The document summarizes a meeting agenda on research into using digestates - the solid residues from anaerobic digestion - in horticultural growing media. It includes an introduction to current research projects using digestates in novel growing media for ornamental plants at Moulton College and Cambridge Eco. Initial results are presented from a glasshouse trial comparing the growth of black pine, cyclamen and ferns in different bark-based growing media amended with various volumes of different digestates. Potential applications for the horticultural industry are discussed, including using digestates as a liquid fertilizer or component of new peat-alternative growing media. Cost analyses show digestate-amended media can have similar or lower costs than peat-

1. Dr Mary Dimambro – Cambridge Eco
Dr Russ Sharp and Sam Brown – Moulton

2. Agenda
10:00 Welcome. Dr Russ Sharp, Moulton College
10:05
Introduction to anaerobic digestion and current research on the use of
digestates in horticulture. Jacks Guinness, WRAP
10:25
Introduction to the project on the use of digestates in novel growing
media for ornamentals. Dr Mary Dimambro, Cambridge Eco
10:35
Trial system design, growing media and results to date for black pine,
cyclamen and fern. Sam Brown – Moulton College
10:45 Potential application to industry. Dr Russ Sharp, Moulton College
11:00 Discussion
11:20 Viewing the glasshouse trials
11:45 Discussion, tea and coffee, networking
12:00 Close

3. About Moulton College
 Moulton is the only land-based college to
teach from ‘special needs’ through to
‘doctoral studies’.
 Graded by OFSTED as Outstanding!
 Campuses/academies in Moulton, Higham
Ferrers, Silverstone, Daventry and
Peterborough

4. About Moulton College
 Horticultural research currently funded on a
diverse range of subjects:
 LED lighting
 Growing media
 Soil compaction (PhD)
 Soil-borne diseases (PhD)
 Plant propagation (PhD)
 Urban cooling (PhD)
 Alternative crops
 Garden History

5. About Cambridge Eco
 Advising Defra, WRAP, industry, growers
 Experts in the use of
 composts
 digestates

6. Use of digestates as a liquid fertiliser for
commercial strawberry production
 6 digestates
 Grow bags with trickle irrigation
Digestate Control Digestate Control

7. Admixtures Trial
bark
+ other sustainable materials
+ digestates
= novel growing media

8. Why bark and digestate?
 Bark = Low bulk density, nitrogen content, and pH
 + an absorbent matrix
 Digestate = High bulk density, nitrogen content, and pH
 + liquid digestate able to be absorbed onto matrix
 Ornamentals chosen = non-food crop with high
tolerance to salinity and ammonium.

9. Use of digestates in novel
growing media for ornamentals
Project phases:
 October-December: Desk study
 January-February: Designing the admixtures
 March-May: Glasshouse trial
 June-August: Report

10. Design Considerations -
Admixtures
 Bark admixtures optimised for
 Fern
 Cyclamen
 Pine
 All can grow in standard hardy nursery stock mix

11. Design Considerations -
Admixtures
 Key parameters:
 Particle size, air filled porosity and drainage
 Electrical conductivity important
 Ferns and cyclamen: up to 200µS/cm with low levels of
nutrients
 Pines: 600µS/cm (pollution and salt tolerant) with base
fertiliser and long term controlled release fertiliser

12. Digestate analysis
Food
Separated
Food
Whole 1
Food
Whole 2
Potato
Whole
Maize
Slurry
Separated
Maize
Separated
Total
solids (%)
2.9 3.7 4.5 2.2 7.1 5.1
C:N 4.0 3.5 3.3 3.3 7.3 6.1
pH 8.5 8.4 8.4 8.2 8.2 8.2
EC (1:6)
(dS m-1)
4.4 5.4 7.0 4.2 4.5 3.8

13. Digestate analysis
Food
Separated
Food
Whole 1
Food
Whole 2
Potato
Whole
Maize
Slurry
Separated
Maize
Separated
mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
N 3700 4900 6000 2400 4200 4100
NH4-N 2990 3784 5260 2039 2044 2175
NO3-N (<) 0.1 0.1 0.1 0.1 0.1 0.1
P 202 315 456 128 514 246
K 1330 1869 1109 4752 4661 3382
Ca 797 2000 1974 126 1862 889
Mg 92.9 93.3 63 46 436 188
S 134 236 342 78 303 171
Fe 75 231 555 57 197 54
Mn 3.4 8.4 5.0 1.3 15.9 6.4
Na 1021 1146 2225 46.4 439 121

14. Can digestates replace
additives?
 Dolodust (magnesium limestone): Raise the pH level &
adds traces of magnesium
 Digestates pH balances the low pH of the bark, and
contain sufficient magnesium
 Nitrochalk (ammonium nitrate & calcium carbonate)
 Digestates have adequate or high levels of ammonia
and calcium
 Fertilisers (base and controlled release)
 Digestates contain sufficient nutrients

15. Trial design
 Controls (industry standards)
 Peat-based control (PC)
 Peat-free control (bark and wood fibre based, BC)
 4 digestates – each with 5 admixtures
 5 replicates = 110 plants per trial

16. Trial design
 Randomised plot design. Rearranged at fortnightly
intervals to minimise edge effects
 Both labels and pot markings used to identify plants
in each treatment.
 Extra plants bought in to allow grading of plants
before allocating treatments.

17. Trial Design
 Plants irrigated to just below drip point to ensure that
digestate did not leak out and move between treated
plants.
 Saucers not used to ensure no stagnant water was
present around the roots.
 Plants monitored for signs of pests and diseases.
None currently found so far.
 Electric heaters in use in early part of the trial and
then vents opened to ensure a cool growing
environment typical of an HONS nursery.

18. Subject species
 Why Black pines? HONS plant highly tolerant to both
salt and heavy metal pollution.
 Why Ferns? A HONS plant that thrives in organic
growing media rich in ammonium.
 Why Cyclamen? A HONS plant that thrives in
organic growing media for which we can obtain
flowering data.

19. Making the admixtures
 The main selection criteria were EC, structure and
consistency (not too wet or too dry)
 Standard ingredients trialled
 The final admixtures:
 60% bark, 30% wood fibre, 10% top soil + digestate

20. Final admixtures
Digestate
feedstock
Digestate
Type
Volume of digestate in 5 litres of
admixture (ml)
100 250 500 750 1000
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

21. pH Cond. at
20 ◦C
Density Ammonia-
N
Dry
Matter
Nitrate-N
NAME uS/cm kg/m3 mg/l % mg/l
BC 6.4 435 468 48.1 52.9 6.9
PC 5.39 311 420 30.3 39.1 131
MS1 6 96 405 38.3 62 18.8
MS2 6.3 150 406 63.2 62.9 21.9
MS3 7 213 462 97 53.3 17.4
MS4 7.41 317 551 146 50.5 19.1
MS5 7.63 431 575 225.8 44.2 6.1
FS1 5.97 102 366 40.1 62 17.6
FS2 6.59 161 414 79.9 61.3 20
FS3 7.3 238 505 128.3 53.8 22.3
FS4 7.72 411 545 246 49 2.3
FS5 7.67 536 607 299.5 43.2 8.4

22. Available nutrients
Total
Soluble
N
Chloride Sulphate Phosphorus Boron Potassium Copper
NAME mg/l mg/l mg/l mg/l mg/l mg/l mg/l
BC 55 77.7 927.8 27.3 0.17 460.5 0.08
PC 161.3 18.7 228.4 78.1 0.19 253.7 <0.06
MS1 57.1 43.7 20 5.4 0.18 88.9 0.07
MS2 85.1 81.4 43.8 10.5 0.16 130 0.13
MS3 114.4 134.4 32.1 26.7 0.2 210.5 0.21
MS4 165.1 181.2 40.5 36.3 0.15 290.6 0.21
MS5 231.9 215.4 41.5 54.3 0.17 410.4 0.31
FS1 57.7 53.3 28.6 6.1 0.16 80.9 0.1
FS2 99.9 119.9 38.1 10.5 0.19 90.4 0.11
FS3 150.6 207.5 41.5 16.5 0.14 108.5 0.12
FS4 248.3 337.1 45.6 24.2 0.15 174.5 0.16
FS5 307.9 424.9 43.7 29.7 0.15 221.9 0.17

23. Monitoring
Fortnightly assessment
 Pines: Plant height and number of branches.
 Ferns: Number of fully extended fronds and length of
longest frond
 Cyclamen: Number of leaves and flowers
Monthly assessment
 Foliage quality:
 SPAD chlorophyll meter (ferns and cyclamen)
 Visual assessment (all)

24. Results to date
 Trials will finish over the next few weeks
 All data collected so far show no significant
difference in the response of all three species to the
different growing media for all parameters tested.
 Analysed via ANOVA and Kruskal Wallace tests.
 Full results in final report, including changes over
time and final harvest measurements.

25. Results: Fern
0
20
40
60
80
100
120
140
160
PC BC FS1 FS2 FS3 FS4 FS5 FW1 FW2 FW3 FW4 FW5 MS1 MS2 MS3 MS4 MS5 PW1 PW2 PW3 PW4
Frondlength/mm
Treatment

26. Results: Fern
0
5
10
15
20
25
30
35
PC BC FS1 FS2 FS3 FS4 FS5 FW1 FW2 FW3 FW4 FW5 MS1 MS2 MS3 MS4 MS5 PW1 PW2 PW3 PW4 PW5
Numberoffonds
Treatment

27. Results: Fern

28. Results: Cyclamen
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
PC BC FS1 FS2 FS3 FS4 FS5 FW1 FW2 FW3 FW4 FW5 MS1 MS2 MS3 MS4 MS5 PW1 PW2 PW3 PW4 PW5
Averageleafnumber
Treatment

29. Results: Cyclamen

30. Results: Pine
0
50
100
150
200
250
300
PC BC FS1 FS2 FS3 FS4 FS5 FW1 FW2 FW3 FW4 FW5 MS1 MS2 MS3 MS4 MS5 PW1 PW2 PW3 PW4 PW5
Averageincreaseinheight/cm
Treatment

31. Results: Pine
0
2
4
6
8
10
12
14
16
18
20
PC BC FS1 FS2 FS3 FS4 FS5 FW1 FW2 FW3 FW4 FW5 MS1 MS2 MS3 MS4 MS5 PW1 PW2 PW3 PW4 PW5
numberofstems
Treatment

32. Cost benefit analysis
 Comparing the costs of the admixtures to standard
growing media

33. Digestate Requirement
 1m3 digestate enough for
 ~ 5 m3 - 50 m3 growing medium
 One lorry load digestate
 ~ 25 t digestate
 ~ 5 – 50 lorry loads of growing media

34. Digestate Storage
Volume Type Cost
/m3/year
over 10
years
1 m3 White 1000 Litre IBC container £ 7
1 m3 New IBC, timber pallet £ 10
10 m3 CT2200JA Tank £ 12
19 m3 V19000 £ 13
36 m3 Elliptical Concrete Tank £ 15
72 m3 Small AD storage unit £ 24
4500 m3 Steel / concrete above ground £ 3.5
4500 m3 Earth bank lagoon £ 2

35. Transport
 Transport cost depends on
 Distance
 Volume
 Frequency of deliveries
 Typical digestate transport costs per 26t lorry:
 £4 /m3 if using infrastructure of digestate producer
 £6 /m3 if 4 lorry loads per day are delivered
 £23 /m3 if 1 lorry load per day is delivered

36. Ingredients
 Cost of peat has increased
 Due to adverse weather conditions during harvest
 World shortage of peat in 2013
 Lacking peat content is replaced with bark and
woodchip and other materials
 Now less than 80% peat in some peat based growing
media
 Cost of growing media ingredients is volatile
 Currently the cost of some peat free growing media is
artificially lowered to encourage uptake
 Cost of raw ingredients is only a snapshot / estimate

37. Growing Media Cost
Peat Free
Nursery
Stock
Peat Based
Nursery
Stock
Digestate / Bark
Admixtures
£60 - £78 /m3 £60 - £177 /m3 Bark fine 60% £ 38.50
Wood fibre 30% £ 18.50
Top soil 10% £ 9.30
Digestate storage £ 1
Digestate
transport
£ 4
Total £71 - £78 /m3 £71 - £177 /m3 £ 71.30 /m3
*Delivery costs included

38. Potential application to industry
 New growing media
 Potential for use as liquid fertiliser (diluted)
 Potential for closer links with plants for disposal of waste
material, especially infected material that should not be
composted.
 Unlike other peat-alternatives, peat and digestates are
produced on in massive volumes, thus able to satisfy a
large proportion of current demand.
 Challenges: the grade of bark to use and optimising
nutritional content of media for different species.

39. Dr Mary Dimambro
mary@cambridgeeco.co.uk
07731451899

41. Heavy metals (mg/kg)