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23 Aug 2022

Making more of digestate

As featured in Arable Farming Magazine

Making more of digestate

by Arable Farming Magazine July 2022 issue

Two recent studies have examined the potential value of digestate in terms of crop nutrition and carbon storage.

A recent scientific paper published by Ghent University in collaboration with the European Biogas Association (EBA) analyses results from a study of the nitrogen and carbon mineralisation rates of five digestates from the most representative feedstock streams in the European biogas sector: pig manure, poultry manure, energy crops, sewage sludge and food waste.

The paper reports that after 127 days, net N mineralisation of digestates ranged from 21% to 39%, highlighting the fertiliser capacity of these organo-mineral products, but also the wide range of N mineralisation results from the different products, as digestate from pig manure (39%) reached almost double the value of digestate from sewage sludge (21%). use-on-scottish-farms More information The carbon:nitrogen and ammonium nitrogen:total nitrogen ratios of the products proved to be good predictors of net N mineralisation and sound indicators of the fertiliser potential of the digestates, although the variability seen in N mineralisation makes it highly advisable to analyse digestate products before use in the field, the papers authors conclude.

For those seeking practical information on digestate use, SAC Consulting, part of Scotlands Rural College, has been involved in a six-month project gathering practical guidance and examples to share with land managers on the benefits of using digestate on-farm.

Great source Fiona Salter, of SAC Consulting, says: Digestate, when used appropriately, can be a great source of nutrients for the farm and through the work we have been doing, we hope to make land managers aware of the risks involved, how to mitigate them and the benefits that are to be had.

We know that many farmers are struggling right now with the rising costs of fertilisers, and we hope that we can provide insight into the nutrient benefit of using digestate appropriately and how to calculate the fertiliser replacement value.

A document has been created for land managers sharing guidance on digestate use on Scottish farms and details of the regulations and requirements involved.

There are also two podcasts now available on Farming and Water Scotlands website, one of which features William Rose, of Mid Coul Farms, who is reaping the benefits of spreading digestate on his farm in Dalcross, Inverness.

Mr Rose says: Using digestate has really transformed our ability to grow bigger yields and more consistent crops, so it has been a big boon and if we can achieve our objective of using no bought-in fertiliser at all, even on relatively small areas, particularly in the current climate, then we would feel very satisfied.

He adds that using digestate has added organic matter into the ground and improved the structure of his soils, but he advises farmers to read the guidance and use the products in the right way.

Spreading digestate well is challenging.

You must have accurate spreading equipment and must at least have a dribble bar or some form of injection.

Being able to spread accurately and only where it is needed, is essential, he says.

EBA study key messages

  • Making the most of the AD process: Digestate recovery should be a systematic part of an AD project to exploit all its potential benefits. Digestate has a double agronomic interest: making mineral elements available to plants and contributing to carbon storage in soils
  • Closing nutrient loops: Nutrients, such as nitrogen, are fully conserved in the AD process. They even become more available, partially changing from an organic to a mineral form, preferred by plants. The nitrogen mineralisation of digestates ranges from 21-39%, underlining the fertilising property of the digestates included in the study
  • Enabling carbon storage: On top of its primary function as organic fertiliser, digestate also has the potential to store carbon in agricultural soils. This increases the amount of CO2 stored in the soil, reducing its presence in the atmosphere. About 50-80% of carbon from digestates applied as organic fertiliser can be stored in the soil

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