Article

What is the impact of digestate on soil?

Published: 2 July, 2018

Anaerobic digestion (AD) is becoming an increasingly attractive technology for farmers, as it adds value to waste material by converting it into biogas. Digestate, the by-product generated through the AD process, is a rich source of nutrients and has emerged as an alternative to chemical fertilisers.

Some of Agri-Tech East’s farmer members have AD plants and regularly apply digestate on their fields. However, nutrient losses, particularly that of Nitrogen, are a common problem and can lead to leaching through the soil or to ammonia gas emissions.

Setting up field labs on a working farm

Having encountered those issues on farm, a group of seven farmers from the East of England, including five Agri-Tech East members, decided to join the Innovative Farmers’ Network to set up a Field Lab and investigate how to maximise the nutrient value of their digestate. From sandy black soil to sandy clay loam, soil types vary greatly from one farm to the next and this is likely to have an impact on the efficiency of the digestate in supplying nutrients.

Anaerobic digester

JF Temple & Son is one of the farms in the project that manages an AD plant as part of his business

Based on their experience of managing an AD plant and utilising its organic output, farmers in the group were keen to focus on nitrogen stability and availability as key areas of investigation in the project. Nitrogen stability is being tested by adding acid to the digester to generate a more alkali digestate. A mix of deep-rooting and mycorrhizal-friendly cover crops (buckwheat, vetch, radish and black oat) and the addition of organic fibre were selected as treatments to increase the nitrogen readily available for crops from the digestate. Overall, these treatments are expected to not only capture more nutrients but also to have a positive impact on soil health, by improving soil structure and living conditions for the proliferation of beneficial micro-organisms.

With support from NIAB and Cranfield University as research partners, farmers opted for a simple split-plot design where a field is ‘split’ into two, with the treatment on one side and the control on the other. This type of design works well with on-farm trials since they can accommodate different soil types and fit around the already busy schedule of a working farm business.

Another area that farmers would like to explore and one that is not often directly considered is the economic impact on each of their farms of applying digestate. Do the costs of production and storage outweigh the agronomic benefits? A cost-benefit ratio analysis will be undertaken to answer this important question.

From challenges to answers

Following digestate application and cover crop drilling in the Autumn of 2017, the first set of results from soil sampling all the different treatments came in earlier this year. Although it is too early to draw any conclusions, differences in nitrogen content was observed amongst farms. The benefits from the digestate and cover crop combination should be observed when harvesting the following cash crops this summer, which will be maize and sugar beet.

NIAB training session at Cereals 2018

The training session on soil measurements attracted a larger crowd than expected

Farmers will be undertaking the next set of measurements, including assessment of soil structure (VESS) and counting worms. To facilitate this, Agri-Tech East organised a training session, delivered by NIAB at Cereals. Although we could not get our spades in the ground, the Soil Pit provided the perfect backdrop for David Clarke and Lydia Smith to explain differences in soil structure under different conditions.

A year into the Field Lab, one of the highlights has been the challenges of undertaking soil research across different farms, with all the environmental variations that this implies. But mostly, it has been fantastic to see farmers engage in all stages of the process, from designing the field trials to accommodate the project around their business, to interpreting and discussing the significance of the results with other farmers and researchers in the group. Now is the time for the next round of soil sampling, if the weather will let us.

Agri-Tech East has been coordinating the Innovative Farmers digestate group, in collaboration with the Soil Association and research partners NIAB and Cranfield University. Farms involved in the project are Allpress Farms, Boxford Suffolk Farms, Euston Estate, Holkham Estate, JF Temple & Sons Ltd, JH Walter Energy and Upton Suffolk Farms. To find out more about the Innovative Farmers Network, visit the website.