Ensuring crops effectively use all available nutrients to maximise growth and minimise losses to the environment is one of the biggest challenges now facing arable producers. Alice Dyer looks at practical steps towards making optimum use of nitrogen fertilisers.
The effectiveness of how a crop uses nitrogen can be determined by measuring Nitrogen Use Efficiency (NUE) - a term used to indicate how much available nitrogen a plant can successfully take up.
Poor establishment techniques, reduced soil health, or applying fertiliser at a time not related to crop growth, leading to denitrification or volatilisation can all have a negative effect on NUE.
Improving nitrogen use efficiency not only reduces greenhouse gas emissions but input costs on farm.
Inorganic fertiliser applications were responsible for 23% of agricultural ammonia emissions in 2016 according to Defra, and increasing political pressure to reduce emissions could mean growers will be forced to rethink their fertiliser plans.
With the Environment Bill highlighting nitrate emissions as a key driver in climate change, fertiliser limits are set to be put in place by November this year, at a level which reflects the costs of fertiliser application to the environment at farm, local, regional or national level.
This is a similar model to Denmark where ammonia emissions were reduced by 40% between 1990 and 2016 after applied nitrogen levels were limited to 18% below the economic optimal level.
The Government’s Clean Air Strategy 2019 proposes a course of action for farmers to switch from urea-based fertilisers to ammonium nitrate, unless used in conjunction with urease inhibitors, or applied by injection.
Improving nitrogen use efficiency all starts with the soil, according to biological soils expert George Hepburn of QLF Agronomy.
“There is an ideal scenario for your soil – we want 45% minerals, 5% organic matter, 25% air and 25% water. If you haven’t got your soils well-structured in the first place, then you’re not providing a good home for the biology which breaks down a lot of applied fertilisers and nutrients to make them readily available for the plant.”
In order to maximise efficiency of inputs, Mr Hepburn says there’s an increased movement towards ‘wrapping’ them in carbon.
“If you’re not applying a carbon source with nitrogen fertiliser, it will use the carbon source that is already in your soil which is organic matter. We’re lucky in this country because we’ve got good inherent fertility in our soils from our history of mixed farming. Our organic matter levels buffer this, but over the years we’ve been burning this carbon off with fertilisers and heavy cultivations.”
Farmers, alongside their agronomist should look at a farm fertility plan which ensures each field, every year has a dose of carbon through inputs like compost, lime, gypsum, digestate or cover crops, says Mr Hepburn.
“All of these things are fertility-building for the soil, whereas nitrogen fertiliser is the opposite. When you apply nitrogen, to actually utilise that nitrogen you need to add carbon with it to balance the carbon to nitrogen ratio. Compost and farmyard manure not only puts NPK on, you’re also getting organic matter and more carbon, and adding some beneficial bacteria and fungi.
“We use Boost, a liquid carbon fertiliser. It’s a molasses-based product with yeast that acts as a food source for the bacteria and fungi in the soil, and at the same time it buffers the effect of some of the harmful things that fertilisers do. We’ve seen nitrogen fertiliser reductions of 10-15% without detriment to yield.”
AHDB-funded ADAS to undertake a review looking at how no-till cultivation methods influence the requirement for autumn application of nitrogen fertiliser in arable cereal crops.
Dr Kate Storer, crop physiologist, ADAS, says: “The average efficiency for spring applied nitrogen is about 60%, so not very efficient. But that could vary from less than 30% to at least 80%, and there are all sorts of factors which can affect it.”
Using data from Denmark and Northern France, the review also considered how different autumn cultivations and tillage systems would affect the uptake of spring nitrogen applications.
Dr Storer says: “We reviewed evidence from previous European studies on the effect of min-till to less than 10cm and how this affected crop N uptake when compared with mouldboard ploughing. There appeared to be limited or no effect throughout the season on the recovery of spring nitrogen applications when you compared different tillage systems.”
With a growing interest in no-till farming, the review delved deeper into nutrient requirements in a zero tillage scenario.
Dr Storer says: “The study found no direct evidence that nitrogen applications in the autumn for cereals were beneficial under no-till situations. It should be recognised that there wasn’t much direct evidence available, but this conclusion was also supported by an analysis of the typical soil supply and crop demand of N in the autumn. Situations with no-till and lots of incorporated straw are likely to have the lowest amount of available N in the soil, and new trials are needed to test the value of autumn N in these scenarios.”
Matching fertiliser supply to the demand of the crop may minimise immobilisation of nitrogen in the soil which might improve N uptake efficiency. Applying more fertiliser than the crop requires reduces uptake efficiency, according to Dr Storer. Over-application can be reduced by estimating how much nitrogen is going to become available in the soil by using lock-up tables or measuring soil N.
To maximise NUE an even balance is needed between application rates and removal of nitrogen by the plant.
The development of satellite imagery technology gives growers the option to review variations in crop canopy development from a series of images, to manage nitrogen applications, says Simon Griffin, technical manager, Soyl.
“Looking at the data, growers can see where a crop is thicker, or thinner and taking up less nitrogen, and then vary nitrogen inputs across the field accordingly.”
Yield benefits of 4-9% were seen when applying nitrogen based on changes in biomass identified by satellite imagery, but last year the company undertook a series of trials in winter wheat across the country, applying variable rates of nitrogen to one half of all fields, and the farm’s uniform rate on the other.
A soil survey was also carried out on each field.
“Using soil conductivity surveys to measure a soil’s physical characteristics showed us where clay content or soil depth changes,” says Mr Griffin. “This is important relating to N supply because a heavy clay soil supplies more than a light soil would, and if that varies across the field we can factor it in.”
All the fields showed a higher NUE for the half that had N applied variably than the half of the field that had N applied at a flat rate.
“This is as a result of matching the nitrogen applications to crop demand and growth,” says Mr Griffin. “The whole crop will get the same amount of N, just some will come from the soil, and some will come from a bag.
“Improving nitrogen efficiency requires a joined-up approach across the whole farming year based on what’s most efficient. It starts with variable seed rates related to the underlying changes in the soil texture to get an even plant population across the field – if we can get an even plant number, it sets the crop up to utilise N in the spring.”
Applying nitrogen in smaller doses and more frequently to match the growth of the crop as it progresses through the season, may contribute to an improvement in NUE by reducing the risk of leaching and volatilisation. This is according to Dr Sarah Kendall, research scientist at ADAS, who is involved in the Yield Enhancement Network.
“The little and often approach has been something that farmers who perform well in the YEN competition often talk about but from a scientific perspective it’s a bit of a knowledge gap.
“When we look at the rate of nitrogen applied there is a weak positive association. This means for N rates above the average nitrogen rate applied in the data set, there is an improvement in yield of 6kg of grain per kg of nitrogen applied.
“When we look at the number of nitrogen applications there seems to be a stronger association with yield, and there’s some indication that potentially applying five or six nitrogen splits could be more beneficial for yield. This will probably come as a surprise to the industry that there isn’t more of a stronger association with increased nitrogen rates and yields.”
However, Dr Kendall warns growers that this might not be the case in all situations.
“It’s important to understand that growers involved in YEN are going to be applying the correct amount of nitrogen, so generally they’re doing best practice or beyond. If sub-optimal rates of nitrogen are applied then this is likely to have a negative effect on yield. By trying to fine tune the nitrogen rate, if you’re already in the right ballpark area the potential for that improving yield is probably less than fine tuning how you apply it.”
A similar relationship has been seen in oilseed rape data.
“The data is more limited, but we think there’s potential for the message to be the same and that managing canopy correctly through optimising nitrogen timings is very important if your nitrogen rate is close to where it should be.”
Getting the most out of bought-in fertilisers will be one of the most important management areas for producers in the coming years, says CF Fertilisers arable agronomist Allison Grundy.
“Increasing NUE improves profitability and protects the environment. While NUE is a good overall efficiency indicator of crop nutrient recovery, it is probably more useful to look at NfUE when making fertiliser decisions,” she says.
Nitrogen Fertiliser Use Efficiency (NFUE) is used to describe the recovery of applied nitrogen fertilisers for a clearer picture of the impact fertiliser choices are having on production efficiency.
In milling wheat trials carried out by arable research contractors Armstrong, applying zero N to trial plots produced 4.2 t/ha whereas applying the optimum N rate of 254kg N/ha produced 10.4t/ha.
“The N offtake - N in the crop at harvest - was 68kg N/ha for the crop with no fertiliser added and 257kg N/ha where the optimum amount was applied giving a final equation of 257 – 68 divided by 254 which equals 74%,” says Ms Grundy.
While season plays a large part in the recovery of nutrients, decisions made during the application period all affect the final recovery rate.
“In reviewing our own trials and ADAS literature to set the correct parameters for the CF N-Calc N application rate calculator, we’re seeing NfUEs of around 70% for winter wheat but many factors can affect this with product choice being important,” says Ms Grundy.
In the Armstrong trials, Nitram (34.5% N) outperformed straight urea in terms of NfUE across all trials conducted in two different production years and at all yield levels, adds Ms Grundy.
“Looking at the data from six trials shows an average NfUE for the AN of 74% compared to 66% for urea.
“This difference of 8% NfUE is the equivalent of an additional 16% total loss of nitrogen from urea and in crops with an application rate of 200kg/ha N this would be equivalent to a loss of 32kg/ha N.
“NfUE analysis dispels the continued sentiment that whilst urea volatilises N to the air, AN leaches to groundwater so the overall environmental impact from both products is similar.
“It’s just not the case. The N losses through volatilisation from urea are far greater than any perceived spring losses with AN through loss via the soil.”
Other nutrients such as phosphate, potash and sulphur not only affect plant health and metabolism, they also have a significant effect on NfUE, adds Ms Grundy
“If indices of P and K are not maintained deficiencies can work against N recovery and utilisation. Adequate sulphur is also required to drive N utilisation.”