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Literature review: Digestate use in protected horticulture

A desk study that collates information relevant to the UK on the use of digestates as a fertiliser or growing media ingredient for protected ornamental and food crop production.

Project code: OMK005-005 Research date: January-March 2014

Date: 4 March 2015

WRAP‘s vision is a world where resources are used sustainably. Our mission is to accelerate the move to a sustainable resource-efficient economy through re-inventing how we design, produce and sell products; re-thinking how we use and consume products; and re-defining what is possible through reuse and recycling. Find out more at www.wrap.org.uk

Written by: Mary Dimambro and Joachim Steiner (Cambridge Eco Ltd) and Document reference: WRAP, 2014, Banbury, Literature review: Digestate use in Francis Rayns (Garden Organic) protected horticulture, Prepared by M E Dimambro, J Steiner and F Rayns

Front cover photography: Strawberries growing in a nutrient solution containing whole food waste digestate at Warwick Crop Centre (Cambridge Eco Ltd) While we have tried to make sure this report is accurate, WRAP does not accept liability for any loss, damage, cost or expense incurred or arising from reliance on this report. Readers are responsible for assessing the accuracy and conclusions of the content of this report. Quotations and case studies have been drawn from the public domain, with permissions sought where practicable. This report does not represent endorsement of the examples used and has not been endorsed by the organisations and individuals featured within it. This material is subject to copyright. You can copy it free of charge and may use excerpts from it provided they are not used in a misleading context and you must identify the sourceName of the material and acknowledge WRAP‘s copyright. You must notWRAP] use this report or material from it to endorse or Document reference: [e.g. WRAP, 2006, Report (WRAP Project TYR009-19. Report prepared by…..Banbury, suggest WRAP has endorsed a commercial product or service. For more details please see WRAP‘s terms and conditions on our website at www.wrap.org.uk

Executive summary The aim of this review is to describe the current state of knowledge worldwide regarding the use of whole digestate, separated liquid digestate (liquor) and separated solid digestate (fibre) from anaerobic digestion (AD) in protected horticulture. Overall, published studies indicate that there is significant potential for digestates (whole, liquor and fibre) to be used in protected horticulture for both ornamental and edible crop production. A number of studies have examined the use of whole and separated liquor as a liquid organic fertiliser in soil-grown crops and also in soil-less production. Evidence was also found of whole, liquor and fibre digestate as growing media ingredients. Several studies have been identified, investigating either composting the digestate fibre alone or co-composting with other ingredients. The composted end-products were then assessed for their potential as growing media or as growing media ingredients. The evidence suggests that whole, liquor and fibre digestate, in combination with other standard industry ingredients, can generally achieve similar or better yields compared to standard growing practices, provided the recommended nutrient and electrical conductivity (EC) levels are considered. Moreover, in some instances improved crop quality and/or taste was reported when using digestates. As the trials reported generally focussed on a limited number of digestate treatments, further optimisation of the treatments may have given better results, and would be recommended, should digestate use be considered for commercial use in protected horticulture. Large scale adoption of digestates within the protected sector could result in a reduction in the use of peat and inorganic fertilisers in this sector. There is evidence that growing media incorporating digestate fibre (from biosolids) are used in commercial growing media products endorsed by US local administration, and winning national awards by the US Environmental Protection Agency. Perceptions around food safety could present a barrier to the uptake of digestates in some markets. Very few studies were found to consider microbiological aspects of food safety with regards to the use of digestates in protected horticulture. No evidence was found to suggest that the use of digestates posed a greater microbiological risk than commercial growing media and fertilisers, although one UK study did recommend that further research was required. Moreover, where overseas studies included an analysis of food safety in terms of microbial contamination or levels of potentially toxic elements, these were not deemed to be greatly different to those in conventional growing media and fertilisers. In addition, PAS110 compliance ensures PTEs, pathogens and organic contaminants are monitored and kept within safe levels. In regulatory terms, the current market limitations imposed by the anaerobic digestate quality protocol (ADQP) present a significant barrier to the uptake of digestates in some markets. The UK regulatory approach should be kept under review, particularly given the apparently successful deployment of digestates in a range of markets elsewhere.


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Contents 1.0 2.0 3.0 4.0

Introduction ................................................................................................. 7 Methodology ................................................................................................. 7 Findings ........................................................................................................ 8 Whole digestate and digestate liquor studies with soil-grown crops ........... 9 4.1 UK research: Use of digestates as a liquid fertiliser in strawberry production ... 9 4.2 Research outside the UK: Using whole digestate and digestate liquor as a fertiliser for soil-grown crops ............................................................................... 10 5.0 Whole digestate and digestate liquor studies with soil-less production .... 13 5.1 UK research: Hydroponic production of tomato and lettuce with three whole digestates .......................................................................................................... 13 5.2 Research outside the UK regarding whole digestate and digestate liquor with soil-less production............................................................................................. 15 6.0 Whole digestate and digestate liquor as a growing media ingredient........ 18 6.1 UK research: Bark admixtures: Formulation and testing of novel organic growing media using quality digestates for the production of containerised plants ... 18 6.2 Research outside the UK regarding whole digestate and digestate liquor as a growing media ingredient ................................................................................... 20 7.0 Digestate fibre use in horticulture .............................................................. 21 7.1 Digestate fibre as a growing medium ingredient ......................................... 22 7.1.1 UK research: The use of cattle slurry digestate fibre in mixtures with coir and pine bark as plant growth substrates in the intensive production of glasshouse tomato crops .......................................................................... 22 7.1.2 Research outside the UK: Digestate fibre as a growing medium ingredient ................................................................................................ 24 7.2 Using composted digestate fibre as a growing media ingredient ................... 31 7.2.1 UK research: The potential of using composted digestate fibre for horticulture for ornamental production ....................................................... 31 7.2.2 Research outside the UK: Using composted digestate fibre as a growing media ingredient ...................................................................................... 33 7.3 Using digestate fibre co-composted with other materials as a growing media ingredient .......................................................................................................... 34 7.3.1 Digestate fibre co-composted with vine prunings .............................. 35 7.3.2 Digestate fibre co-composted with wood chip and flax straw ............. 35 7.3.3 Digestate fibre co-composted with sulphur and almond shell powder.. 38 8.0 Bioassays using digestates as a growing media ingredient ........................ 38 9.0 Commercial studies .................................................................................... 39 10.0 Conclusions ................................................................................................ 40 11.0 References .................................................................................................. 42

Acknowledgements The authors would like to thank Rob Lillywhite and Catherine Keeling of the University of Warwick for assistance with the literature searching and interpretation, in addition to the UK and European academics who provided information on their research.


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Glossary AD

Anaerobic digestion

Admixture

The substance that results from mixing all the ingredients in a growing media ‗recipe‘; more generally a mixture which results when two different materials are combined without occurrence of chemical reactions

ADQP

Anaerobic Digestate Quality Protocol – End of waste criteria for the production and use of quality outputs from the anaerobic digestion of source segregated wastes

Biofertiliser

Biofertiliser is the name adopted for digestates certified as compliant with UK end of waste criteria

Biogas

Mixture of gases produced by anaerobic digestion

Biosolids

Treated sewage sludge

BMW

(Source segregated) biodegradable municipal waste

CAT

Calcium chloride/DTPA

COD

Chemical oxygen demand

cv

Cultivar

DECC

The Department of Energy & Climate Change

Digestate fibre

Fibrous fraction of material derived by separating the coarse fibres from the whole digestate

Digestate liquor

Liquid fraction of material remaining after separating coarse fibres from whole digestate

DM

Dry matter

EA

Environment Agency

EC

Electrical conductivity. Units: 106 μS/cm = 103 mS/cm = 1 S/cm

Fertigation

The application of water-soluble fertilisers through an irrigation system which is principally used in trickle and tape systems

FM

Fresh matter

GRH

Ground rice hulls

GWC

Green waste compost


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HONS

‗Hardy Ornamental Nursery Stock‘. Plants grown on nurseries for use in gardens and managed landscapes. Hardy refers to them being able to survive the winter without significant damage from frosts.

Hygromull

Open-pore hydrophilic PU foam improves WHC and substrate aeration, and is able to adsorb nutrients.

Lecaton

Thermally expanded burnt clay granules improve the physical characteristics of substrates, are able to absorb cations, and may release Ca

MBT

Mechanical Biological Treatment – combination of mechanical and biological treatments for extracting recyclables from mixed household waste

MSW

Municipal Solid Waste

ORG

Organic Recycling Group, part of the Renewable Energy Association

PAS110

The British Standards Institution‘s publicly-available specification BSI PAS 110 is a specification for digestate quality (BSI, 2014). It is also a core requirement for the UK‘s end of waste positions for digestate. PAS110 specifies:  Controls on input materials and the management system for the process of anaerobic digestion and associated technologies  Minimum quality of whole digestate, separated fibre and separated liquor  Information that is required to be supplied to the digestate recipient

Perlite

Expanded volcanic Al-silicate increases the aeration and water-holding capacity of substrates

PTEs

Potentially toxic elements (heavy metals)

Styromull

Closed-pore polystyrene foam which improves substrate aeration

TKN

Total Kjeldahl nitrogen

TOC

Total organic carbon

TS

Total solids

WHC

Water holding capacity

Whole digestate

Material resulting from an anaerobic digestion process that has not undergone post-digestion separation

WRAP

Waste & Resources Action Programme


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1.0

Introduction

In 2012, an estimated 1.44 million tonnes of digestate was produced from anaerobic digestion (AD) in the UK, with approximately 99% of this being used in agriculture and field horticulture, with the very small remainder used for land restoration (WRAP, 2013b). Whilst there are undoubted benefits associated with the use of digestate in agriculture1, the reliance on a single market is risky, and AD industry resilience would be improved if a wider range of markets could be developed. Indeed, the Defra/DECC Anaerobic Digestion Strategy and Action Plan identified the need to find appropriate markets for quality digestates, and one of WRAP‘s aim is to facilitate the increased uptake of digestate use by the horticultural industry (Defra and DECC, 2011). Within this context WRAP has supported a number of field projects to investigate potential non-agricultural markets for digestate. These include landscaping and regeneration, energy crops on previously developed land, sports and amenity turf, and soil manufacture (under projects OMK001 and OMK004). The use of digestates in protected plant horticulture has also been highlighted as an area for potential development (WRAP, 2011b, Zero Waste Scotland, 2010), and this has been progressed via WRAP programme OMK006. This includes four research projects which considered a range of uses of digestates for protected horticultural crops, including edibles and ornamentals (WRAP 2015a, 2015b, 2015c, 2015d). At present, the Anaerobic Digestate Quality Protocol (ADQP) does not cover digestates used in these emerging markets (WRAP and Environment Agency, 2010). WRAP‘s OMK006 trials recommended that a brief review of existing research into the use of digestates in protected horticulture could be of benefit to the industry, highlighting less mainstream potential uses for digestate as well as informing future regulatory approaches to such emerging digestate markets. This desk study aims to address these recommendations. 2.0

Methodology

Search terms were established for the literature searching, including synonyms, spelling variations and different combinations of terms as listed below. Academic searching was largely carried out using the ISIS Web of Science and Coventry University ‗Locate‘. These covered not only the principal agricultural and horticultural databases, but also food-related databases, environmental science, water, pollution, toxicology and general science databases. A number of these databases also index grey literature. The titles retrieved from these searches were screened, and then abstracts were downloaded. These abstracts were subjected to a second round of screening where useful papers were selected. Full text was obtained for these items. In addition, relevant publications cited in reports obtained in the searching were also obtained. In addition to the standardised interrogations of the academic databases mentioned above, unstructured searches were also carried out using internet search engines (Google, Google Scholar, Yahoo, Google.de and Defra Science search). For grey literature searches using internet search engines there are always an almost infinite number of hits, and so only relevant web pages were investigated. Some literature was available from WRAP, EA and ORG websites and sources, as well as the authors‘ databases and libraries.

1

For example, see www.wrap.org.uk/dc-agri


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The searches focussed on publications containing information about the use of digestate in horticultural systems. The search terms used included digestate, anaerobic digestion residue, anaerobically digested, anaerobic fermentation residue, fermented residue, biogas residue, biogas slurry, biogas effluent plus a combination of one or more of the following:  Whole, liquor, liquid, separated, fibre, fiber, solid  Horticulture, protected horticulture, glasshouse production  Hydroponics, growing media, soil-less production, glasshouse production, nutrient solution, horticulture, peat, pot trial  Food crop plants: Including tomato, pepper, cucumber, lettuce, strawberries, pak-choi, raspberries  Ornamentals: Including ferns, cyclamens, chrysanthemums, begonias, primroses, polyanthus, hedera, osteospermum, poinsettias, fuchsias, geraniums, pansies, impatiens, cineraria, hydrangea and various species of trees including pine. This list is not exhaustive but represents the main search terms used, with plant species listed in the Defra glasshouse survey (Defra, 2008) as the main starting point, where individual species searches were undertaken. In addition to literature published in English, articles written in German were also investigated, since AD is well-established in German speaking countries, with the above search terms in German language equivalents. Some literature was also obtained published in Italian, although no extensive searching in the Italian language was undertaken. European digestate experts were contacted to ascertain whether further information could be found regarding work which is not available in English, for an insight into any additional European studies. 3.0

Findings

The review of the academic literature revealed that a range of information was available for a number of crops. The idea of using digestate, either whole or separated, in growing media had been suggested in a number of reports and reviews (for example: (Hogg et al., 2007, WRAP, 2011b, Arvanitoyannis, 2008)) but it is not always clear if this is based on experience or purely recommendations for research. Sometimes the terminology was unclear, particularly in publications from overseas, so it was difficult to be sure if the authors were referring to digestate of the consistency, feedstock type, quality and AD technology of that presently available in the UK. Discussions with European experts revealed a few additional studies which generally focussed on the use of digestates from maize and silage-based systems which tend to have high dry matter content. These findings are collated and discussed below, according to digestate use and type. Unfortunately not all publications clarify whether the digestate has been separated into fibre and liquor fractions or remains whole, but where this is defined this information is included. Some studies were not written in English, German or Italian, and in such cases where only the abstract was in English with insufficient information available for the purposes of this review, this has been highlighted. A range of relevant publications in Chinese were identified, with insufficient information in the brief English abstracts to understand the trial design or specific use of the digestate. These publications included the use of digestates in the production of tomatoes (Tongguo, 2011, Chang, 2010, Li and Zhang, 2001, Xie, 2010), cucumber (Tongguo, 2011), mini Literature review: Digestate use in protected horticulture

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cucumbers (Hao, 2007), peppers/capsicum (Zhao, 2007, Jianxiang, 2007, Wei, 2009, Tongguo, 2011), aubergine (Li and Zhang, 2012), lettuce (AI, 2006), spinach (Li, 2003), Chinese cabbage (Dong-Dong, 2010, Zhang et al., 2010), celery (Yi, 2011) and strawberries (Zhao, 2010, Chen, 2007). 4.0

Whole digestate and digestate liquor studies with soil-grown crops

Crops grown in soil or growing media (which may include peat or coir), are often fertilised with a nutrient solution which may be delivered as part of the irrigation regime (fertigation), or as a separate fertiliser once, or at set time points. Firstly, recent UK work on the use of digestate as a fertiliser for strawberries is discussed. This is followed by a review of studies of whole and liquor digestate use as a fertiliser, with the majority of work in this area focussing on tomato production. 4.1

UK research: Use of digestates as a liquid fertiliser in strawberry production

A feasibility study undertaken in 2013 had the aim of establishing whether six digestates produced in the UK could be used successfully as fertiliser for strawberry production (WRAP, 2015d). The feedstock materials included potato waste, dairy cattle slurry, food waste, maize, and a mixture of maize, manure and milk waste. Some digestates were whole digestates, whilst others were the separated liquor fraction. Overall, the results showed that it is possible to produce strawberries using digestate-based nutrient solutions with yields and quality similar to strawberries grown with traditional mineral fertilisers (see front cover photograph). Table 1. Final nutrient concentrations in the digestate solutions following dilution for the strawberry trial (WRAP, 2015d) Recommendation min N mg/l

Control*

Maize/ Manure (l) 71

Maize

Potato

Slurry

Food

Food

(l)

(w)

(w)

(w)

(l)

Digestate amendments

71

71

71

71

71

49 (as KNO3)

(21x)

(23x)

(27x)

(51x)

(53x)

(36x)

120

P mg/l

120-150 (Req. dilution factor) 40-55

27

11

8

4

4

4

3

K mg/l

250-300

217

175

150

179

72

43

41

100-120 25 1200-1700 500-800 20-50

79 8 417 281 31

1.5-1.8

1.8

52 12 1897 164 0 0.26 2.2

43 9 767 107 0 0.22 2.1

29 6 176 30 0 0.08 2.1

33 8 446 86 0 0.10 1.9

29 6 138 25 0 0.05 2.1

33 6 240 36 0 0.08 1.9

Ca mg/l Mg mg/l Fe µg/l Mn µg/l Mo µg/l Total solids (%) EC (dS/m)ǂ

47 (as MKP) 196 (as KNO3 & KP)

1200** 500 40

The digestate amendments column on the right indicates the additional nutrients added to all six digestates, to ensure a minimum requirement appropriate to the crop. The figures refer to the mineral elements as specified in the left hand column. * The control treatment solution was prepared using a 1-1-3 feed with the addition of calcium and potassium nitrate (figures in this column include all additions) ** No additional iron was added to the Maize/Manure(s) digestate ǂ The ECs given here are of the prepared digestate (and control) feed solutions as delivered to the plants (digestate + water + amendments) (l) = digestate liquor; (w) = whole digestate

Glasshouse trials were carried out with strawberry plants, (variety Elsanta), planted in standard commercial peat grow-bags. Digestates were diluted between 25 and 51 times to bring their nitrogen concentrations down to commercial norms (see Table 1). Some minor amendments were then performed to optimise the profile of the nutrient solution. Solutions


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were delivered to the plants via low pressure trickle irrigation. Fruit yield and quality were compared with fruits from control plants fertigated using conventional nutrient solution. There were significant differences in yield and fruit quality between digestate treatments of various feedstock types: Three of the six digestates performed as well as the control in terms of both total fruit yield and Class 1 yield. Two (food and dairy cattle slurry based) treatments produced fruit that generally out-performed the control throughout the season with respect to taste assessments. Improvement in fruit flavour was the most notable difference. The other three digestate treatments had slightly reduced yields compared to the control, but this reduction was large enough to be significant only in one of the treatments (digestate derived from maize, manure and milk waste). All three of these digestate treatments achieved better results in the taste test than the control. Figure 1. Day 18 showing strawberry plants fertigated with digestate and an industry standard (control). All plants observed to be healthy with no effect of treatment in evidence (WRAP, 2015d).

4.2

Research outside the UK: Using whole digestate and digestate liquor as a fertiliser for soil-grown crops

Tomato trials were carried out over several years at the GBZ Straelen, in Germany, and summarised briefly in a number of research notes (Andreas, 2004a, Andreas, 2005, Andreas, 2006, Andreas, 2003). Tomatoes were grown under glass with white mulch foil as a soil cover. In most cases, no data is available on the digestate type, nutrient breakdown or application timings in these research notes other than what is reported below. In 2002, the trial consisted of two treatments both with an application rate of 300 kg N/ha over the season. The control was two applications of calcium ammonium nitrate, with the second treatment being digestate (from farm residues) applied through drip-feeding. For the three tomato varieties trialled, there were no significant differences in yield between the control and digestate treatments (Andreas, 2003). In 2005, a standard organic treatment of horn meal as the base fertiliser and vinasse (distillation residue) applied as a top dressing (two applications), was compared to a digestate treatment which comprised three applications of digestate after planting (with 1.28 kg N, 0.3 kg P2O5 and 1.28 kg K2O/m³), with both treatments having the same total N applied (340 kg N/ha) (Andreas, 2006). Tomato yield differences between treatments were Literature review: Digestate use in protected horticulture

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insignificant, varying slightly between the five tomato varieties trialled. In 2004, with different tomato varieties and similar treatments, the results were slightly more scattered than 2005, but comparable yields were observed in both the digestate and control treatments for each variety (Andreas, 2005). Figure 2: Tomatoes grown using a digestate fertiliser solution at GBZ Straelen (Andreas, 2004a).

Figure 3: Left: Cocktail tomato ―Oakley‖, grown using digestate (Reintges 2013, pers. comm.). Right: Round ―organic‖ tomatoes grown using a digestate fertiliser solution at GBZ Straelen (Andreas, 2007).

Figure 4: Peppers grown using a digestate fertiliser solution at GBZ Straelen in 2004 (left) and 2007 (right) (Andreas, 2004b, Andreas, 2007).

Furthermore, unpublished studies at GBZ Straelen investigated pepper grown using maize/cereal/sugar beet digestate. No results are publicly available, although photographs


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have been published to show young pepper plants thriving on digestate fertiliser in 2004 and 2007 (Andreas, 2004b). In a study from the USA, digestate produced from pig slurry and wastewater from a pig unit was treated with a trickling nitrification biofilter with polystyrene beads, which converted almost 90% of the ammonium in the digestate into nitrate (Cheng, 2004). The resultant nitrified digestate was subsequently used as both fertiliser and irrigation water for approximately 14,400 tomato plants grown in perlite in greenhouses. All tomatoes were grown using the treated digestate, with no commercial control. Experimental data indicate that the tomato greenhouses used approximately 12 m3 of the effluent and 3.84 kg nitrogen per day. Moreover, the daily yield was 520 kg (37 g/plant) of marketable fruit, which was deemed by the authors to represent a financially viable operation.

Table 2. Average nutrient concentration of the digestate after treatment with the nitrification biofilter, which was subsequently used for tomato production (Cheng, 2004)

TKN NH4-N NO3-N P o-PO4-P K COD TOC pH TS VS

mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l % % of TS

Biofilter- treated digestate 24.2 14.0 109.3 39.8 30.1 398.5 114.9 44.8 7.2 0.336 15.9

A study on cucumber production used digestates produced from pig manure, with one digestate type termed ‗biogas slurry‘ (assumed to be whole or liquid digestate due to the 3% organic matter content), and the other termed ‗biogas residue‘ (assumed to be separated fibre due to the 30% organic matter content) (Duan et al., 2011). One treatment (treatment 1) used digestate in three different ways (see Table 3): Digestate fibre was used as a base fertiliser, providing 1/3 of the nutrients, whole/liquor digestate was used as a top dressing, and whole/liquor digestate was used as a foliar application. In both instances the whole/liquor digestate was mixed 1:1 with water. Inorganic fertiliser was used as the control treatment and also applied in three ways: Base fertiliser, top dressing and foliar application (treatment 2, see Table 3). The chlorophyll content in the leaves was higher in the digestate treatment. Compared with the control, the cucumbers grown with digestate were longer with a lower curvature (both properties deemed an advantage), with a significant total yield increase of 16%. Moreover, the cucumbers grown with digestate had higher concentrations of soluble sugars and proteins, indicating an improved nutritional quality. Both the digestate and control cucumbers had concentrations of Pb, Cd, Hg and As which were lower than the minimal detectable limit of the Chinese national standard. The incidences of aphids and mildew on cucumber plants grown with digestate were both significantly fewer compared to the control.


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Table 3. Fertiliser treatments for cucumbers (Duan et al., 2011)

Nutrient application

Treatment

Digestate fibre (kg)

Digestate ‘slurry’ (kg)

Base fertiliser Top dressing Foliar application

1 2 1 2 1 2

20 0 0 0 0 0

0 0 40 0 5 0

Water (kg)

Diammonium phosphate (kg)

0 0 40 79.69 5 9.95

0 0.9 0 0.18 0 0

Carbamide (kg)

Potassium chloride (kg)

Potassium sulphate compound fertiliser (kg)

0 0.15 0 0.03 0 0

0 0.60 0 0.10 0 0

0 0 0 0 0 0.05

The following study focuses on rye grass, which is not generally grown in the glasshouse, but nonetheless provides additional data on the use of digestates as a fertiliser for pot grown plants. In a German pot trial, perennial rye grass (Lolium perenne) was grown in loamy sand for five months in 21 cm diameter, 25.5 cm high pots (Benzenberg et al., 2011). After 40 days, a one-off fertiliser application was made, with two types of digestates from crops (silage feedstock species not specified in the paper, but most likely grass). One digestate was produced from the separated liquid fraction of silage, and the other digestate from whole crop (unseparated) silage. Inorganic fertiliser was used as the control treatment. The two digestates and the inorganic fertiliser were all applied at five N-rates: 0, 50, 100, 150 and 200 kg N/ha. Increasing the N rate resulted in increasing biomass yield. For each N rate, there was a similar above-ground biomass yield for the two digestates and the mineral N fertiliser. However, for the digestate made from the liquid fraction of silage, the biomass yield response levelled off at 150 kg N/ha. 5.0

Whole digestate and digestate liquor studies with soil-less production

Soil-less production via hydroponics includes a range of systems, from plant roots grown purely in nutrient solution through to plants grown with nutrient solution in inert media such as perlite, rockwool or gravel. Further details of hydroponics systems are discussed in a recent WRAP report (WRAP, 2015b). The majority of work on the potential of digestates in such systems has been undertaken outside the UK, with one recent WRAP funded trial conducted in England, as discussed below. 5.1

UK research: Hydroponic production of tomato and lettuce with three whole digestates

The feasibility of using digestates for the hydroponic production of tomato and lettuce in England was investigated in a recent study (WRAP, 2015b). Three whole digestates were used, with two from food waste and a third from cattle manure and potato waste. Overall, the study showed that digestate was suitable as an amendment in hydroponic solutions, but that diluting the digestate to an appropriate ammonium concentration for the crop is an important consideration. Further research was recommended on the potential for pathogens to be present in the digestates.


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The digestates were all diluted to ensure that ammonium represented only 10% of the required total nitrogen concentration in the final solution (see Table 4). Inorganic nutrients were then added to ensure that there were no nutrient deficits. Two controls were used; an industry standard and also an industry standard plus ammonium (at 10% of total nitrogen). Plants of tomato (cultivar Supersweet 100) and lettuce (cultivar Little Gem) were established in rockwool blocks that were then placed into an open hydroponic system where the aerated nutrient solutions were changed every seven days. Plant growth and (for tomato) fruiting were monitored and compared with plants grown using conventional nutrient solutions.

Table 4. Dilutions required to reduce digestate ammonium to 10% of the total N (WRAP, 2015b). Where there are three sets of numbers, those outside brackets refer to the pre-flowering stage for tomato, those within parenthesis refer to fruitset (tomatoes) and those within square brackets refer to lettuce.

Total-N (mg/l) Ammonium-N (mg/l) Nitrate-N (mg/l) Excess total-N (x) Ammonium-N (% of total) Dilution needed (x) Nitrate amendment required (mg/l)

Cattle manure & potato waste 3359 2846 513 30 (23) [17] 85 252 (198) [142] 100 (127) [176]

Table 5. The final nutrient concentrations mg/l N P Mg Tomato: Pre-fruiting 113 62 50 Tomato: Post-fruiting 144 62 50 Lettuce 200 62 50

of the K 199 199 154

solutions for Ca Mn 122 0.62 165 0.62 247 0.62

Food waste 1

Food waste 2

6912 6654 258 61 (48) [31] 92 589 (462) [333] 102 (129) [179]

4327 4227 100 38 (30) [21] 98 374 (294) [211] 101 (129) [180]

the hydroponics trial(WRAP, 2015b) Fe Cu Zn Mo Cl B 2.5 0.05 0.09 0.03 0.85 0.44 2.5 0.05 0.09 0.03 0.85 0.44 2.5 0.05 0.09 0.03 0.85 0.44

The tomato fruit yield was similar for all five treatments, with sugar content and the results of taste tests indicating that fruit quality was also equivalent. Tomato fruit grown in one of the food waste digestate treatments contained more cobalt than fruit from other treatments, while fruits grown in the inorganic solution amended with 10% ammonium contained less sodium than those grown in the cattle manure and potato waste digestate.

Figure 5. Lettuce and tomato grown using digestate nutrient solution (WRAP, 2015b)

Lettuce yields were unaffected by the digestates, although leaves had higher concentrations of calcium and copper when grown in solutions containing digestates. Salmonella was tentatively identified (using conventional plating techniques) in a number of digestate


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samples, as was E. coli. In consequence, the lettuce plants were tested to ascertain whether internalisation of the pathogens had occurred. Although the lettuce was not contaminated by these pathogens, the report highlights that microbiological safety concerns may limit the usefulness of using anaerobic digestates for lettuce production until further data are available, although at the current digestate dilution levels the microbiological risks involved do not appear to be any greater than those associated with conventional production methods. 5.2

Research outside the UK regarding whole digestate and digestate liquor with soil-less production

In three studies from the USA, Liedl et al. (2004b, 2004a, 2006) compared diluted digestate produced from poultry litter with commercial hydroponic feeds in soil-less tomato, cucumber and lettuce production, and found promising results in all three systems, as described below. In a hydroponic tomato production trial, tomato plants were grown in buckets containing a mixture of 85% coarse perlite and 15% coir (v/v). Two nutrient solutions were used for fertigation: A commercial standard fertiliser (150ppm N, 50 ppm P, 200 ppm K, 150 ppm Ca, 50 ppm Mg, 60 ppm S and various trace elements (not specified)), and diluted poultry litter digestate, with the NH3-N content of the digestate diluted to match the N concentration in the commercial fertiliser treatment. Unfortunately no data on the nutrient content or dilution rate of the digestate is presented in the paper. The pH of both nutrient solutions was adjusted to achieve 5.5-6.8 using 85% H3PO4 and EC levels of 2.2-2.8 mS/cm. With digestate alone, plant growth rate was reduced, and fewer, smaller fruits were produced, with signs of ammonia toxicity. The authors highlighted that tomato plants are sensitive to fertilisers where ammonia is the dominant form of N. To reduce ammonia levels the ammonium/nitrate ratio was balanced by firstly heating and air sparging the digestate to reduce ammonia levels, and then by adding Ca(NO3)2, to return the nutrient solution to the same N concentration as the commercial control. When this solution was fed to the tomato plants signs of Mg deficiency were evident. It was found that when the N forms had been balanced and the Mg concentration supplemented using MgSO4 with the same Mg concentration as the commercial control, the poultry litter digestate solution was found to function as well as the commercial control (Liedl, 2004a). A summary of the results is shown in Figure 6. Figure 6. Summary of the main findings of a series of hydroponic tomato trials (Liedl, 2004a)

In the second trial, four growth trials were conducted on hydroponically grown lettuce transplants (nutrient film technique). In all trials there were four treatments, including a


15

commercial hydroponic solution (150ppm N, 54 ppm P, 250 ppm K, 134 ppm Ca, 33 ppm Mg and various trace elements (not specified) and three concentrations of poultry litter digestate (no raw digestate analysis data provided, but for some information on the diluted digestates see Table 6). The first three trials used 100, 200 and 300 ppm ammonium-N and the fourth 50, 100 and 150 ppm ammonium-N in the digestate. The nutrient solution pH was maintained at 5.5-6.1 using H3PO4, with this addition enhancing P levels of the digestate treatments. The dilution of the digestate was found to be critical, as increasing the concentration above the 100-200 ppm N optimum for lettuce production was detrimental to shoot fresh weight, with higher digestate concentrations also changing the taste by enhancing bitter characteristics. Increasing digestate concentration also increased root fresh weight. The 100 ppm N digestate treatment produced shoot fresh weights not significantly different from those produced using the conventional commercial solution. Tissue N content ranged from 4.3-5.2% (assumed to be DM), with highest N observed in the commercial control treatment, with more desirable plant tissue N levels found in the digestate treatments (Liedl, 2004b).

Table 6. Analysis of the nutrient solutions used in the fourth lettuce trial, in ppm (Liedl, 2004b). Nutrient component Nitrate Ammonium Phosphorus Potassium Calcium Magnesium Iron Manganese Boron Copper Zinc Molybdenum Sodium Aluminium pH Conductivity mS/cm

Commercial control 348.00 0.34 129.55 110.48 223.86 55.41 3.29 0.60 0.34 0.13 1.26 0.09 27.06 0.00 5.69 1.87

Digestate 50 ppm N 9.35 0.49 221.97 238.76 29.53 11.30 0.28 0.01 0.16 0.23 0.20 0.03 35.44 0.01 5.67 0.95

Digestate 100 ppm N 0.00 43.60 410.80 216.06 51.74 15.88 0.55 0.41 0.26 0.35 1.05 0.01 52.00 0.14 5.25 1.31

Digestate 150 ppm N 0.00 126.00 632.37 263.54 63.41 20.51 0.76 0.88 0.40 0.54 0.84 0.03 70.13 0.12 5.51 2.12

In soil-less cucumber production (perlite/coir media), the digestate was diluted to the same mineral nitrogen concentration as that found in the commercial feed, and solution pH maintained at 5.6-6.0. Average fruit weight decreased by 11 % when plants were grown with the digestate, but the percentage of fruits classified as grade 1 increased (33 % grade 1, compared to 26 % with the commercial feed) (Liedl, 2006). In a Canadian study, digestate (‗process liquid wastewater‘) from the anaerobic digestion of mixed municipal solid waste was used as a material for fertigation of three ornamentals (silverleaf dogwood, common ninebark and Spiraea) over a three month period in a growing medium consisting of 73% bark, 22% peat, and 5% pea gravel by volume (Chong, 2008). Four weeks after planting, four fertigation treatments commenced: 1 2 3 4

Recirculated control fertilizer solution based on a nutrient formula with a targeted EC of 2.2 dS/m; recirculated mushroom farm wastewater (diluted 10x with tap water); recirculated digestate from MSW (diluted 20x with tap water); Nutryon (Polyon) 17-5-12 (17N–2P–10K) 6 month controlled-release fertilizer with micro-nutrients (Nutrite, Elmira, Ont.) topdressed at a rate of 39 g/container (nutrients not recirculated).


16

The liquid was recirculated using a computerised recirculating injector system. Chemical monitoring was undertaken with supplementary feeding provided where necessary to ensure a balanced nutrient supply. Dilution ensured that sulphate, chloride and sodium were not oversupplied, with a target EC of 2.2 dS/m for all treatments. Growth, in terms of plant dry weight and height, was as good with the digestate treatment as with standard controlled release fertilisers. A Chinese study investigated the effect of a range of nutrient solutions including animal manure digestate (termed biogas slurry so assumed to be whole or liquor) on growth and leaf nitrate concentration of lettuce, using five treatments (Wenke, 2009): 1 2 3 4 5

Inorganic nutrient solution (control): NO3--N, 15mmol/l Organic nutrient solution: N from diluted digestate (1:5 Organic nutrient solution: N from diluted digestate (1:5 Organic nutrient solution: N from diluted digestate (1:4 Organic nutrient solution: N from diluted digestate (1:4

v/v): 8.69 mmol N/l v/v) + glycerine: 15 mmol N/l v/v): 10.43 mmol N/l v/v) + glycerine: 15 mmol N/l

The composition of inorganic nutrient solution was as follows (all in mmol/l): 0.75 K2SO4; 0.25 KH2PO4; 0.65 MgSO4; 0.1 KCl; 7.5 Ca(NO3)2; 1.0x10-3 H3BO3; 1.0x10-3 MnSO4; 1.0x10-4 CuSO4; 5.0x10-6 (NH4)6Mo7O24; 1.0x10-3 ZnSO4; 0.1 EDTA-Fe(II). The composition of digestate nutrients was: N 0.73 g/l; P 0.028 g/l; K 0.74 g/l; NH4+-N 0.7 g/l; NO3--N 8.62 mg/l; calcium 352 mg/l; magnesium 227 mg/l; SO24- 15 mg/l; iron 4.1 mg/l; manganese 0.48 mg/l; copper 0.96 mg/l; zinc 3.35 mg/l; EC 6.36 (no EC units in the paper); pH 7.8. The EC after dilution was 2.05 for the 1:4 v/v treatments and 1.76 in the 1:5 v/v treatments. The initial pH of all solutions was adjusted to 6. Lettuce seeds were pre-germinated in an incubator and then planted into 5cm high pots (base 3 cm –top 5 cm Ф) filled with sterilised sand, with three seeds per pot. Once the seedlings were present all pots were watered with a 5ml standard inorganic solution of 4mmol/l NO3—N, then 9 and 15 days later the plants were fertigated with 400 ml/pot of the treatment nutrient solutions. On day 20 the plants were harvested. Treatments 2 and 3 significantly increased shoot weight and number of expanded leaves, compared to the control. Compared with the control, all organic treatments (2-5) significantly reduced shoot nitrate content, with the treatments including glycine being lower than the digestate only treatments. The authors concluded that digestate offers potential to replace inorganic fertilisers in the hydroponic production of lettuce, with lower nitrate levels deemed an advantage for consumer health, as excessive intake of nitrate was highlighted as being a potential health hazard. Although not specifically a horticultural crop, but of relevance due to the use of digestate for fertigation, the following Canadian trial is included. The use of digestate for the fertigation of two cool season turfgrass species (green creeping bent grass and Kentucky bluegrass) was investigated (Michitsch et al., 2008). The grasses were grown in plastic containers (8.3cm diameter and 20cm deep) in a 4:1 sand:peat mixture for three months in a growth room, with three cuts being undertaken during the trial period. Digestate from BMW was applied weekly at rates of 25, 50, 100 and 200% of the recommended rate of 25 kg N/ha. This was compared to a standard inorganic fertiliser at the same N rates. It was found that the use of digestate at the recommended N doses performed as well as the inorganic fertiliser at the same N rate. Moreover, the low NO3:NH4 ratio had no observable effect on the growth of the grasses.


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6.0

Whole digestate and digestate liquor as a growing media ingredient

The studies below focus on the mixing of digestates with other growing media ingredients to support the growth of a range of crops. One recent UK study used whole and liquor digestates for the production of three ornamentals as described below. 6.1

UK research: Bark admixtures: Formulation and testing of novel organic growing media using quality digestates for the production of containerised plants

Four different digestates were mixed at five different rates with a base growing medium comprising bark, wood fibre and topsoil (see Table 7) (WRAP, 2015a). The four digestate feedstocks were food waste (whole digestate), food waste (separated liquor, different source), potato waste (whole digestate) and maize (separated liquor). It was found that a suitable base mix for subsequent addition of digestate and the creation of admixtures with an open structure was: 60% bark, 30% wood fibre and 10% topsoil by volume. This was combined in a ratio of 5l base mix to five different volumes of digestate (0.1l, 0.25l, 0.5l, 0.75l and 1l) to create the final experimental admixtures. In addition to the four digestates, each added at five rates, two industry standards were used as controls – one peat based and the other peat free. The plant species investigated were wavy cyclamen (Cyclamen repandum), fern (Asplenium scolopendrium) and black pine (Pinus nigra). All were planted into the admixtures and assessed regularly for at least 90 days.

Table 7. Details of the digestate type and volumes used for the admixtures trial (WRAP, 2015a)

Digestate

Digestate

Volume of digestate in 5 litres of admixture (60% bark, 30% wood fibre, 10% top soil) feedstock Type 100ml 250ml 500ml 750ml 1000ml Food waste Separated liquor FS1 FS2 FS3 FS4 FS5 Food waste Whole FW1 FW2 FW3 FW4 FW5 Potato waste Whole PW1 PW2 PW3 PW4 PW5 Maize Separated liquor MS1 MS2 MS3 MS4 MS5 For black pines there was no significant difference in any growth parameter compared to the controls, including number of stems, plant height and plant quality. A growth vs time analysis showed pines of all treatments grew at comparable rates. Destructive harvest analysis to determine the mean dry weight of the whole plant, of stems and of roots as well as dry matter content and mean root-to-shoot ratio did not generally show statistically significant differences between the digestate-bark admixtures and the controls. For ferns there was mostly no significant difference in any growth parameter compared to the controls, for assessments made of number of fronds, frond length, chlorophyll content and foliage quality. A growth vs time analysis showed ferns of nearly all treatments growing at comparable rates (see Figure 7). Destructive harvest analysis to determine the mean dry weight of the whole plant, of stems and of roots as well as dry matter content and root-toshoot ratio did not generally show statistically significant differences between the various treatments and the controls. Admixtures with high doses of 1l food waste digestate were the exception to the above findings, with a significant reduction in fern chlorophyll content and leaf quality, which was attributed to comparatively high sodium content and EC originating from the food waste digestates.


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Table 8. Control and admixture characteristics (further parameters are listed in the project report) (WRAP, 2015a) pH

EC

Density

Ammonia-N

Nitrate-N

Total N

Total P

Total K

mg/l FW 48.1

mg/l FW 6.9

% w/w DW 0.72

mg/kg DW 3850

mg/kg DW 1330

BC

6.4

435

kg/m3 FW 468

PC

5.39

311

420

30.3

131

0.91

2427

949

MS1

6

96

405

38.3

18.8

0.31

1695

531

MS2

6.3

150

406

63.2

21.9

0.36

1859

508

MS3

7

213

462

97

17.4

0.43

2780

537

MS4

7.41

317

551

146

19.1

0.51

3044

600

MS5

7.63

431

575

225.8

6.1

0.6

3910

648

FS1

5.97

102

366

40.1

17.6

0.39

1624

489

FS2

6.59

161

414

79.9

20

0.38

1669

609

FS3

7.3

238

505

128.3

22.3

0.45

1989

684

FS4

7.72

411

545

246

2.3

0.47

2065

688

FS5

7.67

536

607

299.5

8.4

0.63

2473

767

PW1

5.92

106

419

46.7

19.9

0.36

1874

552

PW2

6.3

122

427

51.6

18.2

0.4

2375

539

PW3

7.22

155

465

75.1

15.3

0.43

3262

581

PW4

7.63

315

599

172.1

10.7

0.44

4303

596

PW5

7.7

397

571

200.9

42.2

0.47

4622

601

FW1

6.2

174

344

66.2

60

0.42

1644

487

FW2

7.16

424

411

127.1

20.6

0.41

1879

661

FW3

7.77

409

461

224.9

19.2

0.52

2302

722

FW4

7.91

579

507

327.8

17.6

0.61

2763

826

FW5

7.9

898

591

441.9

20.2

0.69

2681

796

Admixture

uS/cm

For cyclamen, there was no significant difference in any growth parameter measured compared to the controls. A growth vs. time analysis showed cyclamen in all treatments senescing at similar times. Destructive harvest analysis was carried out on the cyclamen corms, as all plants had senesced at the point of harvest. The mean dry weight of the corm and dry matter content did not generally show statistically significant differences between the various treatments and the control.

Figure 7. Fern plants after 90 days of growth in peat-free control (left) and a maize digestate-bark admixture (right) (WRAP, 2015a)


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No fertiliser supplementation was required during the trial, indicating that the range of digestates investigated was able to provide an appropriate level of nutrients for the three species under test. All four digestates contained essential plant macro- and micronutrients. Analysis of the digestates also showed that the majority of the available nitrogen was in the form of ammonium, with some nitrates also present. The outcome of the experiments showed that using digestate/bark admixtures as a growing medium for all three plant species generally did not significantly affect plant quality. 6.2

Research outside the UK regarding whole digestate and digestate liquor as a growing media ingredient

A Czech glasshouse study focussed on determining the impact of the application of digestate from pig slurry on the yield and dry matter content of a range of cultivars of both tomato and pepper plants in peat-bark growing media (Kouřimská et al., 2009, Kouřimská et al., 2012). The digestate consisted of (per l) 595 mg NH4+, 755 mg PO43-, and 1.1–1.25 g K2O. The four treatments, all in 20 l pots were: 1 2 3 4

Unfertilised control; Inorganic standard 15 g (NH4)2SO4 and 9 g K2HPO4 added to each pot (20 l) prior to planting, and 7.5 g (NH4)2SO4 added 30 days later; Digestate: 0.8 l added to each pot prior to planting, and another 2 l added 30 days later; 50% of inorganic fertiliser (7.5 g (NH4)2SO4 and 4.5 g K2HPO4) and 50% (0.4 l) of digestate added to each pot prior to planting, and 3.75 g (NH4)2SO4 and 1.5 l of digestate added 30 days after planting.

The amendments were mixed into the growing media at the start of the experiment and then more of the same amendments were added after 30 days as a liquid feed. The trials with tomatoes were repeated with new plants and growing media over five years, and for the peppers over two years. The unfertilised control had the lowest yield. There were no significant differences in yields with the organic and inorganic fertiliser treatments (2, 3 and 4), for all cultivars of both tomato and pepper, although the combined treatment showed a slight trend for being the highest yielding. Interestingly, the dry matter contents of both tomato and pepper were in the following descending order: Digestate (treatment 3) > combined (treatment 4) > control (treatment 1)> inorganic only (treatment 2). The higher dry matter content of the crop in the digestate treatment was deemed by the authors to be a positive effect, indicating that the digestate matches the nutritional needs of the plants in the most suitable way. The higher dry matter content can be an important factor for vegetable products (such as ketchups, dry products, puree etc.), improving the shelf life of fresh vegetables. Lower water content inhibits the growth of undesirable microorganisms, which cause decay of the products. Moreover, the heavy metal content remained below permitted limits, and was not significantly different to conventionally fertilised tomatoes. There was no difference between these plants and those grown in peat-bark media that had had a conventional inorganic fertiliser added. An Indian study mixed digestate slurry (assumed to be whole or separated liquor, as feedstock not defined) 1:1 with wheat bran, and after 21 days mixed this with soil (Kichadi and Sreenivasa, 1998). The study also included a treatment with dried digestate at 10% moisture, mixed with soil. The main result was that the digestate, with or without the addition of two beneficial mycorrhizal fungi, increased tomato plant growth and fruit yield, in addition to supressing the soil borne pathogenic fungus Scerotium rolfsii. The following two studies discussed in this section focus on crops not generally grown in the glasshouse, but nonetheless provide additional data on the use of whole and liquor fraction digestates as a growing media ingredient.


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A Norwegian pot study investigated the effect of a range of waste derived amendments including liquid digestate from source separated household waste and nitrified liquid digestate of the same origin, in addition to standard N P K fertiliser, on the growth of spring barley (Hordeum vulgare). Plants were grown in a mixture of sand and peat, with the amendments mixed into the top portion of the soil prior to sowing, at two rates of 80 and 160 kg N ha-1. The liquid digestate was found to be as good as the NPK fertiliser. However, the nitrified liquid digestate treatment caused significantly increased nitrate leaching, and a lower yield (Haraldsen et al., 2010). A one year Czech study (Lošák et al., 2012) investigated the effect of a pig slurry/maize silage digestate on pot grown kohlrabi (a nitrogen demanding crop). All pots contained 6 kg of fluvial soil, with four treatments: 1 2 3 4

untreated control, urea (1.5 g N/pot), digestate (N-P-K-Mg g/pot: 1.5-0.18-0.69-0.08), urea, triple super phosphate, KCl, MgSO4 (N-P-K-Mg g/pot: 1.5-0.18-0.69-0.08).

The N dose was the same in treatments 2-4, being 1.5 g N/pot. In treatment 4 the P, K and Mg doses corresponded to those supplied in the digestate treatment. The treatments were thoroughly mixed and the kohlrabi seeds were sown ten days later. Yields in the digestate treatment were comparable to those achieved with artificial fertilisers (over three times those of the unfertilised control). Both digestate and fertilisers reduced the ascorbic acid (Vitamin C) content of the crop but tissue nitrate concentration (high levels of which can be harmful to health) was much higher in the fertilised treatments 2 and 4. 7.0

Digestate fibre use in horticulture

The separated solid fraction of digestate (termed digestate fibre) is produced by separating the whole digestate via a range of different processes including screw, screw press, centrifuge and membrane filtration (Cavinato, 2013). The use of digestate fibre in growing media has been highlighted as a potential emerging application in the UK (WRAP, 2013a), although this end use is not currently permitted by PAS110 and the ADQP, and as such there are no guidelines to date regarding desired characteristics. As a comparison, the existing guidelines for the specification of quality compost for use in growing media can be used as an important resource for recommended targets and limits. The main quality parameters for compost use in growing media include stability, phytotoxicity, PTEs and physical and chemical properties, with upper limits of 50 mg/l for NH4-N, 150 mg/l for sodium, 1000 mg/l for chloride and 1500 μS/cm for EC (WRAP, 2011a). Interestingly, a recent industry survey regarding the potential use of digestate fibre as a growing media ingredient highlighted the following (WRAP, 2013a): ‗The production of growing media for container growing of plants for professional and amateur use is a highly specialised market with stringent requirements for the physical, chemical and biological characteristics of the product. Companies have internal specifications for components to meet requirements such as moisture content, bulk density, air-filled porosity and available water capacity. There is a requirement for constituents to be low in nutrients and soluble salts, and possibly low pH – to allow crop-specific nutrients to be added in the correct proportions. They must have low levels of physical contaminants, toxic elements and organic contaminants, be free from pathogens and also microbially stable to prevent further physical breakdown during product use. There is a continued need to find suitable peat replacement products that are not only technically robust but also sustainable Literature review: Digestate use in protected horticulture

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and of low cost, and this keeps a general interest in digestate as a possible contender for use in growing media by some producers.‘ Published research has investigated digestate fibre mixed with a range of materials (including other wastes and commercial growing medium ingredients) or the use of composted digestate, on the performance of a range of edible and ornamental crops. In other cases digestate has been composted together with other ingredients to produce a product. These scenarios are discussed below. Digestate fibre is also being used in other (non-horticultural) applications. For example, matured or dried digestate fibre is used for cattle bedding in the US (Alexander, 2012). However such uses are beyond the scope of this literature review, and are not discussed further here. When the digestate fibre is subsequently composted either alone or together with other feedstocks, provided the composting process meets the requirements of PAS100 (BSI, 2011), it would be regulated as a compost, rather than as a digestate. This has implications for the waste status of the material, since ‗end of waste composts‘ can be used for a wider range of applications than ‗end of waste digestates‘. Further information on acceptable markets for the different materials can be found in their respective quality protocols (WRAP and Environment Agency, 2010, 2012).

7.1

Digestate fibre as a growing medium ingredient

Solid digestate (fibre) has been trialled as a growing media ingredient with a range of other constituents, and some examples are summarised in Table 9. A recent UK study focussing on the use of digestate fibre for tomato production is discussed below.

7.1.1 UK research: The use of cattle slurry digestate fibre in mixtures with coir and pine bark as plant growth substrates in the intensive production of glasshouse tomato crops The use of digestate fibre produced from cattle slurry was assessed as a growing media ingredient for tomato production in a recent feasibility study (Challinor, 2014). Overall, the study concluded that digestate fibre may act as an additional nutrient reservoir for the sustained growth and yield of crops, such as tomato. The digestate fibre was left to stand for six weeks prior to mixing. The three treatments were coir only, 50:50 digestate fibre:coir (v/v) and 50:50 digestate fibre:pine bark (v/v). The higher digestate fibre pH was moderated by the lower pH of the coir and the bark, to create growing media deemed suitable for tomato production (see Table 10). The resultant growing media were filled into polythene sleeves, and then planted with tomatoes (cv Dometica). Water and nutrients were delivered via standard drip irrigation. The feed regime was tailored to best meet the nutrient demands of the plants and optimise nutrient availability. It was possible to obtain similar fruit quality in both the digestate mixtures and the standard coir substrate over a period of eight months.


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Table 9. Summary of a range of digestate fibre trials *Studies at the University of Padova with all studies using the following treatments: Increasing rates (0, 33, 67 and 100%) of ground rice hulls (GRH), with or without 20% digestate, with the remainder being peat.

Species Lettuce and cress

Cherry laurel, Spirea ‗Grefsheim‘ and wild privet Roses Rose cuttings (propagation trial)

Mixes used Solid digestate derived from manure or maize and agro-industrial waste mixed with peat Solid digestate mixed with standard growing media Solid maize silage based digestate mixed with standard growing media Distillery fruit waste solid digestate mixed with rice husks and peat*

Geranium cuttings (propagation trial)

Distillery fruit waste solid digestate mixed with rice husks and peat*

Tomato and

Distillery fruit waste solid digestate mixed with rice husks and peat*

Tomato

Fruit and wine waste solid digestate mixed with rice husks and peat*

Salvia splendens

Results Peat and digestate mixes performed as well as peat

Author Crippa (2011)

Very good results using up to 60 % solid digestate in the growing media mix The 20 % digestate treatment improved the quality of roses Digestate improved rose cuttings and did not influence rooting as compared to treatments without digestate Digestate did not affect number of leaves, leaf weight, cutting height, stem weight or % rooting. Digestate reduced cutting diameter and root weight, hence increasing the root:shoot ratio With 20% digestate in the rice hull compost, transplant growth was equivalent to the peat compost control Digestate improved cuttings and did not influence rooting

Wrede pers. comm. Wrede (2012) Tassinato (2011)

(Tassinato, 2011)

Bassan et al. (2012) Bassan et al. (2014)

There were no visible symptoms of plant damage caused by pesticides or herbicides during the trial, demonstrating that there was no such contamination in the digestate mixes. The coir treatment had the highest marketable yield at 30.19 kg/m2, with the digestate and bark mix yielding 27.85 kg/m2, compared with 25.89 kg/m2 from the digestate and coir mix. The author highlighted that it is likely that positional effects of the substrate rows and the absence of replication in the trial glasshouse may have influenced the plot yields. Further research on potential crop contaminants and also the presence of human, animal and plant pathogens in the digestate was recommended, especially prior to future use in intensive cropping. Moreover, it was highlighted that the physical, chemical and biological characteristics of the digestate fibre must be fully quantified before use, as an understanding of the analytical profile will help to avoid any of the potential nutritional difficulties, and assist management decisions during the crop growth period.


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Table 10. Physical and chemical properties including water extractable nutrients (fresh samples) of the three growing media used for the tomato trial (Challinor, 2014) Parameter

Units

pH Conductivity μS/cm 20°C Bulk Density g/l Dry Matter % m/m Moisture % m/m Water extractable Ammonium-N mg/l Nitrate-N mg/l Phosphorus mg/l Potassium mg/l Calcium mg/l Magnesium mg/l Sodium mg/l Chloride mg/l Sulphur mg/l Iron mg/l Manganese mg/l Boron mg/l Zinc mg/l Copper mg/l Molybdenum mg/l

Coir 6.4 267 412 12.9 87.1