By Dr Daniel Murphy, Dr Nui Milton and Gina Pemberton, Centre for
Land Rehabilitation, The University of Western Australia
Understanding the capacity of soil microbes to produce nitrogen naturally
is becoming a key to better nitrogen (N) management in cropping soils.
Anywhere from 40 to 80 percent of the crop N requirements can be met
through microorganisms breaking down residues and organic matter to release
plant-available N (biological soil N supply). The remaining N requirement
is met through fertilisers.
Improving N fertiliser management is therefore a matter of knowing the
timing and location of biological soil N supply, so that fertiliser N
is only applied when and where necessary.
Splitting applications of fertiliser N at strategic plant growth stages
is already becoming common. This maximises crop uptake at the right time
and minimises the risk of nutrient leaching.
Growers can also spatially adjust fertiliser rates in the field using
information from yield mapping (knowing the best/worst performing areas
of a paddock) and by soil type.
The next extension of this approach is to utilise spatial soil maps that
tell us about the soil’s capacity for biological N supply, together
with information on the soil’s chemical and physical fertility.
For example where biological soil N supply is high, less fertiliser N
may be needed to achieve optimum yields. Alternatively where biological
soil N supply is low, there is a greater reliance on fertiliser N.
To develop a practical decision-support tool for fertiliser application
rates, we are exploring the use of midinfrared technology to develop calibration
curves for a range of soil properties – biological, chemical and
The advantage of the midinfrared technology is that once calibrated,
soil samples can be collected from the field and scanned rapidly –
two minutes per sample.
Mid-infrared is not as accurate as measuring each soil property by standard
techniques, but it does have a place in the development of soil spatial
maps to identify sources of crop constraint.
In soil that was collected under an oat crop in 2003 at Dangin, Western
Australia, using a 25m x 25m sampling grid (180 separate sampling points
over 10ha), biological soil N was measured using standard laboratory methods
(figure 1) and also predicted using mid-infrared technology (figure 2).
Figure 1: Standard laboratory detection of biological soil N
supply – data from soil samples (0-10 cm) collected on
a 25m x 25m sampling grid. Colours represent data categorised into 4 ranges,
where red = very low biological soil N supply, yellow = low, light blue
= moderate and blue = high.
Figure 2: Mid-infrared predicted biological soil N supply –
data for the same 10ha area as that shown in Figure 1 (ie, the pattern
of colour on Figure 2 would be identical to that on Figure 1 if the mid-infrared
prediction was 100% accurate). The same colour groupings apply.
There was strong agreement between measured (figure 1) and mid-infrared
predicted (figure 2) values of biological soil N supply.
The data suggests optimum crop yields would require extra fertiliser
in the red and yellow areas. In a good rainfall year, low fertiliser N
application would also benefit the light blue area. In the dark blue areas,
soil N is already sufficient.
This work, however, is still at the development stage, and more research
is needed to test the transferability of calibration curves among different
regions and soil types.
For more information:
Dr Daniel Murphy, 08 9380 7083, email@example.com
GRDC RESEARCH CODE UWA395, program 4