Saturday, March 28, 2020

Extreme winter washout prompts nitrogen advice

February 28, 2020 by  
Filed under Crops

Exceptionally wet winter weather means farmers may need to apply more nitrogen to crops this spring, say independent experts.

The Midlands and East Anglia – usually among the driest parts of the country – are likely to experience downward shifts in soil nitrogen supply (SNS) indices, says the Agriculture and Horticulture Development Board.

The extent of the moderate and high excess winter rainfall zones is already much wider this year, compared with the long-term average, says the AHDB. Farmers should follow the recently revised AHDB nutrient management guide (RB209) and adjust their nitrogen strategy accordingly.

Field capacity

Excess winter rainfall is the amount of rain land receives after the soil profile becomes fully wetted in the autumn – field capacity – and before the end of drainage in the spring, which typically occurs around the end end of March.

Ideally, these calculations also take account of water lost through any growing crop, such as via  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, an understanding of excess winter rainfall is essential for accurate nutrient management planning.

AHDB crop nutrition specialist Sajjad Awan said: “This winter has been phenomenally wet. 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%.”

Ongoing rain

As several weeks of the excess winter rainfall period remains, Mr Awan says it would not be a surprise if all low-excess rainfall regions are washed off the UK map by the end of March (see table).

“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 some farmers to cut back on the total amount of nitrogen applied to crops. Calculations take account of several other critical factors, such as the previous crop and soil type.

To plan your nitrogen applications, visit ahdb.org.uk/ewr

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