applying digestate from an anaerobic digestion plant© Tim Scrivener

Growers should be making the most of the organic nutrients found in food-based digestates to save on bagged nitrogen fertilisers and promote healthy soils, according to a report.

Many organic materials, such as composts, manures and digestates, contain nitrogen that can be used as a crop fertiliser, but it is often unclear how this becomes available to plants.

The results of a five-year project led by the charity Waste and Resources Action Programme (Wrap) will now help growers to quantify the amount of crop-available nitrogen found in food-based digestate.

See also: Solid v liquid fertiliser: Which is best? 

Food-waste liquid digestate, which is a product of anaerobic digestion plants, generally contains more available nitrogen than most other organic materials.

The charity hopes results of its latest research, published in Field experiments for quality digestate and compost in agriculture (DC-Agri), will help growers to use the products more in crop production.

The research also found solid compost builds levels of soil organic matter more quickly than other organic materials, such as farmyard manure.

See also: Why investing in organic matter can sustain future crop yields

Wrap director Richard Swannell hailed the findings as a “significant step” towards increased use of digestate and compost products in agriculture.

“Digestate and compost are valuable renewable products of our food and garden waste recycling processes,” he said.

“These new findings show the benefits using digestate and compost correctly can bring, and for the first time growers have the evidence to make informed decisions about their fertiliser use.”

The study drew on results from 22 experimental site in England, Scotland and Wales.

Indicative nutrient contents for food-based digestate

Dry matter content 4%
Nitrogen (N) 5kg/t
Readily available N 4kg/t
Phosphate 0.5kg/t
Potash 2kg/t
Magnesium 0.1kg/t
Sulphur 0.4kg/t
*Data taken from WRAP’s DC-Agri report. For full project results visit Wrap’s website

The study also found the average nitrogen use efficiency of food-based digestate, when applied in the spring using a bandspreader, was 55% of total applied nitrogen.

However, this was reduced to 15% of total applied nitrogen when the product was bandspread in the autumn, highlighting the effect of nitrogen losses from leaching over winter.

See also: 7 ways growers can improve soil health

Nitrogen benefits aside, the report summary argues that composts and digestates can also provide an important additional source of phosphate, potash and sulphur.

This gave a nutrient boost to crops early in the season, which resulted in higher crop yields in comparison with crops just grown with bagged fertilisers.

The report says this added nutrient benefit was particularly important on shallow soils over chalk and limestone, where it can be difficult to hit or maintain target phosphate levels.