Excess winter rainfall (EWR) and soil nitrogen supply (SNS)
When planning nitrogen applications, it is essential to take into account any loss of nitrogen from the soil.
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, however, they are equally vulnerable.
The amount of winter rainfall has an important influence on the amount of nitrate leached and, thus, the nitrogen availability to a crop.
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).
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). EWR calculations should also take account of the water lost through a growing crop (evapotranspiration) during this period.
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.
Case study: arable crops (RB209 section 4)
The SNS index of a field can be estimated by the Field Assessment Method described in RB209. In addition to information on soil type and the previous crop, this method requires an estimate of the rainfall range for the field, ideally based on EWR. RB209 provides SNS indices in simple look-up tables for the following rainfall categories:
- Low – less than 150mm EWR (annual rainfall less than 600mm)
- Moderate – 150 to 250mm EWR (annual rainfall between 600 to 700mm)
- High – over 250mm EWR (annual rainfall over 700mm)
Each year, AHDB uses Met Office UK rainfall data from (1 October to 31 March) and presents average EWR data over 199 40km by 40 km squares. Three maps are produced, one for winter cereals, one for winter barley and one for winter oilseed rape. Note: the latter crop has a larger canopy and loses more water. EWR deviation from the long-term average (1981 to 2010) is also provided. The map is located at the top of this page.
The EWR information can be used to guide nutrient management planning. AHDB stresses, however, that field-level information should be used to provide the best estimates of SNS indices.
Long-term (1981–2010) average excess winter rainfall (until 31 March)