In response to advanced crops and uncertainty about the amount of available nitrogen in the ground, ADAS has advised farmers to assess crop growth before applying large quantities of nitrogen to crops this spring.
In its latest Crop Action Report, ADAS predicts most wheat crops with fewer than 1,000 shoots per square metre will benefit from 40kg of nitrogen per hectare.
However, it suggests forward crops with high tiller numbers will be at risk of lodging: “These crops will benefit from delayed N to reduce the lodging risk.”
While most oilseed rape crops are in the middle of stem extension, the most advanced are beginning to reach the green bud stage and so farmers should be thinking about nitrogen application.
ADAS says: “Crops with a green area index of 1.5 and above will benefit from having the first N application split, reduced or delayed until green bud to reduce the risk of building an overly large canopy.
“For smaller crops, the first fertiliser application of N should be applied from mid-February, as soon as the crop starts actively growing and when soil conditions allow."
While mild weather has resulted in extensive soil mineralisation since last autumn, high levels of rainfall in some areas has meant the mineralised nitrogen, which would otherwise be available to growing crops, may have been leached out of the soil profile.
Effectively, soil nitrogen levels are wide-ranging, with factors such as soil type and weather playing a big part in this variation.
John Williams, principal soil scientist at Adas, says: “Good growing conditions in many parts of the country have led to many crops taking up significant quantities of soil nitrogen, which would otherwise have been lost from the soil by over-winter leaching.
“Where crops have not established well, over-winter nitrate leaching losses are likely to be higher, particularly where over-winter rainfall has been above average.”
To help ensure nitrogen is not under- or over-supplied, ADAS, recommends using green area index and tiller number assessments, along with soil nitrogen tests, to help plan fertiliser programmes.
With nitrogen levels varying not only from farm-to-farm, but within fields, precision experts Soyl suggests using variable rate application would allow less forward areas to be fed early to encourage growth, while more forward areas could have some nitrogen held back until later in the season.
Yorkshire farmer Graham Potter, who uses variable nitrogen application, says: “Variable rate nitrogen is a necessity, especially in a wet season like this. Crops are showing massive variation in growth and development coming out of winter and would be impossible to manage manually.”