22 March 2002

Scope for cut in N rates this spring – ADAS

LATEST ADAS advice suggests growers can safely reduce nitrogen inputs this spring, following normal winter rainfall.

Lower wheat prices also mean a further 15-25kg/ha can be shaved from rates.

Tests show soil mineral nitrogen levels are near normal for a given soil type, says ADAS nutrition expert Peter Dampney.

So growers can use regional excess winter rainfall (EWR) data to calculate cuts to N rates, based on the soil nitrogen supply (SNS) of individual fields shown in the RB209 Fertiliser Recommendations booklet,

Low grain prices add to the scope for rate cuts. Assuming a grain value of £63/t and ammonium nitrate fertiliser (34.5% N) priced at £110/t (equivalent to 32p/kg N), a 5kg yield response will be needed to pay for each 1kg of nitrogen applied this year. Usually just 3kg is needed.

That means standard RB209 rates can be cut by 15-25kg/ha this year. For cheaper N sources, such as urea costing £120/t (equivalent to 26p/kg N), the break-even ratio is nearer 4, so only 5-10 kg/ha needs cutting.

The regional EWR data collated by ADAS, with DEFRA support, shows:

lMost of East Anglia, Lincolnshire and Central England has had below 150 mm EWR, so use the RB209 SNS table for low rainfall.

lParts of central southern England, the south-west, the south-east, the West Midlands, Yorkshire and the north-east have had between 150-250 mm EWR, so use the SNS table for moderate rainfall.

lMost of the south, the south-west, the north-west and Wales have had over 250 mm EWR, so use the SNS table for high rainfall.

Where organic manures have been used recently SMN levels are much higher, confirming the value of SMN analysis, Dr Dampney notes.

SMN levels are lower on "nitrate leaky" sandy or shallow soils than "nitrate retentive" clay or silt soils , he adds. &#42