Growers planning this year’s nitrogen applications should start with a review of the performance of previous crops, before making field inspections.
Seasonal factors are less important than the characteristics and history of the field when it comes to determining nitrogen requirements, said Peter Dampney of ADAS.
“There’s always some guesswork involved at this very early stage of the season,” he said. “Cereal crops never take up much nitrogen over the winter and this year’s oilseed rape crops are currently behind in terms of development, due to the very cold winter.
“So it’s reasonable to assume that any nitrogen that might have gone into the crops is at risk from leaching, but the DEFRA rainfall maps haven’t been produced yet.”
The first nitrogen application, usually at the end of February or beginning of March, will depend on what the crop looks like and the ground conditions.
“Given the severity of the winter, I would expect that more crops than usual will benefit,” he said. “If they’re backwards and thin, some early nitrogen will give them the boost they need.”
Mike Slater of Frontier Agriculture agrees that many crops are likely to need some help. “Soil temperatures have been so low that very little mineralisation will have taken place. And although crops are coming out of the winter in better condition than expected, there has been leaf loss which will have to be made good.”
His view is that nitrogen and sulphur challenges could be rather different to normal, after the early and abrupt arrival of winter. “Growers should get out and look at crops now, and then again in March,” he said.
By March, a count of shoot numbers will help to determine canopy development. “That information will then help you decide how much nitrogen needs to be applied. A crop with 500 plants/sq m has a Green Area Index (GAI) of 0.5 and contains 10kg of N/ha, whereas a crop with 1500 shoots/sq m has a GAI of 1.5 and has already taken up 45kg of N/ha.”
Allison Grundy of GrowHow warns growers not to go too early with the first application. “Soil temperatures need to have warmed up to 5ºC for the crop to be able to take it up. And although we’ve had the thaw, temperatures have dipped again in the last week.
“If it’s too cold, this application is a waste of time and money. When the time is right, priority should be given to the very thin crops.”
Where N-min sampling is taking place from February onwards, it must be completed before any nitrogen is applied, she said.
“The main dressing can then be adjusted according to the results, so putting on around 40kg/ha early should be fine.”
The nitrogen recommendations for winter wheat in the new Fertiliser Manual are higher than those in the previous RB209, said Dr Dampney, reflecting the yield potential of newer varieties.
“In addition, the break-even ratio has been changed and is now five,” he said. “Although it’s very difficult to predict what grain will be worth next summer, this ratio won’t be far off at all. Any alterations will be very small.”
The nutrient values of different manures have also been updated.
Previous performance, especially grain nitrogen or protein levels, is a very useful indicator, and should be used for any fine tuning of nitrogen applications, he believes.
“They show how successful you’ve been in the past. Then you’re able to work out this year’s crop requirement, by considering expected yield, grain protein and the amount supplied by soil nitrogen. There’s no substitute for farm experience.”
Dr Dampney reminds growers they have to show that they’ve undertaken nitrogen planning as part of the extensions to the Nitrate Vulnerable Zone programme.
“You have to be able to demonstrate that you’ve considered the crop’s needs, what the soil supplies, how much organic manures supply and the fertiliser requirement. This plan can be modified as the season progresses, but it must be evident.”
Don’t exceed the Nmax limit, he warned. “It won’t be a problem for most. Remember that it varies according to the average across the whole area of crop being grown on the farm.”
Applying early N
• More crops than usual will benefit
• Growers urged to check crops
• Don’t go too early with first application
• Prioritise very thin crops
• Previous crop performance a useful indicator