Traditionally ureabased foliar sprays have been applied at milky ripe stage to achieve higher grain protein but more recently, urea polymer foliar products have been developed for application, starting earlier in the season, with the aim of improving the efficiency of applied N.
As government agricultural policy moves to reducing on-farm emissions of greenhouse gases, protecting groundwater from contamination and regenerating soils, there is growing pressure on farmers to adopt more sustainable farming practices.
According to Carl Gibbard, of Agro-Vital UK, careful product choice can help growers achieve more yield and better quality from less nitrogen input without harming the environment.
We know nitrogen is an extremely important nutrient for healthy crop development, but uptake can be limited if applied in sub-optimal conditions and when overused it can have negative consequences for the environment,” he says.
The type of nitrogen growers use and how and when it is applied on-farm should be considered more closely if emissions are to be reduced and greater efficiencies are to be realised, he says.
“Granular nitrogen is the most common form of nitrogen farmers will use, but this is often at very high volumes and is prone to leaching and volatilisation,” says Mr Gibbard.
On the other hand, urea polymer fertilisers, applied as foliar nitrogen, break down to release nitrogen as amide (NH2), which plants can readily use.
“The breakdown phase often happens over a period of four to six weeks, steadily providing the crop with nutrition and avoiding the ‘flush’ of growth often seen with granular fertilisers.
Recent trials have indicated that 7kg N per hectare from EfficieN28-t, a urea polymer applied as a foliar nitrogen fertiliser, works just as effectively as 40kg N/ha from granular nitrogen, producing strong yields and good quality, and does not leach or scorch,” Mr Gibbard says.
Foliar N products offer an opportunity to ‘fine-tune’ N inputs, according to independent crop nutrition consultant Dr Ian Richards, with granular, soil-applied N doing the ‘heavy lifting’ in a fertiliser programme.
“With granular N, you can put the main dressing on for cereals – up to 100-150kg/ha without damaging the crop.” Liquid UAN fertilisers can also be applied in quite large quantities at the beginning of the season and while some are claimed to be absorbed more quickly than granular N, there is little difference between the two forms in this respect, says Dr Richards.
He adds: “Foliar N has a role later in the season where, for example, growers of milling wheat want to boost protein levels.
It can be applied at GS75 – the milky ripe stage.
If you are just wanting N, the cheapest way to do this is a 20% weight by volume solution of urea.
“There are formulated slow-release polymer products and others containing sulphur, magnesium or micronutrients as well as N.
“Some offer reduced risk of scorch but differences in cost then come into the choice of product to use.” Interest in environmental issues relating to fertiliser application, such as reducing ammonium nitrate application, is rising.
“If soil-based fertiliser is applied in spring after the drains have stopped running not much is lost by leaching, if any.
If there is a bit left over in autumn it may be at risk of leaching but is usually taken up by the autumn-sown crop.
“With both soil-applied and foliar urea there can be a risk of nitrogen loss by volatilisation.
The amount lost varies a lot from none to 30-40% in extreme cases.
“The key is to watch the crop N requirement and make sure you do not exceed the optimum rate.” The Government has recently consulted on reducing ammonia emissions from the use of solid urea on farms in England with its preferred option being a total ban, according to NFU.
However, the consultation refers to solid urea only and no ban is proposed for foliar urea, says Dr Richards.
Looking ahead, there may be a greater need for products for fine-tuning and foliar N could help growers meet environmental requirements, he suggests.
“It may be used as part of an effort to make matching N requirement to crop requirement as exact as possible; there may be a bigger role for fine-tuning.
“It would be nice to think that in the future N application could be more accurately defined and applied than it is now.” When deciding on whether to use a foliar product, it is also worth checking buyer and end user contracts, says Dr Richards.
“Some contracts specify you cannot use foliar urea.
Sometimes use of a product can be acceptable technically but may not be commercially so.”