New field lab investigates cover crops & AD

What could digestate do for soil?

It all started at last year’s Royal Norfolk Show.

In the innovation tent, a group of farmers got talking about the potential of using digestate, the by-product of anaerobic digestion (AD) plants, as a natural fertiliser. Soon after, they met with Agri-Tech East and decided to set up an Innovative Farmers field lab to test the idea.

The group are now exploring how to use digestate effectively, both to ensure the maximum amount of nitrogen is available to the crop, and to find the best way to prevent it escaping as ammonia gas. 

A three-pronged approach to increasing soil health

The group will be employing three techniques to their anaerobic digestion, all of which have potential benefits for soil health and structure. The farmers will be using a split-field approach, comparing the treated AD next to their normal AD practice.

  1. Cover crops: to reduce soil erosion, improve structure and composition, improve the ability for soil to absorb more nitrogen, improve soil carbon and organic matter, provide winter cover to protect soil. The group have agreed to use vetches, black oats, fodder radish and a low level of Buckwheat as their cover crops.
  2. Fibre from chopped straw: this will be added after digesting to increase organic matter and to make the soil more 'sticky' to bind in nutrients, increasing the cation exchange capacity (the ability of the soil to retain several nutrients in plant-available form).
  3. Reducing digestate pH with acid: high alkalinity increases the volatilisation of ammonia, therefore more N is lost from the digestate in storage and application. Adding an acid like the acetic acid naturally found in digesting could reduce the pH enough to help reduce volatilisation. It may also provide more suitable conditions for mycorrhizae to start processing the AD into the soil. 

Dr Belinda Clarke, Director of Agri-Tech East, is coordinating the group and said:

“By bringing together farmers and researchers for these on-farm trials, we hope to improve the efficacy of digestate use and understand the costs.”


Taking Part  

Patrick Allpress, Farm Director at Allpress Farms Ltd in Chatteris, Cambridge

“We want to slow things down a bit, and get back to the soils of 20 years ago.”

Allpress Farms invested in a 500kW AD plant in 2014, feeding it 50% onion waste and 50% leek waste. The plant is fed 12,000 tonnes of this feedstock per year, producing around 11,500 tonnes of digestate. In one year, the plant can produce a potential 4.4 million kilowatt hours, which is roughly enough to supply electricity to 900 houses.

“The first point is getting the value out of digestate – we know what it costs, but measuring the value of it as a farm input is difficult…By doing the trials we hope to use digestate like a regular fertiliser, replacing the inorganic fertilisers.”

Patrick Allpress’s trial will be undertaken over two 30-acre fields. The area will be split into:

  • Cover crop with no digestate
  • No cover crop and only digestate
  • Cover crop and digestate
  • Control fallow treatment area
  • A personal plot, with cover crop and an inoculant to help root growth

The group will be gathering baseline data in October, with initial results ready in the spring.

Follow this field lab.

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