Be sure to follow rules when top-dressing, urges ADAS

Winter rainfall to mid-March was heavier than usual across most of the country, and leaching of N residues in soils is likely to have been higher than normal, confirms ADAS.

The simple temptation might be raise application rates slightly to compensate. But legal, economic and environmental considerations require a more considered approach, says soil scientist Peter Dampney.

“Reports of soil N are variable, but are typically average or lower than average.” But if nitrogen has already been applied this spring (as bag fertiliser or organic manure) it’s too late to check soil mineral N (SMN) levels by sampling and analysis, he warns.

“When assessing the soil N supply (SNS) without SMN analysis, you’ll need to use the RB209 Index tables based on soil type, previous cropping and winter rainfall,” says Mr Dampney. “But adjust this assessment if there’s been a history of previous manure applications, or the previous crop had an abnormal yield or nitrogen rate applied.”

Growers should remember that before any nitrogen is applied in an NVZ, the SNS and planned N rate must be assessed and the method used to determine the soil N supply recorded, he adds.


Excess-winter-rainfall-1 Excess-winter-rainfall-2

The figures for excess winter rainfall (mm) show it was wetter than usual across much of the country, so more N is likely to have been leached away

Main N applications should also take account of the farm’s Break Even Ratio – the ratio between the nitrogen and grain price.

“Provided N use has been about right in previous years, reduce this season’s rates by 8-10 kg/ha for each unit change in BER. The grain N/protein content of last harvest’s crops is the best way to tell whether last year’s N use was correct.”

Be sure to allow for the crop-available N supplied by organic manures that have been applied or will be, urges Mr Dampney. “Each dressing can supply significant amounts of N, P and K depending on what manure is applied, when, at what rate and how. Each application can be worth up to £500/ha.

“If you use urea, be aware that there can be large losses of N as ammonia, and it doesn’t spread like ammonium nitrate, so be sure to calibrate your spreader. Try to apply urea just before significant rain, and use it on feed rather than milling wheats, as urea can reduce grain protein content.”

Growers within NVZs must comply with all relevant rules, implemented last January, warns Mr Dampney. Further advice on them, issued by DEFRA and the EA, can be found at http://www.environment-agency.gov.uk/business/sectors/54714.aspx.

Sampling confirms lower residues

Soil-sample

N-min test results from more than 600 soil samples by Lincolnshire firm Farmacy confirm that crop available nitrogen reserves after a range of crops and set-aside are lower than the 2005-2008 average – in some cases considerably so.

Although there are big variations between and within soil types, the amounts are lower than last year’s going into the main cropping season. Reserves after peas are much lower than in past years, according to the findings (see table below).

Mean N-min values are lowest in the north west, notes the firm’s Matt Ward.

“The Farmacy N-Plan gives farmer clients justification for nitrogen application rates and produces N-max reports to ensure they are compliant with the NVZ regulations.”

Farmacy N-min 2009 levels relative to 2005-08 average

  • Combining peas -31.1%
  • Set-aside -21.8%
  • Linseed -16.8%
  • Sugar beet -13.1%
  • Winter OSR -11.1%
  • Beans -9.3%
  • Potatoes -5.8%
  • Winter cereals -2.5%

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