Yara N-Sensor: John Hawthorne's experience of it

The N-Sensor was just three years old when he took delivery of his first unit, the one requiring ambient light to operate.

“We farm about 4,500 acres with cropping which majors on winter wheat and oilseed rape,” he explains. “We alternate crops year-on-year, a regime that suits our heavy ground.”

He adds that, as far as he is concerned, the defining moment for the N-Sensor came in the second year after he had used it to control the nitrogen application on his winter wheat.

“All of my neighbours’ crops went down flat just before harvest while ours remained upright,” he says. “The point is we didn’t apply any less nitrogen, we just put it in the right place in the right amounts. I reckon in that year the N-Sensor paid for itself several times over.”

Since then, an ALS version has been acquired, enabling fertiliser to be applied irrespective of light conditions.

Liquid fertiliser

A user of liquid fertiliser, applications are made using a 5,000 litre Chafer 618S sprayer with a 40m boom - a width that has made Mr Hawthorne keen to adopt a dual-sensing system that would allow different rates to be applied over the width of the boom.

“The unit only looks at a square about 5m wide on each side and that has to be representative of the whole 40m boom,” he says.

To achieve the variation in application rates required by the N-Sensor, without extremes of pressure, the sprayer is fitted with a large diameter single line which feeds a set of low volume nozzles and/or a set of high volume nozzles.

By using a permutation of these nozzles, on their own or in pairs, the application rate can be altered automatically in respect of changes in forward speed and/or rate change instruction from the N-Sensor.

Mr Hawthorne used a Yara’s two-application programme for his oilseed rape for the first time two years ago. It was based on applying the difference between the expected nitrogen uptake of the plant and a target N/ha application rate.

For the first application the expected nitrogen uptake is 60kg/ha and the target is 150kg/ha. If the N-sensor detects the colour and canopy is such that it estimates the uptake has been 60kg/ha then it will proceed to allow 90kg/ha of nitrogen to be delivered from the spreader or sprayer.

If the measured uptake is lower, say 50kg/ha, then the application rate increases to 100kg/ha; a 65kg/ha uptake would result in 85kg/ha application rate and so on.

Limits

“We set minimum and maximum limits for application rate to 40kg/ha and 120 kg/ha respectively,” Mr Hawthorne explains.

For the second application made when the crop is at its ‘booting stage’, the expected uptake of nitrogen is 140kg/ha and the target is now a total of 240kg/ha.

As before, it is a matter of making up the measured differences using the N-sensor with the maximum and minimum rates set at 40kg and 150kg/ha.

There are ways of fine tuning this – for a lower yield of say, less than 4t/ha, the fertiliser recommendation is reduced by 20kg/ha and for a plus 5t/ha yield, it is increased by 20kg/ha.

The same principle is used to assess nitrogen application to wheat, using different totals and in a three-application strategy.

“We only make two applications of nitrogen to wheat so we do not conform quite so well to that programme. Instead, we

apply a target rate with the N-Sensor calculating the actual rate as the job proceeds,”Mr Hawthorne adds. “For oilseed rape, though, we had the best yields ever last year, averaging 4.1 tonnes/ha across 2,300 acres.”

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