N-Sensor put to the test

Allan Mayfield and the N-Sensor

Varying nitrogen application according to the greenness of the crop can lift yields, writes Alan Mayfield

The Southern Precision Agriculture Association (SPAA) is trialling an N-Sensor to see whether a variable rate of nitrogen produces a higher yield than a constant rate when used post-emergence in cereal crops.

This N-Sensor, developed by the European company Yara, is now used commercially in Europe by 350 growers and contractors, but is not yet available for sale in Australia.

Fitted to the roof of the tractor, it scans crops and automatically regulates the rate of nitrogen fertiliser spread according to the greenness of the crop.

Photo: Allan Mayfield and the N-Sensor: promising results.

This multispectral scanner measures the light reflectance properties of the crop canopy. This is in the range of 450 to 900 nanometres (nm). in the visible wavelength range (400 to 700nm) the reflectance is indicative of leaf chlorophyll content and hence the crop nitrogen status. In the near-infrared region (700 to 900nm) higher reflectance is indicative of greater crop biomass.

The N-Sensor consists of two diode array spectrometers: one collects the crop reflectance data on both sides of the tractor, the second measures the irradiance conditions at the same time to correct the reflectance signal for changes in sun angle and cloud cover. Normal operating time is from 10am to 4pm. The system is controlled through a terminal that displays current system information and logs crop and GPS data on a chip card to produce a paddock map.

A pre-determined rate of fertiliser is set in the control box after scanning a small part of the crop. After that the fertiliser rate will vary around this value. More fertiliser is applied where the crop is paler green; less where it is darker. The unit can be reset to work the other way: to apply more fertiliser to darker green areas and less to paler green areas. Alternatively, crops can be scanned earlier; for example, when spraying herbicide. Crops can then be checked to identify causes of yellowing before applying the fertiliser.

The research question is which, if any, is the right decision? This will depend on the cause(s) of the crop yellowing and the ability of different parts of the crop to respond economically to more nitrogen fertiliser.

There is also a low biomass "cut-off" setting, which prevents excessive amounts of fertiliser being applied to parts of the crop that may be thin due to other factors, such as poor emergence or pest damage.

To test this concept, the N-Sensor will be used to apply strips of nitrogen fertiliser at a variable rate and then at a constant rate alongside (using the same total amount of fertiliser). These tests will be across a range of soil types and crop conditions. Plant samples will also be taken from different areas of the crop to check whether nitrogen is the main limiting factor or whether the yellowing is due to other cause(s). crop yields of the variable and constant fertiliser strips will be assessed from yield maps.

Grain samples will also be taken from the different treatments and zones to compare grain proteins. These trials will be repeated in 2006. in preliminary trials at three sites in South Australia in 2004, yield increases with variable compared with constant rate in wheat ranged between zero and four per cent. The average increase in trials in Europe is about three per cent. Grain protein in these European trials was also slightly higher with variable rate application.

The N-Sensor is also being used to map variation across crops in New South Wales, Victoria and South Australia. Information gathered will be used with other crop and soil details to better define zones for differential management across paddocks.

The N-Sensor is being trialled in Europe for other possible uses, including variable rate desiccation of crops with herbicides and variable rate application of fungicides according to differences in the crop biomass across the paddock. That is, more is being applied to thicker parts of the crop, less to thinner parts.

GRDC Precision Agriculture Initiative (SIP09)
GRDC Research Code: SPA00003

For more information: Dr Allan Mayfield, 08 8842 3230, mayfield@capri.net.au