How to use excess winter rainfall data to plan nitrogen applications

Use excess winter rainfall (EWR) data to adapt nitrogen applications to wheat, barley, oilseed rape and grass based on Nutrient Management Guide (RB209) recommendations.

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, EWR should also account for crop evapotranspiration.

EWR data can indicate potential nitrate losses in drainage water.

Such losses need to be accounted for when planning nitrogen applications.

• AHDB uses Met Office data to calculate EWR information
• EWR is displayed in a UK grid (comprising 199 squares, each measuring 40 km by 40 km)
• There are five ground-cover maps (to account for evapotranspiration)
• The maps show three EWR categories:
  • Low EWR: less than 150 mm
  • Moderate EWR: 150 to 250 mm 
  • High EWR: more than 250 mm 
• These categories align with look-up tables in RB209 to help you estimate soil nitrogen supply (SNS) indices
• The maps are updated twice each year (based on interim data in February and full data in April)

Although this information can guide nutrient management planning, field-level information provides the best estimates of SNS indices.

Nitrate, unlike ammonium, is not held on to soil particles. This makes it soluble and vulnerable to leaching. As ammonium-N sources are rapidly converted to nitrate, they are equally vulnerable.

The amount of nitrate leached depends on the quantity in the soil when the soil reaches field capacity, the soil type and the amount of water draining through the soil – excess winter rainfall (EWR).

Light sand soils and some shallow soils can be described as ‘leaky’. Nitrate in these soils can be fully leached in an average winter, even where substantial residues are present in the autumn. The Soil Nitrogen Supply (SNS) Index is nearly always 0 or 1 and is independent of previous cropping, except in low rainfall areas or after dry winters.

Deep clay and silt soils can be described as ‘retentive’. The leaching process is much slower and more of the nitrate residues in autumn will be available for crop uptake in the following spring. Differences in EWR will have a large effect on SNS in these soils. Low levels of SNS (Index 0 and 1) are less frequent than on sandy soils. Other mineral soil types are intermediate between these two extremes.

Because of both regional and seasonal differences, separate SNS Index tables are given in the AHDB Nutrient Management Guide (RB209) sections 3–6 for low, medium or high rainfall situations.