Thin or backward cereal and oilseed rape crops are likely to need early nitrogen in late February or early March when ground conditions are suitable, ADAS says.
But every field will need to be judged individually, and have a N plan in place if the holding is within a NVZ.
Winter rainfall to 27 January (see map) shows that many areas, assuming normal rainfall in February and March, will have had an average or wetter than average winter.
Where this is combined with thin or backward crops with low N uptake, soil nitrogen supply (SNS) will be slightly lower than average.
“But every field must be judged individually,” stresses Peter Dampney, an ADAS soil scientist.
Cold, wet weather has resulted in no significant crop growth since November.
“There are a lot of thin and poorly tillered crops about, and also oilseed rape stands with poor canopies. However inspecting each crop will be essential to judge the need for early spring N.”
Crops likely to benefit from early dressing (40 kg N/ha for winter sown cereals and up to 100 kg N/ha for winter oilseed rape) include thin or backward ones, second cereals at high take-all risk, and cereals intended for bio-fuel. “This N will encourage tillering and canopy development,” he explains.
|Less soil N is likely to be available to crops at the beginning of spring following a wetter than average winter|
Oilseed rape green area indices (GAIs) can be estimated via digital crop photos at www.totaloilseedcare.co.uk.
Early N should not be applied until soils have dried out, and application to waterlogged, frozen or snow covered ground – both financially and environmentally undesirable – is forbidden in NVZs.
Farmers with land in NVZ’s must prepare individual field nitrogen plans before applying manure or N fertiliser this spring, Mr Dampney says.
All holdings in NVZ’s, which account for 68% of England after last year’s zones expansion, should comply with the new rules that came into force in Jan 2009. Failure to do so risks reduced Single Farm Payments, he warns.
To complete the new plans growers must assess the SNS of each field, either via RB209, based on soil type, previous cropping and winter rainfall, but more reliably by soil mineral N (SMN) sampling and analysis.
“SMN analysis is not a precise measure so, for best value, target fields likely to be high, for example where manures have been used.
“It’s crucial to get samples to the lab as quickly as possible. HGCA research shows SMN can increase by up to 10% for each day’s delay- even if samples are kept cool.”
In addition to SNS, crop N requirement, the N available to it from manure dressings and the need for manufactured fertiliser N must all be assessed or calculated, and recorded, Mr Dampney adds. The amount of N already in crops should also be estimated (see table).
A recent amendment to the rules is that the spring N max limit for winter oilseed rape is now 250 kg N/ha, but including any N applied in the autumn up to the limit of 30kg/ha. This was previously 220 kg N/ha irrespective of autumn dressings.
Crop nitrogen content calculator
Crop N content (kg N/ha)
Oilseed rape (crop height, cm)
For more info visit www.environment-agency.gov.uk/business/sectors/54714.aspx