Nitrogen boosts may be necessary following wet winter washout

Wednesday, 5 February 2020

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 towards the middle and eastern half 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. 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 cropped (about 83%) regions currently fall in the ‘high’ rainfall category.

Sajjad Awan, AHDB crop nutrition specialist, said: “This winter has been phenomenally wet for most of the UK. 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 many 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.

*Crop condition

When calculating nitrogen requirements, it is important to consider factors that could limit yield. Under poor growth conditions, crop yield could be limited and may not exhibit a full response to nitrogen.

RB209 includes a method to adjust nitrogen in response to expected yields. In wheat, for example, this is above or below a benchmark yield of 8t/ha.

For low-yielding winter wheat crops, RB209 recommended rates are reduced by 10 kg N/ha for each 0.5 t/ha reduction in expected yield.

The current prolonged wet spell is likely to reduce expected yield. Saturated, cool soils impede crop establishment, resulting in restricted root development and even plant death.

Poorly rooted crops can appear pale or yellow under continued saturated soil conditions; this is likely to be due to the crop’s inability to access nitrogen, rather than nitrogen availability.

To maximise nitrogen utilisation efficiency in such seasons, consider the use of splits to apply the total amount of nitrogen across smaller doses.

Where rooting systems are poorly developed, making fresh P applications (organic materials or synthetic fertiliser), especially on P deficient soils, can be beneficial.

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

Notes to editors

AHDB’s EWR estimates use Meteorological Office Rainfall and Evaporation Calculation System (MORECS) data. Four maps are produced: one for bare soil, one for winter cereals, one for winter barley and one for winter oilseed rape. EWR deviation from the long-term average (1981–2010) is also provided. Updated during the winter months, the current map displays EWR over the period 1 October 2019–31 January 2020.

EWR information was funded by Defra and produced by ADAS as ‘spring N advice’ up until 2015. In that year, AHDB took on responsibility for the publication of nutrient management guidance, through RB209, from Defra. The release of EWR data is part of AHDB’s commitment to promoting best nutrient management practice.

Excess winter rainfall

Excess winter rainfall

Find out how to account for nitrate losses with the EWR tool. Use this information to guide nutrient management planning

Excess winter rainfall (EWR) data presented over 199 40x40km squares. Mid-season (1 October 2019–31 January 2020) results for winter wheat shown.

  • Left: Current winter (2019–20)
  • Right: Long-term average (1981–2010)

Excess winter rainfall (EWR) data presented over 199 40x40km squares. Mid-season (1 October 2019–31 January 2020) results for winter wheat shown.

  • Left: Current winter (2019–2020) compared with the long-term average (1981–2010). 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
  • Right: Difference between current season and long-term average (1981–2010). Despite being associated with high levels of EWR, parts of Scotland, northwest England and Northern Ireland have been drier than the long-term average