
intended for use in growing media and increase sales and use of compost-included growing
media (CIGM) in amateur and professional horticulture. One question not addressed in this
report is that of the potential contamination of composts by herbicide residues. This has
been considered in other research, and changes to published WRAP guidance on the
production of composts for use in growing media have been made.
Conclusions on the nine issues under study
 Physical contamination: technical solutions are available to address this concern
effectively, though around one third of all of the stakeholders contacted believed that the
issue had not yet been effectively resolved and that it limited or prevented their use of
composts or CIGM;
 High bulk density: technical solutions are available to help address this concern. Some
stakeholders felt that they had resolved the issue satisfactorily and that it was possible to
make good CIGM with sufficiently low bulk density. Others felt that it remained a concern
and that it limited or prevented their use of composts or CIGM;
 High pH: technical solutions can partially address this concern and some evidence
suggests that high pH in non-peat growing media may be less of a concern than originally
thought. However, all of the growers and one of the two growing media retailers felt that
the issue had not yet been effectively resolved and that it limited or prevented their use
of composts or CIGM;
 High electrical conductivity (EC): technical solutions can partially address this concern and
the majority of compost producers and growing media manufacturers believed that it had
never been a problem or that if managed with care, it merely limited the inclusion rates of
compost in growing media. All growers and one of the two growing media retailers felt
this issue had not yet been resolved and that it limited or prevented their purchase/use of
CIGM;
 Weed seeds: technical solutions are available to address this concern and most compost
producers and growing media manufacturers believed that it had never been an issue for
them. However, one fifth of compost producers and growing media manufacturers, half of
the growers and one of the two growing media retailers felt that the issue had not yet
been effectively resolved and that it limited or prevented their use of composts or CIGM;
De-mystifying the use of PAS100 compost in horticultural growing media
1
 Sciarid flies and other pests: limited technical solutions are available to address this
concern at present and there is still some concern amongst stakeholders on the subject.
Around half of stakeholders felt that the issue affected them. For example, it limited the
use of compost in growing media by growing media manufacturers and it limited sales of
CIGM to amateur gardeners and professional growers;
 Shelf life: technical solutions are available to address this concern. Most compost
producers and growing media manufacturers believed that the issue had never been of
concern or had been resolved, whereas most of the growers and both growing media
retailers felt that the issue had not been effectively resolved and that it limited or
prevented their use of composts or CIGM;
 Consistency: technical solutions are available to address this concern. Half of the compost
producers and growing media manufacturers believed that the issue had never been of
concern or had been resolved whereas the remaining half, along with all of the growers
and both retailers felt that the issue affected them. Half of the compost producers and
growing media manufacturers consulted felt that it limited their sales or use of compost in
growing media. Retailers felt that it limited their purchase of CIGM and growers felt that it
limited their use of CIGM; and
 Human pathogens: technical solutions are available to address this concern effectively
(although more information may be required on the incidence of Legionella species
bacteria if further outbreaks of Legionnaire’s disease are associated with growing media).
Half of the compost producers and growing media manufacturers believed that the issue
had either never been of concern or had been adequately resolved but the remaining
compost producers and growing media manufacturers, most growers and one of the two
growing media retailers felt that the issue had not been effectively resolved and that it
limited or prevented their use of composts or CIGM.
Comments on information gathered
It became clear through the assessment of information gathered from the literature review
and stakeholder interviews that most of the issues have been wholly or partly resolved,
particularly in the opinions of some of the compost producers who also manufacture growing
media. Effective management of the issues requires an in-depth knowledge of the
composting process and methods of maximising compost quality and also requires
investment of time and money to achieve. It was also clear that several of the issues are
perceived rather than real in the eyes of some stakeholders. The fact that many of the issues
under study are still perceived to represent problems to some is compounded by the fact
that many of the perceptions which stakeholders have relate to experiences from several
years ago.
There is a need to ensure that they understand the great improvements in compost quality
which have been made in recent years. Some of them have considered using or have used
composts in the past which were not even PAS100-accredited. This falls a long way short of
material produced according to any specification at all, let alone a higher specification
specifically designed for compost to be used in growing media (e.g. WRAP, 2011a). Some
leading industry players have formed very poor opinions of composts which are evidently
deeply ingrained. There is a clear need to show stakeholders just how consistent good
composts can be, and that the economics of producing such compost means the right price
must be paid to the composter. There is also a need to show stakeholders how to achieve
and specify the required quality. The WRAP Guidelines and Specification (WRAP 2011a and
b) go some way towards helping achieve this, but they need to be updated and promoted to
ensure that all who might benefit from using them are aware of them.
De-mystifying the use of PAS100 compost in horticultural growing media
2
Recommendations for research, demonstration and knowledge exchange
The following work is recommended in order to address the outstanding issues of concern
identified in this project, to increase production of top quality composts suitable for use in
growing media, to increase the use of composts in growing media and to encourage sales
and use of CIGM in amateur and professional horticulture. The full report indicates which
recommendations relate particularly to the different stakeholder groups. It also shows how
the proposed work relates to the findings of the project and how it will help to meet the
project’s aims. The recommended work includes:
 Study the effect of composting site management practices on compost consistency;
 Comparison of Dutch and UK quality composts – can lessons be learnt from the Dutch
RHP system which is one of the few examples of a scheme which appears to have gained
confidence with the professional horticulture industry?;
 Replicated experimental trials including:
 work to demonstrate that the higher pH values found in CIGM as opposed to traditional
peat media do not necessarily cause problems
 development of methods to prevent and control sciarid (and other) flies in both
amateur and professional situations; and
Nursery-based
comparison trials and demonstration of best practice.

In addition to the above:
 Additional work to improve knowledge of the benefits which composts can bring to
growing media was recommended, including studies on disease suppression and nutrient
management. If CIGM can be shown to be cheaper or better than alternative media, then
growers will be more likely to try it (or try it again)
 The following priorities for knowledge exchange were identified in order to promote the
use of composts in growing media:
 development of new or revised template documents (along the lines of those produced
by ORG and REAL to provide guidance to help producers consistently achieve the
required standards) and updated versions of the WRAP guides (‘Guidelines for the
specification of quality compost for use in growing media’, WRAP, 2011a and ‘Compost
production for use in growing media – a good practice guide’, WRAP, 2011b);
 promotion of new and existing information relevant to increasing the use of quality
composts in growing media at trade shows and in the trade press;
 tailor-made training courses/presentations; and
 demonstration days/grower walks.
De-mystifying the use of PAS100 compost in horticultural growing media
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Contents
1.0
2.0
3.0
4.0
5.0
Introduction ................................................................................................. 8
1.1
Project background .................................................................................... 8
1.2
Benefits of using composts in growing media ............................................. 11
1.3
Project rationale and aims......................................................................... 12
Methodology ............................................................................................... 13
2.1
Interviews of industry stakeholders ........................................................... 13
2.2
Desk-based investigation .......................................................................... 13
2.3
Hyperlinked summary of UK legislation and good practice guidance.............. 14
2.4
Gap analysis to determine where additional research is required .................. 14
2.5
Proposal of knowledge exchange (KE) mechanisms ..................................... 15
Results and discussion ............................................................................... 15
3.1
Interviews of selected industry stakeholders ............................................... 15
3.1.1 Summary of stakeholder views ........................................................ 15
3.2
Desk-based investigation .......................................................................... 16
3.2.1 Physical contamination ................................................................... 16
3.2.2 High bulk density ........................................................................... 20
3.2.3 High pH ........................................................................................ 23
3.2.4 High electrical conductivity.............................................................. 25
3.2.5 Weed seeds ................................................................................... 26
3.2.6 Sciarid flies and other pests ............................................................ 29
3.2.7 Compost shelf life .......................................................................... 31
3.2.8 Compost consistency ...................................................................... 34
3.2.9 Microbiological safety ..................................................................... 40
3.3
Hyperlinked summary of UK legislation and good practice guidance.............. 44
3.3.1 The UK Compost Certification Scheme and BSI PAS100:2011............. 44
3.3.2 The Quality Protocol for Compost .................................................... 45
3.3.3 The Animal By-Products Regulations ................................................ 45
3.3.4 Guidelines for the specification of Quality Compost used in growing
media (2011) ........................................................................................... 45
3.3.5 Compost production for use in growing media – A good practice guide
(2011) 46
3.3.6 Microbiological safety of pot-grown fresh herbs ................................ 46
3.4
Gap analysis to determine where additional research and development is
required ............................................................................................................ 46
3.5
Proposal for knowledge exchange (KE) mechanisms ................................... 49
Conclusions and recommendations ............................................................ 52
4.1
General points ......................................................................................... 52
4.2
Compost producers .................................................................................. 53
4.3
Growing media manufacturers................................................................... 55
4.4
Growers .................................................................................................. 55
4.5
Retailers of growing media........................................................................ 56
References .................................................................................................. 57
A.1.1 BSI PAS100 compost producers and growing media manufacturers...... 63
A.1.2 Growers who may use growing media containing PAS100 compost ...... 69
A.1.3 Retailers’ perception ........................................................................ 73
De-mystifying the use of PAS100 compost in horticultural growing media
4
Glossary
Aeration – The process by which oxygen-rich air is supplied to compost to replace air
depleted of oxygen.
Aerobic – Metabolic process that requires oxygen.
Ammonia (NH3) – A gaseous compound composed of nitrogen and hydrogen, with a
pungent odour.
Anaerobic – Metabolic process occurring in the absence of oxygen.
Animal By-Products Regulations – A set of European regulations designed to ensure
food safety under the ‘farm to table’ approach set out in the EU White Paper on Food Safety
adopted in January 2000. They contain strict animal and public health rules for the
collection, transport, storage, handling, processing and use or disposal of all animal byproducts (ABPs).
Bacteria – A group of micro-organisms with a primitive cellular structure, in which the
genetic material is not retained within an internal membrane (nucleus).
Biodegradable – can be broken down through biological processes.
BSI PAS100:2010 – A publicly available specification which covers the entire production
process for composts and ensures that composts are quality assured, traceable, safe and
reliable.
Bulk density – The mass per unit volume of materials.
Carbon dioxide - a colourless, odourless, tasteless gas that is produced as a result of
respiration (by plants and animals including microorganisms).
CGW (composted green waste) – a term often used by growing media manufacturers for
green PAS100 compost.
CIGM (compost-included growing media) – growing media which contain green and/or
green/food compost.
Compost - a stable, sanitised, soil-like material, which has been made through mixing, selfgenerated heating and aeration.
Compost quality protocol – A standard which describes parameters for the full recovery
of compost in England and Wales which provides user confidence, protects the environment
and eases the regulatory burden on compost producers. It is not applicable in Scotland.
Composting – The natural breakdown of biodegradable materials through mixing, selfgenerated heating and aeration to form a stable, soil-like material.
Density – The weight or mass of a substance per unit of volume.
Ericaceous – Plants belonging to the family Ericaceae including rhododendrons and heaths
and heathers. This family of plants prefers to grow in acid soils or growing media and in this
respect, they differ from most other common commercially grown plant families.
De-mystifying the use of PAS100 compost in horticultural growing media
5
Feedstock – The biodegradable materials present at the start of a composting process.
Green waste – Grass cuttings, leaves and prunings, from parks or gardens.
Growing medium – a material, usually used for potting plants or sowing seeds, which can
be made from single constituents or more usually a mixture of constituents such as peat,
perlite, compost or loam.
In-vessel composting – A diverse group of composting methods in which the materials
are contained in a building, reactor or vessel.
Loam – Soil containing a desirable mixture of sand, silt and clay, suitable for crop
production.
Major nutrient – Essential element required in large quantities from soils by plants.
Maturation – a period (within the composting process) of lower biodegradation than in the
preceding steps of composting. The stabilisation continues but the rate of decomposition
has slowed to the point that turning or forced aeration is no longer necessary. Some
microbial activity and chemical changes, such as the oxidation of ammonium ions to nitrate,
will continue. Beneficial soil micro-organisms that were inhibited or destroyed during the
active composting process will begin to re-colonize the composted materials.
Mature compost – compost in which biological activity (as measured by microbial
respiration) has slowed. All of the easily degradable molecules have been broken down,
leaving the complex organic material behind. It is difficult or impossible to identify the
original feedstock materials. Mature composts usually have a dark colour and a rich, earthy
smell.
Micro-organism – An organism too small to see with the naked eye that is capable of living
on its own.
Moisture content – Percentage of a substance composed of water. Moisture content
equals the mass of the water portion divided by the total mass.
PAS100:2010 - See BSI PAS100:2010.
Pathogen – Any organism capable of producing disease through infection.
pH – A measure of the concentration of hydrogen ions in solution. pH below 7 = acidic, pH
above 7 = alkaline.
Potentially toxic elements – Chemical elements that have the potential to cause harm to
humans, animals and/or plants.
Sanitisation – Biological processes that together with conditions in the composting mass
give rise to a compost in which levels of any human, animal or plant pathogens which may
have been present are reduced to acceptably low levels.
Secondary nutrient – Essential element for plant growth which is required in smaller
quantities than major nutrients, but larger quantities than trace elements.
De-mystifying the use of PAS100 compost in horticultural growing media
6
Sewage sludge - A semi-liquid waste with a solid concentration in excess of 2500 parts per
million, obtained from the purification of municipal sewage. Also known as sludge or
biosolids.
Stabilisation – Biological processes that together with conditions in the composting mass
give rise to compost that is nominally stable (that is a condition whereby biological activity
and biodegradation has slowed and will not resurge under altered conditions such as
manipulation of moisture or oxygen levels, or through the addition of a source of water
soluble nitrogen).
Trace element – Essential element for plant growth which is required in very small
quantities.
Turning – An operation that mixes and agitates material in a windrow, pile or vessel.
Weed propagule – A piece of plant material from which weeds can grow (e.g. seed,
rhizome or root fragment).
Windrow – Elongated pile of composting material.
Acknowledgements
Thanks are due to the many individuals, companies and organisations which helped with this
project.
De-mystifying the use of PAS100 compost in horticultural growing media
7
1.0
Introduction
1.1
Project background
Composts are stable, sanitised, soil-like materials, which have been made through
mixing/shredding, self-generated heating and aeration. This work concerns quality composts
that are PAS100-compliant, which have been made by compost producers accredited
through the UK compost quality certification scheme. This project mainly concerns composts
made solely from source-segregated green or garden wastes, although in some cases the
composts discussed have been made from a mixture of green wastes and source-segregated
food or food processing wastes.
In the UK, the term “compost” is also applied to practically any substance that can be used
to grow plants in. These substances (which are termed “growing media” in many other
countries) often do not contain any true composts at all. Products sold as “John Innes
composts” for example, are mixed using sterilised garden loam (soil) stripped from stacked
turf, along with peat, sand and/or grit and added plant nutrients. In this report, the terms
“compost” and “growing media”/”growing medium” are not interchangeable. Compost is
defined only as in the first paragraph (above) and a growing medium is defined only as a
substance or mixture of substances in which plants can be grown. Since the 1970s plants
grown in pots by amateur gardeners or for retail sale have largely been grown in peat-based
growing media, but this is changing due to pressure on growing media manufacturers,
gardeners and professional growers to use more sustainable growing media.
It is widely acknowledged that the UK horticultural industry is over-reliant on peat
https://www.gov.uk/government/policies/making-the-food-and-farming-industry-morecompetitive-while-protecting-the-environment/supporting-pages/horticultural-peat. It is also
widely accepted that the transition to more sustainable growing media and away from an
over-reliance on just one material, peat – which is also a finite and controversial material –
makes good business sense for all sectors of the horticultural industry and will improve the
long term sustainability of the sector, as well as having environmental benefits.
The Natural Environment White Paper
(http://www.defra.gov.uk/environment/natural/whitepaper/ ), published in June 2011, included an
ambition to reduce horticultural peat use to zero in England by 2030, setting the following
milestones: a progressive 2015 target for new contracts in the public sector, a 2020
voluntary target for amateur gardeners and a 2030 voluntary target for commercial growers.
It also included a commitment to establishing a “Task Force” to advise on how best to
overcome the barriers to reducing peat use.
The Sustainable Growing Media Task Force (SGMTF) was established in 2011, under the
chairmanship of Dr Alan Knight OBE following the commitment made under the National
Environment White Paper. It was made up of representatives from 35 organisations from
across the growing media supply chain, including retailers, growing media manufacturers,
growers and environmental organisations. The Task Force initially aimed to explore how to
overcome barriers to further reduce peat use in horticulture. It has since adjusted its remit
to that of putting the horticultural sector on a long-term sustainable footing by ensuring that
all choices of growing media (or substrate) used for amateur gardening and horticulture are
sustainable. It published several meeting notes and an interim report during 2012, and a
final report to Defra Ministers in July 2012, including a draft roadmap (SGMTF, 2012). Project
P6c, which was conducted as part of the work of the taskforce, was led by WRAP. It aimed
to improve confidence in the use of green compost in growing media.
The final report of the SGMTF (which included the report for Project P6c) acknowledged that
green compost will never be the sole solution to peat replacement. However, it did conclude
De-mystifying the use of PAS100 compost in horticultural growing media 8
that it has an important role to play in sustainable growing media manufacture and that
work is required in order to improve confidence in compost, if it is to be used to a greater
extent in growing media. The SGMTF suggested that the key to improving confidence lay in
understanding and tackling perceived quality issues. Where it is clear that issues have been
resolved, the relevant research must be signposted to industry stakeholders. The SGMTF
also proposed that work should be conducted to tackle the remaining issues in relation to
compost quality. In the final report and in previously published and unpublished reports, the
SGMTF raised the following (real or perceived) issues as being of greatest concern:
 physical contamination;
 high bulk density;
 high pH;
 high electrical conductivity;
 weed seeds;
 sciarid flies and other pests;
 compost shelf life;
 compost variability;
 presence of herbicide residues; and
 the presence of human pathogens in growing media constituents (including composts)
has also been raised recently in several places (e.g. national newspapers and in industry
guidance, AfOR, 2010).
Defra published its response to the SGMTF report in January 2013, broadly endorsing it and
announcing the formation of its successor, the Growing Media Panel, which will oversee
delivery of the roadmap set out by the SGMTF
(https://www.gov.uk/government/publications/government-response-to-the-sustainablegrowing-media-task-force). At the same time, Defra announced over £600k of new
Government funding to address the remaining issues with growing media and peat
alternatives and, in partnership with other funders, a total package worth over £1.1m.
Many companies within the UK horticultural industry are committed to peat reduction and
significant progress has been made towards Defra’s targets. The Growing Media Association
(GMA), which represents the majority of UK and some Irish growing media suppliers for the
UK market, published its commitment to peat reduction in 2010 (GMA/HTA, 2010) and its
members continue to work towards reducing their use of peat.
In 2012, WRAP conducted a project which aimed to help UK growing media retailers identify
peat in their supply chains and take steps to reduce the percentage of peat through the
initiation of action plans. Eight leading UK retailers participated in this programme. They
collectively accounted for approximately 37% of UK peat in bagged growing media and are
now either instigating or strengthening peat reduction in their business through a variety of
strategic and operational initiatives. All retailers identified the need to reduce the use of
peat as a non-renewable, primary resource. The project also provided training on peat
alternatives (including green compost) for 200 garden centre staff.
The Growing Media Initiative is a further industry scheme which was developed by the
Horticultural Trades Association in conjunction with the Growing Media Association, DIY and
garden centre retailers, Defra, the RSPB and the Royal Horticultural Society. It was formed in
an attempt to help the UK horticultural industry meet government targets for the reduction
of peat use. It aims to increase awareness on the need to protect the world's peatlands and
the need therefore to use more sustainable materials for growing plants and improving
garden soil. Provisional GMI membership is open to retailers and manufacturers of growing
media and soil improvers who are committed to achieving 90% peat replacement in their
business and which are currently operating at a minimum of 20% replacement in retail
De-mystifying the use of PAS100 compost in horticultural growing media
9
products. They must produce an annual Action Plan and demonstrate year on year progress.
The GMI is being re-developed to encompass the sustainability criteria and a growing media
performance standard that is being developed as an output of the SGMTF.
At present, the majority of reduced peat and peat-free growing media is sold into the retail
sector; that is, to amateur gardeners as bagged products (AHDB: HDC, 2013; Table 1). Very
few UK professional growers are using peat-free growing media to grow edible or
ornamental crops. The latest data show that the quantity of green compost used in both
amateur and professional growing media was lower in 2012 than in 2011 (Table 1).
Table 1 Volume (million m3) of ingredients used in the UK retail (amateur) and professional
growing media markets in 2011 and 2012 (AHDB, 2013).
Material
Peat
Green compost
Bark
Wood-based
Coir
Loam
Retail media
2011
2012
1.83
1.39
0.35
0.43
0.44
0.43
0.49
0.50
0.13
0.13
0.14
0.12
Professional media
2011
2012
0.93
0.83
0.02
0.01
0.05
0.06
0.04
0.06
0.18
0.19
-
Most of the green compost used in growing media is used in amateur gardening products
(Table 2). For example in 2013, green compost made up 10% of all amateur growing media
(making it the most used alternative in this sector) but only 2% of professional growing
media.
Table 2 Raw materials used (as a % of total materials used) in the amateur gardening and
professional growing media markets in 2012 (AHDB, 2013)
Material
Peat
Green compost
Bark
Wood-based
Coir
Growing media for
amateur gardening
51.8
12.9
8.4
17.8
4.8
Growing media for
professional use
69.1
1.1
5.4
5.2
15.9
Despite the fact that there are now some very successful products on the market, some
users of compost-included growing media have never gained confidence in its use and some
growing media manufacturers have had their confidence dented through past use of poor
quality compost that was unsuitable for inclusion in growing media. Poor uptake of composts
has been attributed by various industry experts as being due to several reasons, of which
poor compost/growing media quality and consistency are only two. Other reasons include
confusion over what the terms ‘compost’ and ‘growing media’ actually mean (which can be
further complicated by poor labelling of products), lack of specialist technical information on
how to use the product(s) to best effect, cost, poor availability of appropriate products in
some geographical areas and a lack of willingness to change.
The fact that peat is becoming more difficult (and probably more costly) to obtain and the
fact that it is increasingly being labelled as a finite resource which should be replaced by
more sustainable products, is likely to result in increased interest in PAS100 composts for
use in growing media. For that reason, it is important to ensure that sufficient composts of
De-mystifying the use of PAS100 compost in horticultural growing media
10
suitable quality are available, along with appropriate development work where required and
technical advice to help develop these relatively new markets.
In an effort to help increase the production and use of PAS100 composts of suitable quality
for use in growing media, WRAP has previously published guidance (WRAP 2011a and b).
These important publications will be further discussed later in this report.
1.2
Benefits of using composts in growing media
Anecdotal evidence from growers and growing media specialists, along with published results
from previously undertaken growing trials using compost-included growing media, have
shown that such media can have several advantages over peat-based media when managed
appropriately. For example:
 Compost provides major and secondary plant nutrients and trace elements. There is the
potential for growers or growing media manufacturers to save on fertiliser costs. Compost
can provide nutrient holding capacity in the growing medium in a similar way to loam;
 Compost has a neutralising value and it will almost never be necessary to add lime to
growing media which include composts, thus providing a further opportunity to save
costs;
 Compost may assist in the suppression of some plant diseases because it is more
biologically active than other growing media constituents. It is thought that the natural
populations of micro-organisms in compost can out-compete, or in some cases directly
attack, plant disease-causing organisms;
 Compost may reduce the surface growth of liverworts, moss and algae when used in
growing medium blends that tend to retain a drier surface than media based solely on
peat;
 Compost-included growing media tend not to require a synthetic wetting agent as peatbased products do in some applications;
 Compost-included growing media tend to slump less over time than peat-based growing
media;
 There is potential to use less water when managing plants grown in compost-included
growing media than in peat-based media if irrigation is appropriately managed;
 Some nurseries have found that it can be faster to pot with compost-included growing
media than with peat-based media. This was found to apply to both hand and machine
potting; and
 Growing media which are based partly on composts generally have a higher bulk density
than those based on peat. This is considered a disadvantage in some applications (e.g.
where there is a requirement for a lot of manual handling of products or a need for
lengthy transport of media or plants). However, it is an advantage in others (e.g. where
relatively unstable top-heavy potted plants, such as some conifer or climber species, are
situated in a windy site – a heavier pot helps to stop the plants blowing over).
Even though the benefits of including composts in growing media have been well
documented, previous WRAP studies and the experiences of individuals within this project
team have shown that there are continuing concerns about compost quality amongst
growing media manufacturers, growers and retailers and these concerns have been reflected
in the conclusions of the SGMTF (as outlined above). Several respondents to our survey said
that they would like to see more scientific proof of some of the benefits of using composts in
growing media.
De-mystifying the use of PAS100 compost in horticultural growing media
11
1.3
Project rationale and aims
For over 10 years, WRAP has been committed to improving the quality and availability of
PAS100 composts. It is committed to developing markets for composts and this includes
increasing the use of composts in growing media and increasing the use of compost-included
growing media in both amateur and professional horticulture. Between 2005 and 2008,
WRAP funded several nursery-based trials of compost-included growing media. In 2011,
WRAP developed guidance intended to help promote and facilitate the use of quality
composts in growing media (WRAP 2011a and b). WRAP has also taken an active part in the
work of the SGMTF (SGMTF, 2012).
One of the goals within the Roadmap is to ‘improve confidence in the use of green compost
such that it is able to fulfil its maximum potential in the growing media market’ (currently
estimated to be around 20% of the market [SGMTF, 2012]). This project addresses one of
the tasks set out under that goal within the roadmap. It aimed to ascertain whether each
issue identified by the Task Force (other than that of herbicide contamination) is current or
has already been resolved. If an issue has been resolved, the means by which it was
resolved has been documented and published in the report. If an issue remains, the nature
of the problem has been discussed and the means by which it might be addressed outlined.
This work is important in order to improve understanding of how to manufacture composts
of appropriate quality for use in growing media and in order to increase the use of quality
composts in growing media.
The project tasks are set out below.
A. To interview a selection of PAS100 compost producers, professional growers, growing
media manufacturers and other relevant stakeholders in order to obtain their views and
information on their experiences in making, supplying and using composts.
B. To conduct a desk-based investigation of previous trials work and research on specific
aspects relating to the quality and use of green composts in growing media and to
produce a short report with bibliography to provide easy access to relevant information.
Aspects studied included:
 physical contamination;
 high bulk density;
 high pH;
 high electrical conductivity;
 weed seeds;
 sciarid flies and other pests;
 compost shelf life;
 compost variability; and
 the presence of human pathogens.
C. To produce a hyperlinked summary of UK legislation and good practice guidance
relevant to the safety and quality issues associated with using composts in growing
media.
D. To complete a gap analysis to determine where additional basic research is required in
order to increase the use of composts in growing media.
E. To propose mechanisms for knowledge exchange (KE) whereby practical industry
experience can be shared without compromising the commercial integrity of individual
businesses.
De-mystifying the use of PAS100 compost in horticultural growing media
12
2.0
Methodology
A range of issues are known - these were checked with industry stakeholders, and deskbased investigation undertaken to determine whether they had actually been addressed.
Where they had been addressed, signposts to relevant research were gathered and are listed
in Section 3.2. Where they have not been addressed (or only partly addressed), the gaps
have been collated and presented in Section 3.4.
2.1
Interviews of industry stakeholders
Industry stakeholders hold important information and opinions on the value of composts in
growing media, the challenges of using them and ways to address these challenges. For that
reason, a range of key stakeholders were interviewed as part of this project. A list of twenty
eight interviewees was compiled including eight compost producers of whom seven also
produced growing media, an additional four growing media manufacturers, ten growers and
six retailers. Three different questionnaires were produced - one for composters and/or
growing media manufacturers, one for growers of horticultural crops and one for retailers of
growing media (Appendix 2).
Each contact was phoned in advance, introduced to the project, asked if they would be
happy to participate and whether they would prefer to complete the questionnaire
electronically (for submission by email) or over the phone. Of those asked to participate in
the project, seven of the compost producers agreed (six of which were also growing media
producers) and three of the four additional growing media manufacturers agreed. Eight of
the growers and two of the retailers participated in the project. Once the interviews were
completed, the results were reviewed and summarised in an overview, a series of simple
tables, graphs where appropriate and summary statements (Appendix 1). Most of those who
responded wished to remain anonymous and so no names of individuals or companies are
mentioned in this report.
2.2
Desk-based investigation
The nine key topics for investigation in relation to the use of composts in growing media
were:
1 physical contamination;
2 high bulk density;
3 high pH;
4 high electrical conductivity;
5 weed seeds;
6 sciarid flies and other pests;
7 compost shelf life;
8 compost variability; and
9 human pathogens.
The project team compiled a list of publications relevant to each of the nine topics. They
then used the references sections within them (where relevant), discussions with
stakeholders, and web-based literature searches to ensure that all additional key relevant
documents were identified. The search engines Google Scholar and CAB Abstracts (which
includes a range of horticultural journals and conference proceedings) were used. These
search engines and Google were also used to obtain final and progress reports from relevant
desk-based studies and experimental projects. It quickly became obvious (as anticipated)
that UK research and development reports were generally most relevant to the project,
although international journals were included in the literature searches. UK journals and
publications were more relevant because UK composts tend to be different from those made
in Europe and the USA in terms of feedstock and product quality. For example, much of the
De-mystifying the use of PAS100 compost in horticultural growing media
13
compost made in other countries contains sewage sludge and/or is based on non-sourcesegregated wastes.
Literature relevant to each of the key topics of investigation was read and assessed, and a
draft report for each topic was prepared, which discussed:
 the background to the issue and reason(s) why it is important; and
 the extent to which the issue has been partly or wholly addressed in the industry.
Each of the nine draft reports included full references to all relevant work (with web links for
on-line readers where possible) including research, development and (where possible) details
of methods by which individual compost producers or growers have addressed the issue
(information obtained during stakeholder interviews).
The draft reports, prepared by individual authors were then reviewed by the other project
team members, sent to the WRAP project management team for their comments and
updated prior to submission of the final version of the report (Section 3.1).
2.3
Hyperlinked summary of UK legislation and good practice guidance
Several pieces of UK legislation and good practice guidance have been developed with the
aim of protecting people, animals, crops and the environment from the risks and challenges
associated with compost production and use. Guidance also exists to help compost producers
make their products as useful as possible in the context of growing media markets.
In this part of the project, a brief description was made of the main pieces of legislation and
good practice guidance which aim to reduce the impact of the main issues (risks and
challenges) associated with the use of composts in growing media (Section 3.3). Hyperlinks
are given to the relevant legislation and good practice guidance. This part of the report can
be read on paper but an internet connection is required for full functionality.
2.4
Gap analysis to determine where additional research is required
Gap analysis was conducted to determine where information was lacking with respect to
knowledge of techniques to minimise problems relating to the nine issues being investigated
in this project.
Gaps in knowledge were first identified during a “brainstorming session” by the project team
members following completion of the main stakeholder interviews in Task A, along with
Tasks B and C and circulation of draft reports associated with these tasks. Notes were taken
during the brainstorming session and a draft gap analysis was completed and circulated to
the team members for comment. This report contained the following information for each
identified issue:
 signposts to recent relevant research/trials work;
 conclusions on whether the research report/results were visible enough




 if so, are they being sufficiently used/put into practice?
 if not, is more work required?
are individuals/organisations solving the issue themselves and if so, can they assist in
moving things forward (what barriers are being put in place due to commercial
sensitivities?);
current practice to minimise risk and/or maximise product quality;
has the issue been resolved or is it still an issue and if so, with which group(s) in
particular; and
work required to resolve the issue.
De-mystifying the use of PAS100 compost in horticultural growing media
14
A limited amount of further literature searching and telephone calls to stakeholders was
conducted in some cases in order to complete the final version of the Task D report.
2.5
Proposal of knowledge exchange (KE) mechanisms
The project team assessed the information gained through completion of Tasks A to D,
considered the options based on their experience in education, training and knowledge
exchange (KE) and suggested a range of KE mechanisms for use in this challenging sector.
These are outlined in Section 3.5.
3.0
Results and discussion
3.1
Interviews of selected industry stakeholders
As part of the project we aimed to gather information through a survey of key players in the
composting, growing media manufacturing, retailing and growing sectors. The full results of
the interviews are presented in Appendix 1 and the questionnaires used as the basis of these
discussions are given in Appendix 2. Of those invited, seven compost producers (of whom six
also produced growing media) took part in the project, along with an additional three
growing media manufacturers, eight growers and two retailers. Despite the fact that not all
invited stakeholders took part, it was felt that the responses fairly represented the views of
the major industry players.
3.1.1 Summary of stakeholder views
The opinions of the 28 stakeholders interviewed as part of this project differed markedly
from one to another in some cases, with some holding largely positive views about compost
quality and the use of PAS100 composts in growing media, and others largely negative
views. While all of the compost producers and growing media manufacturers were aware of
the WRAP specification and guidelines (WRAP 2011a and b) the majority of the growers
were not. The full results of the interviews are presented in Appendix 1 of this report. In
summary:
 The views of the seven compost producers (of whom six also produced growing media)
and additional three growing media manufacturers differed greatly. Some have worked
very hard to meet the needs of the CIGM market and felt that most of the nine issues had
been effectively solved, whereas others felt that none of them had. Most had some
remaining concerns, and most often, those included the presence of physical
contaminants, high compost bulk density and high compost EC;
 Of the eight growers interviewed, two used CIGM to some extent, two did not know
whether their growing media contained composts, and the remaining four did not
currently use CIGM. For the growers, physical contamination and lack of consistency in
the product were the major barriers to use of growing media containing PAS100 compost.
Five out of eight growers listed physical contamination as a major barrier, and six out of
eight cited lack of consistency. However, nearly all of the factors listed were deemed a
barrier to use for one or more of the growers interviewed. The growers who had negative
perceptions of compost had either not tried CIGM themselves or had tried poor quality
media some time ago;
 Of the two growing media retailers who took part in the project, one sold growing media
based partly on composts and one did not, due to concerns over all nine of the issues
under study in this project. The retailer who did sell CIGM felt that physical
contamination, high bulk density, sciarid flies, shelf life/consistency of product, and poor
customer perception limited sales; and
 It is important to state that many of the negative views which stakeholders held about
composts and CIGM were based on previous bad experiences (or the experiences which
De-mystifying the use of PAS100 compost in horticultural growing media
15
colleagues or associates had) with unsuitable PAS100 composts, non-PAS100 composts or
CIGM based on them, rather than with composts made recently according to the WRAP
Specification and Guidelines (WRAP 2011a and b) or CIGM based on these improved
materials.
3.2
Desk-based investigation
Based on the feedback received from the industry stakeholders, a programme of desk-based
investigation was undertaken to determine whether the nine principle areas of concern had
already been addressed. The extent to which the main conclusions from these pieces of
work are understood by industry stakeholders is discussed for each topic along with the
extent to which each issue has been resolved. Recommendations are then made for
minimising the impact of the issue in future.
3.2.1 Physical contamination
Introduction
Contamination of composts with glass, plastic, metal and stones remains a concern for
practically all growing media manufacturers, retailers and growers. Sharps (pieces of glass
and sharp metal fragments) are a particular concern due to their potential for causing injury
to those handling compost or growing media. Stones are less of a problem if the amount
present falls within the PAS100 limits, but visible plastic contamination is unpopular with all
users of compost and CIGM. Many compost producers (including those interviewed in this
project) complain about the amount of physical contaminants within the feedstocks which
they are expected to compost. Several of the growing media manufacturers contacted in this
project complained that green composts typically contain too many physical contaminants.
Some retailers and most of the growers interviewed also complained about the amount of
physical contaminants typically present in CIGM, and writers in the gardening press often
complain about physical contaminants present in bags of reduced peat and peat-free
growing media (e.g. Colborn, 2011). Whether these complaints were due entirely due to
recent experience, to historical experience or even to perceptions based on the bad
experiences of others was sometimes difficult to discern.
Previous and current relevant work
Researchers worldwide have cited the presence of physical contaminants in composts and
have outlined or discussed the problems which these contaminants can pose when seeking
appropriate markets for contaminated composts (e.g. Brinton, 2000; Dimambro et al., 2007;
Ostos et al., 2008). Most of the composts considered as potential constituents of growing
media in the USA or in mainland European research were based on non-source segregated
wastes, and for that reason were likely to contain much higher quantities of physical
contaminants than those principally under discussion in this report (i.e. composts accredited
under the UK compost quality standard, BSI PAS100). Given that the long term aim of this
project is to help improve the quality of UK PAS100 composts, this review will consider
mainly composts made from source segregated feedstocks which do not include sewage
sludge.
No scientific publications (other than one by WRAP [2002] and that by Dimambro et al.,
2007) have reported the percentage of physical contaminants found in UK composts since
BSI PAS100 was first published in 2002. In the former report (which is the only one to have
tested a relatively large number of samples), monthly samples were taken from each of nine
compost sites which were either PAS100 accredited or had been working towards
accreditation for a period of 5 months. The quantities of glass, plastic and metal
contaminants found in composts were extremely low (generally below 0.1%).
De-mystifying the use of PAS100 compost in horticultural growing media
16
No published studies have dealt with the impact of feedstock collection and composting
methods on contamination levels in finished compost. This is probably because many of the
methods which could (and should) be employed to minimise contamination in the finished
product are rather obvious and research is not thought to be necessary. If the feedstock
going into the composting process is free from contamination, then it is logical that the
finished product will also be free from contamination. However, it can be very difficult to
ensure that feedstock is clean, particularly when the composter does not have full control
over the collection process. In reality, a combination of measures aimed at reducing physical
contamination in the finished compost are used in practice on composting sites, including
steps taken to reduce the presence of physical contaminants:
 in incoming feedstocks;
 in feedstocks on site, prior to shredding;
 in compost during the composting process; and
 in the finished product(s).
ORG and REAL (which owns the only UK compost quality certification scheme, BSI PAS100)
has been aware of the problem of physical contamination of composts for many years and
has prepared several templates which provide guidance to help compost producers to
consistently achieve at least the standards required under PAS100:2011. These are available
to new applicants and members on the scheme.
The current (PAS100:2011) standards for maximum levels of glass, plastic, metal and stones
represent a tightening of the rules (from those last published in PAS100:2005) in response to
industry pressure for better quality composts, and the ability of compost producers to meet
these quality requirements. The standards are shown in Table 3.
Table 3 BSI PAS100:2011 standard for physical contaminants.
Parameter
Test method
Unit
Upper limit
Total glass, metal, plastic and any
AfOR MT PC & S*
% mass/mass of
0.25 of which
other non-stone fragments > 2
“air dry” sample
0.12 is plastic
mm
Stones > 4 mm in grades other
“
“
8
than “mulch”
Stones > 4 mm in “mulch” grade
“
“
10
*This method instructs laboratories to determine and report sharps as part of the reporting on
each type of physical contaminant (glass, metal, plastic, stones and other non-stone fragments)
The Organics Recycling Group recognise that the quantity of physical contaminants in
composts remains a problem for some compost producers and users, and has formed a
special interest group on feedstock quality and contamination in order to develop long-term
solutions. They (under their previous name, AfOR) have published a series of template
documents aimed at helping composters tackle feedstock contamination with a view to
reducing the amount of physical contaminants in finished composts. For example, they have
recently updated their template for the standard operating procedure of composting sites,
which includes sections on “Contracts/agreements/communication with waste suppliers”,
“Rejection or acceptance and storage of input materials” and “Traceability of input
materials”. All of these sections aim to help composters design procedures which will
minimise the presence of physical contaminants in feedstock and therefore in the finished
product.
In March 2012, ORG (then AfOR) released two input specification schedule templates that
enable composters and their suppliers to define a standard for the quality and types of input
materials delivered to their composting sites. Versions are available for green waste and for
co-mingled food and green waste composting systems. The templates can be adapted to
De-mystifying the use of PAS100 compost in horticultural growing media
17
include any other biodegradable wastes that are allowed under contractual arrangements
between the composters and their waste suppliers.
This input specification template also provides guidance on how the input materials delivered
to the composting facility should be compared to the input quality standard and defines the
actions to be taken if it is established that the material does not meet the required standard.
Use of the templates is not mandatory under the UK Compost Certification Scheme but ORG
recommends that this, or an equivalent template, is used by the composters and their
suppliers when signing up new contracts or reviewing existing contracts.
BSI PAS100:2011 was developed as a baseline standard. Compliance with PAS100 was never
intended to mean that composts were suitable for all markets. The minimum quality values
stated within PAS100 are simply not high enough for some markets, including growing media
manufacture, although the standard does stipulate that composts must be fit for purpose for
the markets for which they are intended.
In recognition of this, WRAP developed additional specification guidance for specialist enduses of composted green materials in 2004 (WRAP, 2004a). The guidelines aim to assist
producers of composted green materials to better understand and meet the specific
requirements of growing media manufacturers and growers who mix their own growing
media. The guidelines also help provide these customers with a framework for the
establishment of appropriate purchasing specifications as part of supply contracts. They were
developed in partnership with the Growing Media Association, with input from ORG (then the
Composting Association) and other experts.
The guidelines were thoroughly revised and re-published in 2011 following extensive
consultation with industry experts and other stakeholders and were published in two parts.
To help composters use the ‘Guidelines for the specification of Quality Compost in growing
media’ (WRAP, 2011a, Section 3.3.4), WRAP also produced the ‘Good Practice Guide’ (WRAP,
2011b, Section 3.3.5). Amongst other things, the guide aims to inform compost producers
about opportunities in the growing media sector. It also aims to help compost producers
understand the need for measurement of specific quality parameters in green and
green/food derived composts intended for use as constituents of growing media and helps
them achieve the more stringent requirements in terms of compost quality. It also aims to
help growing media manufacturers understand more about composts, how to specify them
and how to use them as constituents of growing media. These two documents have great
potential to help improve the quality and consistency of compost products in terms of their
use as growing media constituents, and there is some evidence (from the survey conducted
in this project) that both compost producers and growing media manufacturers are aware of
them and are using them to good effect. Compost made according to the Good Practice
Guide aims to solve the majority of the issues under study in this report.
The guidelines recommend that compost producers should aim to eliminate all physical
contaminants from their composts, and that growing media manufacturers set lower limits
for the permitted quantities of physical contaminants in composts than those in PAS100:2011
(Table 4). The good practice guide provides advice on how compost producers might achieve
these more stringent specifications.
Two of the leading experts in growing media manufacture (who wished to remain
anonymous) felt strongly that some of the problems with physical contaminants in composts
and growing media in the industry today were perceived, rather than real. They felt that
many growing media manufacturers, CIGM retailers and growers had had bad experiences
with poor quality composts/CIGM in the past and had decided, based on these experiences
that these products were not for them. Those who decided against composts have since
been unaware (and in many cases, unwilling to be made aware) of the significant
De-mystifying the use of PAS100 compost in horticultural growing media
18
improvements which have recently been made in the quality and potential performance of
composts and CIGM. In some cases, these individuals continue to speak out against
composts and CIGM at public meetings and in the press, which may be limiting market
development.
The compost producers who have succeeded in largely solving the physical contaminants
problem have spent considerable amounts of time and money on it. In particular, they:
 have worked with their suppliers in order to reduce the volumes of physical contaminants
coming in with the feedstock;
 have set up manual picking lines (and similar) to remove physical contaminants prior to
shredding;
 have purchased sheds and fences to minimise air-blown contamination of windrows (from
plastics) during composting; and
 employ manual labour to remove contaminants from windrows following turning and
employ a range of mechanical means (e.g. magnets, air density classifiers and screens) to
remove physical contaminants toward the end of the composting process.
These compost producers admit that it costs much more money to make “clean” compost
than compost which contains unacceptable quantities of physical contaminants. However,
this extra cost can be balanced by extra revenue from the more demanding markets, such as
growing media.
Table 4 Recommended targets and limits for the main quality parameters of composted
material that is to be used as a growing medium constituent. (Values obtained from WRAP,
2011a).
Parameter
Sharps
Test
method
AfOR MT
PC & S*
Stones
“
Metal, glass
and plastic
> 2 mm*
“
Unit
Target
Upper limit
Present
or
absent
Absent
Zero
% w/w
dry matter
retained
on lab
sieves
“
Absent
2% stones of >
4 mm, no stones
> 8 mm
Absent
0.2% metal
0.05% plastic
0.1% glass
Comments
Physical contaminants that
are sharp are unacceptable
in any application where
compost is handled and/or
used in growing media.
None > 4 mm shall be
found on visual inspection
at delivery.
None > 2 mm shall be
found on visual inspection
at delivery and a single
presence suggests noncompliance.**See note
below for overall PAS100
total limit for glass, metal
and plastic and any “other”
non-stone fragments.
*This method instructs laboratories to determine and report sharps as part of the reporting on
each type of physical contaminant (glass, metal, plastic, stones and other non-stone fragments)
**Despite the individual limits above, the overall limit in PAS100:2011 that must be adhered to for
“Total glass, metal, plastic and any “other” non-stone fragments > 2 mm is 0.25%. PAS100 also
requires the total level of plastic > 2 mm to be no more than 0.12%. However, this guide
recommends that plastic > 2 mm should be no more than 0.05%.
De-mystifying the use of PAS100 compost in horticultural growing media
19
Recommendations for minimising the impact of the issue in future
The best way to continue to reduce the concentration of physical contaminants in composts
is to ensure that all of those involved in the sector are aware of all relevant best practice
guidance, which details the necessary steps for them to take. There are existing, welldocumented solutions to the problem of physical contamination in composts and some of the
leading compost producers and growing media manufacturers interviewed as part of this
project have said that these solutions, when implemented in full, can be sufficiently effective,
though they take time to implement to the full and do cost money.
Of those consulted as part of this project, all compost producers and growing media
manufacturers were aware of the two recent WRAP publications (WRAP, 2011a and b).
However, those consulted represent the industry leaders, many of whom were involved in
the stakeholder consultation process undertaken during production of the publications. In
short, they would be expected to know about the publications and would be more likely to
put the recommendations within them into practice. On the other hand, members of the
project team working with smaller or newer composters wishing to start supplying composts
into the growing media or other high value sectors have found that managers on these sites
are frequently not aware of recent improvements to the AfOR PAS100 templates (these have
very recently been re-branded as the Renewable Energy Assurance Ltd. (REAL) templates) or
the relevant WRAP publications (2011a and b). For this reason, promotion of the revised and
updated REAL templates within and relating to PAS100:2011 and the WRAP publications
(2011 a and b) through articles in the trade press, targeted emails to members of the
PAS100 scheme and cost-effective training courses are recommended as the most effective
way of continuing to reduce physical contaminant levels in composts. It is important to note
that the margins for cost-effective production of quality composts from green and food
wastes are tight. Not all compost producers can be convinced of the need to invest in their
production systems if the prices that they are achieving for their compost products remain
uneconomic due to poor feedstock quality or distance from markets.
3.2.2 High bulk density
Introduction
A major constraint to the use of compost at high inclusion rates in growing media is that it
has inherently higher bulk density than most other ingredients, with the exception of
sterilised loam, when used in growing media. The reason for the high bulk density is that
compost typically contains around 25% mineral material associated with 75% organic
matter, the mineral fraction being much denser (Schmilewski, 2008). WRAP guidelines
(WRAP 2011b) propose that the maximum bulk density of compost for inclusion in growing
media should be no higher than 550 g/l and preferably within the range of 400-500 g/l.
High bulk density will increase the cost of transport of growing media and may also increase
handling costs at the nursery. Consideration needs to be given to the maximum size of bags
containing compost for both professional and retail customers as well, as bags must not be
too heavy to lift. In the questionnaires one major retailer saw customers having to handle
heavy bags as a major problem. There are however advantages to growing media with
higher bulk density, for example container nursery stock pots are less likely to blow over in
exposed windy sites, and Rainbow (2009) has reported that less firming in of liners was
required when potting in peat-free growing media containing around 33% green compost
compared to standard peat-based mixes.
Previous and current relevant work
Inclusion of high volumes (by percentage) of compost in growing media may also impede
drainage of crops, the main reason being that the mineral material in compost, having a low
particle size will fill in the gaps between particles within the mix and thereby reduce the
De-mystifying the use of PAS100 compost in horticultural growing media
20
volume of pore space. These gaps can contain either air or easily available water depending
on the saturation of the growing medium. Prasad et al. (2001) looked at increasing the
ratio of green compost to peat from 20 to 50% inclusion by volume, and measuring bulk
density and easily available water content. Increased compost inclusion rates increased the
bulk density of the mixes and reduced the pore space of the growing medium. Inclusion of
up to 20% green compost produced only small effects on easily available water content (that
is water which can be easily taken up by growing plants without reaching wilting point), but
when increased to 50% green compost, the easily available water content dropped
significantly (from 25% to 19% easily available water with the reduction of pore spaces).
This is very important when considering crops which stand outside for long periods e.g.
container nursery stock.
Blending materials which are lightweight and very free draining will provide a solution to the
problem of excess water retention. Schmilewski (2008) postulated that to keep the same air
capacity (25%, which would be the target for containerised nursery stock), this could be
achieved either using 0 – 20 mm peat grades alone or a peat-free blend of 40% green
material, 30% composted bark and 30% wood fibre. The air capacity was identical for both
growing media, but the wet bulk density was doubled for the peat-free mix due to the
presence of the compost.
Variability of bulk density between green compost samples is mentioned by many authors
(for example Surrage and Carlile (2008) who sampled 15 sites in Nottinghamshire and found
that bulk density of composts (only some of which were PAS100 accredited) varied fourfold
between 243 and 837 g/l). In work conducted on behalf of WRAP, composts from nine sites
were sampled over a period of 12 months and it was found that the mean bulk density
varied little during the year, with typical average bulk density values ranging from 480 to 620
g/l (WRAP, 2005). These composts were all from PAS100-accredited sites or sites who were
working towards PAS100. Bulk density was consistently higher for some composts than
others, with two of the nine products tested averaging 675 and 629 g/l over the year
(respectively) and a further two products averaging 442 and 444 g/l respectively. There has
been no work conducted to determine the range of bulk density values obtained from
composts produced according to the WRAP Specification (WRAP, 2011a).
The resultant problem of high weight deterring manufacturers of growing media from
including compost in mixes and the public’s reluctance to buy based on heavy bag weights
has also been noted.
To illustrate the typical bulk density of a range of products used in peat-based and peat-free
growing media the data in Table 5 have been derived from a number of sources:
Table 5 Bulk densities of several potential growing media constituents and soil
Product
Irish moss peat
Coir (rewetted)
Wood fibre
Wood fibre (Germany)
Shredded chipboard
Bulk density g/l
200 - 300
250 - 350
50 - 150
160
350
Product
Composted bark
Aged pine bark
Aged conifer bark
Bark fines
Sterilised loam
Bulk density g/l
380
380
390
360 - 420
1200 - 1300
From the above table it can be seen that the addition of green compost with a typical bulk
density of 450 – 550 g/l could increase the final bulk density of mixes considerably.
Examples of mixes containing green compost (which has an assumed bulk density of 500 g/l
for this purpose) and the effect on bulk density of peat, coir and composted bark mixes are
shown in Table 6. A value of 500 g/l for the bulk density of compost was used here because
De-mystifying the use of PAS100 compost in horticultural growing media
21
several of the surveyed composters who were also growing media manufacturers indicated
that it was an achievable target value.
Table 6 Percentage composition (by volume), bulk densities and weights of a range of
typical growing media mixes
Product/bag
% green
% other
Final BD
Wt pack mix
Wt. Pack (kg)
(volume)
compost
(g/l)
(kg)
100% peat
Retail MP (60 l)
25
75 (peat)
313
18.8
15
Retail MP (60 l)
25
75 (coir)
350
21.0
15
Retail MP (60 l)
25
75 (comp. bark)
410
24.6
15
Growbag (30 l)
50
50 (comp. bark)
440
13.2
7.5
MP – multipurpose retail growing medium, 60 litres is a typical pack size
Growbag – 30 litres is typical volume for a number of manufacturers of retail growing media
A 60 litre retail multipurpose growing medium (using for example 25% green compost and
75% bark) has a weight of almost 25 kg, which is the maximum weight of, for example,
bags of fertiliser used in the horticulture industry. There is no maximum weight for lifting
defined by HSE, but an average retail customer might well have difficulty lifting a 60 litre
peat-free mix with the above formulations; a lower volume would need to be used for a
peat-free mix or the amount of green compost in the mix reduced. In the second example,
50% green compost could be used in growbags for retail salad crop production in a peat-free
mix. Compared to the equivalent volume of a 100% peat product it is double the weight and
would cost a lot more to transport, with fewer bags per pallet.
Traditional loam-based products such as the John Innes range, which must contain a mix of
sterilised loam, sand and/or grit and peat, have considerably higher bulk density values than
standard growing media. The bulk density of such mixes is quoted by the John Innes
Manufacturers Association (JIMA) as typically 800-950 g/litre (JIMA, 2010), which is higher
than mixes listed in Table 6. For that reason, following a study of most of the JIMA
members’ 2013 catalogues, typical John Innes mixes are sold in 25 litre bags, compared to
standard peat-based or peat-free mixes of typically 50-60 litres. There is a case for
marketing growing media containing higher amounts of green compost. Such material
would contain more mineral material and would have to be sold in smaller bags due to the
higher bulk density. Product buyers would have to be made aware of why the bags are
heavier and the advantages of adding the green compost (more mineral material means
greater buffering capacity, better water retention etc.). Adding further amounts of sterilised
loam and grit/sand will give characteristics similar to traditional JI mixes but without the peat
content and such mixes are already available for retail sales. It should be emphasised that
no reference can be made to John Innes on the labels of bags containing compost and no
peat, since John Innes mixes must contain peat, and under current rules inclusion of green
compost is not allowed.
Extent to which issue has been resolved
Most of the professional nurseries and at least one major retailer surveyed in this project see
high bulk density as a factor which either prevents or would limit the use of compost in their
growing media. However, two producers of compost who also manufacture growing media
stated that with attention to feedstock management, problems with bulk density can be
overcome. They did accept that high bulk density would generally limit inclusion rates due to
the fact that mineral material was always going to be present, even in well managed
feedstocks. Likewise, growing media manufacturers who used PAS100 composts in their
mixes made the same point.
De-mystifying the use of PAS100 compost in horticultural growing media
22
Matured compost stored under covers (rather than in the open) stays drier and several
growing media manufacturers made the same point that compost needs to be covered
during the maturation stage to ensure it does not get wet, which would increase bulk
density. Due to the nature of compost, unless the mineral fraction can be reduced, the bulk
density will always be high compared to many other potential ingredients in growing media,
such as bark, wood fibre and coir. Soil and small stones may be present in householders’
green bins for example. These often form part of rootballs of dead or unwanted plants. Soil
has a high bulk density (see Table 5, sterilised loam) and won’t be screened out using 0-10
mm screens at the start of the composting process (soil by definition is material below 2
mm). Educating the public as to what can go in to green bins may help here, together with
vigilance at the reception point for green waste before composting.
Recommendations for minimising the impact of the issue in future
Keeping compost as dry as possible during the maturation stage, when higher moisture
levels aren’t required for rapid biological activity will undoubtedly help, either by using
buildings for maturation or covering windrows to minimise rain ingress. It is possible that
green material from certain sites e.g. parks, cemeteries and landscapers is “cleaner” with
less mineral matter present. If such material can be segregated and used specifically for
compost destined for the growing media market, there would be more chance of a lower
bulk density material. In-vessel composting allows air to be blown into the bottom of the
vessel which will help to aerate the material which in turn will be lighter (comment from one
compost producer in the survey).
In an idealised world, educating the public to avoid putting small stones and soil in their
green bins would also help. Such guidance might involve encouraging householders to shake
and knock off as much soil from roots and rootballs as possible before putting dead and
unwanted plants into recycling bins. However, to what extent local authorities can enforce
this is open to debate. Vigilance at the reception points of green waste before composting
may help if it can be seen that quantities of soil are present in a load. WRAP produces
guidelines to local authorities for managing garden wastes at Civic Amenity sites (WRAP
2005d), which includes having dustbins for the public to deposit plastic bags used to
transport the garden waste and trained site operators to explain what can and cannot go
into the waste skips. Walsall Council has written a leaflet which says what can and cannot go
into green waste bins, which can be seen on the WRAP website (Walsall Council, 2008).
WRAP’s link for local authority advice on collections can be found at:
http://www.wrap.org.uk/category/sector/local-authorities.
The WRAP Specification and Guidelines (WRAP, 2011a and b) provide clear information as to
target values for compost bulk density and on methods for achieving these values. It is
important to ensure that compost producers producing, or interested in producing composts
for use in growing media know about these publications and have read and understood
them.
3.2.3 High pH
Introduction
The pH of a growing medium refers to its acidity or alkalinity and is very important as it has
a major impact on nutrient availability (Bunt, 1988). Peat is naturally quite acidic (its pH is
around 4.0), hence lime can be added to raise the pH to the desired level for the type of
plants being grown (generally 5.5-6.0 for general species and 5.0-5.5 for acid-loving
‘ericaceous’ species). High pH in a growing medium commonly causes deficiencies of
manganese and iron and also reduces the availability of phosphorus. It is very difficult in
practice to lower the pH of a growing medium as addition of acid can create toxicities. For
growers in areas of ‘hard’ water (high bicarbonate content), the pH of the growing medium
De-mystifying the use of PAS100 compost in horticultural growing media
23
will rise over time anyway, so for longer-term crops it is sometimes recommended to start
with a slightly lower pH to allow for this (Holmes & Hewson, 2005).
Green compost naturally has a relatively high pH compared with peat (typically 7.0 – 8.5),
and when it is used as a growing medium component, even at modest percentages, it results
in a higher pH than is recommended for peat-based growing media. This has caused
concerns to growers and growing media manufacturers in the past, although it is generally
recognised now that a peat mix with a pH of 7.0 (achieved by addition of lime) is not the
same as a peat/green compost mix with a pH of 7.0 because the latter behaves more like a
soil than a soil-less growing medium and has greater ‘buffering’ (that is, it holds and
subsequently releases nutrients on the surfaces of organic particles). A growing medium
containing green compost can therefore have a higher pH than a peat-based one and still
have good availability of phosphorus and trace elements. It is not the actual pH that is
important for plants, but the effect it has on nutrients and their availability to plants.
There is a significant difference in the effect of pH on nutrient availability between soils and
peat-based growing media (Bunt, 1988). A growing medium containing green compost is
more similar chemically to a loam-based medium than a peat-based one. Less mature
composts contain more ammonium N and therefore have a higher pH than mature composts,
because a higher proportion of ammonium ions in solution will make the pH more alkaline.
This is another reason for only using stable, mature composts in growing media.
Previous and current relevant work
Many trials have demonstrated the increased pH in growing media containing green compost
when compared with peat-based media (for example Maher et al., 2001). However, at
typical inclusion rates (10-30%) this pH increase does not cause problems for most plant
species. Most growers of ericaceous crops that require more acidic growing media have
reservations about the use of green compost, but such crops also need a lower electrical
conductivity so only low rates of green compost would be used anyway. The ‘Peatering Out’
trials commissioned by WRAP (STA0013) demonstrated that, despite growing media
containing green compost having a higher pH than standard peat-based media, the normal
problems associated with high pH (such as iron deficiency) did not occur (only nonericaceous plants were included in the trials). Other WRAP growing trials have also
demonstrated this at Ness Botanic Gardens (WRAP project ORG033-14) and at Aldingbourne
Nurseries (WRAP project ORG0033-18).
The WRAP guidelines for compost for growing media specify a pH of 6.0-8.0 as being
acceptable (WRAP, 2011a). Most green composts have a pH of between 7.0 and 8.0, which
is within the acceptable range. Composts with a very high pH are probably not mature
enough to use in growing media anyway.
Extent to which issue has been resolved
Growing media manufacturers using compost who responded to the questionnaire
commented that the pH of compost is related to stability/maturity and high pH readings are
associated with immature composts.
Compost pH is not a barrier to the use of composts at typical rates in a growing medium for
most species. Factors other than high pH (such as high compost salt concentrations) are
barriers to the use of green compost in media for ericaceous plants. Most growing media
manufacturers do not consider pH in itself as a barrier to the use of compost in growing
media products but half of those surveyed in this project said that the high pH of compost
limited the amount they would use in a mix.
De-mystifying the use of PAS100 compost in horticultural growing media
24
Recommendations for minimising the impact of the issue in future
If mature composts are used in growing media, the issue of pH will not be a factor for most
plant species at typical compost inclusion rates. The WRAP guidelines include information on
compost maturity and how to measure it (WRAP, 2011a).
3.2.4 High electrical conductivity
Introduction
High electrical conductivity (EC) in a growing medium can be a problem, especially for young
plants and salt sensitive species. High EC damages plants because when the concentration
of salts in the soil solution around the roots is higher than the concentration of salts within
the root cells the roots can no longer take up water effectively, causing stunting of growth
and eventually plant death. High EC can be particularly damaging to the roots of young
plants and salt sensitive species, for example ericaceous plants. Certain salts can also be
toxic to plants at high levels (known as the ‘specific ion effect’) and this will also damage
plants (Handreck & Black, 2010). Peat has a naturally low EC which means that fertilisers
can be added to raise the nutrient status with minimal risk of increasing the EC too much.
Green compost has naturally high concentrations of salts such as those of potassium and
chloride, which results in a high ‘background’ electrical conductivity (EC) compared with
peat. Composts using food waste as well as green waste as feedstocks tend to have a
higher EC than those just made from green waste. When composts are blended in a mix
with materials of low EC, such as peat or bark, this issue can be managed, but it is one of
the reasons why optimum rates of inclusion of compost in growing media are generally
below 50%. The effect of high salt levels from higher inclusion rates of compost have been
demonstrated in many studies (e.g. Maher et al., 2001). Variations in EC between batches of
compost are a particular problem for growing media manufacturers who need to amend
fertiliser additions based on the contribution of nutrients and salts from the compost used.
As with pH, growing media containing green compost have a higher chemical buffering
capacity, i.e. the ability to hold and subsequently release nutrients from exchange sites on
particle surfaces. EC levels that would be considered potentially damaging in peat-based
media are therefore less likely to be damaging in a mix containing compost. For example, an
EC of 600 µS/cm (1:5 extract) would be of concern in a peat mix but could be acceptable in
a peat/compost blend.
Previous and current relevant work
Maher et al. (2001) researched the use of compost in growing media and found that the rise
in the EC of a growing medium from the addition of compost could largely be off-set by
reducing the potassium fertiliser addition to the peat it was blended with. Other work
(Mazuela et al., 2012) has shown that the EC of a growing medium containing compost in
grow bags can be reduced by saturating with a nutrient solution which has a lower EC than
the compost does, before using it to grow crops. The above mentioned studies were the only
ones relevant to this report, but neither study was conducted on PAS100 composts, let alone
composts produced according to the WRAP Guidelines for the specification of quality
compost for use in growing media (WRAP, 2011a). Their conclusions therefore have limited
relevance to this project.
Extent to which issue has been resolved
Nearly all the growing media manufacturers surveyed in this project thought that the
inherent high EC of compost was an issue that restricted their use of some composts and
also restricted the amounts that they could safely use in growing media products. They deal
with the problem of high EC in compost by avoiding composts with an EC greater than 1000
µS/cm and limiting the inclusion rate of compost in the mix. One compost and growing
De-mystifying the use of PAS100 compost in horticultural growing media
25
media producer said that avoiding too many grass clippings in the feedstock helped to
reduce the EC of the compost. It was noted by a further manufacturer that minimising the
amount of food waste in the feedstock helps to control high EC in the compost. Guidance to
this effect is given in WRAP 2011b. The WRAP Specification (2011a) recommends that
compost for use in growing media ideally has an EC of less than 600 µS/cm with an upper
limit of 1500 µS/cm (1:5 extract). In practice, not many composts have values of less than
600 µS/cm. A commonly quoted solution to compost with high EC is dilution with low EC
materials such as peat, coir or bark when formulating the growing medium.
Recommendations for minimising the impact of the issue in future
The impact of EC is minimised in practice by limiting the rate at which compost is used in
growing media and using low EC materials as diluents. The impact of EC can more easily be
reduced during manufacture of growing media if the EC is consistent between batches of
compost so that growing media manufacturers can make easy, small adjustments to fertiliser
additions. Green/food composts tend to have higher EC values than green composts and for
this reason, they may have to be included at lower rates.
The two WRAP guides (WRAP 2011a and b) provide clear information as to target values for
compost EC and on methods for achieving these values. It is important to ensure that
compost producers producing, or interested in producing composts for use in growing media
know about these publications and have read and understood them.
3.2.5 Weed seeds
Introduction
One issue of concern raised by the Sustainable Growing Media Task Force was the possibility
of weed seeds from the input materials remaining viable after composting. BSI PAS100 and
the WRAP Good Practice Guide (WRAP 2011b) states that weed seeds and propagules should
be absent and a weed test is part of the suite of analyses for PAS100. Generally the
composters’ response to the questionnaires was that the presence of viable weed seeds in
compost was not an issue. Growing media manufacturers who used green compost in their
mixes generally also said that weed seeds were not an issue. Manufacturers who don’t use
green compost however did feel that weed seeds were an issue. Of the professional growers
who replied, all felt that the presence of weed seeds was an issue which would prevent or
may limit their use of green compost. If it can be clearly demonstrated that weed seeds are
killed at or below temperatures achieved during the composting process, then this will go a
long way to reassuring prospective purchasers of compost that weed seeds have been
eliminated.
Previous and current relevant work
Researchers from many countries have studied the viability of weed seeds in relation to
temperature; for example in Japan, Nishida et al. (1999) studied the survival of ten upland
weed species at 55 and 60oC and found that nine of the species were killed by exposure to
55o C for 72 hours and at 60o C for 24 hours. The remaining weed species was killed after
exposure for 120 hours at 55oC and 30 hours at 60oC. Egley (1990) at the Weed Science
Laboratory at Stoneville, Mississippi, USA heated a range of eight weed seeds in soil in pans
using a range of temperatures between 40 and 70oC for up to 7 days in either dry (2%) or
moist (19%) soil. In moist soil there was enhanced kill of seeds with practically all seeds
non-viable after 3 days at 60oC. It took up to 7 days at 700C for complete kill of the seeds in
dry soil. Soil moisture content clearly influenced weed seed kill: under dry conditions seeds
can survive better. Green compost after maturation typically has a moisture content of
around 40% and it almost always has a higher moisture content (typically 50-65%) during
the active composting process, during which time the majority of pathogen and weed seed
kill will take place.
De-mystifying the use of PAS100 compost in horticultural growing media
26
In Canada there have been a number of studies which added weed seeds in nylon bags to
cattle manure composts in windrows at different depths and looked at seed viability after
different time intervals. Tompkins et al. (1998) placed seeds of 12 weed species in muslin
bags at either 0.3 or 1.0 metre depths in windrows of cattle manure being composted. After
2 weeks composting, most seeds were non-viable and after 4 weeks’ composting at 55-650C
the viability of all seeds was zero. Weed species used were common and included wild oat
and hemp-nettle. The authors recommended sheeting the composted manure with tarpaulins
to stop weed seeds from blowing on the weed-free composted manure. Larney and
Blackshaw (2003) studied five weed species, again using bags placed in beef cattle manure
compost in open air windrows. A temperature above 60OC was required to kill all weed
species, although some were non-viable above 390C. They concluded that factors other than
temperature may play a role in eliminating weed seeds, for example, chemicals present in
the compost.
In Florida, Ozores-Hampton et al. (1999) studied immature municipal separated solid waste
composts of varying ages from 3 days, 4 weeks, 8 weeks and a matured one year old
compost. Extracts were taken using distilled water and added to Petri dishes with seeds of
both weed species and maize. The greatest decrease in seed germination was noted with 8week old compost and the least with the 1 year old compost extract. Further work with 8
week-old compost using 14 weed species was undertaken and the petri dishes incubated in
the dark at 27OC for 8 days. Seeds of nine of the fourteen weed species studied were either
unable to germinate, or showed very low germination. The authors concluded that chemical
exudates from the immature compost, particularly volatile fatty acids such as acetic acid,
were responsible for lack of seed germination. The concentration of acetic acid reduced a
hundredfold between 8 weeks and 1 year. This suggests a further means by which
composting can reduce the viability of weed seeds present.
Finally, Grundy et al. (1998) at Wellesbourne, studied the seeds of eight weed species, which
were buried in moist compost in mesh bags at three different temperatures for 3, 21 and 84
days. This work was done specifically to address concerns of potential contamination by
weed seeds either wind-blown from nearby fields or coming in via the plant material in
feedstocks. Mature compost was placed in 80 litre bins controlled at 35, 45 or 55oC. Moist air
was blown in to stop the composts drying out and moisture content was kept around 40% (a
typical value found in composts during the maturation stage). Eight weed species, including
annual meadow grass, black nightshade, prickly sow-thistle, chickweed and common
speedwell, were mixed with compost and placed in the mesh bags in the composts in the
bins. Samples were removed at intervals as above and seeds placed in petri dishes, sealed
and incubated. Germination was compared with seeds which had not been placed in contact
with the compost. At 55oC, none of the eight weed species germinated or appeared to be
viable even after just 3 days. The authors concluded that as long as all parts of the windrow
reached 55oC for three days all weed seeds from the range studied would be destroyed,
assuming no influx of seeds after the windrow has started to cool. Some of the weed species
in the trial germinated at 35oC even after storage for 84 days. It was concluded that any
viable weed seeds found in compost are likely to have survived due to insufficiently high
temperatures in the windrows or may have blown in on the wind after the compost has
begun to cool.
Extent to which issue has been resolved
Results from many studies around the world have shown that weed seeds will not germinate
when exposed to temperatures of 55-60oC for as little as 3 days. Temperatures during the
sanitization phase of composting typically reach levels of at least 60oC. Turning the compost
will allow all parts from the hot core to the cooler outer extremities to mix and enable the
whole windrow to reach the desired temperature as required by the composter’s Standard
Operating Procedure. PAS100 guidelines recommend that a temperature of 65oC should be
De-mystifying the use of PAS100 compost in horticultural growing media
27
achieved for 7 (not necessarily consecutive) days throughout the compost in order to
eliminate pathogens and weed seeds. It is also recommended that compost moisture content
is maintained at or above 51-65% (m/m) during the sanitisation phase and 40-65% (m/m)
during the stabilisation phase to maximise kill of weed seeds, human and plant pathogens.
In addition, there is evidence that chemicals such as volatile fatty acids released by
immature composts can impact (along with high temperatures) to further reduce the viability
of weed seeds. Providing maturing composts are covered as they cool down or composting is
enclosed, weed seeds should not be a concern when using green compost to make growing
media. In-vessel composting and maturation in buildings would go a long way to eliminate
the likelihood of weed seed contamination. Likewise at delivery to the growing media
manufacturer, green compost, just like any other material should be stored under cover and
surrounding areas kept weed-free.
Data collected by ORG between 2004 and 2008 showed that in a survey of 130 samples in
excess of 90% were free of weeds, indicating that composting at the correct temperatures
will eliminate weed seeds (K Zennaro, personal communication). It is important to note that
some instances of contamination of compost by weed seeds are thought to result from weed
seeds blowing on to the compost after it has been sanitised. Compost producers must
consider all possible reasons for weed seed contamination of composts when they are
developing their standard operating procedures for PAS100.
The compost producers who were also manufacturers of growing media and who replied to
the questionnaire all said that weed seeds were not a problem in the production of growing
media that included green composts due to the temperatures achieved during the
composting process. Some of the manufacturers who would not use compost in their
growing media said that the possibility of viable weed seeds being present prevented them
from using green compost. However, they also said the same about most of the other
potential issues raised in the questionnaire. Professional growers were divided over the issue
of weed seeds, some seeing this as an issue which prevented them using green compost and
others saying that it was not an issue.
Recommendations for minimising the impact of the issue in future
Standard composting times and temperatures are sufficient to render weed seeds nonviable. Contamination of compost could still occur from wind-blown seeds as the compost
cools down during stabilisation and maturation. To minimise this possibility, composts should
either be matured under cover or at the least outdoor windrows should be sheeted for the
composts used to supply manufacturers of growing media. Windbreaks placed upwind of the
prevailing wind (or winds from directions likely to blow seeds in) may reduce the inflow of
seeds to the composting site. All compost sites should control weeds as far as is practicable
and in any event they should prevent weeds from producing seeds that could be blown onto
compost windrows.
The two WRAP guides (WRAP 2011a and b) provide clear information as to how to eliminate
weed seeds from composts. It is important to ensure that compost producers producing, or
interested in producing composts for use in growing media know about these publications
and have read and understood them.
A further option would be to require compost producers to actively demonstrate the phytosanitary robustness of their composting processes. This could be achieved by introducing
‘captive’ indicator organisms (such as weed seeds or selected plant pathogens) at the
beginning of the composting process, and retrieving them for testing at the end of the
process. The Animal Health and Veterinary Laboratories Agency (AHVLA) adopt a similar
De-mystifying the use of PAS100 compost in horticultural growing media
28
approach for demonstrating pathogen kill when composting processes seek approval to
handle animal by-products such as household food waste.
3.2.6 Sciarid flies and other pests
Introduction
Sciarid flies (Bradysia difformis), also known as fungus gnats, and shore flies (Scatella
tenuicosta) are widespread and important pests of protected ornamental and bedding plants,
herbs and nursery stock propagation (Chandler, 2007; Sumption and Lennartsson, undated).
The larvae of sciarid flies feed on organic matter in the growing medium (such as wild fungi
and decaying plant material) but they also feed on plant roots and inside stems, which can
cause plant death and reduces crop quality. The adult flies can also act as vectors of plant
pathogens such as Pythium and Phytophthora species. Shore flies feed on algae and can also
transmit plant diseases and are a particular problem on potted herbs, causing supermarket
rejection as well as nuisance to workers in the nursery.
Biological control of sciarid flies has been successfully used in the mushroom industry for
many years by using agents such as Hypoaspis species mites and the nematode Steinernema
feltiae. Neither of these biocontrol agents is effective against shore flies. The predatory rove
beetle Atheta coriaria has been found to be effective against the eggs and larvae of both
sciarid and shore flies but require breeding boxes to release them, and close control of feed
in the boxes. The three biocontrol agents are available to professional growers and the mites
and nematodes are also available to amateur gardeners by mail order.
Lindquist (1992), cited by Chandler (2007), found that sciarid flies occurred at their highest
numbers in growing media with high microbial activity such as green compost, and at lowest
numbers in older peats, with low microbial activity. Binns (1977) found that sciarid flies
prefer fungal material and also high levels of nitrogen (including ammonia) from organic
fertilisers. Organic slow release fertilisers such as hoof and horn meal are frequently added
to peat-free growing media, which is likely to further attract sciarid flies.
Previous and current relevant work
Replies to the questionnaire from several professional growers indicated that sciarid flies are
attracted to growing media containing green compost and this would prevent them from
using the material. Manufacturers of growing media either felt that this issue limited
inclusion or prevented them using green compost. One growing media manufacturer cites
sciarid flies being the main complaint (from amateur gardeners) about their range of peatfree products, and they provide specific watering advice (on the bags) to keep the surface
dry, and not overwater. Producers of compost generally felt that this was either not an issue
or it had been in the past but now was not.
At least two compost producers have successfully used Hypoaspis species mites at the end
of the composting process as a control measure before delivery to the growing media
manufacturer. Some commented also that it was important to keep the compost dry and
under protection and ensure that it wasn’t immature (since ammonia, which is present at
higher concentrations in immature composts, is a known attractant of sciarid flies). One
manufacturer who uses a lot of green compost in their mixes felt that part of the problem
was overwatering from above (by retail customers) which would encourage flies to come in
and lay eggs in the damp surface. Another manufacturer who uses considerable amounts of
green compost felt that research is required to develop techniques or products to make
green compost less attractive to sciarid flies.
De-mystifying the use of PAS100 compost in horticultural growing media
29
A search of the literature has come up with a small number of references which may be of
help regarding discouraging sciarid flies from growing media in general, including media
containing green compost.
Olson et al. (2002) in the USA studied the emergence of sciarid flies in sterilised and
unsterilized coir and peat at a range of moisture levels between 15 and 90% and also
amended the two growing media components to produce fine, medium and coarse mixes.
The work showed that sciarid flies did not emerge from either coir or peat unless an
attractant (yeast) was present. Sterilising the growing media did not make any difference
either. Moisture content did not affect emergence in coir across all rates of moisture content
and for peat the 15% and highest moisture contents produced fewest flies, the 52%
moisture content producing most. The authors suggested that manipulating the texture and
moisture content of growing media may be a means of slowing down the build-up of sciarid
flies. Plant growth was not affected by the treatments which minimised sciarid fly
emergence.
Cloyd et al. (2010) studied the repellence of commercial fabric softener dryer sheets
(Bounce® Proctor and Gamble) to sciarid flies under controlled conditions in compartments.
It had been observed that carrying the dryer sheets in a top pocket repelled mosquitoes and
was used by professional gardeners in the USA for that purpose. Five experiments were
undertaken, using compartments with and without sheets present; some experiments
included a commercial growing medium with and without sheets present. Overall, sciarid flies
were reduced by two thirds across all treatments which showed the repellency effect.
Analysis of the sheets showed the presence of Linalool (3,7-dimethyl-1,6 octadien-3-ol) a
monoterpene alcohol used in the perfume industry. This chemical is found naturally in plants
such as lavender, marjoram and basil and is known to be toxic to some mites and insect
pests. The second chemical found was citronellol, (3,7-dimethyloct-6-en-1-ol) another
monoterpene alcohol which produces a lemon scent and is known to be repellent to
mosquitoes. This chemical is found naturally in a number of plants. The authors suggested
that sheets of the fabric softener could be placed in containers near plants in glasshouses in
order to deter sciarid adults and thus stop egg laying and reducing larval populations.
The above two potential control measures could be studied in the future, looking at growing
media texture and moisture, and looking at certain chemicals of plant origin which may be
help to deter sciarid flies. Due to the rapid breeding cycles of sciarid and shore flies, total
elimination of these pests is unlikely to be feasible.
Extent to which issue has been resolved
Two of the compost producers surveyed have successfully added Hypoaspis species mites to
green compost before despatch to growing media manufacturers, and they have reported
that this has been a successful control measure. However, absence of sciarid flies at
manufacture and delivery to the nursery or garden centre will only be sufficient for initial
potting of plants, and further applications of biocontrol agents would need to be made for
ongoing control, which is not a cheap option. Hygiene at the nursery and attention to
watering will also be required in order to keep this pest to a minimum.
One compost and growing media producer stated that sciarid flies were an issue in the past
but were now no longer an issue, and they were one of the composters who added
Hypoaspis species to some of their professional growing media mixes. One UK producer of
both composts and growing media felt that sciarid flies were at least an occasional problem
which may be due in part to using not fully matured compost. Another stated that sciarid
flies were not a problem provided that the compost was fully stabilised and that tighter
standards than PAS100 for sampling and checking maturity needed to be in place - see
WRAP (2011 b) for additional maturity requirements. Certain chemicals in green compost,
De-mystifying the use of PAS100 compost in horticultural growing media
30
e.g. ammonia, are known attractants of sciarid and other flies, so ensuring maturity before
use will undoubtedly help. Instructions to growers and advice on retail bags regarding
watering would also help by ensuring that the surface of the pot or seed tray is kept dry by
using smaller amounts of water more often and allowing the surface to dry out between
waterings, and watering from below if possible (reply by a compost manufacturer who also
produces growing media).
Recommendations for minimising the impact of the issue in future
Compost producers need to consider covering windrows, which would help to keep compost
dry and not exposed to sciarids. Maturation under cover in buildings would be a better
option. Shore flies live on algae, so keeping the composting site free from standing water is
important. Dosing matured compost with Hypoaspis mites as a biocontrol measure will
control sciarid fly larvae and eggs, but not shore flies.
Professional growers and also amateur gardeners could adopt the following measures to
minimise sciarid and shore fly breeding:
 Keep areas under benches and gutters free from algae and puddles;
 Don’t water plants from the top; watering pots and trays from below will keep the
growing medium surface dry;
 A layer of sand or fine vermiculite on top of the growing medium will help to discourage
egg laying by sciarid flies (Binns 1977); and
 Applications of biocontrol agents Hypoaspis species and Steinernema feltiae will destroy
larvae. These products are also available for retail purchase but price may be a limiting
factor. Atheta (rove beetle) is also effective, but due to requirements to inspect breeding
boxes and replace pelleted feeds this would currently be suitable only for professional
growers.
The following future work should be considered:
 Investigate the interaction of growing media, particle size and watering regime to control
flies.
 Determine the effect of attractants e.g. yeasts, organic fertilisers impregnated on sticky
traps to lure flies.
 Further study of natural plant products such as Linalool and Citronellol to repel sciarid flies
from around plants and suitable delivery systems including impregnated tissues.
3.2.7 Compost shelf life
Introduction
The shelf life and storage characteristics of growing media containing green compost are
very important because products, particularly retail growing media products, may be stored
for up to 12 months between manufacture and use. Changes to the physical, chemical or
biological properties of the products during storage can have negative effects on plant
growth (Surrage & Carlile, 2009). Retail products may also be rejected by consumers if they
appear to be contaminated with fungal growth (even if the fungus is actually harmless to
plants). The basic requirement for good shelf life is for the product to be chemically
stable/mature. If active composting occurs after a compost has been incorporated in a
growing medium, it will have a negative effect on plant growth (Raviv, 2011a). Biological
processes occurring in a growing medium after manufacture can cause various problems
(Verhagen, 2007) including:
 loss of volume and particle size change (reduction in air-filled porosity, which can lead to
a lack of air around plant roots);
 nitrogen immobilisation (which will results in less nitrogen for crops);
De-mystifying the use of PAS100 compost in horticultural growing media
31
 formation of phytotoxic compounds;
 formation of water repellent colloids;
 increase in pH (leading to decreased plant availability of most trace elements and
phosphate);
 change in Cation Exchange Capacity (CEC, which may mean that nutrients are held less
strongly within the medium and are more easily leached);
 increase in salinity due to mineralisation (which can adversely affect plant roots); and
 reduced shelf-life due to the above phenomena.
There is some contradiction between different requirements because composts with high
biological activity are undesirable for use in growing media, due to competition with roots for
oxygen and possible nitrogen immobilisation, yet compost disease suppressiveness is a
biological phenomenon and requires microbial activity (Boulter-Bitzer et al., 2006).
Most growing media manufacturers currently using compost in their products do not regard
shelf life as a barrier to the use of compost in growing media (some have never considered it
a problem), although it was cited as an issue by the manufacturers who are not using it.
Previous and current relevant work
Early research by WRAP (WRAP, 2005) indicated some problems with storage of non-peat
growing media but these have largely been overcome.
In the above project, the storage stability of retail growing media based on green compost
was evaluated in a 12 month storage trial. The trial simulated commercial conditions as
closely as possible: treatment mixes were prepared and bagged using modern production
facilities, then palletized prior to storage outdoors. The project compared peat-free, peatreduced and all-peat mixes, included two peat types (Irish and Finnish), two alternative
substrates (matured forest brash and matured bark fines) and two rates of mature PAS100
certified green compost (20% and 33% v/v). Representative, replicated samples were taken
at bagging (time 0) and after 1, 3, 6 and 12 months of storage. Samples were tested for
ammonium nitrogen, nitrate nitrogen and other water-extractable nutrients, and were
subjected to two stability tests: nitrogen drawdown index (NDI) and the Dewar (self-heating)
test.
At sampling, no bad odours, flies or weeds were encountered in any treatment and obvious
fungal growth, although occasionally observed in all treatments, was insignificant. All mixes
started with a similar amount of nitrogen (N) fertiliser. The key change observed was
immobilisation of water-soluble N (also known as ‘lock-up’ or ‘drawdown’), as evidenced by
reductions in levels of ammonium N and nitrate N, which varied markedly with treatments.
Losses of available N were highest in green compost-based mixes, especially in the peat-free
formulations, where both bark fines and brash were present, and almost as high in the two
Finnish peat plus green compost mixes. By contrast, even at 12 months, N losses in the
100% Irish peat mix were virtually nil. Where Irish peat was mixed with 20% v/v green
compost, losses were small but, where mixed with 33% v/v green compost, they were
marked. After 12 months, only three of the eleven mixes evaluated were considered fit-forpurpose, namely, 100% Irish peat, Irish peat plus 20% green compost and 100% bark fines.
The project recommended the evaluation of the use of slow-release N sources in growing
media containing green compost and the inclusion of such sources is now common practice.
In a separate trial, the storage properties of peat-free growing media containing green
compost were studied in the UK by storing media with increasing percentages of green
compost for 12 months under different conditions (Surrage & Carlile, 2007). These trials
showed that the bulk density, organic matter content, pH, electrical conductivity,
De-mystifying the use of PAS100 compost in horticultural growing media
32
ammonium-N, magnesium, calcium, zinc and manganese concentrations remained fairly
constant over 6 months, with some variation in nitrate-N levels and an increase in available
phosphorus concentration (presumably due to release of available phosphorus from the
mineral component of the green compost over time). A dehydrogenase activity assay
indicated some activity in the mixes containing green compost in the initial month but then a
decline in microbial activity during the 6 month trial.
Work in Ireland (NiChulain & Prasad, 2007) looked at different methods of evaluating the
stability of green compost intended for use in growing media. The three methods tested
were the Dewar self-heating test, the ‘Solvita’ TM test (which is based on the evolution of
CO2 and free ammonia) and a pressure sensor method (OUR/Oxitop) which measures the
uptake of oxygen and hence the biological activity of the material. The Dewar self-heating
test only really distinguishes between very mature and very immature compost so was not
considered precise enough. The research recommended the use of the oxygen uptake rate
(OUR) method for determining the stability of compost for use in growing media.
The standards body in The Netherlands (RHP) has also researched stability (and hence shelf
life) of growing media and the OUR method has been adopted. The standard for respiration
of green compost for use in growing media in The Netherlands is <15 mmol of oxygen per
kg of organic matter per hour.
WRAP published guidelines for compost to be used in growing media (WRAP, 2011a), which
include guidance on compost maturity and stability. A C:N ration of 15 (not greater than 20)
is recommended and a Nitrogen Drawdown Index of 1 (not >0.7 ideally). In addition to this,
some growing media manufacturers use the ‘Solvita’ test for maturity and specify a score of
7 for compost to be used in growing media products, which is the score typically achieved by
mature compost.
Extent to which issue has been resolved
Storage characteristics and shelf life do not appear to be a major barrier to the use of green
compost in growing media at current inclusion rates (typically 10-30%). Respirometric
methods have been shown to be good for assessment of biological activity and a CEN
(European Committee for Standardisation) method using CO2 evolution/O2 uptake is
proposed as a European standard.
The storage conditions under which composts and growing media are kept (both composts
prior to incorporation into growing media and post-manufacturing) are important, however.
Shrink wrapping pallets is not recommended since this can restrict the amount of oxygen
which can get into the compost or media and storage conditions must not be too cold (not
below freezing) or too hot (e.g. storage within glasshouses is not ideal [anonymous industry
expert, personal communication]). Screened compost must be turned prior to bagging to
prevent it becoming anaerobic and the finer the grade of compost the higher the risk of
anaerobic conditions occurring.
One of the growing media manufacturing respondents to the questionnaire did not consider
shelf-life of compost a problem as long as the storage conditions of the end product were
right. They also commented that incorporation of an appropriate slow release nitrogen
fertiliser avoids the problem of depletion of nitrogen reserves in the product during storage.
Recommendations for minimising the impact of the issue in future
Shelf life is not a major issue if strict quality control measures are followed in relation to
compost stability, so that only mature composts are used and they are consistent from batch
to batch. Details on how to achieve good compost shelf life are provided in WRAP, 2011a
and b. It is important to ensure that compost producers producing, or interested in
De-mystifying the use of PAS100 compost in horticultural growing media
33
producing composts for use in growing media know about these publications and have read
and understood them.
3.2.8 Compost consistency
Introduction
Several authors have reported that lack of consistency of composts is a key/major problem
in relation to their use in growing media (Carlile, 2008; Mazuela et al., 2012; Prasad &
Maher, 2001; Surrage, 2007; Surrage & Carlile, 2008). Some of this work was undertaken
prior to the widespread accreditation of composts to PAS100 (e.g. Prasad & Maher, 2001),
but most of it does relate to PAS100 composts. Composts have been reported to vary both
within UK sites (between batches) and between UK sites (Wallace, 1999, WRAP, 2005) and
consistency of composts does remain a barrier – whether perceived or not. Several of those
who responded to the request for their views in this project felt that lack of compost
consistency either prevented them from using composts in growing media at all, or
prevented them from using more of it than they did in manufactured growing media.
Compost characteristics can vary both within a particular composting facility (for example,
between batches across the course of a year) and between composting facilities. The former
is of concern to users requiring small quantities of compost from a single site, as they cannot
be sure that compost performance will be consistent. The latter is of concern to users
requiring larger quantities, who may source compost from more than one supplier.
There is an acknowledgement amongst some scientists and growing media manufacturers
that with careful choice of feedstocks and process control, compost producers can maximise
the quality and minimise the variability of their products in order to satisfy the requirements
of growing media producers (Surrage, 2007; Raviv, 2011a). Relevant work, the current
situation within the industry in relation to this topic and recommendations to minimise the
impact of the issue in future are discussed in the following sections.
Previous and current relevant work
Shortly after the BSI PAS100 specification for composts was first produced in 2002, the
DETR cited that compost variability was a key barrier to widespread use of composts in the
horticultural sector (DETR, 1998). For this reason, WRAP commissioned a study in 2003 to
examine the nature and extent of compost variability over a 12 month period (WRAP, 2005).
Finished compost and feedstock from nine composting sites were sampled and tested for a
range of quality parameters monthly for 12 months. Most of these composts were PAS100,
whereas others were working towards PAS100 accreditation.
Feedstocks were tested for dry matter content, chloride, total potassium (K), carbon (C),
nitrogen (N) and C:N ratio.
Finished composts were tested for a wide range of parameters including:
 Physico-chemical parameters:
 Bulk density
 Dry matter and moisture content
 pH
 Electrical Conductivity (EC)
 Chemical parameters
 Water extractable
 Chloride
 Nitrate-N (NO3-N), Ammonium-N (NH4-N) and the ratio between them, total N, total
C and C:N ratio
De-mystifying the use of PAS100 compost in horticultural growing media
34
 Phosphorus (P), K, Magnesium (Mg), Sodium (Na), Boron (B), Manganese (Mn)
 CAT extractable
 P, K, Mg, Na, B, Mn
 Total N, C, P, K
 % Loss on ignition
 Volatile solids content
 Physical contaminants
 Plastic
 Glass
 Metals
 Stones
 Microbial parameters
 Carbon dioxide (CO2) evolution.
The total weight of stones (> 2 mm) was reported for the months March to July inclusive;
from August onwards, stones from 2-4 mm and stones > 4 mm were reported separately.
Physical contaminants were only reported for the first four months.
The project showed that there were some pronounced seasonal variations in the
characteristics of the feedstocks tested. Nitrogen and K concentrations were higher during
spring/early summer, when greater quantities of green, leafy materials were accepted for
composting. Carbon concentrations were higher during autumn/winter when a greater
proportion of leafless, woody materials were accepted for composting. There were no great
differences between different green waste feedstocks (i.e. kerbside v civic amenity site).
Seasonal variations in feedstocks did not necessarily translate into variations in compost
quality. Total C and N concentrations were consistent within sites over the course of the
year, despite there being pronounced seasonal variability in these parameters in feedstocks.
Potassium levels did show a degree of variation, but local climatic factors may have played a
role in this.
Many compost parameters showed no significant geographic variation across sites or
seasonal variation within sites. Site specific factors, such as screen size, maturation period
and storage arrangements were thought to have had the greatest impact on compost
variability. Having looked at all of the processes from which samples were taken during the
project, the authors of the study felt that compost producers who maintained a consistent
composting process regime produced more consistent composts.
Process management was thought to play a critical role in reducing product variability. The
following practices were reported as likely to help maintain product consistency:
 increasing the length of time that composting is actively managed;
 increasing maturation period;
 storing finished compost under cover;
 using microbial respiration assay to ensure composting is complete; and
 ensuring composting process parameters (turning regimes, active composting periods
etc.) are consistent over the whole year.
In general, the composts tested in the WRAP project were found to have performed well
against the growing media specifications of the time (WRAP, 2004a), particularly those
composts aimed at the growing media market. All composts met the guideline value for pH,
and most of the composts were within or close to the guideline values for EC. Extending the
maturation period was thought likely to result in composts with lower EC. All of the composts
De-mystifying the use of PAS100 compost in horticultural growing media
35
met the guideline values for contaminants and most also met with guideline values for
chloride, sodium, stone content, moisture content and bulk density.
There was no evidence of significant geographical differences between composts made from
green wastes. There were slight differences in the levels of a few elements (including Cl and
Mg), but this variability was no more than that found in different batches of composts made
at the same site. The composts tested were largely from composters supplying other, less
stringent markets such as agriculture. Variation in composts working to the WRAP spec
guides is unknown but may be less.
More recently, Surrage and Carlile have conducted work, based at Nottingham Trent
University, to investigate the variation in quality of composted green wastes and to suggest
ways by which the consistency and quality of composts can be improved. The work is
reported in greatest detail in Surrage’s PhD thesis (Surrage, 2007).
This comprehensive piece of work included studies on the nature of green waste
treatment/recycling in the UK, the storage properties of peat-reduced media, and the
performance of peat-free growing media as well as work on the variability of composts. It is
important to place the work in context: it was conducted during the years 2004 to 2007 (as
was the WRAP project discussed above), during a period when the UK composting sector
was still in a phase of fairly rapid development. The study tested 15 samples of green
compost, but there was no indication of how many of these were PAS100-accredited or
working towards accreditation, and limited information was provided on the feedstock
collection method, the exact list of feedstocks from which each was made, or the composting
system by which each was produced (e.g. process duration, sanitisation criteria etc.).
The fifteen samples came from twelve different compost producers (including both local
authority and commercial producers) located throughout England. Samples were tested for:
bulk density, organic matter content, ash content, moisture content, pH, electrical
conductivity and total N, P and K content. Considerable variation was found between
samples for every parameter tested. This variation was thought to be due to wide variations
in feedstock types, feedstock collection methods and to differences in composting processes.
Surrage pointed out that the quality of one of the products tested was particularly high and
that this quality was likely to be due to the care taken to select appropriate feedstocks and
to design and manage an appropriate composting process. She suggested a range of
measures which could be taken in order to improve the consistency of all composts intended
for the high value growing media market, where both quality (in terms of both safety and
useful horticultural characteristics) and consistency is vital. The measures suggested
included:
 development of best practice guidance in composting (e.g. the Composting Association
Code of Practice, 2005) and dissemination of best practice guidance amongst
practitioners. This guidance should include information on:
 source segregated green waste collections (essential in PAS100 composting
processes);
 blending of feedstocks in order to obtain an appropriate C:N ratio for optimal
composting; and
 longer composting processes for composts intended for use in growing media;
 standardisation of best practice throughout the composting industry including:
 adherence to the requirements of appropriate best practice guidance;
 adherence to the requirements of and accreditation to PAS100:2005 (which has since
then been superseded by PAS100:2011).
De-mystifying the use of PAS100 compost in horticultural growing media
36
In his work at Horticulture Research International (HRI) at Warwick on the RECOVEG project
(Recycling horticultural wastes to produce pathogen suppressant composts for sustainable
vegetable crop production), Noble showed that the results of scientific work to determine
composting best practice could be applied to industrial scale processes in order to produce
commercial quantities of high quality composts with predictable characteristics (Noble,
2005).
In his recent review, Raviv (2011a) felt that the variability of both nutrient content in
composts and nutrient availability from composts merited further scientific study, since
better understanding of the subject would help growers to optimise plant nutrition and
minimise nutrient leaching. He also recommended the formulation of rigid quality control
measures for composts intended for use in growing media.
There are a number of controls in place to ensure that compost ‘quality’ and safety are high
in the UK. Similarly, there are several controls in place to make sure that composts are
applied in order to provide benefits to agriculture, soil-grown horticulture and landscaping
without harming humans, wild or domestic animals, crops, amenity vegetation or the
environment. These controls, which take the form of legislation and guidance, are
summarised in Section 3.3. In fact, most of the controls documented in that legislation and
guidance relate to safety rather than quality (as defined by compost users). Compost
producers must therefore find ways of maximising all aspects of product quality and must
document the results from testing appropriate parameters additional to those required by
PAS100 in order to satisfy customer requirements. For example, parameters such as total
nutrient content and nutrient availability are of interest to growing media manufacturers but
are not required under the PAS100 specification.
ORG (under their old name, AfOR), has worked hard to improve the quality and consistency
of composts during the past decade. In 2002, the first version of BSI PAS100 was produced
(see Section 3.3). The scheme provided a baseline quality standard for compost and ensured
that compost producers certified through the scheme manufactured a product that was
consistent, safe and reliable to use. It also provided a foundation upon which producers
could further develop their compost products. It was revised and republished in 2005 and
again in 2011, when the upper limits for physical contaminants were reduced in response to
requests from some industry stakeholders, and the increased ability of compost producers to
meet these lower limits.
The quantity of feedstock recycled through composting and accredited through the UK
compost certification scheme is steadily increasing and it is estimated that in 2010 48% of
feedstock which was treated through composting was treated by sites compliant with or
having applied to join BSI PAS100 (WRAP, 2010). The percentage of feedstock treated in
Scotland at sites compliant with or having applied to join BSI PAS100 is thought to be much
higher, due to work by Zero Waste Scotland to reduce volumes of organic waste to landfill
and encourage PAS100 certification of Scottish composting sites.
In Scotland, compost produced to BSI PAS100:2011 is considered a product rather than a
waste and is therefore not subject to waste management licensing regulations. In England,
Wales and Northern Ireland, compliance with the Compost Quality Protocol (CQP) is also
required if compost is to be classed as a product (rather than a waste). The CQP builds on
BSI PAS100:2011 and clarifies which waste materials can be used in quality compost
production, reinforcing traceability throughout the production process by ensuring accurate
record keeping. Together, the BSI PAS100 standard, the CQP and the requirements of the
Animal By-Products Regulations (all described in more detail in Section 3.3) have helped to
improve considerably the safety and quality of composts since their initial publication
(starting in 2002) (K Zennaro [ORG], personal communication). Although there have been no
De-mystifying the use of PAS100 compost in horticultural growing media
37
further published studies on the consistency of UK compost products in terms of their
variability within and between sites, there is clear evidence that increasing numbers of
producers are achieving the revised (stricter) standards for physical contaminants set out in
PAS100:2011.
In 2005, The Composting Industry Code of Practice was published (The Composting
Association, 2005). It dovetailed with the requirements of PAS100 and amongst other things,
it aimed to:
 identify good site management practices that could act as a benchmark for industry and
regulatory authorities; and
 provide greater confidence in the composting process and end product (presumably since
it aimed to help compost producers develop effective, consistent composting processes).
Whilst it was recognised during the first few years of the 21st century that the BSI PAS100
specification, the CQP (in England, Northern Ireland and Wales) and the Animal By-Products
Regulations were all helping compost producers to focus on the development of composting
processes which could result in the production of safe, quality, general purpose products,
there was still a lack of guidance for compost producers as to how best to produce specialist
composts for high value markets such as growing media. The WRAP specification and
guidelines are therefore of particular value in this respect (WRAP 2011a and b).
Extent to which issue has been resolved
The stakeholders interviewed as part of this project were divided on whether they felt that
consistency of PAS100 composts was an issue for them. In brief, of the ten responses
obtained from compost producers and growing media manufacturers (some of whom were
also compost producers), one said that compost consistency had never been an issue for
them, three said that it had been an issue but was not any more, two said that it limited
their use of PAS100 composts for growing media production and three said that it prevented
them from using composts in growing media (one did not respond to the question).
Of the eight growers interviewed, six said that lack of compost consistency was a key reason
why they did not want to use growing media based partly on composts in their production
system. The remaining two growers interviewed did not respond to the question. Of the two
major retailers interviewed, one said that their company did not sell growing media
containing green compost, because they believed that all nine of the issues addressed in this
project (including consistency of composts) remained a problem. The other simply did not
have confidence that CIGM could perform as well as other types of amateur growing media.
The second felt that compost consistency was one of the issues which continued to limit
their sales of (CIGM).
One compost producer and growing media manufacturer who felt that his company had
largely solved the issue of compost consistency had strong opinions on the subject. He felt
that it was perfectly possible to make good, consistent compost products for use in CIGM
providing the entire process from feedstock selection and preparation to product preparation
was managed with care and attention to detail. He felt strongly that some compost
producers could improve the consistency of their products through more careful
management of their processes and that the perception of poor consistency of both
composts and CIGM related more to earlier bad experiences (their own or those of
colleagues) than to experiences of the best recently produced composts/CIGM.
It is clear that although some stakeholders felt that the issue of poor compost consistency
has been solved, many do not, and there is a widespread perception amongst both growing
media manufacturers and growers that compost consistency (and therefore the consistency
of CIGM) remains an issue. More work therefore needs to be done to address the issue.
De-mystifying the use of PAS100 compost in horticultural growing media
38
Recommendations for minimising the impact of the issue in future
Between them, four major growing media manufacturers said that the following were
particularly important when trying to improve consistency of composts for use in CIGM:
 careful selection of input materials; for example, tightening of control over the C:N ratio
in feedstocks going forward for composting (by mixing of green and woody substrates to
get an ideal blend with appropriate moisture content);
 Very careful management of the composting process in line with best composting
practice, to ensure good aeration, appropriate moisture content and perhaps more
frequent turning;
 Effective screening, using good quality machinery at the end of composting to remove
large woody fragments;
 Investment in frequent testing (both during AND after the composting process) and
blending facilities to allow mixing of different batches of compost in order to improve
characteristics of batches and consistency of batches throughout the year; and
 Investment in a longer composting process than that required for composts intended for
use as soil conditioners, with covered composting areas for sanitisation, stabilisation,
maturation (and storage prior to sale if required), better management, greater scale of
production.
If compost producers were to implement all of the points listed above in order to maximise
compost consistency, they will clearly spend more money than they would operating a basic
process intended to make good quality, safe composts for use as soil conditioners. However,
some compost producers and growing media manufacturers have said that it is possible to
balance this extra cost through extra revenue gained from sales.
Although there is some evidence that the above points will help compost producers make
better, more consistent products, and there is good guidance available to help them achieve
best practice (WRAP, 2011a; 2011b), there is still a serious problem in terms of the
perceptions of both growing media manufacturers and growers that composts and CIGM are
not consistent.
For that reason, several approaches are recommended. Firstly, the WRAP good practice
guidance (2011a; 2011b) should be promoted more widely. Rather than only being available
online, it should be printed and sent to all compost producers who are known to sell into
high value markets (e.g. turf production, turf management and growing media) with a
covering letter. It may also be a good idea to run two dedicated seminars (in both the North
and South of the UK) for compost producers interested in selling to the growing media
market. The WRAP guidance could be promoted, and the difference in mindset required for
top quality compost production explained in detail. It is worth noting that only half of the
growers who responded to the survey were aware of the WRAP guidance. Whilst most
growers now buy in their growing media and rely to a large extent on their growing media
supplier to recommend the best products for them, several were interested to learn about
the guidance and were interested to hear of the significant improvements currently being
made in the quality of both green compost and CIGM.
For the above reason, there is also a need for further work to demonstrate the performance
of CIGM on nurseries. Although there has been a considerable amount of (mainly WRAPfunded) work (e.g. WRAP, 2005c; 2006a; 2006b; 2007; 2008; 2009) done on nurseries in
the past, some of it has been conducted over relatively short time-scales with limited
technical input from those (trained in experimental work using growing media on nurseries)
who really understand growing media, CIGM in particular and how to use CIGM to best
effect. There is a clear need for simple trials to be conducted on the nurseries of growers
De-mystifying the use of PAS100 compost in horticultural growing media
39
who are genuinely keen to try CIGM again, or for the first time, using the very best CIGM
products and the help of trained horticultural scientists/growing media professionals who
understand both the science and practicalities of using growing media on nurseries. These
individuals should work with the growers and the growing media manufacturers during the
trials to identify and solve challenges relating to the use of the media (e.g. watering issues,
sciarid fly problems, and nutritional difficulties) as they occur.
The two WRAP guides (WRAP 2011a and b) provide clear information as to target values for
compost parameters and on methods for achieving these values. It is important to ensure
that compost producers producing, or interested in producing composts for use in growing
media know about these publications and have read and understood them.
3.2.9 Microbiological safety
Introduction
Under the requirements of PAS100, producers of both green and green/food composts are
obliged to run processes that achieve defined thresholds for indicator organisms. Producers
of green/food composts (whether accredited to PAS100 or not) are also legally obliged to
achieve limits for indicator organisms (under the terms of the Animal By-Products
Regulations [ABPR], see Section 3.3.3). This means that the absence of Salmonella must be
demonstrated, whilst E. coli (or enterobacteriaceae in the case of ABPR-approved facilities)
are permitted below a limit of 1000 colony-forming units per gramme of material tested –
which is line with other European approaches to compost quality.
The conditions present within PAS100 (and ABPR-compliant) composting processes are
sufficient to minimise risks from human pathogens – not only as a result of thermal
inactivation but also due to the chemical and microbiological environment during composting
(Gale, 2002, WRAP, 2003, 2004b).
There have been recent concerns with a specific group of human pathogens, namely
Legionella species, and L. longbeachae in particular. This review summarises earlier work to
determine recommended sanitisation limits for PAS100 composting processes and more
recent developments in relation to the issue of Legionella species in growing media.
Previous and current relevant work
Much previous work has looked at the effect of (non-PAS100) composting processes on
human pathogen numbers, including coliforms and indicator pathogens, in materials other
than green waste (for example, sewage sludge or animal manures, which tend to contain
much greater numbers of human pathogens prior to composting [WRAP, unpublished]).
Physical and chemical conditions in heaps or windrows of composting materials are generally
unsuitable for the growth of enteric (gastro-intestinal) bacteria, although growth has been
reported in some wastes (Wichuk & McCartney, 2007) including green wastes (Brown et al.,
2000; WRAP, 2003). Where such growth occurs, it tends to occur prior to the hottest
(thermophilic) phase of the composting process (Brown et al., 2000). In general, sustained
temperatures of more than 55oC will kill off most enteric bacteria (WRAP, 2003; Wichuk &
McCartney, 2007) and it is likely that this temperature regime should kill off many other
pathogens too (Gale, 2002, WRAP, 2003).
Studies looking into the presence of microorganisms in growing media were undertaken at
Nottingham Trent University from 1995 until (at least) 2008 (Carlile & Hammonds, 2008).
Work showed that peat-based media tended to contain fewer coliform bacteria than peatfree media but pathogen numbers were low in both cases and would present a very low risk
to human health, especially when compared with coliform numbers considered to present a
risk when found in food (Forsythe, 2005). Following extensive literature searching, the
De-mystifying the use of PAS100 compost in horticultural growing media
40
authors of this review could find no reports of people suffering ill effects due to the presence
of enteric bacteria in PAS100 composts or in CIGM. Bearing in mind relevant experimental
work and the conclusions of the authors considered in this report, the probability of human
illness caused by Salmonella species, E. coli. or other enteric bacteria present in composts or
CIGM is thought to be very low.
Whilst the problem with salmonella and E. coli in CIGM may be perceived (by a limited
number of stakeholders) rather than real, the issue with L. longbeachae in composts and
CIGM is a real one. However, given the underlying presence of the organism in the wider
environment, there are questions about how common the bacterium is in growing media and
how serious an issue its presence actually is.
Legionnaires’ Disease was originally named after the outbreak in Philadelphia 1976 when a
type of pneumonia affected a large number of members of the American Legion, a military
veterans association, which held a gathering at a hotel there (De Jong, 2010). Legionella
species – the bacterium causing the disease – was identified several months after this
outbreak for the first time. Now we know that there are around 50 different species of
Legionella and that many are not pathogenic to humans. The great majority of reported
cases are a result of infection by L. pneumophila through inhalation of aerosols (water
droplets) containing the bacteria. However, some cases are a result of infection by other
Legionella species, for example, L. longbeachae, which is well recognised in Australia and
New Zealand, and where cases have been epidemiologically associated with the use of
growing media (termed ‘potting compost’, Pravinkumar et al., 2010). In a soil survey
performed in 1989 to 1990 in Australia, 33 (73%) of 45 ‘potting soil’ (growing media)
samples tested positive for Legionella species; 26 (79%) of the 33 contained L. longbeachae
(Steele et al., 1990b).
For a long time, contamination of growing media by Legionella species bacteria was
considered largely to be limited to Australia. For example, Steele et al. (1990a) found that
more than two thirds (33 out of 45) of Australian samples and no samples (out of 19 tested)
of European growing media tested positive for Legionella species. However, during the past
13 years, associations between Legionnaires’ Disease cases and gardening or use of potting
mixes have been reported from a range of countries other than Australia, including New
Zealand (Whiley & Bentham, 2011), Japan (Koide et al., 2001), USA (Anon., 2002, cited in
Casati et al. (2009a), The Netherlands (den Boer et al., 2007) and more recently the UK
(Lindsay et al., 2012).
L. longbeachae has only recently been detected as a cause of respiratory illness in the UK.
Pravinkumar et al. (2010) and Lindsay et al. (2012) suggested a possible association
between handling ‘potting soil’ (growing media) and infection with L. longbeachae in three
Scottish cases. Two of these three cases had underlying health problems which were
thought to predispose them to infection by L. longbeachae. A more recent Health Protection
Scotland report (Health Protection Scotland, 2013) provides a useful overview of the
underlying incidences of Legionnaires’ Disease in Scotland, and examines the potential
relationship between nine cases of L. longbeachae and gardening activities. It reaches the
following conclusions over risk:
However, the incidence of L. longbeachae infection in Scotland is very low: less than one
confirmed case per million total population per year since 2008 with only one death in a
confirmed case in the same period. There is no evidence from Scotland of horticultural
workers, who have continuous workplace exposure to compost and growing media, suffering
from legionellosis caused by L. longbeachae. Most cases are aged over 55 years of age and
most have underlying, chronic diseases. In population terms, the burden of disease (i.e.
years of expected life lost, years of life with added disability and years of poor quality of life)
De-mystifying the use of PAS100 compost in horticultural growing media
41
resulting from the infection is comparatively small. Given the volume of growing media
products and compost sold and the number of gardeners in Scotland, the risk of exposure to
this organism resulting in diagnosed, severe disease appears to be very low.
A major difficulty with much of the published research on L. longbeachae is that it rarely
states whether or not the organism has been related specifically to one or more ingredients
within the tested growing media. In addition, the terms ‘potting soil’, ‘potting mix’, compost
and growing media are often used interchangeably, making interpretation tricky. Similar
difficulties are encountered in another report from 2013 (Currie et al., 2013), although this
clearly states that no Legionella species were isolated from the one sample of PAS100 green
compost tested. Further research in this area would be helpful, particularly as there are
precedents for (non PAS100) green compost containing Legionella species elsewhere:
Hughes & Steele (1994) isolated several Legionella species (including L. longbeachae and L.
pneumophila) from six commercial composting facilities (33 samples) and private garden
compost heaps (80 samples) in Australia. Casati et al., (2010) isolated six Legionella species
(not including L. longbeachae.) from samples of green compost taken during and after
composting from compost sites in Switzerland.
The question of L. longbeachae is certainly not confined to green composts and CIGM
though. It has been detected in other types of growing media including those based on coir,
wood fibre, composted bark and more rarely peat (Casati et al., 2009; Whiley & Bentham,
2011).
While growing media (including CIGM) are now thought to be a potential source of
L. longbeachae bacteria, the exact mechanisms whereby infection takes place are not as yet
clear; it may be as a result of aerosols being created during gardening or watering of the
growing medium, perhaps most likely in an enclosed space. Other modes, such as ingestion
via contaminated hands, have also been postulated (Steele et al., 1990b).
It is very important to place in context the risk of human infection from Legionella species
bacteria present in CIGM. Evidence shows that it is not specific to CIGM and, in fact, most
commonly used types of growing media could potentially contain Legionella species bacteria.
Extent to which issue has been resolved
The representative of the company interviewed which produced composts for the growing
media sector (but did not themselves manufacture growing media) had no problems in
achieving the PAS100 limits for indicator pathogens. This company had not been given
other, stricter criteria (by their customers) with which to comply and they did not feel that
pathogens of relevance to human health were an issue for them, since their customers were
happy with their products. The six companies producing both composts and CIGM were
divided. Three complied with the PAS100 standards by following the recommended
sanitisation criteria in PAS100:2011 (and no additional higher standards), had no problems in
doing so and felt that pathogens of relevance to human health were not an issue for them. A
fourth had had problems in the past with elevated levels of pathogens of relevance to human
health in their composts, but no longer did. That company complied only with the PAS100
standard (i.e. no additional, higher standard) and also felt that pathogens of relevance to
human health were no longer an issue for them. The fifth company which complied only with
the Dutch RHP standard (the Dutch compost quality accreditation scheme, which has similar
limits to those in PAS100:2011) had no problems in achieving the standard, but felt that the
issue of pathogens of relevance to human health in composts was one reason why they did
not sell more CIGM than they did. The sixth company worked with the PAS100 limits only,
but said that they felt that the issue of pathogens of relevance to human health was one
reason why they did not sell CIGM into the professional sector at all and sold less CIGM into
the retail sector than they might do.
De-mystifying the use of PAS100 compost in horticultural growing media
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In summary, there was no evidence that any of the companies interviewed as part of this
project had trouble reaching the standards for numbers of indicator pathogens in their
national compost quality certification scheme (PAS100:2011 for all of the UK companies).
Nor was there any evidence that any of the companies were requesting (or aiming for)
compliance with a higher standard for human pathogen content.
None of the companies mentioned testing for Legionella species either in compost or in
CIGM and none said that they had had any problems with incidence of Legionella species in
their products. However, despite the above, several growing media manufacturers
acknowledged that potential presence of pathogens of relevance to human health was one
reason why they did not sell more CIGM. Of the growers interviewed, almost all were
concerned about the potential presence of these organisms in CIGM and several cited it as
one of the reasons why they did not use it.
In summary, the issue of pathogens of relevance to human health in CIGM has been
resolved as far as some companies are concerned and they do not cite these organisms as a
problem for them. Others feel that the issue has not been resolved, although they did not
state whether they were concerned about specific pathogens or pathogens in general. Some
questions remain around Legionella, although the growing media sector has recently agreed
to standard handling advice on bagged product, following the recommendations of Health
Protection Scotland (2013):
Health and Safety Advice:
 Use in a well-ventilated place and avoid breathing in dust
 Always wear gloves when gardening and then wash hands after use
The same report (cited above) suggests that risks from Legionella are very low.
Recommendations for minimising the impact of the issue in future
Given that there have been no human health questions due to the presence of
microorganisms other than Legionella species in CIGM, and that PAS100 composts should
contain no Salmonella species and low numbers of E. coli, it can be concluded that the
problem (for pathogens other than Legionella species) in CIGM is mainly perceived rather
than real (and it is only perceived by some, but not all, of those interviewed). For those who
perceive that enteric human pathogens still are a problem, the main challenge will be to
improve confidence in CIGM through provision of clear information to stakeholders, rather
than to set stricter limits for sanitisation of compost or for numbers of enteric pathogens in
compost or CIGM.
Given that Legionella species can potentially be present in all types of commonly used
growing media, is native to the UK and may be present naturally in soils and on plant
material, worries about limiting any impact of the pathogen apply to all growing media (not
just CIGM) and potentially to gardeners with compost heaps and workers on compost sites.
General (common sense) warnings that users of growing media should wear gloves and
wash hands afterwards should certainly remain on all bags of growing media, and compost
producers should consider Legionella species in their risk assessments, particularly with
regard to bioaerosols.
It is important to conduct further research in order to gain a greater understanding of the
sources, life cycle and epidemiology of Legionella species (and in particular L. longbeachae).
Studies of the incidence and behaviour of Legionella species in green waste during and after
the composting process will be particularly valuable. Only then can judgements be made as
to how best to minimise the already very small risk to those handling composts and CIGM.
De-mystifying the use of PAS100 compost in horticultural growing media
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It would also be wise to ensure that those researchers working on Legionella bacteria fully
understand the relevant legislation, rules and working practices relating to the manufacture
of composts and growing media: some clearly do not. For example, Lindsay et al. (2012)
state that there has been a move to “replace peat (in growing media) with green waste”
(PAS100 compost is, of course, not a waste). They also state that “Multipurpose Compost
(by which they mean multipurpose growing media as defined in this project, although it is
often called multipurpose compost in the UK) in the UK is subjected to the PAS100 standard,
which involves a sanitisation phase”. In reality, generally only the PAS100 component of
growing media will have been sanitised, and many growing media do not contain that
component at all. Lindsay et al. (2012) also use the term “compost” without defining it,
although they appear to be referring to growing media which may or may not include
PAS100 compost.
3.3
Hyperlinked summary of UK legislation and good practice guidance
3.3.1 The UK Compost Certification Scheme and BSI PAS100:2011
The UK Compost Certification Scheme aims to independently certify compost producers who
are producing compost according to the BSI PAS100:2011 specification. PAS100:2011 is a
baseline quality standard, which ensures that compost is consistent, safe and reliable to use
in a range of markets. The UK Compost Certification Scheme is an independently-certified
scheme, which is aligned to the requirements of the PAS100:2011 standard.
http://www.wrap.org.uk/category/materials-and-products/compost
BSI PAS100 specifies the minimum requirements for the process of composting, the selection
of input materials, and the quality of composted materials, as well as for the marking and
information labelling of the product. In countries, such as the USA, sewage sludge is used in
compost, and is largely accepted by the public. In the UK, sewage sludge in not an
allowable input under the CQP and is not allowed in PAS 100 compost.
Certification involves a series of steps. In January 2013, the Association for Organics
Recycling (AfOR) became the Organics Recycling Group (ORG) within the Renewable Energy
Association (REA). AfOR’s Compost Certification Scheme and the partnership certification
services it provided are now operated by Renewable Energy Assurance Limited, a wholly
owned subsidiary of the REA. Advice and assistance on gaining certification can be sought
from ORG, the contracted independent certification bodies and approved consultants.
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Further information can be found at the ORG website for more detail.
http://www.organics-recycling.org.uk/category.php?category=991&name=Certification
3.3.2 The Quality Protocol for Compost
The Compost Quality Protocol (CQP) was launched in England and Wales to provide a clear
framework for the production and supply of quality compost; it was updated August 2012.
https://www.gov.uk/government/publications/quality-protocol-for-the-production-and-useof-compost-from-waste
The Protocol clarifies the point at which waste regulatory controls on composted sourcesegregated biodegradable waste are no longer required.
The compost producer must demonstrate compliance with the Protocol and approved
standard (currently only PAS100:2011 is accepted as a standard).
Compliance with the Quality Protocol means that quality compost can be used without the
need for waste management controls from the point at which it is dispatched to the
customer.
The Quality Protocol does not change the regulatory requirements that apply to the
manufacture of compost from waste materials. The production and storage of compost can
take place only with an environmental permit.
3.3.3 The Animal By-Products Regulations
Animal by-products (ABPs) can present a risk to human and animal health if not used or
disposed of safely. The type of ABPs used in composting is restricted and the way in which
they are processed (in more specialised, contained systems than those permitted for nonABP materials), is tightly controlled.
ABPs are classified into three categories based on their potential risk to animals, the public or
to the environment https://www.gov.uk/using-animal-by-products-at-compost-and-biogassites . Only the lower risk material (Categories 2 and 3) can be used within an in-vessel
composting process. (Categories 2 and 3 cannot be used within open windrow composting).
3.3.4 Guidelines for the specification of Quality Compost used in growing media (2011)
In addition to these regulatory requirements for the production of compost used in growing
media, WRAP has produced a guidance document to help compost producers who want to
supply the horticultural growing media market meet the quality standards required by this
sector (WRAP, 2011a, updated 2014). The WRAP guidelines are an update of those
De-mystifying the use of PAS100 compost in horticultural growing media
45
previously published by WRAP in 2004 (Guidelines for the specification of composted green
materials used as a growing media component) and are designed to assist producers of
composted green materials to better understand and meet the specific requirements for
composts to be used in growing media. They are not intended to be prescriptive; the actual
detailed specification for compost will need to be agreed with the growing media
manufacturer or grower who is purchasing the compost, and will depend on the types of
plants to be grown in the mix it is used for.
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Specification.pdf
3.3.5 Compost production for use in growing media – A good practice guide (2011)
To help compost producers to use the ‘Guidelines for the specification of Quality Compost in
growing media’, WRAP has also produced a ‘Good Practice Guide’ (2011, updated 2014).
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Good_Practice_Guide.pdf
The guide covers key areas on feedstock mixes, monitoring and acceptability for inclusion of
compost to be used within growing media. It explains process control and screening
additional requirements for specific markets such as long term containerised stock or
ericaceous media. There is also guidance on testing requirements both broadly for inclusion
within growing media, and for specific end uses. Concerns over legionella and herbicide
residues are also discussed.
3.3.6 Microbiological safety of pot-grown fresh herbs
The use of fresh herbs (loose, pre-packed or grown in a pot) is becoming increasingly
popular and is covered by Regulation (EC) No 2073/2005 amended 2007 on the
microbiological safety of foodstuffs:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2007:322:0012:0029:EN:PDF
Strategies to prevent fresh vegetables including herbs from being contaminated with
microorganisms of concern (e.g. Salmonella spp.) during production are based on control
measures taken during production, harvesting and processing. When used as a constituent
in growing media for pot production of herbs, the risks of pathogen transfer from the
medium itself (rather than just the compost fraction) must be considered. As outlined
above, risks from pathogens of relevance to human health are considered acceptably low in
PAS100 composts.
3.4
Gap analysis to determine where additional research and development is required
Work conducted during this project has led the authors to conclude that several gaps still
exist in the knowledge of how to produce top quality composts for use in growing media,
and how best to use CIGM in horticultural production systems. However, a general problem
is that the few stakeholders who have achieved partial or whole success with one or more of
the issues under discussion are unwilling to divulge their findings. With the severe reduction
in government funding for horticultural research that has happened over the past 20 years
the research is now largely done by commercial companies and, understandably, those who
have spent the money achieving success want to keep their valuable knowledge to
themselves. The requirements that have, nevertheless, been identified for literature reviews,
laboratory work, replicated trials and simple development/demonstration work are
summarised below.
 Study on the effect of management practices on compost consistency
 Some compost producers/growing media manufacturers felt that they had solved the
problem of lack of compost consistency, but many others felt that it had not been
De-mystifying the use of PAS100 compost in horticultural growing media
46
solved. It may be possible to find a compost producer (or more than one) who is keen
to improve product consistency and work with them over a period of 12 to 18 months
to implement known best practice in terms of feedstock choice, feedstock management
and process management in order to develop a process which will minimise variation in
compost products. The resulting case study and documented best practice could help
many more compost producers improve the consistency of their products. Provision of
one-to-one technical support for these compost producers may help them improve
their products more quickly and to a greater extent.
 Comparison of UK and Dutch composts
 It would be useful to compare the properties of RHP-accredited (Dutch) ‘humic
composts’ with the best UK PAS100 composts which are being produced for the
growing media market. UK growers often trust Dutch growers and Dutch compost
products and some view their “humic composts” as superior to composts produced in
the UK. A short project could usefully:





conduct a short literature review comparing the Dutch RHP standard with PAS100;
compare existing data (chemical, physical and biological compost parameters) from
Dutch RHP accredited ‘humic composts’ with the best UK PAS100 composts
intended for growing media;
compare (if data permit) the consistency of RHP-accredited composts throughout
the year and between suppliers and determine whether (and if so why) the RHP
composts are superior to UK PAS100 composts;
(where data are lacking) test a range of recent samples of both RHP accredited and
PAS100 accredited composts for key parameters (not only parameters which are
known to be critical to the performance of composts as components of growing
media – e.g. conductivity, stability, pH – but also microbial respiration, the
presence of specific microbial species, microbial diversity and other parameters
which might make RHP composts superior to UK PAS100 composts in terms of their
suitability for use in growing media); and
report on key differences between RHP accredited (‘humic’) and PAS100 accredited
composts and determine whether these differences are likely to result in real
differences in performance when the two differently accredited composts are used
as components of growing media.
 Replicated experimental trials at research facilities
These should be conducted at research facilities where genuine expertise in growing
media and/or container-based crop production exists. In some cases, trials could be
conducted on professional nurseries where the grower understands the need for
replication and standardisation of management across the trial (other than for the
differing treatments). Key challenges needing to be addressed include:
 Development of methods to reduce salt concentrations in composts (experiments to
test leaching/flooding techniques which do not adversely affect compost properties);
 Trials to demonstrate that the higher pH values found in CIGM (as opposed to
traditional peat media) do not necessarily cause problems. Some or all of this work
could be conducted on professional nurseries, as outlined in the second major bullet
point below; and
 Development of methods to prevent and control sciarid (and other) flies in both
amateur and professional situations. Although some growing media manufacturers
felt that this problem had been solved to some extent, several growers disagreed and
there is little doubt that some work remains to be done on this subject, Some work
should focus on biological control agents, attractants and repellents, but work must
De-mystifying the use of PAS100 compost in horticultural growing media
47
also address the impact of management practices (in particular with regard to
irrigation practices) on the incidence and severity of sciarid fly attack. It must be
realised that different production systems may require different solutions (e.g.
depending on the size of the container, type of irrigation systems available and
physical/chemical properties of the growing medium). Some of this work could be
conducted on professional nurseries, as outlined in the second major bullet point
below.
 Validation of composting system sanitisation regime for kill of plant pathogens
of importance
 Some growing media manufacturers felt that some plant pathogen species may be
able to survive the composting process and that this was a key reason why they
chose not to include composts in their growing media. Although plant pathogens
were not a key issue under study in this programme, some growing media
manufacturers wished to see further work done to show that named plant pathogens
of concern were reliably killed under the commonly used time/temperature/turning
regimes commonly used in UK PAS100 composting systems.
 Nursery-based comparison trials and demonstration of best practice on
professional nurseries
 These are needed in light of the general issue of ownership of research and the need
to demonstrate and disseminate good practice for use of CIGM. Ideally this should
follow completion of essential work in the previous major bullet point. Simple trials
should be conducted on the nurseries of growers who are genuinely keen to try CIGM
again, or for the first time, using the very best CIGM products and the help of trained
horticultural scientists/growing media professionals who understand both the science
and practicalities of using growing media on nurseries. These individuals should work
with the growers and the growing media manufacturers during the trials to identify
and solve challenges relating to the use of the media in ways which are appropriate
to the growing systems in place (e.g. watering issues, sciarid fly problems, nutritional
difficulties) as they occur;
 The main aim would be to demonstrate the benefits of compost, such as nutrient
supply and chemical buffering (e.g. it would be useful to prove that growers can use
a lower rate of controlled release fertiliser if compost is used in the growing medium
and hence money can be saved during the production of longer term crops like
nursery stock); and
 Choice of participating grower(s) would be key to the success of this work, since the
growers chosen must be prepared to share the results widely.
 Studies on disease suppression

Several growers said that if certain composts or CIGM could be shown to suppress
disease then they would be interested in trying these products again, since the
number of pesticides available to professional growers continues to diminish year on
year and there is a pressing need to develop effective alternative disease prevention
and control strategies. There is clear evidence that composts and CIGM can suppress
disease in container-grown crops: both are widely used in the USA for this purpose,
but are not currently used in the same way in the UK. Whilst compost teas have
become popular on many ornamental plant nurseries in both the UK and Holland and
many growers believe that they help to suppress disease, very little work has been
done to develop disease suppressive composts for use in ornamental UK container
production systems. Detailed work is required to identify the characteristics of
disease suppressive composts and to develop methods for producing reliable and
consistent products.
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 Sampling and laboratory testing - further study on Legionella species bacteria

The GMA considers (probably correctly) that Legionella species pose an extremely
low risk and are therefore best not mentioned at all on bags of growing media.
However, we do not really know for sure whether PAS100 composts contain
Legionella species, whether they can survive and proliferate in CIGM and whether
they can then go on to cause infection and disease. Further information will almost
certainly be called for if further outbreaks of Legionnaires’ disease are associated with
growing media. In that case, independent research will be required to gain a greater
understanding of the sources, life cycle and epidemiology of Legionella species (and
in particular L. longbeachae). Studies of the incidence and behaviour of Legionella
species in green waste and during and after the composting process will be
particularly valuable. Only if further scientific work is done can judgements be made
as to how best to minimise the already very small risk to those handling composts
and CIGM.
3.5
Proposal for knowledge exchange (KE) mechanisms
It became clear when working on this project that many of the issues under study have been
resolved, at least in the minds of some stakeholders. Effective knowledge exchange is vital in
order that mechanisms for minimising the impact of the issues under study are disseminated
and fully understood. Knowledge exchange mechanisms are therefore proposed as follows.
 Development of new or revised template documents, information pamphlets
and guidance
 Review the ORG templates on minimising physical contamination in feedstocks and
composts to ensure that they cover all relevant points;
 Develop a simple, detailed guide for local authorities, waste handlers/suppliers and
compost producers on why contamination in feedstocks and composts must be
minimised and how to achieve composts which are consistently free from
contamination. The guide could be produced in the traditional leaflet/pdf file but also
as a more interactive, simplified, web-based decision tree. A short video clip of a
leading gardener who is pro green compost inclusion could also be produced, but
showing the technical aspects of the composting process;
 Review good practice guidance (WRAP 2011a and b) to ensure that the latest and
best information is included on the issues discussed in this report;
 Produce a user-friendly, practical grower fact-sheet on using compost in ‘own mix’ or
ready-mixed professional growing media (similar to other WRAP fact-sheets). Several
of these could be produced following completion of grower trials on different plant
types. They would include information on sourcing the best quality compost,
reference the WRAP guides for specifying compost, and detail what to ask for; and
 Improve the labelling and product information on growing media bags and product
information/dispatch sheets. There is still a widespread lack of understanding of the
difference between a growing medium (often labelled compost in the UK) and
PAS100 compost (which will never on its own make a suitable growing medium for
young plants). This is without doubt the reason behind some amateur and
professional growers’ mistrust of PAS100 composts.
 Promotion of new and existing information relevant to increasing the use of
quality composts in growing media
 Promotion of relevant (new and old) templates and guidance aimed at solving the
challenges (real and perceived) which high value compost producers and growing
media manufacturers have in relation to the use of composts in growing media:
 Topical, colourful, easy-to-read, short trade press articles (production horticulture,
garden centre/retail) drawing attention to relevant (new and old) templates and
De-mystifying the use of PAS100 compost in horticultural growing media
49


guidance aimed at high value compost producers and growing media
manufacturers;
 Targeted emails to compost producers’ known to (or likely to) want to reduce
contamination in their products. Aim: to draw attention to the latest ORG
recommended procedures and rules for reducing contamination in feedstocks and
to recent new and revised publications on how to optimise compost properties in
composts intended for use in growing media; and
 Trade stands at key shows including Four Oaks (http://www.fouroakstradeshow.com/) and the IPPS conference (http://www.ipps.org.uk/).
Improve confidence in CIGM through provision of clear information to stakeholders
(particularly growing media manufacturers and growers):
 on the safety of current PAS100 composts (rather than to set stricter limits for
sanitisation of compost or for numbers of enteric pathogens in compost or CIGM);
 by drawing attention to the latest ORG recommended procedures and rules for
reducing contamination in feedstocks and to recent new and revised publications
on how to optimise compost properties in composts intended for use in growing
media;
 by drawing attention to future project reports/case studies where compost
producers had greatly improved their products through careful management
regimes or where compost products had been used successfully on professional
nurseries;
 by ensuring that researchers working on Legionella species bacteria fully
understand the relevant legislation, rules and working practices relating to the
manufacture of composts and growing media. This will require a short study to
determine who is working on the issue, followed by a simple targeted letter
explaining the facts.
Two key methods are suggested for the above confidence-building:
 A telephone/e-mail ‘campaign’, which could be launched with an invitation to
come and discuss issues in an open forum, with three technical experts providing
talks on the main issues of concern.
 Provision of an internet discussion forum (for use only by invitees). In Year 1,
the title page could provide links to important technical information. Individuals
would be able to post questions to at least three technical experts who will
provide evidence-based answers to discussion points. In Year 2, the knowledge
exchange could be summarised from the discussion traffic to produce improved
answers and technical information.
 Tailor-made training courses/presentations based on existing information

Provision of training courses on production of high value composts for use in growing
media. Courses should be tailored to the target audience and could use video clips,
online training or in-house training as vehicles to get the message across. The
training could be built into existing training platforms rather than be stand-alone.
 BASIS (registration) Ltd are working with the Horticultural Trades Association
(www.the-hta.org.uk/) and have a dedicated training Continual Professional
Development (CPD) registered course for garden centre staff called ‘Guardian’.
This is largely pesticide use information. They plan to migrate this short course
to a web-based training module with a multi-choice questionnaire. A member of
this project team discussed with BASIS whether they would be happy in principal
to host a similar style training module on CIGM. BASIS are very amenable to
progressing this. Garden centre staff are often very negative about reduced peat
and peat-based media (including CIGM). If they better understood the
advantages and disadvantages of using CIGM and knew how best to use them,
they may be more likely to sell them with enthusiasm;
De-mystifying the use of PAS100 compost in horticultural growing media
50



Producers: Courses should explain how producers can overcome all of the major
issues, with reference to the most recent and best information and guidance.
Training should concentrate on how to overcome the genuine issues associated
with production of composts for growing media (e.g. physical contamination, bulk
density, consistency of composts), but other (to a large extent perceived) issues
which remain a concern for some compost producers, growing media
manufacturers and retailers should also be discussed;
Growing Media Manufacturers: Development and delivery of a dedicated seminar
or slot within a seminar for growing media manufacturers on the key chemical,
physical and biological aspects of using composts in growing media. It is
acknowledged that some may be reluctant to take part for reasons of commercial
confidentiality; and
Commercial Growers: Development and delivery of a dedicated seminar for
growers wishing to use CIGM. This course should be developed and delivered
following completion of the simple development work and nursery-based trials
outlined in Section 3.4 and would cover solutions to the main questions/issues
which growers currently have relating to the use of CIGM, including all nine of
those addressed in this project. These training courses may be localised but could
also be filmed in a series of short clips to make them more accessible to a wider
audience.
 Demonstration days/grower walks




Grower walks/demonstration days should be organised either as standalone, short
events, or in association with dedicated events (for example in association with
existing trade shows or conferences/seminars of membership organisations such as
the International Plant Propagators Society) in order to explain how to overcome the
major issues associated with using CIGM in theory and in practice;
Grower advocate days could be hosted at RHS sites around the country as they have
good geographical distribution (Yorkshire, Essex, Surrey, and Devon) and have
excellent facilities. These events would be similar to the “Farmer to Farmer” events
currently being organised by WRAP. Speakers might include growing media technical
experts and/or senior gardeners from the Royal Horticultural Society (RHS), who
could be trained up to spread the message. Well-respected horticulturists/growing
media specialists must be used for this work and it is likely that their ability as both
speakers and advocates will be critical to success;
The Horticultural Trades Association (HTA) National Plant Show - This takes place
annually in June in the Midlands. It showcases the very best of British plant suppliers
all under one roof and would therefore be a good opportunity for producers of CIGM
to reach nursery stock growers (and potential users of CIGM); and
Engagement with key partnership organisations such as the RHS, the National Trust
or The HTA should be considered. For example: our survey indicated that the RHS
were in favour of the use of CIGM and they may be prepared to run events for
growing media retailers and/or gardeners to publicise their views and methods for
using CIGM in practice. Both the RHS and the National Trust already run programmes
of events and talks and may be prepared to consider running events on the subject
of CIGM.
De-mystifying the use of PAS100 compost in horticultural growing media
51
4.0
Conclusions and recommendations
4.1
General points
Nine issues of concern have been addressed in this project: physical contamination, high
bulk density, high pH, high electrical conductivity, weed seeds, sciarid flies and other pests,
compost shelf life, compost variability and the presence of human pathogens.
 A review of relevant literature including peer-reviewed papers, conference papers and
current legislation and guidance documentation showed that a significant amount of
(mainly UK) work has been done in efforts to solve these nine issues. In most cases,
evidence from the literature suggested that it was possible to minimise the impact of
these issues in composts and compost-included growing media (CIGM) to levels which
were likely to be acceptable, at least for some stakeholders.
 However, in two instances, further research may be warranted. In the case of sciarid
flies, available evidence suggested that further scientific work was required in order to
address the issue effectively (for users of growing media). Further work may also be
required in future to determine the frequency and typical distribution of Legionella species
in PAS100 composts and CIGM.
 A range of twenty eight stakeholders were interviewed as part of this project, in an effort
to determine the extent to which the nine issues of concern represented problems for
them in terms of their sales or use of growing media. A broad range of views was
obtained (and listed in Appendix 1) and it was clear that there were strongly opposing
views on whether each issue remained a barrier to sale/use.



Seven compost producers (of whom six also produced growing media) and an
additional three growing media manufacturers participated in the study. Their views
differed greatly, with some feeling that most of the nine issues had been effectively
solved, and others feeling that none of them had! Most stakeholders had some
remaining concerns, and most often, those included the presence of physical
contaminants, high compost bulk density and high compost EC.
Of the eight growers interviewed, two used CIGM to some extent, two did not know
whether their growing media contained composts, and the remaining four did not
currently use CIGM. The survey clearly showed that physical contamination and lack
of consistency in the product were the major barriers to growers using growing
media containing PAS100 compost. Five out of eight growers listed physical
contamination as a major barrier, and six out of eight cited lack of consistency.
However, nearly all of the factors listed were deemed a barrier to use for one or
more of the growers interviewed. It is fair to say that the views of some respondents
were based on preconceived ideas rather than on actual experiences of using CIGM.
Six growing media retailers were contacted, but only two replied. Of these, one sold
CIGM and one did not, due to concerns over all nine of the issues under study in this
project. The retailer who did sell CIGM felt that physical contamination, high bulk
density, sciarid flies, shelf life/consistency of product, and poor customer perception
limited sales.
 The great majority of the compost produced in the UK is made for and sold into the
agricultural market, which is considerably less demanding than the growing media
market. It is important to note that much of the bad press which composts have received
in relation to their potential for use as growing media constituents and many of the poor
reports from growers and growing media manufacturers relates to non-PAS100 composts
or PAS100 composts produced for the agricultural market, rather than those produced
De-mystifying the use of PAS100 compost in horticultural growing media
52
specifically for higher value markets according to higher standards (such as those
recommended in the WRAP Guidelines for the specification of quality compost for use in
growing media, WRAP, 2011a). It is therefore vitally important to inform growing media
manufacturers, growers and retailers of growing media of the great strides which have
been made in recent years in improving compost quality and consistency through
effective knowledge exchange, in addition to conducting relevant research and
development. The most important knowledge exchange, research/development and
demonstration requirements for each of the major stakeholder groups in the context of
the subject of this report are summarised below.
 The extent to which the nine issues of concern have been solved for stakeholders in the
growing media sector and the further work and knowledge exchange required (based on
both a study of relevant literature and stakeholder opinions) are summarised in Table 7
and in the following sections.
4.2
Compost producers
 It is clear that some compost producers felt that they have solved the nine key
issues addressed in this project through hard work, attention to detail and
investment in their systems and processes, whereas others still have concerns
about some or all of them, particularly physical contaminants and high compost
bulk density.
 The most pressing need for compost producers in relation to the issues raised in
this project is for effective guidance documentation, effective distribution and
promotion of this guidance documentation and appropriate knowledge transfer.
These are outlined in more detail in Section 3.5.
 Compost producers would also benefit from publications released following
completion of recommended research and development outlined in Section 3.4.
 Most compost producers will continue to prefer to produce composts for less demanding
markets (e.g. agriculture) or may be forced to do so due to inappropriate feedstock
quality or a lack of growing media manufacturers local to them. However, some are well
placed (both geographically and in terms of their composting systems) to supply high
quality products for growing media manufacture. Dialogue should be encouraged between
growing media manufacturers and compost producers new to that sector or considering
involvement in it, with a view to setting up new working relationships between the two
groups. It is important that compost producers understand that additional investment in
procedures and infrastructure is likely in order to satisfy the demands of this sector.
However, it is also important for the growing media manufacturers to understand that
compost producers must be paid a viable price for an improved product;
De-mystifying the use of PAS100 compost in horticultural growing media
53
Table 7 The extent to which the nine issues of concern have been solved for stakeholders in the growing media sector and whether further
work and knowledge exchange is required for each issue.
Does literature suggest
that:
Issue
partial or
complete
solutions
exist?
further
R&D
required?
No. of stakeholders (out of the total contacted)
who felt that issue has been resolved (though it
still may limit % inclusion of compost in
growing media or sales of compost or growing
media)1
Compost
Growers
Retail
producers and
growing media
manufacturers
Stakeholder opinions
Further
R&D
required?2
Physical
Yes
no
8/11
2/8
2/2
no
contamination
High BD
Yes
no
8/11
4/8
2/2
no
High pH
Yes
no
8/11
5/8
1/2
yes
High EC
Yes
no
8/11
3/8
1/2
yes
Weed seeds
Yes
no
7/11
4/8
1/2
no
Sciarids etc.
Yes
yes
8/11
5/8
2/2
yes
Shelf life
Yes
no
8/11
6/8
2/2
no
Consistency
Yes
yes
7/11
0/8
2/2
yes
Human pathogens
Yes
possibly
7/11
5/8
1/2
possibly
1
NB: Ten compost producers and growing media manufacturers, eight growers and two retailers provided their views during
stakeholder interviews.
2
For details, see Section 3.4 (or summary of gap analysis above)
3
For details, see Section 3.5 (or summary of priorities for knowledge exchange below)
KE
required
?3
yes
yes
yes
yes
yes
yes
yes
yes
yes
the
De-mystifying the use of PAS100 compost in horticultural growing media
54
 Compost producers interested in supplying composts for use in growing media could
benefit greatly from forming a group or co-operative, in order to share knowledge and
promote their products in a cohesive way to potential buyers;
 ORG has expressed a strong interest in providing certification (for PAS100 compost
producers through REAL) to a higher specification for composts intended for growing
media. A standard equivalent to PAS100 would be prohibitively expensive to run.
However, it would be possible to certify PAS 100 accredited producers to additional
criteria such as those set out in a specification such as the WRAP one (WRAP, 2011a).
Compost producers interested in supplying composts for use in growing media could
benefit significantly from such a scheme. If there is sufficient demand, REAL could
extend the scope of the compost certification scheme to include assessment for
conformance with the WRAP specification for compost use in growing media. On
composters’ request and at an additional fee, REAL’s certification bodies could check
compliance with the WRAP specification for compost use in growing media in addition to
PAS 100 and the Compost Quality Protocol. This will only be possible if REAL’s
Certification Bodies agree to provide this service; and
 Compost producers must be prepared to develop a close working relationship with the
growing media manufacturer(s) to which they supply. Only by working closely together
can both parties develop a profitable, successful partnership.
4.3
Growing media manufacturers
 It is clear that some growing media manufacturers felt that they have solved the nine key
issues addressed in this project, whereas others still have concerns (in some cases serious
concerns) about some or all of them, particularly physical contaminants, high compost
bulk density and lack of consistency amongst composts and produce no CIGM or limited
amounts of CIGM as a result.



There is a clear need to ensure that growing media manufacturers understand the
great improvements in compost quality which have been made in recent years. Many
of them have considered using or have used composts in the past which were not
even PAS100-accredited, let alone produced according to a modern, higher
specification such as that produced by WRAP (2011a). Some leading industry players
have formed very poor opinions of composts which are evidently deeply ingrained.
Growing media manufacturers would benefit from publications released following
completion of recommended research and development outlined in Section 3.4.
It may be possible to improve the understanding which growing media manufacturers
have about composts as constituents of growing media through improvement of and
promotion of the latest guidance documentation as outlined in Section 3.5.
4.4
Growers
 Most of the growers contacted in this study had serious reservations about the use of
CIGM and most had a poor understanding of its characteristics and how to use it to best
effect in their cropping systems. Some of the growers have used (or have heard about
colleagues using) CIGM in the past which was of poor quality. There products were
unlikely to have been based on PAS100 composts, let alone on composts produced
according to a modern, higher specification such as that produced by WRAP (2011a).
However, some leading growers have formed very poor opinions of CIGM, which are
evidently deeply ingrained.
 Nursery stock growers are the most relevant target grower group, since the crops they
grow tend to be less sensitive than those produced in other sectors (e.g. pot herbs,
De-mystifying the use of PAS100 compost in horticultural growing media
55
bedding plants and vegetable transplants). Nursery stock growers could provide a
significant market for CIGM and are worth working with in order to increase use of CIGM
in professional horticulture.


There is a clear need to ensure that growers understand the great improvements in
the quality of CIGM which have been made in recent years, though that may be
difficult to achieve. Some of the reservations which growers have over the use of
CIGM for some of the more sensitive ornamental crops may be justified.
Limited research and development work (including work to look at potential benefits
of using CIGM) and considerable efforts in relation to demonstration and knowledge
exchange are recommended if UK nursery stock growers are to comprehend the
recent improvements in CIGM and consider using them in their production systems.
Recommended research, development, demonstration and knowledge exchange
mechanisms are outlined in Sections 3.4 and 3.5.
4.5
Retailers of growing media
 Only two of the six retailers contacted took part in the project, therefore it is difficult to
provide a balanced view of what UK retailers feel in general. The views of the two
retailers who did take part differed greatly, with one cautiously positive about the use of
composts in growing media and sales of CIGM and the other whose company was not
prepared to sell CIGM at present due to concerns about its quality, consistency and
safety.
 There is a clear need to ensure that retailers of growing media understand the great
improvements in the quality of CIGM which have been made in recent years, though
that may be difficult to achieve before the recommended research, development,
demonstration and knowledge exchange have been undertaken;
 Reports from the recommended research, development and demonstration activities
set out in Section 3.4 may encourage retailers of growing media to increase their
purchase of CIGM;
 Updating, publication, distribution and promotion of guidance documentation and
appropriate knowledge transfer as outlined in Section 3.5 will also help retailers realise
the potential for CIGM sales; and
 Challenges remain, but if the above recommendations are implemented there should
be greater success with CIGM.
De-mystifying the use of PAS100 compost in horticultural growing media
56
5.0
References
AHDB: HDC (2013) Tracking Peat usage in Growing Media Production. Grower Summary
CP100
AfOR (2010) Guidance on Legionella longbeachae. Association for Organics Recycling (now
REA, London).
Binns, ES (1977) Fungus Flies. The Garden 102 (2): 76-78.
Boulter-Bitzer, JI; Trevors, JT & Boland, GJ (2006) A polyphasic approach for assessing
maturity and stability in compost intended for suppression of plant pathogens. Applied Soil
Ecology 34(1): 65-81.
BSI (2011) BSI PAS100:2011 Specification for composted materials. British Standards
Institution, London, UK.
Brown, G; Goulder, R & Paget, T (2000) Survey of pathogens in composted materials. Final
report, Landfill Tax Credit Project, University of Hull.
http://www.mondegreen.org.uk/documents/SurveyofPathogensinCompostedMaterials.pdf
Accessed 08.02.13
Bunt, AC (1988) Media and Mixes for Container-Grown Plants. Pages 79-81, Unwin-Hyman,
UK.
Carlile, WR (2008) The use of composted materials in growing media. Acta horticulturae
779: 321-328.
Carlile, WR & Hammonds, SJ (2008) Micro-organisms of human health importance in
growing media. Acta horticulturae 779: 67-73.
Casati, S; Conza, L; Bruin, J & Gaia, V (2010) Compost facilities as a reservoir of Legionella
pneumophila and other Legionella species. Clinical Microbiology and Infection 16: 945 – 947.
Casati, S; Gioria-Martinoni, A & Gaia, V (2009) Commercial potting soils as an alternative
infection source of Legionella pneumophila and other Legionella species in Switzerland.
Clinical Microbiology and Infection 15: 571 – 575.
Chandler, D (2007) Protected ornamentals: investigation of fungal pathogens infecting larvae
of sciarid and shore flies. Final Report from HDC Project no. PC 277.
Cloyd, RA ; Marley, KA; Larson, RA & Arieli, B (2010) Bounce® fabric softener dryer sheets
repel fungus gnat, Bradysia sp.nr. coprophilia (Diptera:Sciaridae), adults. HortScience 45:
1830 – 1833.
Colborn, N (2013) What’s in the bag? It’s a media secret. The Garden February: 19: 32.
Currie, SL; Beattie, TK; Knapp, CW & Lindsay, DSJ (2013) Legionella spp. in UK composts –
a potential public health issue? Clinical Microbiology and Infection
http://dx.doi.org/10.1111/1469-0691.12381
De-mystifying the use of PAS100 compost in horticultural growing media
57
Defra (2010) Monitoring the horticultural use of peat and progress towards the UK
Biodiversity Action Plan target (SP08020).
http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Project
ID=17045&FromSearch=Y&Publisher=1&SearchText=soil&SortString=ProjectCode&SortOrde
r=Asc&Paging=10 Accessed 03.02.13.
de Jong, B & Zucs, P (2010) Legionella, springtime and potting soils. Euro Surveillance 15
(8) http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19497 Accessed 06.02.13.
den Boer, JW; Yzerman, EP; Jansen, R; Bruin, JP; Verhoef, LP; Neve, G & van der Zwaluw, K
(2007) Legionnaire’s disease and gardening. Clinical Microbiology and Infection 13: 88 – 91.
DETR (1998) Less waste, more value: consultation paper on the waste strategy for England
and Wales. Department for the Environment, Transport and the Regions, London, UK.
Dimambro, ME; Lillywhite, RD & Rahn, CR (2007) The Physical, Chemical and Microbial
Characteristics of Biodegradable Municipal Waste Derived Composts. Compost Science &
Utilization 15 (4): 243-252.
Egley, GH (1990) High temperature effects on germination and survival of weed seeds in
soil. Weed Science 38: 429-435.
Forsythe, SJ (2005) The microbiology of safe food. Blackwell, UK.
Gale, P (2002) Risk assessment: use of composting and biogas treatment to dispose of
catering waste containing meat. Final Report to the Department for Environment, Food and
Rural Affairs
http://www.organics-recycling.org.uk/dmdocuments/Risk_assessment_2002.pdf. Accessed
02.02.13.
GMA/HTA (2010) Committed to peat reduction. The Growing Media Association and
Horticultural Trades Association, Theale, Berkshire, UK.
Grundy, AC; Green, JM & Lennartson, M (1998) The effect of temperature on the viability of
weed seeds in compost. Compost Science and Utilization 6: 26-33.
Handreck, K & Black, N (2010) Growing media for ornamental plants and turf. Pages 316322, University of New South Wales Press Ltd.
Health Protection Scotland (2013). Increased incidence of Legionnaires’ disease caused by
Legionella longbeachae in Scotland.
http://www.documents.hps.scot.nhs.uk/respiratory/legionella/longbeachae_report2013_final.
pdf Accessed 16.01.14
Holmes, S & Hewson, A (2005) Nutrition of container-grown nursery stock. HDC Factsheet
05/05, Horticultural Development Council, UK.
Hughes, MS & Steele, TW (1994) Occurence and distribution of Legionella species in
composted plant materials. Applied Environmental Microbiology 60: 2003 – 2005.
Koide, M; Arakaki, N & Saito, A (2001) Distribution of Legionella longbeachae and other
Legionella species in Japanese potting soils. Journal of Infection and Chemotherapy 7: 224 –
227. (Abstract only, available at http://www.ncbi.nlm.nih.gov/pubmed/11810588) Accessed
10.02.13.
De-mystifying the use of PAS100 compost in horticultural growing media
58
Larney, FJ & Blackshaw, RE (2003) Weed seed viability in composted beef cattle feed lot
manure. Journal of Environmental Quality 32: 1105 - 1113.
Lindquist, R (1992) Fungus gnat population dynamics in potting mixes differing in
suppression to Pythium root rots. Ohio Florists Association Bulletin 748: 7 – 8.
Lindsay, DS; Brown, AW; Brown, DJ; Pravinkumar, J; Anderson, E & Edwards, GF (2012)
Legionella longbeachae serogroup 1 infections linked to potting compost. Journal of Medical
Microbiology 61 (2):218–222.
Maher, MJ; Prasad, M; Mahon, MJ; Campion, J & Walshe, PA (2001). Utilisation of compost
in horticultural growing media. Project 4751. ISBN 1 84170 265 X.
Mazuela, P; Urrestarazu, M & Bastias, E (2012). Vegetable waste compost used as substrate
in soilless culture. In: Crop Production Technologies, Dr. Peeyush Sharma (Ed.), ISBN: 978953-307-787-1, InTech.
NiChualain, D & Prasad, M (2011) Evaluation of three methods for determination of stability
of composted material destined for use as a component of growing media. ISHS Acta
Horticulturae 819: 303-306.
Nishida, T; Kurokawa, S; Shibata, S & Kitahara, N (1999) Effect of duration of heat exposure
on upland weed seed viability. Journal of Weed Science and Technology 44: 59-66.
Noble, R (2005) Recycling horticultural wastes to produce pathogen suppressant composts
for sustainable vegetable crop production. Final Report EU FP5 Project RECOVEG
QLK5-2001-01458.
Noble, R & Roberts, SJ (2004) Eradication of plant pathogens and nematodes during
composting: a review. Plant Pathology 53: 548 – 568.
Olson, DL; Oetting, RD & van Iersel MW (2002) Effect of soilless potting media and water
management on the development of fungus gnats (Diptera:Sciaridae) and plant growth.
HortScience 37: 919 – 923.
Ostos, JC; Lopez-Garrido, R; Murillo, JM & Lopez, R (2008) Substitution of peat for municipal
solid waste- and sewage sludge-based composts in nursery growing media: Effects on
growth and nutrition of the native shrub Pistacia lentiscus L. (2008) Bioresource Technology
99 (6): 1793-1800.
Ozores-Hampton, M; Stofella, PJ; Bewick, TA; Cantliffe, DJ & Obreza, TA (1999) Effect of
age of cocomposted MSW and biosolids on weed seed germination. Compost Science and
Utilization 7 (1): 51 – 57.
Prasad, M & Carlile, WR (2011). Practical experiences and background research on the use
of composted materials in growing media for the UK market. ISHS Acta Horticulturae 819:
111-124.
Prasad, M & Maher, MJ (2001) The use of composted green waste (CGW) as a growing
medium component. ISHS Acta Horticulturae 549: 107 – 113.
Pravinkumar, SJ; Edwards, G; Lindsay, D; Redmond, S; Stirling, J; House, R; Kerr, J;
Anderson, E; Breen, D; Blatchford, O; McDonald, E & Brown, A (2010) A cluster of
Legionnaires' disease caused by Legionella longbeachae linked to potting compost in
De-mystifying the use of PAS100 compost in horticultural growing media
59
Scotland, 2008-2009. Euro Surveillance 15 (8)
http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19496 Accessed 06.02.13.
Rainbow, RWA (2009) The Use of Green Compost in the Production of Container Nursery
Stock in the UK: Challenges and Opportunities. Proc. ISHS on Growing Media. Acta
Horticulturae 819: 27 – 32.
Raviv, M (2011a) The future of composts as ingredients of growing media. ISHS Acta
Horticulturae 891: 19-32.
Schmilewski, G (2008) The role of peat in assuring the quality of growing media. Mires and
Peat 3 Article 2.
SGMTF (2012) Towards sustainable growing media – Chairman’s report and roadmap
(http://www.defra.gov.uk/publications/files/pb13867-towards-sustainable-growing-media.pdf)
accessed 31.01.13.
Steele, TW; Lanser, J & Sangster, N (1990a) Isolation of Legionella longbeachae sero-group
1 from potting mixes. Applied and Environmental Microbiology 56 (1):49-53.
Steele, TW; Moore, CV and Sangster, N (1990b) Distribution of Legionella longbeachae
serogroup 1 and other Legionella in potting soils in Australia. Applied and Environmental
Microbiology 56 (10):2984-8.
Sumption, P & Lennartsson, M. (Undated) Research topic review: Organic Plant Raising.
Institute of Organic Training and Advice OFO 347 (DEFRA) 1-29 (http://orgprints.org/5973/)
Accessed 08.02.13.
Surrage, VA & Carlile, WR (2008) Variation in Quality of Composted Green Wastes of UK
Origin and Their Suitability for Inclusion in Growing Media. Proc. ISHS on Growing Media.
Acta Horticulturae 779: 631-6.
Surrage, VA (2007) Composted green material and its use in growing media. PhD Thesis
(Nottingham Trent University.
Surrage, VA & Carlile, WR (2007) Development and storage properties of a green compostbased peat-free growing medium: challenges and solutions. ISHS Acta Horticulturae 819:
395-402.
Tompkins, DK; Chaw, D & Abiola, AT (1998) Effect of windrow composting on weed seed
germination and viability. Compost Science and Utilization 6 (1): 30-34.
Verhagen, JBGM (2011. Stability of growing media from a physical, chemical and biological
perspective. ISHS Acta Horticulturae 819: 135-141.
Wallace, P (1999) A study to assess the quality of waste derived composts produced by a
range of processes. Environment Agency R&D Technical Report P229, pub. WRc, Swindon.
Walsall Council (2008) Your garden waste collection service. Available on the WRAP website
at http://www.wrap.org.uk/sites/files/wrap/Walsall%20-%20%20Garden%20Waste%20%20Message.pdf Accessed 19.03.13.
Whiley, H & Bentham, R (2011) Legionella longbeachae and legionellosis. Emerging
infectious diseases 17: 579 – 583.
De-mystifying the use of PAS100 compost in horticultural growing media
60
Wichuk, KM & McCartney, D (2007) A review of the effectiveness of current timetemperature regulations on pathogen inactivation during composting. Journal of
Environmental Engineering and Science 6: 573 – 586.
WRAP (2011a) Guidelines for the specification of quality compost for use in growing media.
WRAP, Banbury, UK.
(Updated: 2014 http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Specification.pdf)
WRAP (2011b) Compost production for use in growing media – a good practice guide.
WRAP, Banbury, UK. (Updated 2014:
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Good_Practice_Guide.pdf)
WRAP (2010) A survey of the UK organics recycling industry in 2010. WRAP, Banbury, UK.
WRAP (2009) Recycled compost produces quality results for Jack Moody – Case study,
WRAP, Banbury, UK.
WRAP (2008) Peat-free growing medium produces ornamental nursery stock. Fact Sheet on
trials at Darby Nursery stock, WRAP, Banbury, UK.
WRAP (2007) Container production of trees in growing medium based on green compost –
Barcham Trees. Final Report from Project ORG033-015. WRAP, Banbury, UK.
WRAP (2006a) Ness Botanic Gardens – Nursery stock production in reduced peat growing
media. Final Report from Project ORG033-014. WRAP, Banbury, UK.
WRAP (2006b) Aldingbourne Nurseries. Final Report from Project ORG033-018. WRAP,
Banbury, UK.
WRAP (2005a) Storage trials of retail growing media products containing composted green
materials. Final Report from Project ORG0019. WRAP, Banbury, UK.
WRAP (2005b) Assessment of the potential for site and seasonal variations of composted
materials across the UK. Final Report for Project ORG 0005. WRAP, Banbury, UK. ISBN: 184405-159-5.
WRAP (2005c) Compost plays royal role at Duchy of Cornwall nursery – Case study, WRAP,
Banbury, UK.
WRAP (2005d) Managing Garden Waste at Civic Amenity Sites – Good Practice Guide. WRAP,
Banbury, UK. ISBN: 1-84405-138-2.
WRAP (2004a) Guidelines for the specification of composted green materials used as a
growing medium component. WRAP, Banbury, UK. ISBN 1-84405-112-9.
WRAP (2004b) Investigation of the Effect of the Composting Process on Particular Plant,
Animal and Human Pathogens known to be of Concern for High Quality End-Uses. WRAP,
Banbury, UK. ISBN: 1-84405-141-2.
WRAP (2003) A review of the literature on the occurrence and survival of pathogens of
animals and humans in green compost. Waste and Resources Action Programme, Banbury,
UK. ISBN: 1-84405-063-7
De-mystifying the use of PAS100 compost in horticultural growing media
61
WRAP (2002) Assessment of the potential variation of composted materials across the UK.
Final report from Project ORG005. Waste and Resources Action Programme, Banbury, UK.
WRAP (unpublished) Risk Assessment of the Use of PAS100 Green Composts in Scottish
Livestock Production. Report currently being prepared for publication by WRAP, Banbury, UK.
De-mystifying the use of PAS100 compost in horticultural growing media
62
Appendix 1- Interviews of selected
industry stakeholders
A.1.1 BSI PAS100 compost producers and growing media manufacturers
Seven compost producers (of whom six also produced growing media) and an additional
three growing media manufacturers participated in the study. All of them supply to the UK
market and one of them is based in Germany. They are either PAS100 compost producers
who supply to the growing media industry or use PAS100 compost within their growing
media mixes or are both a PAS100 compost producer and a growing media manufacturer
using their own composts in their growing media production. The exception is the German
company, which produces composts to the Dutch RHP specification and is a major supplier of
growing media to the UK professional grower market. (It does not use UK PAS100 composts
in its media).
The Environment Agency accepts that imported waste derived compost may cease to be
waste provided it has been produced in compliance with:
 a relevant standard or code of practice of a national standards body or equivalent body of
any European Economic Area (EEA) State; or
 any relevant international standard recognised for use in any EEA State; or
 any relevant technical regulation with mandatory or de facto mandatory application for
marketing or use in any EEA State.
These must give levels of product performance and protection of human health and the
environment, equivalent to those required by the Compost Quality Protocol.
All respondents who expressed an opinion felt that their sector was undergoing expansion
and provided valuable insights into their views on the questions posed; though almost all
preferred not to have their views directly attributed to them.
Factors which affect the sales/use of PAS100 compost in growing media
production
The ten compost producers and growing media manufacturers who took part in the project
were asked to indicate the extent to which the nine issues considered affected their sales or
their use of PAS100 compost in growing media. Responses were rated in terms of the extent
to which each issue remained a problem for the interviewees and these responses have been
summarised in Figure A.1.1
De-mystifying the use of PAS100 compost in horticultural growing media
63
Figure A.1.1 The extent to which compost properties affect sales and use of PAS100
compost in growing media
A broad range of views was obtained and it was clear that there were strongly opposing
views on whether each issue remained a barrier to sale/use. Some growing media
manufacturers/compost producers felt that they had resolved most of the issues of concern,
whereas others felt that most of the issues listed above remained a problem for them. The
issues of greatest concern included physical contamination, high bulk density, high EC and
customer requirements which preclude or limit the use of green compost. Some of those
interviewed also felt that fungi and pesticide residues were potential barriers to the use of
PAS100 compost in growing media.
The markets for the growing media produced have differing sensitivities. Professional
growing media is the most demanding market, followed by retail growing media, whereas
the use of PAS100 compost for inclusion within landscaping projects is, broadly speaking,
deemed the least demanding market.
Extent to which issues are perceived to have been resolved
Opinions as to whether the issues of concern had been resolved varied amongst
stakeholders. One compost producer and growing media manufacturer felt that they had
put a number of measures in place throughout the compost production process to resolve
the issues of concern, but would like to see:
 removal of aminopyralid & clopyralid from the supply chain, starting with the removal of
retail approvals for clopyralid;
 improvement in understanding of best practice with regard to contaminant removal and
screening techniques at compost sites; and
De-mystifying the use of PAS100 compost in horticultural growing media
64
 more compost producers keeping maturing and finished composts dry and ready for
effective screening.
Others felt that the perceived issues were unlikely to be overcome.
Perception of potential benefits from inclusion of PAS100 compost in growing
media
The nine respondents were also asked to review a list of potential benefits of including
PAS100 compost within growing media and to rate their responses from the following
choices: agree, disagree, neither agree nor disagree. Figure A.1.2 shows the results
compiled from these responses.
Figure A.1.2 Stakeholder opinions on the potential benefits of PAS100 compost inclusion
within growing media mixes
Range of values sought for key parameters
The characteristics of PAS100 compost can vary between suppliers and a stakeholder’s
perception of PAS100 compost might be affected by the particular supplier they use or have
encountered in the past. We were therefore keen to identify the range of values our
stakeholders expect from PAS100 compost that they would consider to be suitable for use in
growing media. Seven of the stakeholders replied to this question in detail (five produced
both compost and growing media and two produced only growing media). The responses
are provided in Table A.1.1.
De-mystifying the use of PAS100 compost in horticultural growing media
65
 The BSI PAS100:2011 limits relevant to this project are:
 Physical contamination:
 total glass, metal, plastic and any other non-stone fragments > 2 mm must not be
present at more than 0.25% (mass/mass) of ‘air-dry’ sample.
 plastics must make up not more than 0.12% (mass/mass) of ‘air-dry’ sample.
 stones > 4 mm must make up not more than 8% (mass/mass) of ‘air-dry’ sample in
compost grades other than mulches.
Weed
propagules must not be present in 1 litre of compost;

 Salmonella species must be absent and E. coli numbers must be ≤1000 cfu/g fresh
compost.
De-mystifying the use of PAS100 compost in horticultural growing media
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Table A.1.1 Stakeholder (S) specifications for parameters of concern
Parameter
Physical
contamination
Units
% m/m
S1
expect lower
than PAS100
specification.
pH
High bulk
density
Electrical
conductivity
Weed seeds
pH Units
g/l
7–8
hope to get 450
g/l max
<1000 µS/cm
Sciarid flies and
other pests
Compost shelf
life
Consistent
product?
Microbiological
safety
Present/
absent
Months
EC µS/cm
%
yes/no
no. of
CFU/ml
0
absent ideally
S2
< 10 mm.
No sharps.
PAS100 for stones
and plastic. Glass
trace specified by
internal method 0.1
piece /litre max.
Zero preferred
7-9
450 - 550
<1000
(Target 600)
0
(Fungal spores
more of a problem
than weeds)
absent
S3
zero
S4
nil (further
screening is carried
out prior to
inclusion in growing
media) <8 mm
screen but still get
occasional glass
contamination
<7.5
<550
S5
zero (RHP norm)
S6
low but not too low
to achieve
S7
<0.01
<8.0
<700
<8.0
<500
<7.0
<500
<1500
<2500
<1200
<1500
0
0
0 (RHP norm)
0
0
absent
absent
absent
absent
absent
6
Stable (RHP norm)
12
12
yes
yes
yes
always
PAS100 limits
Salmonella &
Camphylobacter
<1,000
None (?)
<500 g/l
fresh
almost indefinite
No (not deemed
necessary)
No
yes
(Solvita used)
1000
yes
species nil
E.coli <1000
Enterobacteriaceae
Other (if named
above)
Odour low – an
particle size <10mm
earthy rather than
putrid smell. No
smell of ammonia
NB: where boxes have been left blank, the stakeholder did not provide a response for the parameter in question.
<10,000
nitrate >50 mg/l
NH4 to NO3 ratio (no
ratio given)
De-mystifying the use of PAS100 compost in horticultural growing media
67
It is clear from the compost producers’ answers that the expectation is for a lower physical
contamination level in compost destined for inclusion within growing media than that
required by the PAS100 standard, with zero or nil often stated as a requirement. The
microbiological safety levels for indicator pathogens (E. coli and salmonella) seem adequate
within PAS100 and match user expectations, as does the weed seed limit of 0%.
One growing media manufacturer’s representative indicated in some detail whether he felt it
was achievable to source compost which would meet market needs for all nine parameters
of concern. This representative has a thorough knowledge of the physical, chemical and
biological properties of composts and a comprehensive understanding of the needs which
plants have with regard to growing media. He is well-respected within the growing media
sector, is arguably one of the most technically able practitioners in the field and his views are
thought to represent those of the industry as a whole. His views are summarised in Table
A.1.2.
Table A.1.2 Comments of one stakeholder about the potential for sourcing compost for use
in growing media which satisfied the requirements for parameters of concern
Parameter
Comments on the growing media manufacturer’s
ability to source compost with appropriate value
for the parameter
Physical contamination
Whatever the limits set, it only takes one object, e.g. a
biro top or bottle top, to convince the consumer that they
have purchased rubbish.
pH
Always high, impossible to reduce.
High bulk density
Inevitably high, an issue because of the carbon footprint.
High electrical conductivity
Not just high EC, but certain salts such as chloride.
Weed seeds
Very few samples reach specification and it is evident from
trials with competitor products that this is an issue.
Sciarid flies and other pests
If not already present they will “join the party”.
Compost shelf life
This is not defined in PAS100a; most manufacturers will be
looking for 6-12 months storage stability. The WRAP
storage trial showed poor results with growing media
containing green compost that did not contain a high
percentage of peat.
Consistent product?
There is no consistency in PAS100 composts; this is
inevitable, given the material used.
Microbiological safety
An area of great concern when supplying food retailers.
a
Shelf life is not defined in either the PAS100 standard or in the WRAP specifications or
guidance (WRAP 2011a and b). Shelf life would depend on the conditions under which
compost is stored, therefore if composts are to be compared and/or the shelf-life defined
for specific products, the storage conditions under which they were to be kept would also
have to be defined.
This producer felt strongly that very few suppliers can currently achieve the necessary high
standards in order to supply large manufacturers. He didn’t dismiss the possibility of using
compost in the future, but at present the PAS100 compost on offer is in small quantities, of
mediocre quality and variable consistency, which requires excessive quality control
procedures on each batch.
Discussion points on using PAS100 compost in growing media
Finally, discussion points were raised by the PAS100 compost producers and growing media
manufacturers. These have been summarised below:
De-mystifying the use of PAS100 compost in horticultural growing media
68
 Should standards for production have been focused on the end product (market





requirements) rather than the requirements of organics recyclers?
Has the BSI PAS100 standard encouraged the government and other bodies to have too
much faith in the ability of green compost to fill the gap in the market caused by the need
to stop using peat?
Recent publicity relating to the presence of herbicides in composts has not helped the
case for using green compost in growing media. If anything, this publicity has
strengthened opposition from the professional growing sector.
Is there enough green compost of sufficient quality to justify the use of such material in
UK growing media and would effective quality control measures render their use noncost-effective?
Current growing media production has complete traceability and is supplied to high
auditing standards. Could this be achieved with compost-included growing media?
The Defra SGMTF determined that reduced peat and peat-free growing media must be “fit
for purpose” if consumers were to be convinced of their usefulness. However, tests
conducted by some growing media manufacturers showed that compost-included growing
media was not fit for their purposes.
Asked what would be required to encourage further inclusion of PAS100 compost in growing
media, the following comments and suggestions were raised from several stakeholders:
 Reduction of soluble salts, particularly Na, K, Cl, in the compost media;
 Reduction of contamination, particularly glass contamination, to virtually zero;
 Consistency in the output of [each] composting site and less variation between sites;
 Improved control on particle size;
 Publicly available growing trial data is useful but of more use would be to get the basics
right;
 Guidelines are of no use and should be changed to requirements;
 Compost bulk density should be lower;
 Trials carried out through the new Growing Media Panel. Work should be conducted
according to the new trial protocols, developed by this group, firstly to prove that BSI PAS
100 composts can be part of growing media that are fit for purpose and secondly that
there will be a sustainable source of sufficient quantity of this material available; and
 Education, since management of compost-included growing media is different to peatbased, and users (whether amateur or professional) need to understand how to use it in
order to gain from their advantages and minimise their disadvantages.
A.1.2 Growers who may use growing media containing PAS100 compost
Eight leading growers were interviewed to gain their views on using growing media
containing PAS100 compost. All of the growers interviewed obtained growing media from
named manufacturers, which they then used ‘straight’ or mixed with additional materials on
site.
Factors which might affect the use of growing media containing PAS100 compost
Growers were asked to indicate the extent to which the nine issues considered in this project
affected whether they would use growing media containing PAS100 compost growing media.
Their responses are summarised in Figure A.1.3.
The results of the survey clearly showed that physical contamination and lack of consistency
in the product are the major barriers to growers using growing media containing PAS100
compost.
De-mystifying the use of PAS100 compost in horticultural growing media
69
Extent to which issues are perceived to have been resolved
Responses varied considerably. One grower said that his supplier currently does not offer
growing media based on composts and that in any case, they did not want it. He felt that
PAS100 standards are not stringent enough for growing media for commercial production
and to some extent are questionable even for amateurs.
Other growers said they would consider the inclusion of compost in their growing media if
quality could be improved, if there was a guarantee of sufficient supply and if it did not entail
additional cost. Another grower commented that they were currently using a medium which
included 10-20% compost by volume. They went on to comment that the compost-included
mixes were useful to them.
Figure A.1.3. The extent to which listed factors affect whether growers use growing media
which contains PAS100 compost.
Perceived benefits of including PAS100 compost within growing media
The growers were also asked whether they agreed, disagreed or neither agreed nor
disagreed with a list of potential benefits to including PAS100 compost in growing media.
Their responses are summarised in Figure A.1.4.
Range of values expected within the growing media used
All growers were asked to list the range of values required for key parameters in the growing
media they use. Seven out of eight growers provided these details, with the eighth grower
relying on the technical knowledge of his supplier to provide the correct characteristics. The
information is provided in Table A.1.3 below.
Discussion points on using growing media containing PAS100 compost
In summary, of the eight growers (from nine nurseries) interviewed, two used growing
media containing PAS100 composts (although one used only a very small amount in large
containers). Two growers did not know whether their growing media contained composts or
De-mystifying the use of PAS100 compost in horticultural growing media
70
not, and the remaining four clearly stated that the growing media they used did not contain
PAS100 compost.
To encourage them to use compost within their growing media, reassurances on compost
traceability, safety, consistent quality, lack of physical contaminants and acceptable price are
needed. Good technical trials were also requested.
Figure A.1.4. Benefits (perceived by growers) when PAS100 compost is included in growing
media
De-mystifying the use of PAS100 compost in horticultural growing media
71
Table A.1.3 Characteristics required in growing media (as recorded from grower responses to survey)
Characteristic
Units
Grower 1
Grower 2
Grower 3
Grower 4
Grower 5
Grower 6
Physical
contamination
pH
% m/m
0
0
Nil
Nil
0
negligible
pH Units
5.0-6.0
3.0-5.0
High bulk
density
High electrical
conductivity
Weed seeds
Sciarid flies
and other
pests
Compost shelf
life
g/l
5-6.25
5.8
(depending
on mix)
250-500
EC µS/cm or
<550
mS/cm
%
0
Present/absent Absent
Months
Consistent
Yes/No
product?
Microbiological no. of CFU/ml
safety
12
(without
controlled
release
fertiliser)
Yes
0
absent
Yes
Number of
standards to
follow
5.5-6.5
5.0-6.5
low
500
<1000
0.7
nil
Accepted but
needs control
0
absent
Medium lifetime
3
yes
yes
Yes
Must be safe
Safe as
possible, staff
use latex gloves
for potting
?
200-300
0.5-1
low
nil
nil
nil
absent
1
3
Yes
Yes
Grower 7
0
absent
De-mystifying the use of PAS100 compost in horticultural growing media
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A.1.3 Retailers’ perception
Two major retailers responded to a survey designed to find out their views on the inclusion
of PAS100 compost within growing media.
The first has a significant share of the UK retail market, selling to mainly amateur gardeners.
PAS100 compost is included within a wide range of growing media products blended to meet
specific plant needs, at an inclusion rate of 25-90%. This retailer said that although they
used PAS100 certified compost in their growing media they are still seeing too many
problems with it. He felt that physical contamination, high bulk density, sciarid flies and
pests, shelf life and consistency of product, and poor customer perception all limit their sales
of growing media containing PAS100 compost. He also said that compost pH, high electrical
conductivity, weed seeds and microbiological safety have never been an issue for them. He
felt that in order to encourage further use, good price, public pressure and excellent trial
results would all help with both the internal decision-making processes and eventual sales.
The second retailer sells to amateur gardeners within the UK and the Republic of Ireland.
Whilst their representative agreed with all the potential benefits of PAS100 compost inclusion
within growing media mixes, he felt that all of the factors listed prevented them from selling
such a product. He stated that they had ticked all of the issues in section 3 (of the
questionnaire) as factors of concern if they were to use PAS100 compost in their own brand
compost. He felt that all of the issues would need to be considered as they would with any
other type of growing media sold, along with the product performance. He did understand
that PAS100 production systems give an element of control over the concerns raised, but he
felt that they still need to be further addressed. He said that good growing trial results and
consistency of products were key factors for his company when looking at alternative
growing media, and that through years of their own trials, they were simply not seeing the
required quality coming through. He said that customer perception remained a concern to
his company, and he felt that customers would not be able to manage such media, since
they behave differently to peat and coir-based media. Another major concern was the
relatively heavy weight of growing media based partly on composts, which is especially
important when selling in retail bags.
De-mystifying the use of PAS100 compost in horticultural growing media
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Appendix 2 - Questionnaires used
A)
Questionnaire on the use of BSI PAS 100 Compost in
growing media for PAS 100 compost producers and
growing media manufacturers
WRAP (Waste and Resources Action Programme) has commissioned a short project (Project
OMK005-002) which is looking at the use of PAS 100 composts in growing media. The
purpose of the work is to improve awareness in the use of composts within the growing
media industry by collating and/or signposting existing information on its safe, effective use
and by identifying the nature of further research action, if this is required. The work is also
intended to support an increase in the use of compost as a constituent of growing media.
A number of questions around compost use in growing media have arisen within the Defra
Sustainable Growing Media Taskforce (SGMTF). These concern issues including physical
contamination, pH, bulk density, electrical conductivity, weed seeds, sciarid flies, compost
shelf life, compost consistency and microbiological safety. In addition to a short literature
review, Project OMK005-002 aims to gather information through a survey of key players in
the composting, growing media manufacturing, retailing and growing sectors. Each industry
stakeholder interviewed will be asked questions relating to their involvement in the
composting and/or growing media sector, with particular reference to their attitudes and
experiences with the above issues. The results of the survey will be collated and summarised
in a report which will aim to help improve the quality of both composts and growing media
which contain composts (as required under the SGMTF roadmap).
Availability of PAS 100 quality composts has increased in recent years as a result of
government targets for landfill diversion and recycling and due to increasing pressure to
reduce use of peat in their products, many growing media manufacturers use PAS 100
composts as a constituent of one or more of their products at varying inclusion rates.
Although most growing media manufacturers use green composts (which have been made
purely from source-segregated garden wastes) in their products, this project concerns both
green composts and food/green composts which also include food wastes.
Please note: if you do not want to answer one or more of the questions in this
questionnaire, please leave them blank and move on to the next question. The questionnaire
is designed to be completed electronically by a project team member, during or following an
interview with a compost producer/growing media manufacturer, or electronically by a
compost producer/growing media manufacturer. The boxes will expand as they are being
completed. If the questionnaire is to be filled in on paper, then the document should be
altered (boxes should be expanded by adding lines within them) prior to printing in order to
allow sufficient space for writing in them.
Organisation Details
Organisation Name
Respondent Name
Contact details
De-mystifying the use of PAS100 compost in horticultural growing media
74
Nature of business or organisation (please tick those that apply)
PAS 100 Compost producer
Growing Media Manufacturer
1. Scale of Production
Cubic metres produced of PAS 100
compost (different grades) and/or
growing media (GM) types or range
Geographical supply
area
Proportion of UK market?
(%) if known
e.g. 11,000 m3 of 0 – 10 mm green
compost;
15,000 m3 of peat-free multipurpose
GM;
3,000 m3 of reduced peat-free
propagation GM.
e.g. UK (not
Scotland) and
republic of Ireland
e.g. ~ 5% of retail market
for bagged growing media
Sector:
Expanding (E)
Declining (D)
Static (S)
e.g. E
2. Do you use sell PAS 100 compost in its own right or as an ingredient in growing media, and on what scale?
(Please describe what you do briefly)
3. Please give an indication of the extent to which the following factors or issues affect your sales or use of
PAS 100 compost in growing media. Tick the box which matches your current thoughts.
This issue
prevents me
from
selling/using PAS
100 compost in
growing media
This issue
limits my
sales/use of
PAS 100
compost in
growing media
This has been an
issue for us in the
past but is not
any more
This has never
been an issue for
us
Physical contamination
pH
High bulk density
High electrical conductivity
Weed seeds
Sciarid flies and other pests
Compost shelf life
Consistency of product
Microbiological safety
Customer requirements
Other (please specify)
De-mystifying the use of PAS100 compost in horticultural growing media
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4. The following is a list of potential benefits of including PAS 100 compost in your growing media mix.
Please tick the one box per row to indicate which matches your current thoughts.
Agree
Neither agree
nor disagree
Disagree
Source of slow release nutrients
Less moss/liverwort on pot tops
Lower cost per m3 compared to other ingredients
Soil-borne disease suppression
Better moisture retention
Lower lime and base fertiliser requirement
Better buffering of nutrients
To meet recycling targets
Carbon footprint
Other benefits, please name
5. The characteristics of PAS 100 compost can vary between suppliers. Could you please tell us the range of
values you expect from the PAS 100 compost you supply and/or use (they could be more stringent than
the requirements of the PAS 100 certification scheme). NB: If you do not have the correct figures to hand,
simply comment on any problems you have in sourcing composts with appropriate characteristics for the
parameter of concern).
Parameter
Units
Physical contamination
% m/m
pH
pH Units
High bulk density
g/l
High electrical conductivity
EC µS/cm
Weed seeds
%
Sciarid flies and other pests
Present/absent
Compost shelf life
Months
Consistent product?
Yes/No
Microbiological safety
no. of CFU/ml
Value you
expect for
parameter
Comments on your ability to source
compost with appropriate value for the
parameter
Other (if not named above)
De-mystifying the use of PAS100 compost in horticultural growing media
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6. Do you have any further comments on your (or your compost supplier’s) ability to achieve the above values
7. If you have managed to resolve one or more of the key issues that were causing problems in the past OR if
you have ideas as to how they might be overcome, please comment below.
How you managed to resolve
undernoted issue (if relevant)
How do you think this issue could be
resolved in future?
Physical contamination
pH
High bulk density
High electrical conductivity
Weed seeds
Sciarid flies and other pests
shelf life of end product
Consistency of product
Microbiological safety
Other (if not named above)
8. If you have not used PAS 100 compost and do not intend to do so please let us know why? (Please give as
much detail as possible)
9. Do you have any other comments/discussion points you would like to raise on compost use in growing
media:
De-mystifying the use of PAS100 compost in horticultural growing media
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10. Are you aware of the following publications
WRAP good practice guide (Compost production for use in growing media)
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Good_Practice_Guide.pdf
WRAP Guidelines for the specification of quality compost for use in growing media
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Specification.pdf
(yes or no)
11. What would allow or encourage you to consider recommending/using compost or more compost in growing
media? – for example, data or information on specific benefits from using PAS 100 composts, trials results
where composts have been used successfully to grow specific plant types, further details of the PAS
scheme, changes to the PAS scheme, changes to the WRAP good practice guide, improved Guidelines for
the specification of quality compost for use in growing media, recommendation from a respected authority,
or something else?
Please tick this box to indicate that you are happy for us to attribute the comments
below directly to you within the final report.
Please tick this box if you are happy for us to use your comments but would like to
remain anonymous
Thank you for taking the time to communicate your views on the use of compost
within growing media. Your help is very much appreciated.
Please return to audrey@earthcaretechnical.co.uk
De-mystifying the use of PAS100 compost in horticultural growing media
78
B) Questionnaire on the use of BSI PAS 100 Compost in
growing media (for growers)
WRAP (Waste and Resources Action Programme) has commissioned a short project (Project
OMK005-002) which is looking at the use of PAS 100 composts in growing media. The
purpose of the work is to improve awareness in the use of composts within the growing
media industry by collating and/or signposting existing information on its safe, effective use
and by identifying the nature of further research action, if this is required. The work is also
intended to support an increase in the use of compost as a constituent of growing media.
A number of questions around compost use in growing media have arisen within the Defra
Sustainable Growing Media Taskforce (SGMTF). These concern issues including physical
contamination, pH, bulk density, electrical conductivity, weed seeds, sciarid flies, compost
shelf life, compost consistency and microbiological safety. In addition to a short literature
review, Project OMK005-002 aims to gather information through a survey of key players in
the composting, growing media manufacturing, retailing and growing sectors. Each industry
stakeholder interviewed will be asked questions relating to their involvement in the
composting and/or growing media sector, with particular reference to their attitudes and
experiences with the above issues. The results of the survey will be collated and summarised
in a report which will aim to help improve the quality of both composts and growing media
which contain composts (as required under the SGMTF roadmap).
Availability of PAS 100 quality composts has increased in recent years as a result of
government targets for landfill diversion and recycling and due to increasing pressure to
reduce use of peat in their products, many growing media manufacturers use PAS 100
composts as a constituent of one or more of their products at varying inclusion rates.
Although most growing media manufacturers use green composts (which have been made
purely from source-segregated garden wastes) in their products, this project concerns both
green composts and food/green composts which also include food wastes.
Please note: if you do not want to answer one or more of the questions in this
questionnaire, please leave them blank and move on to the next question. The questionnaire
is designed to be completed electronically by a project team member, during or following an
interview with a compost producer/growing media manufacturer, or electronically by a
compost producer/growing media manufacturer. The boxes will expand as they are being
completed. If the questionnaire is to be filled in on paper, then the document should be
altered (boxes should be expanded by adding lines within them) prior to printing in order to
allow sufficient space for writing in them.
Organisation Details
Organisation Name
Respondent Name
Contact details
De-mystifying the use of PAS100 compost in horticultural growing media
79
1. Nature of business or organisation
Main type of operation
Geographical spread and scale
Type of clients? (e.g. wholesale [e.g. to
multiple retailers or other nurseries] or
retail [e.g. to gardeners, landscapers etc.]
Where do you get your growing media
(GM) from (e.g. mix your own or buy from
named GM manufacturer)?
If you buy in your GM, does it contain PAS
100 compost? (yes, no or don’t know)
If it does not contain PAS 100 compost,
why not? (e.g. because supplier does not
offer it, or because you do not want it, or
because your supplies does not recommend
it to you)
If your GM does contain compost, can you
comment on why you are using this (or
these) blend(s) (e.g. because you want to
reduce peat use or because your supplier
recommended it)
2. Crops covered
No. of pot plants/plant types (e.g. propagation material
[seeds/cuttings], liners, larger pot sizes and approximate
types grown (e.g. bedding, pot plants, alpines,
herbaceous, shrubs, trees, edible crops in modules/pots)
Proportion of UK
market? (%)
Market Value if
known? (£)
Sector:
Expanding (E)
Declining (D)
Static (S)
3. Please give an indication of the extent that the following factors affect whether you use growing media
which contain PAS 100 compost. Tick the box which matches your current thoughts.
This factor
prevents me
from using PAS
100 compost in
my growing
media
This factor
limits my use
of PAS 100
compost in
growing media
This factor has
been an issue for us
in the past but is
not anymore
This factor has
never been an
issue for us
Physical contamination
pH
High bulk density
High electrical conductivity
Weed seeds
Sciarid flies and other pests
Growing medium shelf life
Consistency of product
Microbiological safety
Other (please name)
De-mystifying the use of PAS100 compost in horticultural growing media
80
4. The following is a list of potential benefits of including PAS 100 compost in your growing media mix. Please
tick ONE box on the right for each row to indicate that you agree, disagree or are not sure whether the
statements on the left represent genuine benefits to you when compost is included in your growing media.
Agree
Neither agree
nor disagree
Disagree
Source of slow release nutrients
Less moss/liverwort on pot tops
Lower cost per m3 compared to other ingredients
Soil-borne disease suppression
Better moisture retention
Lower lime and base fertiliser requirement
Better buffering of nutrients
To meet recycling targets
Improved carbon footprint
Improved sustainability of growing medium
Other benefits, please name
5. The characteristics of growing media which contain compost can vary between suppliers. Could you please
tell us the range of values you expect from the growing media you use? NB: If you do not have the correct
figures to hand, simply comment on any problems you have in sourcing growing media with appropriate
characteristics for the parameter of concern).
Characteristic
Units
Value
Comments
Physical contamination
% m/m
pH
pH Units
High bulk density
g/l
High electrical conductivity
EC µS/cm
Weed seeds
Sciarid flies and other pests
%
Present/absent
Compost shelf life
Consistent product?
Microbiological safety
Other (please name)
Months
Yes/No
no. of CFU/ml
De-mystifying the use of PAS100 compost in horticultural growing media
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6. If you have not used growing media which are partly based on PAS 100 compost and do not intend to do
so please let us know why? (Please give as much detail as possible)
7. Do you have any other comments/discussion points you would like to raise on compost use in growing media:
8. Are you aware of the following publications
WRAP good practice guide (Compost production for use in growing media)
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Good_Practice_Guide.pdf
WRAP Guidelines for the specification of quality compost for use in growing media
http://www.wrap.org.uk/sites/files/wrap/Growing_Media_Specification.pdf
(yes or no)
9. What would allow you to use or encourage you to use compost (or use more compost) in your growing media
– for example, data or information on specific benefits from using PAS 100 composts, trials results where
composts have been used successfully to grow specific plant types, further details of the PAS scheme,
changes to the PAS scheme, changes to the WRAP good practice guide, improved Guidelines for the
specification of quality compost for use in growing media, recommendation from a respected authority, or
something else?
Please tick this box if you are happy for us to attribute your comments directly to you
in the report of our findings which will be submitted to WRAP and the SGMTF.
OR
Please tick this box if you are happy for us to use your comments but would like to
remain anonymous
Thank you for taking the time to communicate your views on the use of compost within growing
media. Your help is very much appreciated.
Please return to audrey@earthcaretechnical.co.uk
De-mystifying the use of PAS100 compost in horticultural growing media
82
C) Questionnaire on the use of BSI PAS 100 Compost in
growing media (for retailers)
WRAP (Waste and Resources Action Programme) has commissioned a short project (Project
OMK005-002) which is looking at the use of PAS 100 composts in growing media. The
purpose of the work is to improve awareness in the use of composts within the growing
media industry by collating and/or signposting existing information on its safe, effective use
and by identifying the nature of further research action, if this is required. The work is also
intended to support an increase in the use of compost as a constituent of growing media.
A number of questions around compost use in growing media have arisen within the Defra
Sustainable Growing Media Taskforce (SGMTF). These concern issues including physical
contamination, pH, bulk density, electrical conductivity, weed seeds, sciarid flies, compost
shelf life, compost consistency and microbiological safety. In addition to a short literature
review, Project OMK005-002 aims to gather information through a survey of key players in
the composting, growing media manufacturing, retailing and growing sectors. Each industry
stakeholder interviewed will be asked questions relating to their involvement in the
composting and/or growing media sector, with particular reference to their attitudes and
experiences with the above issues. The results of the survey will be collated and summarised
in a report which will aim to help improve the quality of both composts and growing media
which contain composts (as required under the SGMTF roadmap).
Availability of PAS 100 quality composts has increased in recent years as a result of
government targets for landfill diversion and recycling and due to increasing pressure to
reduce use of peat in their products, many growing media manufacturers use PAS 100
composts as a constituent of one or more of their products at varying inclusion rates.
Although most growing media manufacturers use green composts (which have been made
purely from source-segregated garden wastes) in their products, this project concerns both
green composts and food/green composts which also include food wastes.
Please note: if you do not want to answer one or more of the questions in this
questionnaire, please leave them blank and move on to the next question. The questionnaire
is designed to be completed electronically by a project team member, during or following an
interview with a compost producer/growing media manufacturer, or electronically by a
compost producer/growing media manufacturer. The boxes will expand as they are being
completed. If the questionnaire is to be filled in on paper, then the document should be
altered (boxes should be expanded by adding lines within them) prior to printing in order to
allow sufficient space for writing in them.
Organisation Details
Organisation Name
Respondent Name
Contact details
De-mystifying the use of PAS100 compost in horticultural growing media
83
1. Nature of business or organisation
Main type of operation (Single site/multi-site)
Geographical spread (county, single country UK,
multi-country UK, multi country, Europe)
Type of clients (e.g. amateur gardeners,
professional growers)
2. Growing media (containing composts) sold
Approximately what percentage of total UK growing media sales is your company
responsible for (including your own branded and independent branded products)?
Do you consider the market for growing media based partly on PAS 100 composts to be
expanding (E), declining (D) or static (S)?
Products sold (please fill in a separate line below for each product sold, and the
percentage of compost in each)
%
E/D/S
% of compost
included
3. Please give an indication of the extent to which the following factors affect whether you are prepared to
sell growing media which contain PAS 100 compost. Tick the box which matches your current thoughts.
This factor
prevents me from
selling growing
media containing
PAS 100 compost
This factor limits
our sales of
growing media
containing PAS
100 compost
This has
been an
issue for us
in the past
but is not
any more
This has
never
been an
issue for
us
Am not
aware of
the issue
Physical contamination
pH
High bulk density
High electrical conductivity
Weed seeds
Sciarid flies and other
pests
Shelf life of product
Consistency of product
Microbiological safety
Poor customer perception
Other (please name)
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4. Please give an indication if the following potential benefits might make you consider asking for PAS 100
compost to be included within your growing media mixes. Tick the box which matches your current
thoughts.
Agree
Neither agree
nor disagree
Disagree
Am not aware
of this
potential
benefit
Source of slow release nutrients
Less moss/liverwort on pot tops
Lower cost per m3 compared to other ingredients
Soil-borne disease suppression
Better moisture retention
Lower lime and base fertiliser requirement
Better buffering of nutrients
To meet recycling targets
To improve carbon footprint/sustainability of growing
media
Specific requests from customers
Other benefits, please name
5. If you do not sell growing media which contains PAS 100 compost and do not intend to do so please let us
know why?
6. Do you have any other comments/discussion points you would like to raise on growing media which contain
PAS100 composts in growing media:
7. What would allow or encourage you to consider buying and selling more growing media which contain
PAS100 compost – for example, customer pressure, promotion of compost-based media from television
programmes or gardening publications, readily available information on specific benefits from using growing
media based on PAS100 composts, trials results where composts have been used successfully to grow
specific plant types or something else?
Please tick this box if you are happy for us to attribute your comments directly to
you in the report of our findings which will be submitted to WRAP and the
SGMTF.
OR
Please tick this box if you are happy for us to use your comments but would like to
remain anonymous
Thank you for taking the time to communicate your views on the use of compost
within growing media. Your help is very much appreciated.
Please return to audrey@earthcaretechnical.co.uk
De-mystifying the use of PAS100 compost in horticultural growing media
85
www.wrap.org.uk