Farmers may need to apply more nitrogen to crops this year, following an exceptionally wet winter across most of the UK.

According to AHDB, the extent of the ‘moderate’ and ‘high’ excess winter rainfall (EWR) zones is already much wider this year, compared with the long-term average.

Historically drier areas of the country, particularly the middle and eastern parts of England, are the most likely to experience downward shifts in soil nitrogen supply (SNS) indices.

To determine the precise impact on nitrogen management strategies, farmers should follow the guidance published in the recently revised AHDB Nutrient management guide (RB209).

EWR is 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 (around the end of March). Ideally, the calculations also take account of water lost through any growing crop (i.e. via evapotranspiration) during this period.

Because nitrate is soluble, any water moving through a field takes nitrate out with it. As this affects soil nitrogen supply (SNS), an understanding of EWR is essential for accurate nutrient management planning.

To help farmers select the appropriate SNS look-up table in RB209, AHDB uses Met Office data to create EWR estimates, across 199 (40 x 40 km) UK regions, for the following categories:

• Low – less than 150 mm EWR (annual rainfall less than 600 mm)
• Moderate – 150 to 250 mm EWR (annual rainfall between 600 to 700 mm)
• High – over 250 mm EWR (annual rainfall over 700 mm)

Based on mid-season (1 October 2019–31 January 2020) estimates, most (about 83%) cropped regions currently fall in the ‘high’ rainfall category.

Sajjad Awan, AHDB crop nutrition specialist, said: “This winter has been phenomenally wet. Currently, only about 3% of cropped regions remain in the low-rainfall category. This is exceptional: long-term average data would put the typical low-rainfall figure closer to 25%.

“As several weeks of the EWR period remain, it would not be a surprise if all low-EWR regions are washed off the UK map by the end of March. In fact, without the drying effects of crops, no regions would fall into the low category at all. With many farmers forced to leave land bare this winter, it is even more important to consider a lack of evapotranspiration.”

The current season is in stark contrast to the 2018/19 winter, which was relatively dry and allowed some farmers to cut back on the total amount of nitrogen applied to crops.

SNS calculations take account of several other critical factors, such as the previous crop and soil type.

To plan nitrogen applications, consult RB209 and access the latest EWR maps via ahdb.org.uk/ewr