Smart use of renewable fertilisers can increase yields and reduce costs with no negative impact on crop quality or safety, according to a report published by the Waste & Resources Action Programme (WRAP) today (9 February).

Research for the ‘Digestate & Compost in Agriculture’ (DC-Agri) programme, a five-year project led by WRAP, found that the 1.5 million tonnes of food waste processed by the UK’s anaerobic digesters (AD) every year is a valuable source of readily available nitrogen, which the report describes as ‘the single most important nutrient influencing crop yields’.

WRAP says that data derived from the study will enable farmers to predict how much nitrogen digestate will supply their crops, allowing them to confidently reduce the amount of bagged fertiliser used without affecting yields.

According to the report, compost made from recycled green garden waste and food waste builds levels of organic matter more quickly than other organic materials such as farmyard manure.

High levels of organic matter in soil mean that mineral particles are bound together, producing soil aggregates that make cultivation easier, therefore, the study concludes, delivering stronger and more resilient crops.

In addition, the research found that both digestate and compost provide increased levels of phosphorus, potassium and sulphur, which help to maintain soil fertility.

The DC-Agri project was created to form a ‘robust evidence base’ to support the confident use of renewable fertilisers. It was commissioned in 2010 and saw core experiments run over three growing seasons, with supplementary research completed in 2015. The project has also established a programme of guidance tools, events and learning materials.

Good practice guides

Following the completion of the research programme, WRAP has created guides to good practice that explain how to use digestate and compost products safely, and to help farmers and growers make the most of renewable fertilisers, both economically and environmentally.

Extensive field studies carried out over the course of the DC-Agri project found that crops grown with an integrated nutrient plan that combines bagged fertilisers with digestate or compost resulted in both higher yields (grain yields from winter cereal crops increased by 7-10 per cent) and better financial returns.

The timing and method of digestate application, it found, is critical to gaining the maximum value from crops. If, for example, digestate is used in the spring on actively growing crops, nitrogen value can significantly increase.

The report also argues that compost should be seen more as a means to build up long-term soil nitrogen, rather than a short-term replacement. The value in compost, it states, lies in its ability to build organic matter more quickly than other organic materials, with retention of the organic matter in soils supplied by green compost almost double that of farmyard manure.

‘Significant step forward’ for AD and composting industries

Following the publication of the DC-Agri report, Dr Richard Swannell, Director at WRAP, said: “Digestate and compost are valuable renewable products of our food and garden waste recycling processes. These new findings show the benefits that using the products correctly can bring, and for the first time farmers and growers have the evidence to make informed decisions about their fertiliser use. This is a significant step forward for both the anaerobic digestion and composting industries and for farmers and growers.”

Resources Minister Rory Stewart added: “Sustainable farming is increasingly important for our environment and food supply. I welcome this report which will allow farmers to make better decisions about renewable fertilisers, helping to grow crops more smartly and efficiently.”

Making informed digestate decisions

Commenting on the research, Martin Rogers, Environment Policy Advisor at the National Farmers Union, said: “Digestate is an excellent biofertiliser which can help farmers to reduce their costs. With the recent rapid growth in AD infrastructure, it is becoming increasingly available to farmers across the UK.

“It is therefore important that anaerobic digestate is managed appropriately to maintain quality standards and achieve the best results. The DC-Agri experiments provide evidence to help farmers make informed decisions about its use.”

The full DC-Agri report and good practice guides, plus more information on the project, can be found at WRAP’s website.