Nitrogen nuance: Drier March tempers excess winter rainfall

Relatively dry conditions during March prevented a complete winter washout across the UK. Jason Pole examines the final excess winter rainfall (EWR) data and explains how to use it alongside our Nutrient Management Guide (RB209).

Geopolitical tensions and volatility in global energy markets have pushed up nitrogen costs.

We provide a range of products, tools and services (based on RB209) to guide nutrient management and buffer the impact of elevated nutrient prices on margins.

This includes publishing excess winter rainfall (EWR) data to help indicate the potential amount of nitrate washed out from fields.

The EWR figures are based on accumulated rainfall over October to March, which is a proxy for the actual EWR period.

The actual EWR period, which depends on local weather and site conditions, starts when the soil profile becomes fully wetted in the autumn (field capacity) and ends when drains stop flowing in the spring.

We show the EWR results in a UK grid (comprising 199 squares, each measuring 40 km by 40 km) using three categories:

• Low EWR: less than 150 mm
• Moderate EWR: 150 to 250 mm
• High EWR: over 250 mm

We also publish maps that account for evapotranspiration from winter crops (wheat, barley and oilseed rape) and grass.

The EWR categories align with RB209 look-up tables to estimate soil nitrogen supply (SNS) in various cropping situations.

Each year, we release interim EWR results in February to highlight emerging trends.

As usual, this showed that most UK grid squares were already in the ‘high’ category by the end of January, especially in a bare-soil scenario.

The final bare-soil data shows that almost all squares (189/199) map as ‘high’, with just 10 in the ‘moderate’ category and none in the ‘low’ category.

Results are similar for wheat and barley. They are also similar for oilseed rape and grass, which are associated with a greater drying effect (compared to winter cereals).

When accounting for evapotranspiration in oilseed rape, the number of ‘high’ squares reduces to 163, with more in the moderate (19) and low (17) categories.

Consistent with long-term trends, the driest areas mainly cluster around the East of England.

Met Office rainfall data shows that large parts of the UK experienced below-average rainfall during March 2026.

This tempered EWR in many of the areas that were not already in the high EWR category in the interim data set.

However, the end of the EWR period was not as dry as the previous winter (which saw an extremely dry end).

As a result, last winter was generally wetter than the 2024/25 winter.

The AHDB Nutrient Management Guide (RB209) 2026 edition marks the first revision since the 2023 ‘golden anniversary’ edition and the 2024 strategic review of the guide.

Based on feedback from over 900 people, the review’s findings both informed the latest revision and prioritised investment in nutrient management research to feed into subsequent editions.