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With price differences between Ammonium Nitrate and urea narrowing in recent months how do you make the best Nitrogen fertiliser choice for your farm?


Here's an article to help you answer those very questions...

Relatively low grain prices continue to make cost of crop inputs one of the most important issues in the minds of UK growers, but there is much more to making the right decision about which Nitrogen source to use than simply looking at price per tonne.

Understanding the factors affecting efficiency of Nitrogen utilisation and their impact on production economics are critical to make the right decision.

With more competitive prices for UK produced Ammonium Nitrate (AN), the arguments for using urea are increasingly under question with the 2017 fertiliser application season to date underlining this.

Dry conditions in much of the country have resulted in urea sitting on top of the land with volatilisation meaning much of the available Nitrogen being lost to the atmosphere with a detrimental effect on cost of production as well as the environment.

With Nitrogen fertiliser creating the greatest return on investment of all inputs at around 5:1 on cereals, correct choice and use of Nitrogen source are, therefore, some of the most important decisions a grower has to make.

Availability of Nitrogen

A key factor to consider is Nitrogen utilisation efficiency – the relationship between what comes out of the back of a spreader and what is actually taken up by the plant.

According to the definitive Defra NT26 research programme into UK fertiliser use, the process of volatilisation to the air reduces the amount of urea Nitrogen available to cereal crops by on average of 22%.

In the worst case scenario recorded, this was 43% and was even higher in grassland. This means that nearly half of the Nitrogen paid for in urea can end up in the atmosphere and not on your crops.

Volatilisation is compounded by the way in which the Nitrogen contained in urea becomes available to the plant. This is a complex process known as hydrolysis which is reliant on interaction with soil bacteria and the environment.

The inefficiency and variability of the processes involved in this combined with the inherent volatilisation, seriously affects the amount of Nitrogen available to the plant from urea-based fertilisers.

It’s a very different situation with AN where Nitrogen is directly available to the plant as soon as it is applied.

In fact, the Defra researchers worked out that to maintain crop yields, the optimum N rate using urea would have to be, on average, 10% higher than with AN.

On average, winter cereals using urea were shown to have yields 0.3 – 0.4 t/ha less than those which received AN-based fertiliser N.

2017 underlined the problem

Traditionally it was thought that if you managed to get urea on early enough in the season the economics might stack up but this has been largely disproved in recent years.

Even with early application and ideal weather conditions, the current lower prices for AN have largely negated any perceived advantages.

Latest thinking suggests the success of urea applications depends very much on air temperature and rainfall in the three days after application.

Without significant rain in this period, urea is not washed into contact with the microbial enzymes in soil that allow hydrolysis and the resultant Nitrogen losses to air will offset any potential cost savings.

You only have to look at the warm and dry conditions of recent weeks to understand why this is such a problem. Very few urea applications across the country will have been effective this year with significant risk of low yields and poor quality without further investment in the crop.

In fact for grassland, looking at Spring weather over 15 sites from Cornwall to Glasgow over the last five years gives a good indication of the risks involved long-term.

At 0oC urea needs 5mm of rainfall in three days after application to work effectively but the chance of getting this is just 1 in 3 (33%) whilst at 5oC, 7mm rainfall is needed and the likelihood of this falls to 1 in 4 (24%).

At 10oC – not an uncommon Spring temperature in the UK and certainly one that many regions have experienced this year - urea needs 10mm rainfall in the three days after application to work and there is only a 15% chance of this occurring.

In contrast, AN is much less dependent on the weather for it to be effective plus it is stable in use and is easier to apply.

The end result is Nitrogen applied as AN is far more likely to be taken up by the plant at the time it is most responsive to it and in the quantities it needs.

Optimum application

Uneven spreading of fertiliser has been estimated to reduce wheat margins by around £50/ha through a combination of yield loss and nutrient wastage.

Under-application in any area can result in plants not achieving their full harvest potential whilst higher localised rates mean a proportion of nutrients are not used and the risk of nitrate leaching is increased.

Efficiency of spreading depends on many factors including density of material, granule or prill shape and consistency of manufacture.

Whatever the fertiliser source, considerable energy has to be transferred to material in the application process and the wider the spread width the higher this needs to be. This puts significant demands on product hardness and strength.

Solid urea is available in prills or granules, both of which have a lower density than AN. Although urea has a higher % of Nitrogen than AN, this lower density means that a spreader hopper of urea will contain no more Nitrogen than one filled with AN.

The lower density also works against urea in the application process, making crop coverage far less reliable. Furthermore, the wider the spread width used, the greater the variability.

In addition, quality fertiliser manufacturers like CF Fertilisers spend a lot of time developing prill and granule structures to ensure optimum granulometry and this is largely absent in the global commodity production of urea.

Not surprisingly, the Defra researchers concluded that prilled urea did not achieve a good spread pattern. Whilst quality AN products gave a reliable and consistent pattern up to 24m with a well calibrated spreader, urea did not.

Environmental risks

Using higher levels of urea-based fertilisers would undoubtedly lead to serious harm to the environment, the Defra researchers believe.

In contrast, AN has conclusively been shown to be ‘the best option for British Farming’ due to the low levels of emissions, Defra says.

Trials using twenty eight purpose-built wind tunnels quantified ammonia emission from granular urea at on average 22% on arable land, compared to just c. 2% from N applied as AN.

Ammonia emissions from urea ranged from 2% to nearly 43% in cereals and 58% in grassland leaving the researchers to make their inevitable conclusion that ‘urea is thus considered to be a less reliable source of fertiliser N for crop production purposes than AN.’

These increased losses of ammonia not only impact directly on the UK’s ability to meet its global commitments to reducing ammonia emissions, compensatory increases in Nitrogen application rates with urea could make individual growers in breach of their individual NVZ requirements, so they are not without consequence.

Whilst urease inhibitors and coatings can reduce volatilisation from urea, concerns exist with regard to their shelf-life with some reports suggesting these are active for only a matter of weeks after manufacture.

With all urea used in the UK being imported, the effectiveness of these by the time product reaches the farm, is stored and used is increasingly under question.

Achieving quality output

A review of all available data by ADAS in 2015 concluded use of AN increased grain protein content by 0.4% compared to urea, thus illustrating its superior performance in milling wheats.

Defra has also reported that urea use lowers protein levels in wheat by around 0.3%. This has been replicated in other trials on many occasions.

Replicated experiments funded by Defra since 2003 have shown that at a rate of 200kg N/ha, AN yielded more than urea in 77% of cases and gave a yield advantage of more than 1t/ha in one case.

Overall, the conclusion reached by the Defra funded researchers in their summary to the extensive three years of in-depth work behind their NT26 research programme is that ‘AN remains the best option for UK agriculture’.

For more advice on this important decision contact our expert team of FACTS qualified staff who are on hand to provide you with the latest advice in fertiliser management and nutrient plans.