Friday, March 22, 2019

Winter nitrate leaching losses below average, says AHDB

March 5, 2019 by  
Filed under Crops

Lower than average winter rainfall levels this year has implications for nutrient management throughout the important spring growth period and beyond, say researchers.

Mid-season excess winter rainfall (EWR) values suggest relatively low nitrate leaching losses this season. The figures are published by the Agriculture and Horticulture Development Board on the AHDB Cereals & Oilseeds website.

Farmers and contractors making the first nitrogen applications last month were reminded to estimate soil nitrogen supply (SNS) to ensure crops received what they required, rather than in excess of requirements.

The SNS index of a field can be estimated by the Field Assessment Method described in the AHDB Nutrient management guide (RB209). As well as information on soil type and the previous crop, this requires a rainfall estimates for the field in question.

Ideally, this should be based on EWR, as it provides a good indication of the potential loss of nitrate through leaching. The mid-season estimates are based on EWR data from 1 October 2018 to 31 January 2019, averaged over 199 (40 km by 40 km) regions.

For each of these regions, a colour-coded UK map highlights the RB209 EWR category:
• Low: less than 150 mm EWR (annual rainfall under 600 mm)
• Moderate: 150-250 mm EWR (annual rainfall of 600-700 mm)
• High: over 250 mm EWR (annual rainfall over 700 mm)

Only 11 of the 199 regions have EWR in excess of the long-term average (1981–2010) for the equivalent measurement period, according to AHDB figures. These were all situated on the west coast of England, Wales and Scotland.

The extent of the ‘low’ and ‘moderate’ rainfall zones is much wider this year, spreading further up the eastern coastline and more inland. Consequently, many regions are in a drier rainfall category than usual, with some regions experiencing relatively large swings from high to low.

Sooner not later

AHDB nutrient research manager Sajjad Awan suggested this meant more farmers than usual made early nitrogen applications during February rather than waiting until later.

“The conditions last year could encourage more to act. The spring of 2018 was cold and wet, followed by a prolonged dry period, and this made nitrogen management challenging.

“However, the winter rainfall situation so far shows that people should not rush to make applications. SNS is likely to be highly variable across the UK right now. It’s even more important in these years to test how much nitrogen is in the soil before fertilisers are applied.”

Updated maps based on the full EWR period (1 October 2018 to 31 March 2019) will be published by the AHDB at the beginning of April at ahdb.org.uk/ewr

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