Leaving a small part of land untouched by the fertiliser spreader this spring could give you an invaluable insight into how to manage your nitrogen to the benefit of the environment and your business profitability in the future, CF Fertilisers told The Scottish Farmer.

Its national trials programme, involving 50 arable producers up and down the country, is encouraging all of them to leave just four square metres 'free' from the fertiliser spreader so they can get an accurate assessment of their nitrogen fertiliser use efficiency (NfUE).

“If you really want to know how effectively your N fertilisers are being used, you need to start by knowing what happens to your production if you don’t use any,” explained CF Fertilisers head of agronomy, Dr Sajjad Awan.

“NfUE is the critical measure of how much of every kg of N fertiliser you apply actually ends up in the plant where it can stimulate growth, rather than getting lost from the system. It’s expressed as a simple percentage and with the higher the value, the more N is finding its way into plants. In a UK wheat crop, it’s typically in the range of 50% to 80%," he pointed out.

“But an NfUE of 50%, which does exist in some situations, means for every 1kg of N you buy, half of it is effectively not being utilised and that’s a not a good result for the environment or your farm’s profitability.”

Table 1

Typical range of NfUE in UK arable crops

Lower than 50% Poor

50 – 60% Fair

60 – 80% Good

Over 80% Excellent

Encouraging early results

The ongoing trials programme being carried out by CF Fertilisers, in conjunction with Agrii and Frontier, is focused specifically on setting a benchmark for current levels of NfUE achieved in the UK and looking at how these can be improved, he explained.

“The sample size represents some 20,000ha of UK wheat production across the country and the first year of trials has already shown some very interesting results. NfUE can be influenced by a number of factors including how good your soil testing is, the type of fertiliser you apply, how often you apply it and of course, how much you actually put on.

“Ammonium nitrate (AN) fertilisers typically achieve the highest NfUEs as they minimise loss of N to the atmosphere, but it’s becoming increasingly apparent how important the accuracy of the soil testing you use is.

“CF N-min soil tests produced a level of accuracy approaching 80%, whereas with estimates of soil mineral nitrogen (SMN) this was just 45% and as low as 33% for RB209-based soil N estimates.”

Table 2

Production and environmental shown in NfUE trials

This accuracy of soil analysis combined with encouraging the growers taking part in the trials to adopt some basic N management principles have translated into significant performance improvements, he added.

“In one trial, we saw fertiliser savings of up to £180/ha being recorded and the average savings across all trials was £40/ha. The average reduction in carbon footprint was nearly 70kg/ha CO2.

"In terms of the all-important NfUE, most farms taking part saw an improvement in this with an average of 7% achieved but with the highest lift being 22%. Despite an average reduction of 11% in N fertiliser rates, there was no yield penalty, indeed the average yield lift resulting from this improvement in NfUE was 90kg/ha.

“From an environmental perspective, the improved NfUE led to a reduction in the soil N balance of on average 23kg N/ha, meaning there was much less risk of N escaping to water and air, compared to lower NfUE systems.”

How NfUE is calculated

Leaving a patch of land free of any fertiliser applications is key in accurately establishing NfUE, said Dr Awan.

“The vital knowledge we can gain from understanding more about NfUE hinges on being able to remove all other N sources from the more commonly used nitrogen use efficiency (NUE) calculation. In that way, growers can see precisely how their bought-in fertiliser is performing.

“At the optimum N application, we’re looking at the amount removed by the crop, ie grain and straw, minus that would have been taken up if no N fertiliser had been applied,” said Dr Awan.

NfUE can be calculated using the following method, when you have a ‘Nil-N’ plot included in your trial: “This is where the four square metres comes in. You can simply place a tarpaulin over your desired patch whenever fertiliser spreading takes place and this ensure your ‘Nil-N’ plot receives no fertiliser application through the season. It’s very simple,” he added.

“Harvest 1m2 grains and straw from this plot. The N content of the grain and straw from this plot can be compared to that of the crop where the full application has been used and, working with your agronomist or FACTS qualified adviser, you can use this to calculate relative N uptake by the crop and establish your NfUE.

“If you don’t have a ‘Nil-N’ plot, NfUE can also be calculated by measuring total crop N offtake at harvest and then subtracting your soil nitrogen supply, and then dividing this by the amount of fertiliser applied,” pointed out Dr Awan.

Moving forward

Much is already known about how to maximise NfUE but with the environmental need to use N more efficiently and recent price rises in all fertiliser sources, learning more about it is essential, Mr Awan believed.

It is possible to get to 80% or higher NfUE and the systems doing this are based on accurate soil N measurement through CF N-Min, using AN fertiliser, making little and often applications, and ensuring other nutrients, including sulphur, are optimised.

“But there is much more to learn. NUE is important in understanding how all your N sources contribute to crop production but only NfUE can help you make the right fertiliser choice and help you fine-tune your management.

“The four square metres of land that had no fertiliser applied could be the start of thinking completely differently about their future N management for many UK growers, as we hope our ongoing NfUE trials will continue to show,” argued Dr Awan.