Varying nitrogen application according to the greenness of the crop
can lift yields, writes Alan Mayfield
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
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
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, firstname.lastname@example.org