Yara Technical Manager, Phil Burrell describes the reasons behind choosing a variable rate nitrogen strategy and looks at some specific case studies of the N-Sensor in action.
Site-specific fertilisation is one of the main objectives of precision agriculture. Variable-rate nitrogen application requires accurate and efficient tools to determine the actual crops nitrogen status. Remote sensing techniques offer the opportunity to deliver this information quickly, precisely and cost-efficiently. The N-Sensor has been developed to determine the crop nitrogen status by measuring the light reflectance properties of crop canopies and to enable variable-rate fertilisation “on-the-go”.
The N-Sensor determines the nitrogen demand by measuring the crop’s light reflectance covering a total area of approximately 50m2 / sec. Measurements are taken every second with the system designed to operate at normal working speeds and all bout widths. Most sensing technology applied to agriculture is based on the typical light reflectance curve for vegetation (NDVI). N-Sensor measures light reflectance at specific wavebands related to the crop’s chlorophyll content and biomass. It calculates the actual N-uptake of the crop. Optimum application rates are derived from the N-uptake data and sent to the controller of the variable rate spreader or sprayer, which will adjust fertiliser rates accordingly.
The whole process of determining the crop’s nitrogen requirement and application of the correct fertiliser rate happens instantaneously, with no time delay. This enables “real-time agronomy and application” to be possible.
Following development coordinated by Yara’s Research and Development Centre, Hanninghof in Germany, the first N-Sensor (Classic) was introduced in 1999 for use on cereals.
Work to develop the N-Sensor to keep up with changes in cereal production as well as for use on a wider range of crops and has been a continuous part of Yara’s R&D Programme. More than 250 trials have been carried out between 1997 and 2010 to refine its performance and add new programs such as the Absolute-N calibrations for oilseed rape.
In 2006, Yara launched the new N-Sensor ALS (Active Light Source), which works in a similar way to the classic N-Sensor to determine a crop’s nitrogen demand by measuring the crop’s light reflectance. Both systems make use of the same field trial based agronomic algorithms for optimum site-specific fertilisation and both are connected to a vehicle terminal where crop and GPS data is stored for processing.
The major difference between the two N-Sensors is that the ALS Sensor has its own built-in light source. Instead of using daylight for the measurement, the N-Sensor ALS is constantly beaming its own source of light at the crop, using Xenon flash lamps, and recording the reflectance. This enables N-Sensor ALS operation independent from ambient light conditions, even at night.
In 2018 the ALS 2 was launched. During research and development, Yara had recognised the issue of damp leaves (dew) that affects all sensors and the reflectance accuracy achieved. Research was targeted to overcome this issue, culminating in the requirement to capture extra waveband data during sensing. This ‘dew suppression’ is unique amongst sensors and is a feature in the ALS 2 N Sensor. A new modular design and improved connectivity gives the ALS 2 more flexibility and easier mounting options.