Understanding nitrogen release from cover crops

Joanna McBurnie provides an update on a review of the nutrient uptake and release patterns of cover crops.

Our letterbox gathers views from the ground on the key issues facing growers and our in-house team tracks the hottest topics.

Currently, crop nutrition leads the race (369 mentions), with crop diversity also featuring strongly (160 mentions).

Crop diversity includes cover crops, where crop nutrition is one of the biggest unknowns – specifically, nutrient release (how much and when).

To help plug this knowledge gap, we commissioned a short desk study in February (funded by AHDB, Anglian Water, Southern Water and Wessex Water).

We are currently putting the finishing touches on the report for this study, but we can discuss some of the initial findings.

Part of the ADAS-led study reviewed research from temperate regions that looked at nutrient uptake and release from cover crops.

Study facts

• 51 studies (12 from the UK)
• 549 measurements (with the most from the UK, closely followed by Germany and Denmark)
• 453 measurements were from field experiments (165 from the UK) 
• Most measurements were on single cover crop species (about three quarters)
• UK data only included winter cover crops (mostly grown in clay loam and loamy sand soils)
• There was little data for phosphate (three studies) and no data for potash
• As a result, the project focused on nitrogen

Many studies found that cover crops supplied up to 30 kg/ha of nitrogen to the following crop. However, 31–60 kg/ha was also commonly observed.

A small number of studies recorded greater contributions (over 61 kg/ha). In the UK studies, rye and mustard species were frequently associated with the 31 to 60 kg/ha nitrogen category.

UK commercial practice – based on our Nutrient Management Guide (RB209) – largely aligns with the findings of these studies, according to a survey (conducted as part of the project).

The survey had 151 participants (mainly farmers and advisors in England) with many (46%) accounting for nitrogen release from cover crops in nutrient management plans.

Most increased the soil nitrogen supply (SNS) index by 1 and typically reduced nitrogen applications by up to 30 kg/ha. Only three respondents increased SNS by 2 indices.  

RB209 states that destroyed cover crops may increase SNS by up to two indices. Although this aligns with the survey findings, nitrogen reductions in practice appear to be on the conservative side.

This may be because there is a general feeling that nitrogen release is too variable and difficult to quantify, which is probably why many respondents preferred not to account for nutrients from cover crops.

Currently, RB209 recommends that SNS should only be increased if there is confidence that the:

• Previous crop received the recommended rate of nitrogen
• Previous crop grew (and took up nitrogen) as expected
• Cover crop grew well
• Cover crop was terminated by the end of February (after which nutrient release is more uncertain)
• Nitrogen released was not at risk of leaching from the field

On nutrient-release timing, there were fewer studies, with most suggesting a relatively quick release of nutrients following cover crop destruction.

In fact, the UK-based studies found most nutrients are released almost immediately (within seven days).

The duration of the release (all nutrients) is uncertain and is affected by many factors (including the weather and the carbon-to-nitrogen ratio of residues).

The review has identified the potential to tighten RB209 guidance. For example, an SNS index increase of 2 in spring barley is associated with a larger reduction in the recommended nitrogen rate than in sugar beet.

It is important to revisit the evidence (including information on following crops) to ensure that RB209 guidance is based on best-available information.

Findings from the project will be used to propose potential changes to RB209, which will be considered by the Crop Nutrient Management Partnership to determine the need to update the guidance.

The evidence will also be used to update online guidance relatively rapidly, such as the AHDB-supported Cover Crops Guide (in time for nutrient management planning this winter).

The project also investigated the potential for a decision support tool in the UK, with various tools already available in other countries.

The survey identified a clear demand, but it is likely to require relatively costly development to create a robust tool for the UK.

We will consider the next steps for cover crop research later this year.