Nitrogen management: High excess winter rainfall already recorded

Over half of the UK is already classified as having high excess winter rainfall (EWR), which has implications for nitrogen management.

The interim EWR data, which is based on the first four months (October–January) of the full EWR period (October–March), is available to view on an interactive UK map.

Although most of the UK is already classified as having high EWR, many areas are still in the moderate and low EWR categories (after accounting for crop evaporation).

The driest areas are mainly clustered towards the eastern coastline (which is consistent with long-term rainfall trends).

We will release data for the full EWR period in April, with many more areas expected to enter the highest EWR category.

The EWR categories correspond to three look-up tables in Section 4 of our Nutrient Management Guide (RB209).

By also considering the impact of soil type and the previous crop, the tables can help estimate a field’s soil nitrogen supply (SNS) index and determine recommended nitrogen rates.

EWR reflects the amount of rainfall the land receives after the soil profile becomes fully wetted in the autumn (field capacity) and before the end of drainage in the spring. It can be used to indicate how much nitrate has leached into drainage water.

We use Met Office data to map EWR across the UK (201 40 km by 40 km grid) using three categories:

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

Two map types are created: for bare soil and for cropped land. The latter maps (for winter wheat, winter barley and winter oilseed rape) account for water lost through the crop.

Emma Willis, Environment and Agronomy Manager at AHDB, said:

“Excess winter rainfall is important when using the field assessment method to calculate soil nitrogen supply, because it is closely related to drainage and nitrate loss.

"Soil nitrogen supply information can be used to help refine nitrogen rates to meet crop requirements.

“Early nitrogen is particularly important for cereal tiller survival. Any backward cereal crops with sub-optimal shoot numbers and less root biomass need to be carefully fertilised.

"Even in forward crops, early nitrogen will help maintain yield potential. As yield is set relatively early in winter barley, maintaining tillers is especially important in this crop.

“When nitrogen is applied as urea, soils need to be above 6°C with sufficient time allowed for it to convert to nitrates. Typically, applications need to go on about 10 to 14 days earlier than ammonium nitrate.

"It is also important to ensure other nutrients do not constrain yield, including sulphur, which plays a key role in the efficient utilisation of nitrogen in cereals.”

We will release the final EWR 2024/25 data in April. We are also exploring the potential to extend the EWR maps to cover permanent pasture and update the long-term rainfall average range to 1991–2020.

To plan nitrogen applications, use the nitrogen calculator, follow RB209 guidance and access the EWR maps.