Increasing nutrient efficiency from anaerobic digestate

This field lab is investigating how to stabilise N and reduce loss in AD. This includes improving absorption and soil structure using cover crops.

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Field Lab Timeline

    12/1/2016 12:00:00 AM
  • Idea formed

    Idea formed
  • 4/24/2017 11:00:00 PM
  • Field lab planning

    Field lab planning
  • 5/31/2017 11:00:00 PM
  • Submit proposal and grant application

    Submit proposal and grant application
  • 6/21/2017 11:00:00 PM
  • Finalise experimental design

    Finalise experimental design
  • 6/29/2017 11:00:00 PM
  • Materials purchased

    Materials purchased
  • 7/14/2017 11:00:00 PM
  • Pre-treatment measurements

    Pre-treatment measurements
  • 7/29/2017 11:00:00 PM
  • Cover crop establishment

    Cover crop establishment
  • 8/31/2017 11:00:00 PM
  • Trial monitoring

    Trial monitoring
  • 9/10/2017 11:00:00 PM
  • Digestate applications

    Digestate applications
  • 12/11/2017 12:00:00 AM
  • Sampling and measurements - early winter

    Sampling and measurements - early winter
  • 2/1/2018 12:00:00 AM
  • Sampling and measurement - late winter

    Sampling and measurement - late winter
  • 3/31/2018 11:00:00 PM
  • Analyses against baseline data

    Analyses against baseline data
  • 6/13/2018 11:00:00 PM
  • Farmer training for VESS

    Farmer training for VESS
  • 6/17/2018 11:00:00 PM
  • Cash crop early establishment measurements

    Cash crop early establishment measurements
  • 7/8/2018 11:00:00 PM
  • Cash crop fast growth phase measurements

    Cash crop fast growth phase measurements
  • 8/12/2018 11:00:00 PM
  • Cash crop maturity measurements

    Cash crop maturity measurements
  • 8/19/2018 11:00:00 PM
  • Full farm surveys

    Full farm surveys
  • 8/30/2018 11:00:00 PM
  • Cash crop results discussion

    Cash crop results discussion
  • 9/2/2018 11:00:00 PM
  • Year 2 treatments and monitoring

    Year 2 treatments and monitoring
  • 9/10/2018 11:00:00 PM
  • Earthworm counts

    Earthworm counts
  • 1/1/2019 12:00:00 AM
  • Sampling and measurement

    Sampling and measurement
  • 1/30/2019 12:00:00 AM
  • Impact of cover crop species on Arbuscular Mycorrhizal Fungi activity

    Impact of cover crop species on Arbuscular Mycorrhizal Fungi activity
  • 6/3/2019 11:00:00 PM
  • Cash crop measurements

    Cash crop measurements
  • 9/11/2019 11:00:00 PM
  • Results discussion

    Results discussion
For further information hover over the above milestone marks
  • Discussion

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  • Achievements

    October 2019

    Field lab results: key findings

    Key findings from this field lab have been identified as follows:
    - Cover crops stabilise Nitrogen levels. Analysis of Nitrogen (N) content in soil post digestate application and cover crop establishment indicated that a cover crop treatment was associated with a decrease in N content in the deeper soil layers, suggesting that cover crops retain the increased N provided by digestate. The extent of retention varied between sites.
    - Organic Matter content between sites was variable, reflecting differences in soil type. More mineral soils tended to have a higher organic matter content. The addition of a cover crop treatment improved organic matter, with noticeable variation between sites.
    - Digestate application has an effect on AMF population levels. The addition of digestate appears to have decreased AMF levels in colonised cover crops but this was inconsistent between species and sites.
    - Visual soil assessments and GAI were inconclusive. Assessing soil structure with the VESS method was particularly challenging as scores are very subjective. In contrast, the worm count and GAI assessments were affected by weather conditions at the time of sampling. This was similar to yield.

    This project has enabled farmers to address risks of diffuse pollution following an autumn application of digestate. By using cover crops, they have been able to stabilise nutrient levels throughout the growing season. Scientists and farmers have learnt from jointly developing the trial designs and assessment protocols to ensure that they are achievable on farm.

    The full summary is available under the Field Lab Documents tab, use your free Innovative Farmers log-in to download the Results Summary or Final Results documents on the top right of this page.

    Milestone: Results discussion

    August 2019

    Results discussion

    This field lab has investigated the impact of digestate application to soils, and the role of cover crops in improving efficacy of digestate by helping retain nutrients, maintain soil organic matter and help maximise effective nutrient application within NVZ rules.

    Join us at 2.30pm on September 12th at NIAB Park Farm, Impington, for the opportunity to learn about some of the research findings resulting from the project and hear from participating farmers. For those interested, there will be discussions about a new field lab that will build on the results from this initial trial, which is open to new farmers to join.

    To book you ticket go to:

    or email

    3 BASIS points available.

    Milestone: Results discussion

    July 2019

    GAI measurements

    Farmers in the group are starting to take Green Area Index assessments of their cash crop. Some of them are also using satellite/drone imaging to quantify differences between treatments.
    The protocol devised earlier in the year will be used for the maize harvest.

    Milestone: Cash crop maturity measurements

    January 2019

    Are cover crops influenced by Arbuscular Mycorrhizal Fungi activity?

    Within the cover crop seed mix used, oat and vetch were tested for colonisation by AMF at four trial sites. Results showed that digestate application decreased AMF colonisation of cover crops but this was inconsistent between cover crop species and trial sites.

    Milestone: Impact of cover crop species on Arbuscular Mycorrhizal Fungi activity

    January 2019

    Cash crop and soil measurement results

    Triallists grew maize as a cash crop following the cover crop. With the drought, maize was maturing really quickly so harvesters and trailers were being pushed to their limit. As a result, it wasn’t always possible to harvest smaller trial plots, which usually require more time and logistics in place. At one of the trial sites, digestate application in combination with cover crops resulted in a higher yield compared to cover crop only. However, the control treatment (no digestate and no cover crop) showed the highest yield compared to the other treatments. Conclusions cannot be drawn from single site results but this unexpected finding highlights that many other environmental variables, such as the extremely hot weather will have likely had an effect.
    VESS, worm count and GAI scores did not show significant differences between treatments. Again, the weather conditions made the assessment difficult and will have impacted on results.
    During the meeting, farmers and researchers drew up a protocol that makes harvesting smaller trial plots more practical and easier to integrate into ongoing operations on farm.

    Milestone: Cash crop results discussion

    September 2018

    Progress update

    The triallists were due to do green area index (GAI) assessments over the summer, however due to dry conditions and slow establishment, only one was able to. The results from this unreplicated measurement showed that GAI was highest in the control plot, which is to be reviewed and discussed.

    The yield data will be of more importance for farm decisions however, and these are due to be collected and shared soon.

    There will be another round of visual soil assessments made now that the soil is softer after some rain.

    All triallists are proceeding to a second year of trials, with cover crop seed ordered and an agreement to trial on a different field / plot to the first year. The treatments will be as last year: Control, cover crop only, cover crop and digestate, digestate only. One triallist has also decided to trial mycorrhizal fungi applications in addition to these treatments.

    Milestone: Year 2 treatments and monitoring

    August 2018

    Farm background survey

    The triallists are in the process of filling out surveys on the history and practice of their farm management, which will help with interpreting results.

    Please log-in for free and see the attached 'Farm survey sheet' in the Field Lab Documents section, which shows the full details of the information being collected.

    Milestone: Full farm surveys

    July 2018

    Green cover assessments

    The farmers are starting to take green area index (GAI) measurements of their crops for analysis. With this being a dry summer many farmers are already busy and the coordinator is checking that they are still able to take measurements, there may be some discussion of how to cope with this if it is not feasible.

    Milestone: Cash crop fast growth phase measurements

    April 2018

    First year results cont.

    Nutrients and SOM

    pH varied greatly among the farms but in general the 'digestate only' plots had the highest pH which is expected due to the digestate itself. Farm B in particular had very high pH among all the plots.

    A question was posed that if a farm which already has a high pH is applying digestate, is this the best thing to do? Another question was posed around when leaching may occur.

    Phosphates were high at Farm F, which was attributed to them being potato growers (therefore there is a tendency to overload the soil with P).

    Farm C has a mineral soil and so the group were expecting to see a big difference in SOM between plots, however this was not the case all plots had very similar percentage.

    Available N

    In the combined results the 'cover crop only' trials had the lowest available N due to the cover crops utilising it. There was a lot of variation in available N and depth at which the available N was located.

    At Farm A's 'field 47' (one of two trial sites on this farm), there was more available N in the control. This could be due to the large amount of SOM at the site disguising the N. Most N was found in the top 30 cm.

    At the second trial site at Farm A the digestate only plot had a lot more available N in all zones (0-30cm, 30-60cm, 60-90cm).

    On Farm C the 'digestate only' plot had more available N in total but most was found at the 60-90cm depth, indicating leaching through the sandy soil.

    Farm G also had the highest available N in the digestate plot.

    Farm H had high N in all 4 trial plots at the 60-90cm depth. Highest available N was found in the 'cover crop only' plot (most at every depth). It was every wet when sampling took place here which could have played a part in this result.

    More results are to come, please revisit the page soon.

    Milestone: Analyses against baseline data

    April 2018

    First year results

    Green Area Index (GAI), leaf area and establishment.

    Combined results from all trial farms:
    - Control: 0.38
    - Cover crop and Digestate: 1.14
    - Digestate Only: 1.01
    - Cover crop Only: 0.99

    The combined results were a little skewed due to those from Farm G, which drilled cover crop very late (October) and did not establish well. They also had weeds in the control.

    In all other farms the trials with the highest GAI were the cover crop and digestate and cover crop only, which were significantly higher than the control and digestate only.

    'Cover crop and digestate' treatment plots had the highest GAI at Farm F, and Farm A's first trial site (one of two). The second site at Farm A had the same GAI readings for the 'cover crop and digestate' and 'cover crop only' treatments.

    Farm C had a higher GAI in the 'cover crop only' treatment and it was noted that the digestate was applied in very hot weather which may have had negative consequences on its effect upon the soil.

    VESS tests

    The farmers will do further VESS tests themselves on farm in late April / early May. All measurements taken by the researcher over winter were subject to the same sampler to make sure there was no differences in the assessment. Variation exists between farms due to many reasons such as soil type.

    Combined VESS test scores:

    - Control: 1.52
    - Cover crop and digestate: 1.63
    - Digestate only: 1.65
    - Cover crop only: 1.63

    Milestone: Analyses against baseline data

    December 2017

    Winter baseline data analysis

    In early December the research team undertook a full day sampling (block soil cores of the top 30 cm and other soil samples) to assess soil structure, root penetration and N, P, K, pH and TOC by NRM. This sampling is due to continue in late January / February. Microbial analysis will be undertaken later in the year by Lydia’s PhD student. Having visually assessed the soil of the trial area with digestate applied and cover crops grown, it is looking in better condition (very good root penetration and structure) than the control and the trial plot where digestate has been applied but no cover crops sown. The trial strip with only digestate looks to have had no impact on soil structure so far.

    Please see the attached field lab documents for photos of the sampling undertaken in December at the control site, cover crop with no digestate application and cover crop with digestate application.

    Milestone: Sampling and measurements - early winter

    September 2017

    Digestate applications

    Digestate was applied after the cover crops were sown in summer. Pre-application soil tests have also been taken for analysis to compare against those post-treatment later in winter.

    Milestone: Digestate applications

    August 2017

    Cover crops sown

    Cover crops including black oat, buckwheat, vetch and fodder radish were sown in august before applications of digestate. These established well, with frost killing off the buckwheat in October / November.

    Milestone: Cover crop establishment

    June 2017

    Methods finalised

    The group decided that the methods will be as follows:

    1. Cover crops will consist of: buckwheat (20kg/ha), fodder radish (3kg/ha), vetch (30kg/ha) and black oat (30kg/ha) – this mix has been chosen to provide a combination of deep rooting, nitrogen fixing and disease tolerance and will be provided by Cotswold Seeds.
    2. Those using chopped straw will apply it with the digestate with some gentle incorporation.
    3. Each farmer will identify a field of about 10 ha for the project which will be split into 4 for the different treatments:
    • No treatment
    • Cover crop, no digestate
    • Digestate, no cover crop
    • With cover crop and digestate
    4. Cover crop seeds to be drilled, not broadcast
    5. Timing of drilling must fit in with current farm practice – although some need to drill later (BS Farms) while others will need to put theirs in sooner. Ideally if possible, drilling should be no later than 3rd week in July and soil sampling Feb 2018 – however again, BS Farms will have their trials delayed to start and end to fit with their cropping regime.
    6. Application rates will be based on nitrogen and will be corrected so that all use the same levels of nitrogen – as high as possible but still within the NVZ regulation.
    7. Yield meter readings from combines are acceptable for yield data in the test areas.
    8. The first pre-treatment soil sampling needs to be done before drilling (ideally by 3rd week in July). This poses a logistical challenge as the students who will be helping to do this won’t be in place until later in 2017 / early 2018.

    Milestone: Field lab planning

    December 2016

    Field Lab Aims

    The farming group wishes to explore options to optimise the use of their ADDs; maximising its nutrient value to meet crop demand and abiding by the NVZ regulations.

    “By doing the trials we hope to use digestate like a regular fertiliser, replacing the inorganic fertilisers.”

    Milestone: Idea formed

    November 2016

    Initial key field lab questions

    The group identified key questions that they wanted the field lab to answer:
    • How to stabilise nitrogen in the digestate
    • Impact on other soil microbiology (including beneficial and pathogenic microbes) and the wider rotation
    • Influence of cover crops on digestate impact
    • Impact of timing of the application – possibly combining autumn application of digesate with cover crops?
    • Differences between impact of liquid vs fibre digestate on soils

    Milestone: Idea formed

    November 2016

    Initial meeting continued

    • Would an autumn application vs a spring application of digestate, potentially in conjunctions with cover crops, help reduce loss of nitrogen? There are some unknowns about the influence of adding bacteria to the soils in terms of how pathogens might be affected – need to understand the interaction between soil-dwelling bacteria and those being applied in digestate and how this would impact the rotation.
    • What is the impact on soil leaching of nitrogen by combining cover crops and digestate? Work at the University of East Anglia on the Salle Estate has proved valuable when using livestock manure and cover crops.
    • How does the type of organic matter (liquid vs fibre) being applied to the soil influence the soil micro-organisms, especially mycorrhizal fungi? How could this influence availability of nutrients to the crop (NB Muntons is already doing some work on this with the University of Lincoln and has discovered increased in sugar concentrations in lettuces fed with digestate).
    • How could digestate compare with farmyard manure in terms of how the soil microbiota is affected?
    • It was agreed better monitoring, replication and sampling is needed, and all delegates were willing to be part of such a regime.

    Milestone: Idea formed

    November 2016

    Initial meeting

    Following a presentation by Dr Ruben Sakrabani of Cranfield University, the following unanswered questions / challenges were identified by those in the room:
    • There is lack of clarity of the legal definition of what is described as “digestate” (ie outputs from livestock slurry, vegetable waste, waste whey, municipal food waste etc)
    • There is a lack of consistency of the reporting format of those firms who analyse digestate – ideally this would also be done in real-time, according to industry standards based on agreed sampling and reporting
    • The composition of some digestates are very consistent (yearly testing / analysis is adequate – others are much more variable). Does the quality of samples change in transit on its way to being analysed (possibly not if it is transported in a cool box?)
    • Loss of nitrates through ammonia is potentially a major issue – the high pH favours conversion of ammonium to ammonia and hence loss to the atmosphere. How can digestate be stabilised (would lowering the pH help?) to prevent this? How can the effective availability of nitrogen be assessed – especially if some is gaseous? Would it be helpful / possible to acidify the digestate – organic acid was thought to be more appropriate that any others, but it is no known if this would work.
    • Some products are already available (eg from DOW – N-Lock) to help keep nitrogen in the root zone for longer – however one delegate who had tried it saw no benefits.

    Milestone: Idea formed

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