Effective uptake of nitrogen fertilisers to optimise yields, maximise spending and minimise losses is a key challenge for the arable sector. Alice Dyer explores how growers can make their nitrogen work harder.
Although the term Nitrogen Use Efficiency (NUE) is a term many growers will be familiar with, there is sometimes a lack of understanding of what it actually means. Nitrogen use efficiency includes all forms of nitrogen that are available to the plant, both organic from the soil, and mineral fertilisers from a bag.
With no single solution to improving NUE, Natalie Wood, arable agronomist at Yara, says growers must undertake a complex number of actions.
“In terms of organic nitrogen, it is a bit of a guessing game as to how much will be mineralised over winter from the soil and therefore you can’t give a definitive figure on this without soil mineral N analysis.”
The amount of mineralisation to occur depends on the soil and weather conditions over the winter period with warmer weather lending itself to higher mineralisation, but large amounts of rainfall leading to anaerobic conditions through waterlogging, which will decrease potential mineralisation.
“As a grower we can’t really affect the rate of mineralisation apart from ensuring we have good levels of organic matter and nice aerated soils to get the best chances,” says Ms Wood.
This means it is all the more important to focus on factors that can be influenced. These are: right product, right timing and right rate, says Ms Wood.
“Starting with product, we need to choose a product that has the characteristics we require. We want to get the product on to the crop as soon as it starts growing at the end of February – if it is growing then there is an immediate demand for nitrogen.”
Using a product that contains nitrate nitrogen means it is immediately available to the crop and does not have to undergo any conversion processes first.
Urea, for example, has to undergo a conversion to ammonium and nitrate before the plant can utilise it, which can take up to six weeks in cold conditions, says Ms Wood.
“Another important part of product choice is whether you require sulphur – with up to 97% of UK soils being deficient in sulphur then it is likely that you do, so growers should consider a nitrogen sulphur (NS) or an NPKS product.”
There is a close relationship between nitrogen and sulphur within the plant meaning that there needs to be sufficient amounts of both for each to be utilised efficiently within the crop.
“If you don’t have enough sulphur then the nitrogen you have applied won’t be taken up as efficiently, therefore your NUE percentage will start to drop,” Ms Wood says.
“If we need sulphur, it makes sense that sulphur is applied little and often with nitrogen to improve the NUE.”
The first application should go on as soon as the crop starts to grow, usually around the end of February.
If conditions are fit to travel, then getting the first nitrogen and PKS on early means that the crop can get off to a good start in the spring and build plenty of biomass before the ‘biomass cut-off’ date of around the end of March when the plant switches to its reproductive phase.
The rate of nitrogen will depend on crop type and market; however, it is important not to apply more than the crop requires.
Taking feed wheat as an example, Ms Wood says: “Usually 220kgN/hectare is a good target to base your calculations on. The first application at the end of February should be about 70-100kgN to influence that biomass growth. Second dressing will be similar at about 100kgN/ha.
“The last dressing is where you are able to fine tune the rate as the majority of the nitrogen has already gone on. There are various tools that can help with this final application, including the Yara Bluetooth N-Tester. This device measures how much nitrogen is in the crop and gives a recommendation for that final dressing based on this – it’s a great way to take the guesswork out of that final application rate.”
Another option is variable nitrogen applications. Yara’s N-Sensor scans the crop and identifies real-time what the nitrogen levels are and then adjusts the spreader/sprayer accordingly.
“This gives a much more even yield across the field,” says Ms Wood.
Variable application can also be done using Yara’s AtFarm software - a currently free, web-based programme that allows users to map their fields and get variable application maps based on the NDVI biomass; which then can be imported to variable-rate spreaders/sprayers.
It uses the algorithm that is in the N-Sensor to get results not solely based on NDVI, which can be saturated at certain growth stages.
Once the crop is harvested, grain analysis will give a rundown of the levels of each nutrient within the grain as well as the N:S ratio.
Ms Wood says: “The results are a good way to look back on your nutrition strategy and see what worked and potentially what didn’t. For example, low levels of a particular nutrient could indicate that more is required next season to rectify this deficiency.”
Jorin Grimsdale, of Mountfair Farming, Berwickshire, used AtFarm for the first-time last spring alongside his N-Sensor.
He says: “I loaded up the boundary information which was surprisingly quick and simple and decided to try and create some variable rate nitrogen maps with it to see what it would do.”
Growing winter wheat, spring oats, winter OSR and spring vining peas and beans across 2,200ha, Mr Grimsdale used the software to generate prescription maps for nitrogen using the N-Sensor’s data.
“I also used it to look at the biomass of the crops. It shows you where the crop biomass is through the spring, and then you can apply Yara’s N-sensor protocol information. This creates algorithms and tells you where it thinks you should apply the nitrogen, just like our tractor does which has an N-Sensor in real time. It was very useful for monitoring the growth and to see what was going on.”
Having used the N-Sensor for over 10 years, Mr Grimsdale says it has been a valuable tool for managing early growth and evening of crops.
He adds: “It is also excellent in oilseed rape in the way it applies nitrogen using the absolute mode by measuring the crops nitrogen status. This is simple to use and precise.
"Atfarm is another tool to aid in crop agronomic decisions; bringing together the N-Sensor and satellite technology in an easy to use format allowing both crop biomass and growth motoring with the ability to tailor nitrogen applications.”