Could lower excess winter rainfall help farmers save nitrogen?

Although most UK areas received less precipitation over the 2021–22 excess winter rainfall (EWR) period, it has not resulted in widespread changes in the EWR categories used to help determine soil nitrogen supply (SNS) to cereals and oilseeds crops.

The EWR categories for most of the UK are the same as would be observed in an ‘average’ winter, with only some areas, mainly in England, entering the next driest EWR category.

With fertiliser prices at historic highs, farmers should use AHDB’s EWR data, nutrient management guide (RB209) and nitrogen calculator to determine the implications for nutrient management strategies.

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.

Once the soil is fully wetted, drainage water may leach nitrate from fields. It is important to consider the extent of such losses in nutrient management plans.

AHDB uses Met Office data (1 October to 31 March) to indicate EWR categories (low, moderate and high) over 199 40 km by 40 km UK squares:

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)
Two map types are created – bare soil and cropped land. The cropped-land maps, which are produced for winter wheat, winter barley and winter oilseed rape, account for water lost through the crop.

The 2021–22 EWR period was turbulent, ranging from very wet to very dry months. However, across the entire period most areas were generally drier. In contrast, the 2020–21 EWR period was relatively wet.

Information on the EWR category can be used to identify the appropriate SNS look-up tables in RB209. These tables, which also account for the soil type and the previous crop, provide an estimate of SNS.

For example, in relatively nutrient retentive soils (deep clay and silt soils), with cereals grown as a previous crop, a move from the moderate to the low rainfall category increases the estimated SNS index from 1 to 2. However, not all changes to EWR status affect the estimated SNS status. For example, if the soils were medium or light in the previous scenario, then no change to the SNS status would be anticipated. Additionally, although conditions were generally drier in many areas, reductions in rainfall were not low enough to trigger a change in the EWR category.

Even in situations where the SNS index is anticipated to be higher than usual, careful consideration is required to determine if it is appropriate to reduce nitrogen rates.

AHDB’s crop nutrient management specialist, Alice Sin said “In light of current fertiliser prices, any boost to SNS is welcome. However, this may only represent a few more kilos of nitrogen per hectare in some situations. Although there is potential to shave nitrogen rates in some situations, careful consideration is needed to determine the best rate. This is particularly true for milling wheat, where it is important to consider all factors that influence grain protein content.”

Over the winter, AHDB published results from a review of nitrogen management in cereals and oilseeds. This found that, despite high fertiliser prices, it may be economically justified to target milling quality, particularly if premiums are around £30/t or more and there is a low risk of not meeting milling wheat specifications.

Alice said “Any reduction in nitrogen rate shifts crop responses towards a steeper part of the response curve – where there is less room for error. Consequently, adjust nitrogen rates carefully and consider making major changes gradually over a few seasons.”

In situations where reduced nitrogen rates are used, it is important to monitor crops for signs of deficiency, assess any potential change to expected yield, and adjust nutrient management plans accordingly.

To plan nitrogen applications, use the nitrogen calculator, follow RB209 guidance and access the EWR maps on the AHDB website: ahdb.org.uk/ewr

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