Soil nitrogen supply is very variable

EXTRA care with fertiliser management will be required this spring following one of the driest winters on record.

The lack of rainfall, combined with difficult growing conditions last year and a mild autumn, means soil nitrogen supply (SNS) levels will be extremely variable, making decisions on applications more difficult than usual.

This is the latest advice from ADAS, which says in areas where there has been low rainfall, nitrogen leaching losses are likely to have been less than average so a high proportion of autumn soil N residues are already likely to be in the current crop or awaiting uptake this spring.

In addition, 2011’s dry spring weather compromised crop performance and reduced nitrogen uptake, particularly on light and shallow soils, leaving more nitrogen in the soil post-harvest.

Mild weather may also have caused more of the potential mineralised N to have appeared by early spring, although the overall quantity released during the growing season is likely to remain unchanged.

ADAS senior soil scientist Dr Paul Newell Price says Central and Eastern England have been particularly bad in terms of a lack of rain and even growers further west, such as parts of Wales and Somerset, may have to use the low rainfall table in the Fertiliser Manual when making application decisions this season.

“Where crop growth is advanced because of the mild conditions, these crops will have taken up much of the available soil mineral N left in the soil last autumn but weak and poorly established crops will contain little N,” he says.

The advice is to target soil mineral nitrogen (SMN) sampling to fields likely to have high N residues, such as where regular organic manure has been applied in the past.

“In areas that have been particularly dry this autumn and winter, and last year’s crop was ‘poor’, then it may be worth sampling on some light, shallow soils,” says Dr Newell Price.

Unchanged

The advice for early nitrogen applications remains unchanged from previous years, he says, and growers should inspect each crop to judge the need for early applications, applying 40kg N/ha in winter sown cereals now to encourage tillering and canopy development in thin or backward crops.

Applications could also be made in second cereals, where take-all risk is high, and cereals intended for biofuel where starch, not protein, is wanted.

“Early N applications should not be made to crops with lush canopies, or oilseed rape crops with a green area index (GAI) of over 2,” he says.

“Where winter oilseed rape crops emerged late and are small, and where SNS is low to moderate, apply an early application of up to 100kg N/ha.”

According to Dr Newell Price, sulphur applications must be factored in where needed, particularly as deficiency is becoming more common on light and medium textured soils. As well as oilseed rape, many cereal crops are potentially sulphur deficient, he points out.

“Have a good think about how the previous crop has performed, how much rainfall you have had in your area over the winter, and how the current crop is looking before making final decisions on spring fertiliser applications,” he says.

New NVZ regulations from January 1, 2012

  1. On farms that produce slurry or poultry manure there must be at least six months storage capacity for pig slurry and poultry manures, and at least five months storage capacity for cattle and other types of slurry. The minimum storage requirement must be calculated using the specified calculation method (Guidance leaflet 4).
  2. The livestock manure nitrogen efficiency values that must be used in the Nmax compliance calculation have increased (see attached table) – these updated values must be used when calculating the supply of crop available N from livestock manure applications made after January 1, 2012. This means more of the nitrogen from applied livestock manures will be considered as available for crop uptake, which will reduce the potential amount of additional manufactured fertiliser that can then be applied. Details of the Nmax rule are described in NVZ Guidance leaflet 7.

Sulphur applications

  • Plant tissue analysis is the best way to confirm deficiency, says ADAS, but aim to anticipate deficiencies by applying sulphur.
  • Always use water soluble sulphur for rapid and effective uptake.
  • For oilseed rape – apply 50-75 kg SO3/ha in early spring
  • For cereals – apply 25-50 kg SO3/ha in early spring

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