Anaerobic digestates as a nutrient source for organic farming (RELACS Practice abstract)

Summary

Outcome
Anaerobic digestion maintains the ratios of nutrients in the feedstock by avoiding nutrient losses, thus enabling more balanced nutrient flows. Furthermore, the end product has a higher nitrogen fertiliser value than compost of the same amount of waste.
Practical recommendations
• Keep the fertiliser in a closed environment for as long as possible. Digestates have a high nitrogen loss potential after treatment when exposed to air.
• Incorporate digestates directly after field application, e.g., through slurry injection (Picture 1), instead of using a broadcast spreader such as a traditional (liquid) manure spreader. This helps to reduce ammonia losses.
• Nitrogen use efficiency from digestates is higher in spring crops than in winter crops when incorporated into the soil before crop establishment.
• For crops with wide row distances (e.g., maize), concentrated application by strip-till is more efficient in terms of nitrogen and phosphorus fertiliser value.
• Solid-liquid separation can increase the applicability and versatility of digestates. The liquid fraction is high in nitrogen and potassium and low in phosphorus, while the solid fraction is high in organic matter and phosphorus.
• The solid fraction has a high risk of nitrogen loss. Apply it as soon as possible after separation or store it in a closed container. Composting it increases the risk of nitrogen loss and does not provide an advantage on soil organic matter compared to the application of non-composted solid digestates.