Backing a winner
In the first trial of its kind in the UK, ammonia losses from a range of fertilisers were assessed to see what fertiliser type and application method best suit UK conditions
How do you pick a winner? Do you study the form and assess the runners and riders in the paddock? Or, do you prefer to look at the odds and choose an outsider; one which just might give you a better return on your stake?
"If we're talking about a modest bet at the local races or point-to-point then it probably doesn't matter," suggests GrowHow's arable agronomist Allison Grundy. "At the end of the day while backing a winner feels pretty good, you should only bet what you can afford to lose."
But, what if you are gambling with the yield and quality of your customers' crops? That's a very different matter and it is one where studying the form and gaining an in-depth understanding of all the options will help minimise the risk. When it comes to crop protection chemistry such an approach is second nature to every adviser and agronomist but what about crop nutrition and, particularly, nitrogen? How do you decide whether to
recommend urea or ammonium nitrate?
"That decision is actually a much easier one that you might think," suggests Miss Grundy. "Comprehensive and ground-breaking research from a consortium of topnotch R&D organisations has provided solid evidence about the relative risks of using urea and ammonium nitrate. The problem is that the findings from this research, which was funded by DEFRA, have not been particularly widely reported," she comments.
One reason for this was that the research wasn't actually done to help farmers and advisers. It was primarily commissioned to find out if there were any viable alternatives to
ammonium nitrate fertilisers as part of the Government's anti-terrorism activities. "The
simple answer was nothing comes close to AN under UK climatic conditions and, I guess, that wasn't really the answer that the Government was hoping for!" research ADAS led an impressive consortium of independent research organisations - Rothamsted Research, North Wyke Research, Edinburgh University, Warwick HRI, Queens University Belfast, SAC and the Silsoe Research Institute. The project, known as NT26 ran from 2002 to 2005 with the field work conducted in 2004 and 2005.
ADAS senior principal scientist, Professor Brian Chambers was one of the experts involved. "Everyone knows that ammonia can be lost by volatilisation following urea application, reducing the amount of crop available nitrogen," he says. "There is, however, a widely held belief that if urea is applied early when the soil is moist and temperatures relatively cool
this tendency to lose ammonia will be minimised. Quite simply this research did not support this theory."
The scientists measured a 58% loss of ammonia from urea applied to a clay grassland soil in Devon on 2nd March 2005. At the nearby sandy soil site the loss was 43% and that went on even earlier on 28th February.
The situation was very similar in winter cereals. "From the 'early' season urea applications to winter cereals ammonia emissions ranged from 10% up to 43% (application dates were between 23rd March and 19th April). The mean was 22%. Going early prevents volatilisation? Not according to this data.
LOSSES
The benefit of going early is not the only myth that this research uncovered. Accepted wisdom is that ammonia losses are only a significant problem on calcareous soils. "Over the whole project, ADAS Gleadthorpe and Rothamsted recorded the two highest average ammonia losses at 35% and 43% respectively and these can hardly be described as calcareous soils," explains Professor Chambers.
Prior to this new research, accepted figures suggested that the average loss of nitrogen as ammonia from urea in arable crops was 11.5%. Once again the research findings disagreed with the accepted wisdom. The research showed that the average loss across all arable sites was almost twice that figure at 22%.
So are these losses a problem? Surely all you have to do is apply 22% more urea and crops will get all the N they need. There are two difficulties with this argument. First of all the average figure is just that, an average. What it hides is the huge variation in the losses. These ranged from just 2%, which is about what you can realistically expect to lose from an AN application, right up to 43%. The scale of the losses depends on factors which are largely outside the adviser and farmer's control - soil moisture, temperature and rainfall.
So working out when to increase the rate and by how much is a huge challenge. The second is the environmental impact of using urea. At a time when farmers are being asked to 'produce more and impact less', how can the use of an input which might release almost half of its ammonia to the atmosphere be justified. Ammonia losses are a key environmental concern because ammonia contributes to acid rain and the enrichment of sensitive habitats such as heathlands. In addition, under the National Ceilings Directive the UK has committed to reducing its ammonia emissions to less than 297,000 tonnes ammonia (NH3) by this year.
NT26 RESEARCH PROJECT
- DEFRA funded project, 2002 to 2005
- Conducted by a consortuim of independent experts, led by ADAS
- Compared ammonium nitrate (AN) with urea, liquid urea/AN (UAN) and calcium ammonium nitrate (CAN)
- Also examined the effect of urease inhibitors on ammonia losses from urea and UAN
- Accurately measured ammonia losses in field for the firs time using sophisticated wind tunnel devices
- Also measured crop yields and N off-takes
- Measured emissions at 10 winter cereals sites with seven different N rates used on all sites so optimum N rate could be calculated
- Measured emissions at five grassland sites
For the first time in the UK ammonia losses from a range of fertilisers have been accurately measured using sophisticated wind tunnels. There were three replicates of each treatment and equipment was moved daily to ensure that the fertilisers were exposed to ambient environmental conditions
To try to mitigate the losses from urea a number of products are available to slowdown
the breakdown process by which urea releases ammonia. The research looked at one of these urease inhibitors, Agrotain.
Inhibitors slow down hydrolysis and so reduce the pH rise around the urea granule which causes ammonia to volatilise. The research concluded that they do definitely help. However, as the chart shows (see figure below), even with an inhibitor the average loss of ammonia was still considerably higher than the losses associated with ammonium nitrate.
There are two other misconceptions about urea which can cloud the decision making process. Because urea is 46% N compared to ammonium nitrate which is 34.5% N, the perception is that by choosing urea there will be less product to handle and store.
GrowHow's marketing analyst David Beck explains: "What farmers are worried about is the volume of product, not the weight. To assess the volume you need to look at bulk density. Nitram has a density of 980kg/cubic metre whereas urea only has a density of 750kg/cubic metre. This means that a 2000 litre fertiliser spreader, when full, will contain 676kg N with Nitram and 690 kg N with urea, so there is hardly any difference between the two."
Last but by no means least urea is generally thought to be a cheaper source of N than ammonium nitrate. "But the difficulty is that we are not comparing apples with apples," says Mr Beck. "The unpredictable nature of urea means that there are often, but not always, yield losses associated with its use. By applying additional urea N these losses can be avoided but then applying more, costs more."
The DEFRA research concluded that to maintain yields and quality the optimum N rate for urea was on average 20% higher than for AN. Mr Beck continues: "Looking at this another way, all the figures suggest that the average yield loss associated with using urea in winter wheat is around 4% but it could be as much as 15%. Based on a wheat price of £90/t and Nitram at £200/t the financial penalty of using urea rather than AN is anywhere between £0/ha and £115/ha loss."
Miss Grundy has the last word: "In many parts of the world urea is an excellent choice but under UK climatic conditions going down the urea route is a high risk strategy. While on average 22% of the ammonia in urea will volatalise and pollute the atmosphere, it could be as much as 43% of ammonia that is lost to the crop. Losses at these levels will have significant implications for crop yields and quality. On the other hand if everything goes right with the weather and soil conditions urea can perform as well as ammonium nitrate. At the end of the day I suppose it all comes down to just how much of a gambler you are."
Reproduced by kind permission of Farm Business Agronomist
