Digestate is a by-product from the anaerobic digestion (AD) process used on farms to generate biogas from waste. It provides a rich source of nutrients and organic matter. Estimates suggest it could save £110* per hectare on artificial fertiliser application. An Innovative Farmers Field Lab led by Agri-Tech East, with NIAB taking the research lead, suggests this estimate is achievable and the results are to be discussed at an open meeting on 12 September.
Lydia Smith at NIAB comments: “AD Digestate is a good source of nitrogen and other nutrients (phosphorus, potassium, sulphur) and repeated applications of digestate can improve nutrient levels in the soil, but must be managed to ensure they are used by crops rather than pollute water courses.
“The field lab was set up to answer a number of questions and in particular the farmers were interested in the use of cover crops to stabilise the nutrients present in the digestate, so that crops benefit.
“It is important to make nutrients available to crops and the Innovative Farmers project has shown that nitrogen levels are considerably increased in the trial plots, and could reduce the requirement for chemical fertilisers, and hence costs.”
Farmer-led
The digestate field lab is a consortium of six farmers that all have AD plants within their farming businesses. The lab is coordinated by Agri-Tech East, with research led by NIAB and Cranfield University. It is facilitated by Innovative Farmers, the not-for-profit network that enables farmer-led research.
Liz Bowles from Innovative Farmers said: “It has been great to see the farmers and researchers working together to make this field lab both scientific and practical, and we look forward to hearing their observations at the meeting which will hopefully encourage other farmers to get involved with the next stages of the research.”
Nutrient efficiency
With help from NIAB, the farmers designed the trial and chose analyses and measurements that would be taken. Yield data was collected and used to determine Nutrient Use Efficiency (NUE). Other measurements included worm count, VESS (Visual Estimates of Soil Structure) and GAI (Green Area Index) of cover crop and following crop. Soil samples were analysed for selected nutrients (available N, P, K, Mg), pH and organic matter.
Reducing loss to watercourses?
One of the concerns about using digestate is that if soluble nitrates are not taken up by crops, they can leach through the soil into watercourses or be lost to the atmosphere as ammonia - avoiding this was a key question the farmer-led group wanted to answer.
Laura Bouvet, Knowledge and Innovation Facilitator for Agri-Tech East (pictured), commented: “Farmers must comply with NVZ (Nitrate Vulnerable Zones) regulations: where a farm is within those zones there are restrictions on use, storage and spreading periods. The farmers wanted to gain a better understanding of how to use digestate effectively within these regulations.
“Some treatments used cover crops and these were found to reduce the available nitrogen in the soil. This suggests that digestate is best applied within a rotation to avoid loss into groundwater.”
Reducing volatile gas?
One of the farmers and speaker at the event, Stephen Temple, also experimented with applying sulphur as the digestate comes out of the AD plant. Acidification is thought to prevent ammonia volatilisation but care needs to be taken if it is spread to prevent foaming.
Role of microorganisms?
The project benefited from a parallel funded project which is looking at the populations of beneficial mycorrhizal fungi that form a synergistic relationship with the plant. They colonise the root zone and help to make key nutrients more available to the plant.
Studies by a NIAB PhD student have found that inoculation with these fungi resulted in a greater uptake of nitrogen by the crop, and that the amount and type of roots were affected if certain nutrients were in abundance. This could indicate that over-supply of nutrients may make the crop plant less resilient to changing adverse environmental conditions if its root system development was depressed.
Next steps
The field lab is producing promising results and these will be discussed at the Agri-Tech East event. A decision on extending the project to look at dosage rates will be made shortly.
The farmer-led trial has also been a learning experience for all involved. Each of the plots was on a working farm and the soil preparation, planting and monitoring had to fit within other farming priorities.
One farmer noted: “As a group, we have also learnt about the importance of establishing protocols ahead of data collection, to make sure they are realistic and achievable for everybody. I am looking forward to a good discussion about next steps.”
*Using the typical values for whole food-based digestate set out in the AHDB Nutrient Management Guide and applying 30cu m/ha to a winter wheat crop in spring on a sandy soil with a P index 2 and K index 2-, growers can save a significant £110/ha on artificial fertiliser application.
The event: Ruminating on the role of digestate in managing healthy soils will be held at NIAB Park Farm, Cambs CB24 9NZ on 12 September 2019 from 15.00 – 19.30. Farmers and others with an interest in digestate are welcome to attend.
Registration bit.ly/ATEeventDigestate